Soil salinization is one of the most common abiotic stresses of rice, which seriously affects the normal growth of rice. Breeding salt-tolerant varieties have become one of the important ways to ensure food security and sustainable agricultural development. However, the mechanisms underlying salt tolerance control still need to be clarified. In this study, we identified a mutant, termed salt-tolerant and small grains(sts), with salt tolerance and small grains. Gene cloning and physiological and biochemical experiments reveal that sts is a novel mutant allele of Mitogen-activated protein Kinase Kinase 4 (OsMKK4), which controls the grain size, and has recently been found to be related to salt tolerance in rice. Functional analysis showed that OsSTS is constitutively expressed throughout the tissue, and its proteins are localized to the nucleus, cell membrane, and cytoplasm. It was found that the loss of OsSTS function enhanced the salt tolerance of rice seedlings, and further studies showed that the loss of OsSTS function increased the ROS clearance rate of rice seedlings, independent of ionic toxicity. In order to explore the salt tolerance mechanism of sts, we found that the salt tolerance of sts is also regulated by ABA through high-throughput mRNA sequencing. Salt and ABA treatment showed that ABA might alleviate the inhibitory effect of salt stress on root length in sts. These results revealed new functions of grain size gene OsMKK4, expanded new research ideas related to salt tolerance mechanism and hormone regulation network, and provided a theoretical basis for salt-tolerant rice breeding.

土壤盐渍化是水稻最常见的非生物胁迫之一，严重影响水稻的正常生长。选育耐盐品种已成为保障粮食安全和农业可持续发展的重要途径之一。然而，耐盐控制的机制仍需阐明。在这项研究中，我们鉴定了一个突变体，称为耐盐小粒（sts），具有耐盐性和小粒。基因克隆和生理生化实验表明，sts是丝裂原激活蛋白激酶4（OsMKK4 ）的一种新型突变等位基因，它控制着水稻的籽粒大小，最近发现其与水稻的耐盐性有关。功能分析表明OsSTS在整个组织中组成型表达，其蛋白定位于细胞核、细胞膜和细胞质。研究发现，OsSTS功能的丧失增强了水稻幼苗的耐盐性，进一步的研究表明，OsSTS功能的丧失提高了水稻幼苗的ROS清除率，且与离子毒性无关。为了探究sts的耐盐机制，我们通过高通量mRNA测序发现sts的耐盐性也受到ABA的调控。盐和ABA处理表明，ABA可能减轻盐胁迫对sts根长的抑制作用。这些结果揭示了粒号基因OsMKK4的新功能，拓展了耐盐机制和激素调控网络相关的新研究思路，为水稻耐盐育种提供了理论依据。