Kinase domain-targeted isolation of defense-related receptor-like kinases (RLK/Pelle) in Platanus × acerifolia: phylogenetic and structural analysis
© Pilotti et al.; licensee BioMed Central. 2014
Received: 18 March 2014
Accepted: 18 November 2014
Published: 8 December 2014
Plant receptor-like kinase (RLK/Pelle) family regulates growth and developmental processes and interaction with pathogens and symbionts.
Platanaceae is one of the earliest branches of Eudicots temporally located before the split which gave rise to Rosids and Asterids. Thus investigations into the RLK family in Platanus can provide information on the evolution of this gene family in the land plants.
Moreover RLKs are good candidates for finding genes that are able to confer resistance to Platanus pathogens.
Degenerate oligonucleotide primers targeting the kinase domain of stress-related RLKs were used to isolate for the first time 111 RLK gene fragments in Platanus × acerifolia. Sequences were classified as candidates of the following subfamilies: CrRLK1L, LRR XII, WAK-like, and LRR X-BRI1 group. All the structural features typical of the RLK kinase domain were identified, including the non-RD motif which marks potential pathogen recognition receptors (PRRs). The LRR XII candidates, whose counterpart in Arabidopsis and rice comprises non-RD PRRs, were mostly non-RD kinases, suggesting a group of PRRs. Region-specific signatures of a relaxed purifying selection in the LRR XII candidates were also found, which is novel for plant RLK kinase domain and further supports the role of LRR XII candidates as PRRs. As we obtained CrRLK1L candidates using primers designed on Pto of tomato, we analysed the phylogenetic relationship between CrRLK1L and Pto-like of plant species. We thus classified all non-solanaceous Pto-like genes as CrRLK1L and highlighted for the first time the close phylogenetic vicinity between CrRLK1L and Pto group. The origins of Pto from CrRLK1L is proposed as an evolutionary mechanism.
The signatures of relaxed purifying selection highlight that a group of RLKs might have been involved in the expression of phenotypic plasticity and is thus a good candidate for investigations into pathogen resistance.
Search of Pto-like genes in Platanus highlighted the close relationship between CrRLK1L and Pto group. It will be exciting to verify if sensu strictu Pto are present in taxonomic groups other than Solanaceae, in order to further clarify the evolutionary link with CrRLK1L.
We obtained a first valuable resource useful for an in-depth study on stress perception systems.
In animals, cell surface receptor tyrosine kinases (RTKs) and serine/threonine kinase receptors (STRKs) are of primary importance for the perception of intercellular signals and the transduction of signals at an intracellular level. In general the domain composition of these receptors is defined by an extracellular ligand-binding domain, a single-pass trans-membrane domain and a cytoplasmic tyrosine or serine/threonine kinase domain [1, 2]. In the plant lineage, the counterpart of RTKs and STRKs is represented by the family of receptor-like kinases (RLKs), which share the same domain composition . The RLK kinase domain, a serine-threonine kinase, belongs to the same gene family as the Drosophila melanogaster Pelle and mammalian interleukin receptor-associated kinases. Thus the term RLK/Pelle was established [3, 4].
With regard to RLK/Pelles, a striking difference between the land plant and non-plant eukaryote genomes is the size of this family, in fact 329 genes have been identified in moss (Physcomitrella patens), 610 in Arabidopsis, 647 in tomato, 1192 in poplar (Populus trichocarpa), 1070 in rice, 1 and 4 in the animals Drosophila melanogaster and Homo sapiens respectively, and none have been identified in fungal organisms [5, 6]. It has been suggested that one of the evolutionary reasons for the gene family’s expansion derives from the need to cope, as sessile organisms, with an ever-changing environment, including a multitude of mutating pathogenic and would be-pathogenic microbes [4, 5, 7]. This would account for the variety of RLK/Pelle types - 53 subfamilies categorized in Arabidopsis - for the genetic redundancy present in part of the subfamilies, and for the up-regulation of hundreds of RLK/Pelle members under biotic stress [3–5, 7].
Some members within the plant RLK/Pelle family, have been shown to be transmembrane receptors [8, 9] and it is likely that this is the main functional contribution of these proteins . Two functional groups are recognized in plants: RLKs controlling growth and development processes, and RLKs regulating interactions with microbes, both pathogens and symbionts. The rice (Oryza sativa) Xa21 is one example of the latter class, which confers resistance to the bacterium Xanthomonas oryzae pv. oryzae. Other examples include EFR (EF-TU RECEPTOR) of Arabidopsis thaliana which is a PAMP (Pathogen Associated Molecular Pattern) receptor , and Arabidopsis RFO1 (RESISTANCE TO FUSARIUM OXISPORUM 1) which confers broad spectrum resistance to diverse Fusarium oxisporum races and Verticillium longisporum[12, 13].
