Comparative characterization, expression pattern and function analysis of the 12-oxo-phytodienoic acid reductase gene family in rice

The 12-oxo-phytodienoic acid reductases (OPRs) belong to the old yellow enzyme family of flavoenzymes and form multiple subfamilies in angiosperm plants. In our previous study, a comparative genomic analysis showed that five OPR subfamilies (subs. I-V) occur in monocots, and two subfamilies (subs. I and II) in dicots. Here, a comparative study of five OsOPR genes, representing five subfamilies (I-V) in rice, was performed to provide insights into OPR biochemical properties and physiological importance. Comparative analysis of the three-dimensional structure by homology modeling indicated all five OsOPR proteins contained a highly conserved backbone with (alpha/beta)(8)-barrels, while two middle variable regions (MVR i and ii) were also detected and defined. Analysis of enzymatic characteristics revealed that all five OsOPR fusion proteins exhibit distinct substrate specificity. Different catalytic activity was observed using racemic OPDA and trans-2-hexen-1-al as substrates, suggesting OsOPR family genes participate in two main branches of the octadecanoid pathway, including the allene oxide synthase and hydroperoxide lyase pathways which regulate various developmental processes and/or defense responses. The transcript profiles of five OsOPR genes exhibited strong tissue-specific and inducible expression patterns under abiotic stress, hormones and plant wounding treatments. Furthermore, the transcriptions of OsOPR04-1 (OsOPR11) and OsOPR08-1 (OsOPR7), representing subs. I and II, respectively, were observed in all six selected tissues and with all above-stress treatments. This suggests that these two subfamilies play an important role during different developmental stages and in response to stresses; while the expressions of OsOPR06-1 (OsOPR6), OsOPR01-1 (OsOPR10) and OsOPR02-1 (OsOPR8), representing subs. III, IV and V respectively, were strongly up-regulated with abscisic acid (ABA) and indoleacetic acid (IAA) treatments in roots, suggesting these three subfamilies play an important role in responding to hormones especially ABA and IAA signals in roots.