Ca2+ Deficiency Triggers Panicle Degeneration in Rice Mediated by
Ca2+/H+ Exchanger OsCAX1a
Abstract
Increasing rice yield has always been one of the primary objectives of
rice breeding. However, panicle degeneration, a complex phenomenon
regulated by many genetic and environmental factors, often occurs in
rice-growing regions and severely curbs rice yield. In this study, we
obtained a new apical panicle degeneration mutant named ym48, which
induces a marked degeneration rate and diminishes the final grain yield.
Cellular and physiological analyses revealed that the apical panicle in
ym48 undergoes programmed cell death, accompanied by excessive
accumulations of peroxides. Following, the panicle degeneration gene
OsCAX1a was identified, which was involved in Ca2+ transport in the ym48
mutant. In OsCAX1a, a relative conserved T to A substitution was noted
at the 64th amino acid, which disrupted Ca2+ transport. Hydroponics
assays and Ca2+ quantification confirmed that Ca2+ transport and
distribution to apical tissues were restricted and over-accumulated in
mutant sheath. Ca2+ transport between cytoplasm and vacuole was
affected, and the reduced content of Ca2+ in vacuole and cell wall and
the decreased of Ca2+ absorption were appeared in ym48 mutant. RNA-Seq
data indicated that the abnormal CBL (calcineurin b-like proteins)
pathway mediated by deficient Ca2+ might occur in mutant, resulting in
the burst of ROS and programmed cell death in panicles. Our results
explained the key role of OsCAX1a in Ca2+ transport and distribution and
laid a foundation to further explore the genetic and molecular
mechanisms of panicle degeneration and the efficiency of Ca2+
fertilization in rice.