A functional dualism may involve BRI1 (BRASSINOSTEROID-INSENSITIVE1). This RLK/Pelle has an important role in the regulation of growth and development through the perception of brassinosteroids (BRs) [14, 15]. Its involvement in stress responses is suggested by the fact that BRs are also strongly related in stress signaling . Furthermore, a tomato BRI1 ortholog, SR160 (SYSTEMIN RECEPTOR 160), is able to bind systemin, a solanaceous specific protein which induces systemic wound responses following attack by feeding insects  although it does not represent the functional systemin receptor [18, 19].
A lack of extracellular domain is a characteristic of another class of the plant RLK/Pelle family, the receptor-like cytoplasmic kinases (RLCKs) . Pto (for Pseudomonas syringae pv. tomato) is a well-known member of this class and confers resistance to the bacterial pathogen, P. syringae pv tomato in wild tomato species, according to a host genotype/pathogen race specificity fashion . Great interest in Pto orthologs in plant species has arisen from the fact that the overexpression of Pto has been shown to confer a broad spectrum resistance in tomato .
Platanaceae is a tree family that is considered as one of the earliest branches of Eudicots  (http://www.mobot.org/MOBOT/research/APweb/.). In contrast, Platanus × acerifolia (Ait.) Willd. is a species of this family which originated in the 17th century from a cross between individuals belonging to the P. occidentalis L. species group and P. orientalis L. [23, 24]. Therefore, P. × acerifolia (Pac) combines genomic contributions from the two parental species, which evolved separately for long geological periods . The high haploid chromosome number, n =21, suggests that the current Platanaceae had ancient polyploid origins  which adds to the overall complexity. Given these features, investigations into the RLK/Pelle family in Pac can provide information on the nature of this gene family in the Dicots, especially when compared with the Arabidopsis counterpart.
Platanaceae suffer from several diseases including canker stain, a vascular disease caused by the fungus Ceratocystis platani (J.M. Walter) Engelbr. & T. C. Harr., and antrachnose, caused by the fungus Apiognomonia veneta (Sacc. et Speg.), which are particularly destructive and have heavily conditioned the diffusion and cultivation of Platanus spp. [26, 27]. Thus unravelling the resistance response of Platanus to pathogens is necessary in order to sustain and speed up current time-consuming genetic selection programmes [24, 28]. The plant RLK/Pelle family is thus a good candidate for finding genes that are able to confer resistance to pathogens.
The aim of this study was to isolate RLK/Pelle genes in Pac which, based on knowledge of Arabidopsis, play a potential role in pathogen perception and signal transduction. We successfully applied a PCR-based strategy, which targeted a representative region of the kinase domain.
We isolated for the first time RLK/Pelle gene fragments in Pac, with an uninterrupted open reading frame (ORF). Sequence analyses clearly showed that the genes were, in most cases, candidates of the following RLK/Pelle subfamilies: CrRLK1L, LRR XII, WAK-like, LRR X-BRI1 group. A number of structural motifs and the analysis of non-synonymous versus synonymous substitution rates, confirmed the phylogenetic identification and provided an insight into their potential functions. In fact, the detection in Pac of signals of a completely relaxed purifying selection in sub-regions of the kinase domain, is another important finding of this study.
As we isolated CrRLK1L-L using primers designed on Pto of tomato, we also wished to clarify the relationships between Pto-like of several plant species available in the GenBank, the bona fide Pto family of solanaceous species and CrRLK1L. As a result a close phylogenetic relationship was revealed between Pto and CrRLK1L.
Plant material, nucleic acid extraction and primer design
Genomic DNA was isolated from young leaves collected in the spring from a potted Pac tree (accession MS.12) using a Plant Midi Kit (Qiagen, Hilden, Germany) according to the manufacturer’s instructions.
Degenerate primers were designed in order to isolate RLK/Pelle candidate genes for pathogen perception and signal transduction: 1) known R genes (Pto, Xa-21, Xa-26, RFO1); 2) PAMP receptor genes (EFR); 3) the whole BRI1 gene group. The primers were designed within the kinase catalytic domain. They were generated from the nucleotide stretches conserved in the model gene and several plant homologs, available from the NCBI GenBank database. In the case of RFO1, a WAK-like gene, collected sequences were used to infer a phylogenetic tree in order to identify clades of homologous WAK-like genes from which distinct sets of primers were generated. We then used a similar strategy in order to search for BRI1-like genes.
