Metabolic coordination of rice seed development to nighttime warming: In-situ determination of cellular redox states using picolitre pressure-probe electrospray-ionization mass spectrometry

Abstract

High night temperature (HNT) at the ripening stage severely affects both rice yield and quality. HNT accelerates embryo growth and chalky formation in the developing grains, accompanying with a diminishment of endosperm cell size. Although these responses may be physiologically interacted each other in the grains, what signals are involved in the accelerated embryo development remains undetermined. In this work, we have used picolitre pressure-probe electrospray-ionization mass spectrometry (picoPPESI-MS) to conduct single-cell metabolomics at several regions in HNT-treated grains, embryonic scutellum and outer endosperms in the basal (‘chalky region’ at maturation) and middle (‘translucent region’ at maturation as a reference) positions. Microscopic observations showed that HNT promoted cell expansion rate in the scutellum. When embryonic cell expansion rate reached the maximum, spatial differences in several metabolisms including ascorbate-glutathione (ASC-GSH) pathway and purine were detected, together with considerable sugar and amino acid accumulations in embryonic scutellum cells. There was no treatment difference in GSH content during active cell expansion in HNT-treated embryos, although an increase in GSH/GSSG ratio due to a reduction in oxidized glutathione (GSSG) content has been contrastingly observed. In the endosperms, greater ASC accumulation with a difference in ASC/dehydroascobic acid ratio has been also detected under HNT conditions. Since dormancy is often correlated with GSSG concentration, it is concluded that spatial regulation of GSH redox homeostasis detected at cell-level might be essential for dormancy alleviation and embryo growth accelerated in HNT-treated seeds.