Flooding is one of the major abiotic stresses on rice production. To overcome stress, some rice varieties employ two distinct strategies, submergence tolerance, and submergence escape. Deepwater rice takes the escape strategy to avoid flooding stress by initiating immediate and rapid stem elongation upon submergence. We have been searching for genes responsible for submergence escape from a deepwater rice cultivar. After a series of time-course microarray analyses under submergence, we found a small gene family consisting of five hypoxia-responsive genes. Among the genes, OsHIGD2 showed rapid induction by hypoxia as well as submergence. We confirmed that OsHIGD2-GFP fusion proteins are localized to mitochondria under a confocal microscope by comparing Mitotracker fluorescence. Coexpression analysis using the database (RiceFREND) revealed that OsHIGD2 is highly co-regulated with hypoxia responsive genes such as those encoding alcohol dehydrogenase 1 (ADH1), pyruvate decarboxylase isozyme 2 (PDC2) and ethylene responsive element binding protein (AP2-EREBP). Yeast two-hybrid screening suggested that at least six proteins may physically interact with OsHIGD2, most of which are related to regulation of redox potential or fermentation. All results indicate that OsHIGD2 is a mitochondrial protein and plays an important role in hypoxia response by participating in the regulation of redox potential.