All the information regarding primer is reported in Additional file 1.
Amplification and cloning of gene fragments
PCRs were performed in a 50-μl reaction volume containing each dNTP at 200 μM, each primer at 1 μM, 2.5U of Platinum Taq DNA Polymerase High Fidelity (Invitrogen), 2 mM MgSO4, and 100–200 ng of genomic DNA as a template, in the buffer supplied by the manufacturer. The initial denaturation step was carried out at 94°C for 2 min, followed by 35 amplification cycles (94°C for 1 min, annealing at 47°C for 50 s, an extension at 68°C for 1 min) and a final extension step at 68°C for 15 min. Analysis of PCR products, cloning and sequencing was as previously described . A variable number of clones per cloned DNA band were sequenced in both directions (Bio Fab Research) (Additional file 2). RLK/Pelle-like sequences were deposited in the GenBank database [GenBank: EU722764-EU722869, EU722871-EU722900, HQ425329-30] (Additional file 3).
Alignments and phylogenetic analyses
Clone sequences were trimmed of vector and primer sequences and were then considered for further analysis. To select gene fragments representing the kinase-catalytic region, the amino acid sequences were compared with UniProt using blastp . Multiple sequence alignments were performed with MAFFT . Pairwise local alignments were performed using the EMBOSS Pairwise Alignment suite  (gap open: 10; gap extend: 0.5; matrix: Blosum62).
Since we isolated portions of the full length ORF, we classified the Pac sequences according to the nomenclature by Shiu and Bleecker  or the name of the gene for which a potential orthology relation was evident, followed by the code -L (= − like).
Phylogenetic analysis was based on the MAFFT alignment of the amino acid sequences of the kinase-catalytic domain. Trees were inferred using the Neighbor-joining (NJ) method , with 1,000 bootstrap replications. The evolutionary distances were computed using the Poisson correction method with rate uniformity among sites  and were in the units of the number of amino acid substitutions per site. Analyses were conducted in Mega version 6 .
Phylogenetic analyses were also conducted with two maximum-likelihood (ML) methods: i) Whelan And Goldman substitution model - WAG , with 1,000 bootstrap replications and rate uniformity among sites (analyses were conducted in Mega version 6). ii) ML approach implemented in the TREE-PUZZLE software package , with the following parameters: VT (variable time) substitution model , gamma-distributed rates of heterogeneity among sites, amino acid frequencies inferred from the data set, branch lengths estimated without the molecular-clock assumption, 50,000 puzzling steps to infer branch support values for the relative majority consensus tree.
To classify Pac sequences we made a phylogenetic comparison with representative members of all the subfamilies of the Arabidopsis RLK/Pelle complement . Then sequences were divided into homology groups and each group was compared with the closest whole RLK/Pelle subfamily.
We also performed a comparative phylogenetic analysis in order to shed light on the relations between the Pto group , Pto-like of Solanaceae, Pto-like of other dicot and monocot plants, CrRLK1L, and the Pto-primer-derived sequences obtained in this work.
Apart from Arabidopsis, genes from nine additional full genome-sequenced species and partial sequences from a number of plant species were included in the phylogenies for comparison. All the reference sequences and those used to root the trees are reported in Additional file 4.
In the phylogenetic trees, sequence names contain: I) the acronym of the protein or, secondly, the locus tag or the accession number, II) the Latin name of the species, and III) the RLK/Pelle subfamily to which the protein belongs. Latin names were abbreviated as follows: Arab. thal. = Arabidopsis thaliana, Caps. = Capsicum, Cath. = Catharantus, Frag. = Fragaria, Mu. = Musa, Nicot. tab. = Nicotiana tabacum, Ory. = Oryza, Phas. = Phaseolus, Pop. = Populus, Pot. = Potentilla, Ric. = Ricinus, Sol.pimp. = Solanum pimpinellifolium, Sol.lyc. = Solanum lycopersicum, Vit. = Vitis, Physc.patens = Physcomitrella patens.
Domain detection and determination of other structural features
The phylogenetic classification at the subfamily level was validated by comparing some representative Pac sequences with long isotigs generated from a 454 GS-FLX transcriptome data set of the same species, though using a different genotype (Pilotti, Brunetti, Iacono and Pindo, unpublished data) (Additional file 5). Putative extracellular domains of the isotigs were identified as described above.
We compared the activation segment of Pto and BRI1 with the corresponding region of the deduced protein of the entire Pac RLK/Pelle-like sequence complement and the corresponding RLK/Pelle subfamilies of Arabidopsis.
