Overexpression of the aspartic protease ASPG1 gene confers drought avoidance in Arabidopsis

Drought is one of the most severe environmental stresses affecting plant growth and limiting crop production. Although many genes involved in adaptation to drought stress have been disclosed, the relevant molecular mechanisms are far from understood. This study describes an Arabidopsis gene, ASPG1 (ASPARTIC PROTEASE IN GUARD CELL 1), that may function in drought avoidance through abscisic acid (ABA) signalling in guard cells. Overexpression of the ASPG1 gene enhanced ABA sensitivity in guard cells and reduced water loss in ectopically overexpressing ASPG1 (ASPG1-OE) transgenic plants. In ASPG1-OE plants, some downstream targets in ABA and/or drought-signalling pathways were altered at various levels, suggesting the involvement of ASPG1 in ABA-dependent drought avoidance in Arabidopsis. By analysing the activities of several antioxidases including superoxide dismutase and catalase in ASPG1-OE plants, the existence was demonstrated of an effective detoxification system for drought avoidance in these plants. Analysis of ProASPG1-GUS lines showed a predominant guard cell expression pattern in various aerial tissues. Moreover, the protease activity of ASPG1 was characterized in vitro, and two aspartic acid sites, D180 and D379, were found to be key residues for ASPG1 aspartic protease activity in response to ABA. In summary, these findings suggest that functional ASPG1 may be involved in ABA-dependent responsiveness and that overexpression of the ASPG1 gene can confer drought avoidance in Arabidopsis.