Development and Characterization of Rice Lines Carrying both Sub1 and Anaerobic Germination Tolerance: SUB1A does not Inhibit AG

Accelerated coleoptile elongation in flooded soils allowed seedling shoots to reach the water surface to maintain gas exchange and CO2 fixation. Rice genotypes with tolerance of flooding during germination (anaerobic germination, AG) exhibited fast coleoptile elongation as an escape mechanism. On the other hand, tolerance of complete submergence during the vegetative stage, conferred by SUB1A, involved growth retardation to conserve energy for maintenance metabolism, with resumption of growth upon de -submergence. Combining genes controlling flood tolerance at these two stages with contrasting mechanisms was necessary in flood-prone areas for protection during germination under direct seeding and during vegetative stage. Breeding lines combining AG + Sub1 were used to determine the timing of the expression of SUB1A and elucidate its impact when combined with tolerance of flooding during seed germination and to evaluate seedling performance under complete submergence. Time-points for SUB1A expression in IR64-Sub1 and AG + Sub1 lines germinated under hypoxia or submerged for 30 h at vegetative stage showed that during flooding treatment, SUB1A and SUB1C expression was inhibited in young seedlings (< 6 d old) in IR64-Sub1 and at 2 and 4 d of seedling growth under hypoxia in AG + Sub1 materials. SUB1A was weakly expressed in AG + Sub1 starting at seedling age of 6 d following 1 d of complete submergence, indicating that expression of SUB1A may be stage-specific. Physiological tests showed higher percentage of survival of AG + Sub1 lines under hypoxia, comparable to that of AG tolerant check and significantly different from intolerant check IR42, indicating that the presence of SUB1A does not affect tolerance of flooding during germination. When seedlings were completely submerged, Sub1 expression was not inhibited by the presence of AG and tolerant genotypes showed suppressed elongation that was significantly different from intolerant check IR42. Despite their contrasting mechanisms of tolerance, combining AG with Sub1 provides multiple flooding tolerance from early crop establishment using direct seeding through to the vegetative stages.