Non-synonymous versus synonymous substitution rates
The ratio of non-synonymous (Ka) to synonymous (Ks) nucleotide substitutions was calculated for pairwise comparisons using a 75 bp sliding window with a step size of 15 bp. The analysis was based on codon-delimited nucleotide alignments and performed with JCoDA  which uses the PAML (yn00) suite  and the Yang and Nielsen substitution model . JCoDA was also used to identify the individual sites under positive selection (site by site): the site models M7 and M8, which account for transitional rate bias and unequal codon frequencies, were applied, and the differences were assessed with a likelihood ratio test (d.f. = 2). The tree topology required by the two models was inferred from the amino acid sequence by the maximum likelihood approach using the Phylip package , with a JTT substitution matrix  and rate heterogeneity approximated using a discrete γ-distribution with four categories. The posterior probabilities of individual sites belonging to the class Ka/Ks >1 were calculated using the Bayes empirical Bayes approach implemented in PAML.
Codon-delimited alignments were also used to map those positions that were subject to non-synonymous variations in order to: i) identify those that were shared among the different phylogenetic LRR XII-L groups of Pac and LRR XII of Arabidopsis, ii) to describe the amino acid variability at each position.
Amplification pattern and oligonucleotide efficiency
Generally PCRs based on degenerate oligonucleotide pairs always gave a clear amplification pattern with minimum or no smearing. Evident expected-sized DNA bands were always present and a variable number of clones were sequenced for each band. In some cases additional non expected bands of variable intensity were also present but these were not investigated further.
RLK/Pelles obtained in Pac
Homologous RLK/Pelle subfamily
I → IX (27)
II → IX (1)
I → VIII (33)
I → VII/VIII (1)
I → VIII/IX( 7)
I → IX (17)
I → XI (4)
LRR X-BRI1 group
I → IX/X (4)
I → XI (3)
I → VIII/IX
I → VIII
I → VIII
I → VIII/IX
I/II → VIII
Phylogenetic analyses for RLK/Pelle subfamily identification
By using reference sequences of full genome-sequenced species, we initially verified that highly/well supported clades did not change between phylogenies based on the partial kinase domain and those inferred with the complete kinase domain or the full ORF. A comparison with CrRLK1L and WAK-like phylogenies in other studies [49, 50] further confirmed the robustness of our methodology (data not shown).
A highly supported clustering showed that close homologs of CrRLK1L, LRR XII, WAK-like and LRR X-BRI1 group, had been isolated (Additional file 6A and B).
All ML analyses - Mega 6 and TREE-PUZZLE implemented - confirmed results of the NJ analyses (Additional file 10).
To further confirm the reliability of the phylogenetic classification at the subfamily level, we compared selected sequences of Pac, for each homology group, with ten isotigs generated from a 454 transcriptome dataset derived from P. × acerifolia. Nearly all the isotigs spanned the whole region represented by PCR RLK/Pelle fragments and matched them with a high/maximum nucleotide identity score. In the isotigs all the putative extracellular domains typical of CrRLK1L, LRR XII, WAK-Like and LRR X were detected thus confirming the phylogenetic identification (Additional file 5).
Re-classification of plant Pto-like genes
The apparent failure to collect potential orthologs of Pto gene family members, prompted us to investigate the existence of sensu strictu Pto orthologs (or close homologs) in plant species using NJ and ML phylogenetic analyses. We used two approaches.
In the second approach we compared the complete CrRLK1L subfamily of those species in which this gene set had been characterised (i.e. arabidopsis, poplar, tomato, and rice) with Pto locus (see Additional file 4 for sequence sources). Moss (Physcomitrella patens) was also included in the analysis as a representative of basal lineages of land plant. CrRLK1L sequences of moss were retrieved from NCBI GenBank using CrRLK1L of Arabidopsis and Pto as query. The vast majority of the sequences grouped together in highly supported multi-species sub-clades confirming that in CrRLK1L, orthologs are more similar than paralogs. On the contrary Pto and paralogs and the other ortologs from tomato database formed a highly supported Solanaceae-specific clade. The phylogenetic analysis was performed with NJ and two ML methods, which gave the same results (Additional file 12).
