Characterization of rice aspartic protease genes and induced expression by phytohormones and Xanthomonas oryzae pv. oryzae

Aspartic proteases (APs) are present in both monocotyledonous and dicotyledonous plants. APs consist of nine different families of proteolytic enzymes. APs play important functions in several physiological processes including defense. However, there are limited reports on APs in disease resistance in rice (Oryza sativa). Here, we report that 104 APs were discovered in rice genome. They can be classified into three clades (I, II and III) with APs from Arabidopsis thaliana, and Nymphaea colorata according to phylogenetic relationships. Tandem and segmental duplications were observed to lead to the expansion of OsAP genes in rice. Analysis of gene structure revealed that numerous OsAPs contain multiple exons and introns, however, several OsAPs harbor a single exon and no introns. Synteny analyses demonstrated that four AP genes among O. sativa, A. thaliana, and N. colorata may evolve from a common ancestor of land plants. Ten conserved motifs of amino acids were uncovered from the OsAPs, and two of the motifs are the catalytic sequences, with the conserved Asp-Thr and Ser-Gly amino acids. Within the OsAP promoter sequences, cis-regulatory elements responsive to light, abscisic acid, auxin, gibberellins, methyl jasmonate, and salicylic acid were identified. Stress-responsive elements and tissue-specific expressed elements were also found. Majority of OsAP genes were found to be expressed in diverse tissues and organs. Some of the OsAP genes were observed to be induced by brassinosteroids, jasmonic acid, and salicylic acid. Further, some OsAP genes can be induced by Xanthomonas oryzae pv. oryzae (Xoo), the cause of rice bacterial leaf blight. Among them, OsAP21 was strongly activated by the pathogen, suggesting its potential function in response to the bacterial agent. The results shed light in the potential functions of OsAP genes in rice disease resistance to Xoo.