Early inhibition and late reversal of harpin initiated cell death program by growth regulator auxin. (A) Inhibition of HR cell death by auxin. Harpin (panel 1), Harpin in 50 μM 2,4-D (panel 2), or 50 μM 2,4-D (panel 3) was infiltrated into the apoplastic space of leaves of Nicotiana tabacum CV. Samsun NN, and symptom development was recorded 36 h later. (B) HR cell death program can be reversed till late in the process. Auxin (50 μM 2,4-D) – (panel 1) or buffer (panel 2) was infiltrated in the same area where harpin was infiltrated 6 hours earlier. The difference in appearance of the symptoms between Fig. 1(A) and 1(B) is due to the differences in the way the tissue dries after second infiltration of buffer. Harpin was used at 1 – 2 μM and purified as described earlier . Symptoms were recorded 24 h after the first infiltration.