Comparison of non-solanaceous Pto-like partial sequences with Pto and close relatives of CrRLK1L
Close phylogenetic relative
chilo-4 Fragaria chiloensis
pot-6 Potentilla tucumanensis
paj-1 Fragaria × ananassa
Pot-5 Potentilla tucumanensis
SG5 Phaseolus vulgaris
OG5 Phaseolus vulgaris
SG2 Phaseolus vulgaris
Tg-10 Musa acuminata
Tg-11 Musa acuminata
Tg-12 Musa acuminata
Tg-13 Musa acuminata
Tg-21 Musa acuminata
Tg-67 Musa acuminata
Structural features of the kinase domain: typical, atypical, RD and non-RD kinases
The RLK/Pelle-like fragments of Pac spanned subdomain I to subdomain VIII-XI (Table 1). Invariant residues of kinases were all conserved except in five sequences (belonging to LRR XII-L, WAK-like-L and LRR VII-L), which were therefore classified as atypical kinases  (for details see Additional file 13).
Based on the crucial role of the RD motif in the catalysis , we assessed the nature of Pac sequences as RD or non-RD kinases, by verifying the presence/absence of the conserved Arg (R) immediately preceding the invariant catalytic Asp (D) in subdomain VIB of the catalytic domain (Additional file 13). Most possessed the RD-Arg. Thirty-nine sequences out of 111 (35%) showed the feature of non-RD kinases. Of the 34 LRR XII-L sequences, 31 were non-RD kinases, and the vast majority (29) had a Cys in substitution of Arg (Figure 2). A single WAK-like-L sequence was also a non-RD kinase (Figure 3). Seven additional non-RD kinases were found among the RLK-like belonging to the unwanted subfamilies (Additional file 8). Features of the non-RD motif of Pac sequences were strongly conserved in the reference sequences of the other plant species, monocot and dicot, with which they grouped (Figure 2, Additional file 8). Specifically Arabidopsis LRR XII were nearly all non-RD, with a Cys substituting Arg.
In the CrRLK1L(−L), WAK-like(−L) and LRR XII(−L) groups of Pac and Arabidopsis, the MEME-based consensus motifs were clearly different from the tyrosine motif and instead there was a clear vicinity with the serine/threonine or the dual specifity motif (Additional file 14).
Analysis of the activation segment
In protein kinases, the activation segment is the catalytic region and is made up of two recognizable regions: the T-loop in which regulatory autophosphorylation often occurs and the P + 1 loop, which forms the primary binding site for the substrate . The function of several residues, in the activation segment of Pto and BRI1, has been clarified [56, 57]. Our objective was to compare the activation segment of Pto and BRI1 with the corresponding regions in the Pac and Arabidopsis CrRLK1L(−L), WAK-like(−L) and BRI1(−L). The LRR XII(−L) sequences were not analyzed as they did not span the entire region of the activation segment.
A search for the motifs in the above cited sequences, highlighted that many or all residues recurred in the sequence complements (Figure 6, Additional files 16, 17 and 18). The motifs were also partially conserved in BAK1, ERECTA and FLS2, which were included in the analysis for comparison (Figure 6).
Given the results of the phylogenetic reconstruction and the analyses of structural features (RD, non-RD, typical, atypical motifs and the activation segment), we defined the RLK/Pelle-like of Pac as bona fide RLK/Pelles.
Non-synonymous versus synonymous substitution rates, non-synonymous positions and their amino acid variability
In the site by site analysis, the likelihood ratio test did not indicate an overall significantly better fit of the M8 site model (positive selection) versus the M7 model (P > 0.05). The Bayes empirical Bayes method revealed a total of six sites with Ka/Ks ratio >1, in the three sequence groups of Pac, although with small posterior probability values (range 56–73%) when accounting for the standard error.
Genome-wide analysis for the characterization of gene families in fully-sequenced species is invaluable for an in-depth comprehension of the information contained in the genomes (see for example: [4, 65, 66]). It is also a necessary support for gene candidate analyses such as those based on domain-targeted isolations of gene groups, in species lacking genome sequencing. We thus based our study on a detailed comparison with RLK/Pelles of fully-sequenced species, especially Arabidopsis, and devised a method for the correct identification of RLK/Pelle gene fragments.
In this study we present some substantial advances on plant RLK/Pelles. We isolated for the first time a set of RLK/Pelles in Pac. Interestingly, the analysis of sequence data also revealed: i) a novel phylogenetic relationship between the Pto group and the CrRLK1L RLK/Pelle subfamily; ii) clear signatures of relaxed purifying selection in the LRR XII-L, which had not been previously revealed for plant RLK kinase domain.
The representativeness of the kinase domain for RLK/Pelle classification has been widely shown [3–5, 7, 67, 68]. Kinase-domain targeted isolation of RLK/Pelles has also been found to be successful [69–75]. In this work we confirmed its representativeness for a correct classification, by matching the cloned sequences with long 454 isotigs from Pac, which displayed the expected extracellular domain signatures (Additional file 5).
