Ca 2+ deficiency triggers panicle degeneration in rice mediated by Ca 2+ /H + exchanger OsCAX1a.

Increasing rice yield has always been one of the primary objectives of rice breeding. However, panicle degeneration often occurs in rice-growing regions and severely curbs rice yield. In this study, we obtained a new apical panicle degeneration mutant, which induces a marked degeneration rate and diminishes the final grain yield. Cellular and physiological analyses revealed that the apical panicle undergoes programmed cell death, accompanied by excessive accumulations of peroxides. Following, the panicle degeneration gene OsCAX1a was identified in the mutant, which was involved in Ca 2+ transport. Hydroponics assays and Ca 2+ quantification confirmed that Ca 2+ transport and distribution to apical tissues were restricted and over-accumulated in the mutant sheath. Ca 2+ transport between cytoplasm and vacuole was affected, and the reduced Ca 2+ content in the vacuole and cell wall of the apical panicle and the decreased Ca 2+ absorption appeared in the mutant. RNA-Seq data indicated that the abnormal CBL (calcineurin b-like proteins) pathway mediated by deficient Ca 2+ might occur in the mutant, resulting in the burst of ROS and programmed cell death in panicles. Our results explained the key role of OsCAX1a in Ca 2+ transport and distribution and laid a foundation to further explore the genetic and molecular mechanisms of panicle degeneration in rice.