Effects of exogenous sorbitol and its analogs on iron transport and phytohormone contents in peach leaves and fruit

Abstract：【Objective】Sorbitol, a polyol, is an important carbohydrate metabolite which plays important roles in photoassimilate translocation and carbohydrate storage in species of Rosaceae. In peach, a known sorbitol-synthetizing species, osmotic tolerance can be improved by synthesizing and accumulating polyols. Application of exogenous sorbitol has been shown to alter carbohydrate allocation in peaches. However, the metabolic changes induced by exogenous sorbitol and their physiological relevance in peach are still poorly understood. This study aims to investigate the effects of exogenous sorbitol and its analogs on iron transport and hormone contents in peach, and to uncover the underlying molecular and biochemical mechanisms.【Methods】‘MX14-1’peach trees cultivated in an orchard located in Dahu town, Minhou county, Fuzhou, China, were chosen for the study. The peach trees were divided into four groups sprayed twice with 1 mmol · L^-1 sorbitol, mannitol, isosorbide solution, or clean water (as control). After the 2nd spray, leaves and fruit were harvested and transported in dry ice to the laboratory within one day. Sodium, potassium and calcium contents and endogenous phytohormones (ZA, ZR,GA, IAA and ABA) concentrations in peach leaves and fruit were determined. Based on the results, realtime RT-PCR was performed to quantify and verify the transcriptional changes of genes involved in ion transport and zeatin biosynthesis.【Results】Compared to the control group, the contents of sodium, potassium and calcium in the fruit increased significantly. The highest sodium, potassium and calcium contents were found in the fruit of isosorbide treatment group, being 53.89%, 35.17% and 46.99% higher than in control fruit, respectively. In contrast to the accumulation of sodium, potassium and calcium in the fruit, no significant change in sodium, potassium, or calcium contents were observed in the leaves in various groups. Gene expression analyses using real-time RT-PCR showed that the spraying treatments had triggered an increase in PpNHX7/SOS1 (a NHX-type Na+ /H+ antiporter) transcripts in the leaves and in the fruit. On the other hand, potassium uptake- and transport- associated genes (PpKUPs and PpKEAs) were different in the leaves and fruit. Comparing to the control, the expression of PpKUPs and PpKEAs were upregulated in the leaves, whereas PpKUPs were upregulated and PpKEAs was downregulated in the fruit. Notably, among PpKUPs and PpKEAs, PpKUP5 and PpKEA2 genes had the highest expression level and showed significant expression changes in both the leaves and the fruit in the treatment groups, suggesting that these two genes encode dominant potassium transporters in response to exogenous sorbitol or its analogs in peach. PpNHXs, PpKUPs and PpKEAs that responded to sorbitol and analogs treatments were likely involved in maintaining Na + /K+ homeostasis to withstand the osmotic stress. In addition, dramatic increases in zeatin contents in the fruit of the treated groups were found. The significant upregulation of the rate-limiting enzyme genes, PpIPT and PpCYP735A, further confirmed the enhancement of zeatin and zeatin-nucleoside synthesis in the fruit. GA3 and IAA, also increased to different degrees. However, no similar changes were found in ABA contents.【Conclusion】Sorbitol or its analog treatments enhanced sodium, potassium and calcium accumulation in the fruit, but there were no significant changes in the leaves. The peach leaves may transport sodium and potassium ions from the leaves to the fruit. It is suggested that some members of PpNHXs and PpKUPs have participated in the transport and homeostasis of sodium and potassium ions and that their absorption and translocation capacity was increased. Meanwhile, the declined expression levels of PpKEAs may limit the secretion of potassium ions accumulated in the fruit, thereby maintaining a high potassium ion concentration and significant accumulation of potassium in the fruit. Additionally, we observed a significant rise in expressions of key zeatin synthesis genes (PpIPT and PpCYP735A), coinciding with the increased contents of zeatin and zeatin nucleoside, which is supposed to be a response of peach fruit to osmotic stress.