CrRLK1L candidates and the relationship between CrRLK1L and Pto
All the analyses highlighted our apparent failure to collect sensu strictu Pto-like genes in Pac and showed that the analysed non-solanaceous plant genomes lack a bona fide Pto gene group. Interestingly, in the phylogenetic analyses, Pto group was closest to CrRLK1L than any other RLK subfamily. Note that i) best matches of Pto, belonging to six fully-sequenced dicot species and to rice, were revealed as CrRLK1L; ii) all non-solanaceous Pto-like partial sequences were classified as CrRLK1L-like; in this regard it is explicative the high closeness of Pto-like partial sequences from bean and cultivated/wild strawberries to CrRLK1L extracted from bean and F. vesca genomes (Figure 5, Additional file 11). All the dicot species that we used in the Pto/CrRLK1L comparative analysis were Rosids, except the solanaceous species which belong to Asterids, and Pac, whose formation predates the split giving rise to Rosids and Asterids (http://www.mobot.org/MOBOT/research/APweb/). All this clearly points to the restriction of Pto gene group to a range of taxa within Dicots, possibly due to a relatively more recent origin than other ubiquitous RLK/Pelle subfamilies. On the other hand a more precise evaluation of the taxonomic restriction of Pto-like genes is needed and would require sequencing additional plant taxa representative of Asterids. The taxonomic restriction of an RLK group is not surprising as species-specific RLK/Pelle subfamilies have previously been reported [5, 7].
The CrRLK1L subfamily is an ancient group of RLK/Pelles with a conserved extracellular domain and whose formation predates the divergence of vascular plants [5, 50]. This was confirmed by our phylogenies which clearly showed that orthologs are more similar than paralogs, and suggests that events giving rise to family members occurred prior to the monocot-dicot split and after the divergence rate was low (Figures 1 and 5, Additional files 7 and 12).
Strong evidence thus suggests that Pto genes evolved from ancient CrRLK1L, through the loss of the extracellular domain: i) the ubiquity of the CrRLK1L in the land plant and the conservation of this family over a long geological period [5, 50]; ii) the restriction of Pto and its paralogs to asterid species (i.e. Solanaceae, based on current knowledge) (Figure 5, Additional files 11 and 12); iii) the close vicinity between the kinase domain of the two RLK/Pelle groups (Figures 1 and 5, Additional files 6 and 12) . Interestingly, the structure of the Pto group is also similar to CrRLK1L in that orthologs are more similar than paralogs . Recently the tomato RLK/Pelle family has been characterised and the phylogenetic comparison with Arabidopsis showed that a CrRLK1L subfamily exists in tomato . Interestingly we noted that Pto group was included in the CrRLK1L clade. Though this was not discussed by the authors it represents a full confirmation of the results of our phylogenies . Phylogenetic vicinity between the kinase domains of RLK and RLCK subfamilies has been reported, and domain reassortment has been repeatedly emphasized as an ancient mechanism of RLK/Pelle evolution in the land plant lineage [4, 5, 77]. CrRLK1L is one of the Arabidopsis RLK/Pelle subfamilies which has corresponding RLPs (receptor–like proteins, i.e. proteins resembling the extracellular domain of RLKs), suggesting that these RLKs and their RLPs gave rise or were each derived from their counterpart . In this scenario, our conclusion on the formation of RLCKs - Pto and paralogs - from RLKs - CrRLK1L - is thus in agreement with the accepted RLK/Pelle gene evolution model.
The vicinity between Pto group and CrRLK1L raises the question if they have to be classified as bona fide CrRLK1L or represent the founding members of a novel RLCK/Pelle subfamily, related to CrRLK1L. This will be a matter of investigation and debate in the future.
The analysis of the activation segment was also useful for shedding light on the relationship between CrRLK1L(−like) and Pto (Figure 6). The functional motifs identified in the activation segment of BRI1 and Pto (Figure 6, Additional file 15), were partially overlapping and were conserved among a wide range of RLK/Pelles. This highlights an interesting point which fits the special case of CrRLK1L/Pto well: the conservation of functional residues can be useful for supporting identification at a high hierarchical level but it is not sufficient to suggest a relationship of putative orthology, even when this conclusion is sustained by phylogenetic vicinity - as is the case of the non-solanaceous Pto-like vs solanaceous Pto group [73, 74].
