Post-anthesis saline-alkali stress inhibits grain filling by promoting ethylene production and signal transduction

Saline-alkali stress is one of the major abiotic stresses that severely affect rice yield. However, the mechanism by which saline-alkali stress regulates grain filling in rice is still unclear. In this study, Oryza sativa L. spp. Indica cultivar Chaoyou1000 (C1000) was exposed to post-anthesis saline-alkali conditions at 6 days after anthesis, which significantly reduced the grain weight by suppressing the accumulation of starch and non-structural carbohydrates in grains. Further analysis found that 1-aminocyclopropane-1-carboxylate (ACC), a precursor for ethylene, was increased by saline-alkali treatment. qRT-PCR results showed that several key genes involved in ethylene biosynthesis, including the OsACS and OsACO genes, were upregulated in saline-alkali-treated grains. In addition, genes involved in the ethylene signalling pathway were also induced by saline-alkali stress. Exogenous ethylene application reduced grain weight and both starch and NSC contents in grains of C1000, suggesting that saline-alkali-induced ethylene has a negative effect on grain filling. Furthermore, the gene expression levels of OsSUS, OsAGPL, OsAGPS, OsSSI and OsSSIIIa, which are key genes in the starch biosynthesis pathway, were downregulated in saline-alkali-treated grains. In agreement, assays on these enzymes further revealed that saline-alkali stress decreased the activities of sucrose synthase (SUS), adenosine diphosphate glucose pyrophosphorylase (AGP) and starch synthase (StS). Together, our results indicated that saline-alkali stress suppressed the enzyme activities involved in the conversion of sucrose to starch by elevating ethylene production, which led to inhibition of grain filling.