Involvement of brassinosteroids and abscisic acid in spikelet degeneration in rice under soil-drying during meiosis.

Spikelet degeneration in rice (Oryza sativa L.) is a serious physiological defect, and can be regulated by soil moisture status and phytohormones. This study investigated the possibility that brassinosteroids (BRs) in collaboration with abscisic acid (ABA) involve in mediating the effect of soil-drying during meiosis on spikelet degeneration in rice. Three rice cultivars were field-grown and three irrigation regimes including well-watered (WW), moderate soil-drying (MD), and severe soil-drying (SD) were imposed during meiosis. The MD significantly decreased spikelet degeneration in comparison with the WW, due mainly to the alleviation in oxidative damage via enhancing ascorbae-glutathione (AsA-GSH) cycle activity in young panicles, and the SD exhibited the opposite effects. Enhanced AsA-GSH cycle strength, decreased oxidative stress and spikelet degeneration rate were closely associated with the synergistically elevated BRs and ABA levels in young panicles in the MD. In contrast, low BRs and excessive ABA levels led to an increase in spikelet degeneration in the SD. The three cultivars exhibited the same tendencies. The intrinsic link among AsA-GSH cycle, oxidative stress, spikelet degeneration rate, and BRs and ABA levels was further verified by using transgenic rice lines and chemical regulators. BRs or ABA play a unique role in regulating spikelet degeneration. Synergistically increased BRs and ABA levels in the MD could work together to strengthen AsA-GSH cycle activity, leading to the reduction in oxidative damage and spikelet degeneration. On the other hand, a severe imbalance between low BRs and excessive ABA levels contributes to the opposite effects in the SD.