The recovery of a CrRLK1L(−L) gene set in Pac, is still an interesting finding, also in the context of defense-related RLK/Pelles. For example THE1 mediates the response of plant cells to the perturbation of cellulose synthesis by triggering growth inhibition, ectopic lignin accumulation and the expression of a set of defense-related genes. THE1 is thus considered as a sensor of cell wall integrity and a role in pathogen defense has been postulated [78, 79]. Another suggestion for the role of CrRLK1Ls in pathogen defense is that, in Arabidopsis, the bacterial effector AvrPto directly binds to the CrRLK1L member AT2G23200, suggesting a significant, yet undiscovered role, in innate immunity .
LRR XII candidates: nonRD motif and relaxed purifying selection highlight a potential link with pathogen resistance
In Arabidopsis and rice the LRR XII subfamily includes PRRs functioning as PAMP receptors and R genes [10, 11, 80] and it has been suggested that this is the general functional contribution of the subfamily. An LRR XII-L group was clearly identified in Pac (Figure 2, Additional file 6). The wide nucleotide diversity range among all Pac sequences (nucleotide score: 62–98) and a comparison, in the phylogenetic tree, of the branching pattern of Pac and Arabidopsis sequences, suggest that an undetermined number of paralogs had been isolated (Figure 2).
An important feature shared by the Arabidopsis and Pac sequence complements is their nature as non-RD kinases and the high frequency of Cys in substitution of the RD-Arg (Figure 2), which strongly supports the phylogenetic identification. Non-RD kinases are present in the kinomes of a wide range of Eukaryotic organisms and a statistically significant and positive correlation was found between kinases that function in the innate immunity and the non-RD motif . This finding, together with the fact that Arabidopsis and rice LRR XII contain PRRs, suggests that we have identified a group of genes that is potentially involved in the perception of pathogens.
Further support for this hypothesis is that we also detected signatures of a relaxed purifying selection in Pac sequences (Figure 7). This finding is new for plant RLK/Pelles. In fact in Arabidopsis and rice, signatures of positive selection were detected in the extracellular domains [7, 82, 83]. In contrast, the kinase domain has been shown to be under a strong purifying selection .
Interestingly, only specific regions of the kinase domains were subject, in Pac, to an evidently relaxed purifying selection and the substitutions occurring in the the non-synonymous positions were largely shared by the different sequence groups of Pac (Figures 7 and 8, Additional file 20). This suggests the action of a selective pressure that tends to keep specific non-silent variations in regions which might be relevant for the fitness. Indeed these regions span zones which are crucial for the function of the kinase catalytic core: i) I-II subdomain, which is involved in anchoring the ATP phosphates; ii) VII-VIII subdomain, the activation segment; iii) V-VIA subdomain, which is the site of catalysis [84, 85]. The fact that the strength of purifying selection can be differentially modulated in the distinct functional regions of a protein coding gene is well known (e.g. [86, 87]), however it is novel for plant RLK/Pelles.
Purifying selection generally eliminates any mutations in order to preserve the exact function of essential genes. On the other hand, a relaxation of purifying selection and positive selection are considered as two non-mutually exclusive forces that drive the protein evolutionary rate towards divergence. In particular in the relaxed purifying selection, random mutations affect genes that are dispensable or redundant, therefore there is no stringent constraint to impede their retention. By chance, these mutations may turn out to be beneficial for morphological and functional specification [88–90]. In fact a relaxed selective constraint plays a role in the expression of phenotypic plasticity, which is one of the most important ways by which organisms adaptively interact with the environment . Specifically, the evolution of caste systems in polyphenic social insects , the fast evolution of lineage-specific genes in humans  and a variation in freezing tolerance among natural accessions of Arabidopsis have all been attributed to a relaxed purifying selection as the main driving force.
It is well known that effectors of pathogenic bacteria target the kinase domain of several RLK/Pelles with a role in plant immunity, giving rise to an “arms race” ending with host resistance or susceptibility [20, 62, 63, 93–96]. Thus in Pac, a relaxation of purifying selection in the kinase domain of LRR XII-L, may have enabled the recruitment of variants which are useful for the resistance response of Pac to some microbial (would be) pathogens. This might be an exciting research theme for the future.
