Characterization of two novel subtilases from common bean (Phaseolus vulgaris L.) and their responses to drought.

Protein breakdown by proteases is basic to the plant response to abiotic stresses such as drought. A large number of genes encoding proteases or putative proteases exist in plants. Only a few of those involved in the response to drought have been characterized, and their regulation is poorly understood. We have identified two new subtilases from leaves of Phaseolus vulgaris L. cultivar Zorin, PvSLP1 and PvSLP2. PvSLP1 was identified at the gene level, using primers based on the gene sequence of the putative drought induced serine protease from Arachis hypogaea L. In P. vulgaris, expression of the PvSLP1 transcript did not change on water withdrawal. PvSLP2 was isolated and characterized at the protein level, together with complete gene and cDNA sequences. The deduced amino acid sequences of both PvSLP1 and PvSLP2 are characteristic of plant subtilases of the S8 family of clan SB. PvSLP2 shows 33% sequence identity to PvSLP1. Expression of the PvSLP2 transcript did not change on withdrawal of water, but its proteolytic activity in leaves increased, depending on the age and position of the leaf. In addition, the level of activity in senescent leaves of well watered plants was higher than in mature or young leaves. These results, together with the fact that PvSLP2 cleaves peptide bonds following an Arg residue, point to regulation of PvSLP2 subtilase activity at translational and/or post-translational levels and suggest a specific role in the response to drought and senescence.