Exogenous myo-inositol increases salt tolerance and accelerates CAM induction in the early juvenile stage of the facultative halophyte Mesembryanthemum crystallinum but not in the late juvenile stage.

Mesembryanthemum crystallinum L. (ice plant) develops salt tolerance during the transition from the juvenile to the adult stage through progressive morphological, physiological, biochemical, and molecular changes. Myo -inositol is the precursor for the synthesis of compatible solute D-pinitol and promotes Na+ transport in ice plants. We previously showed that supplying myo -inositol to 9-day-old seedlings alleviates salt damage by coordinating the expression of genes involved in inositol synthesis and transport, affecting osmotic adjustment and the Na/K balance. In this study, we examined the effects of myo -inositol on physiological parameters and inositol-related gene expression in early- and late-stage juvenile plants. The addition of myo -inositol to salt-treated, hydroponically grown late juvenile plants had no significant effects on growth or photosynthesis. In contrast, supplying exogenous myo -inositol to salt-treated early juvenile plants increased leaf biomass, relative water content, and chlorophyll content and improved PSII activity and CO2 assimilation. The treatment combining high salt and myo -inositol synergistically induced the expression of myo -inositol phosphate synthase (INPS ), myo -inositol O -methyltransferase (IMT ), and inositol transporters (INTs ), which modulated root-to-shoot Na/K ratio and increased leaf D-pinitol content. The results indicate that sufficient myo -inositol is a prerequisite for high salt tolerance in ice plant.

Myo-inositol transport and metabolism participate in salt tolerance of halophyte ice plant seedlings.

Myo-inositol and its metabolic derivatives such as pinitol, galactinol, and raffinose affect growth and development and are also involved in stress adaptation. Previous studies have identified myo-inositol transporters (INTs) as transporters of Na+ from root to shoot in the halophyte ice plant (Mesembryanthemum crystallinum). We found that the supply of myo-inositol could alleviate the dehydration effects of salt-stressed ice plant seedlings by decreasing the Na/K ratio in roots and increasing the Na/K ratio in shoots. Analyses of the uptake of exogenous myo-inositol revealed that ice plant seedlings contained intrinsic high-affinity transporters and inducible low-affinity uptake systems. The presence of Na+ facilitated both high- and low-affinity myo-inositol uptake. Six INT genes were identified from the ice plant transcriptome and named McINT1a, 1b, 2, 4a, 4b, and 4c, according to the classification of the Arabidopsis INT family. In seedlings treated with myo-inositol, salt, or myo-inositol plus salt, the expression patterns of all McINT members differed in shoot and root, which indicates organ-specific regulation of McINTs by salt and myo-inositol. The expression of McINT2, 4a, 4b, and 4c was induced by salt stress in shoot and root, but that of McINT1a and 1b was salt-induced only in shoot. The expression of pinitol biosynthesis gene IMT1 was induced by salt and myo-inositol, and their combination had a synergistic effect on the accumulation of pinitol. Supply of myo-inositol to salt-treated seedlings alleviated the detrimental effects by maintaining a low root Na/K ratio and providing precursors for the synthesis of compatible solute to maintain the osmotic balance.