WAK-like and BRI1 group candidates
WAK-like proteins link the extracellular matrix to the cytoplasm for appropriate signal transmission . Wak-like have been found to be involved in many functions including pathogen resistance [12, 98, 99] and development [100, 101]. Despite our initial aim to isolate a single WAK-like member - namely an RFO1 homolog - a WAK-Like-L gene complement was isolated in Pac (Additional file 6) and seemed highly representative of the phylogenetic structure of the Arabidopsis counterpart (Figure 3). Importantly, results of WAK-like phylogeny agree exactly with the phylogenetic comparison between WAK-like of Arabidopsis and rice, in which sequences clustered in a species-specific manner and with only one multi-species clade headed by Arabidopsis WAKL14 and WAKL21 (Figure 3) . However it remains difficult to predict the level of completeness of the gene complement isolated in Pac. In fact, compared with the Arabidopsis genome, an extensive expansion of the gene family has occurred both in rice and in poplar suggesting a lineage-independent evolution [5, 67]. In any case the formation of species-specific and multi-species clades, in the Arabidopsis/Pac (this work) and Arabidopsis/rice phylogenies , suggests that opposing evolutionary forces shaped this RLK/Pelle subfamily, which produced both genetic divergence and conservation. This is in line with the involvement of these RLKs in very different functions, i.e. development and pathogen resistance, which exploit genetic conservation and diversification, respectively.
In the light of our results, as RFO1 belongs to a species-specific clade, phylogeny-based identification of the potential RFO1 homologs/orthologs was not possible. However based on the branching pattern of the phylogenetic tree, clade I appears to be the group of genes in which it might be possible to search for homologs with RFO1-related functions, i.e. resistance to vascular pathogens (Figure 3). This is particularly important in Platanus species which are hyper-susceptible to the vascular pathogen C. platani. In Arabidopsis, a number of additional WAK-like are involved in stress responses, namely WAK1, WAK2, WAKL5 and WAKL7, which were found to be inducible by salicylic acid (SA) and by wounding, and to protect plants from detrimental effects during pathogen responses, such as high levels of SA [98, 100, 101]. In terms of finding orthologs of these WAK-like, clade II of Pac might be a candidate source, as it shares a 4-residue motif exclusively with the WAK1-headed clade, suggesting a genetic correlation (Additional file 9).
The topology of LRR X-BRI1-L phylogeny was the same as that found by Cano-Delgado et al.  based on full-length proteins of BRI1(−like) from plant species, which confirms once more that the kinase domain is of primary importance in RLK/Pelle subfamily identification and phylogeny. Similarly to CrRLK1(−L) phylogeny, the three major clades included the potential orthologs from different dicot and monocot species, implying that diversification within this gene group predates the monocot/dicot split (Figure 4). Our analyses suggest that potential orthologs of BRI1 group members were isolated in Pac. In Arabidopsis, BRLs are expressed in the vasculature where they have a role in provascular cell growth and contribute to establishing the xylem/phloem pattern through perception of the BR signal [102, 103]. In addition, BRLs also seem to be important in stress responses. In fact, after confirmation that BRI1 was not the functional receptor of systemin [18, 19] a BRL was suspected to be the true systemin receptor since BRLs match the highly specific localization of prosystemin and systemin perception in the vasculature [104, 105]. Several Pac diseases target vascular tissues, cambium and living bark, and the pathogens gain entry through wounds . Therefore, LRR X-BRI1-L of Pac are important candidates in the study of the host response to wound pathogens, such as C. platani.
The study of the RLK/Pelles of Pac led to interesting findings, highlighting some aspects of Pac as a model species. This is also supported by the representative position of Platanus within Dicot phylogeny, one of the earliest branches, temporally located before the split which gave rise to the main groups, Rosids and Asterids, which comprise the majority of cultivated Dicots (http://www.mobot.org/MOBOT/research/APweb/.).
In addition the region-specific signatures of relaxed purifying selection in the non-RD LRR XII-L genes, highlights that this gene group is a good candidate for investigations into pathogen resistance.
With regard to CrRLK1L/Pto, it will be exciting to verify in which taxonomic groups, apart from solanaceae, sensu strictu Pto genes are currently present in order to further clarify the evolutionary link with CrRLK1L.
Above all, the main objective of our work in Pac was to create the first valuable resource to analyze the RLK kinome expression of genotypes that resist or are overwhelmed by pathogens. We believe that this represents the first step in an in-depth study on stress perception systems and also in the identification of useful molecular markers for genetic mapping and selecting resistant genotypes.
Availability of supporting data
The data set supporting the results of this article is available in the NCBI GenBank repository, (see also Additional file 3 for accession numbers and the text of nt and aa sequences).
This research was supported by MIPAF—Ministero delle Politiche Agricole e Forestali, project: “Proteine e geni per la protezione delle piante dagli stress biotici e abiotici, Proteo Stress—Genomica e Proteomica per lo studio della risposta di resistenza in specie arbore, ciliegio e platano”. We thank Adrian Wallwork for language editing.
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