Comprehensive physio-biochemical and transcriptomic characterization to decipher the network of key genes under waterlogging stress and its recuperation in Prunus persica.

Waterlogging is a major abiotic stress that plants encounter as a result of climate change impacts. Peach is very sensitive to hypoxia during waterlogging, which causes poor tree vigor and huge economic losses. The molecular mechanism underlying the peach response to waterlogging and reoxygenation remains unclear. Here, the physiological and molecular responses of 3-week-old peach seedlings under waterlogged and recovery conditions were comprehensively analyzed. As a result, waterlogging significantly reduced plant height and biomass with inhibition of root growth when compared with control and reoxygenation. Similar results were observed for photosynthetic activities and gaseous exchange parameters. Waterlogging increased lipid peroxidation, hydrogen peroxide, proline, glutamic acid and glutathione contents, while superoxide dismutase, peroxidases and catalase activities were decreased. The glucose and fructose contents were accumulated, contrary to sucrose which was reduced remarkably throughout the stress periods. The level of endogenous indole acetic acid (IAA) was increased in waterlogging but decreased after reoxygenation. However, the change trends of jasmonic acid (JA), cytokinins and abscisic acid (ABA) levels were opposite to IAA. In transcriptomic analysis, there were 13,343 differentially expressed genes (DEGs) with higher and 16,112 genes with lower expression. These DEGs were greatly enriched in carbohydrate metabolism, anaerobic fermentation, glutathione metabolism and IAA hormone biosynthesis under waterlogging, while they were significantly enriched in photosynthesis, reactive oxygen species scavenging, ABA and JA hormones biosynthesis in reoxygenation. Moreover, several genes related to stress response, carbohydrate metabolism and hormones biosynthesis were significantly changed in waterlogging and reoxygenation, which indicated unbalanced amino acid, carbon and fatty acid pools in peach roots. Taken together, these results suggest that glutathione, primary sugars and hormone biosynthesis and signaling might play key roles in plant response to waterlogging. Our work provides a comprehensive understanding of gene regulatory networks and metabolites in waterlogging stress and its recuperation, which will facilitate peach waterlogging control.

Reduced expression of CsPH8, a P-type ATPase gene, is the major factor leading to the low citrate accumulation in citrus leaves.

Citrate is an important intermediate product for the biosynthesis of several metabolites in plants. As two important organs of the citrus plant, fruits and leaves have their own metabolites characteristics; among them, citrate is normally high in fruit juice sacs (JS) and low in leaves. In this study, citrate content and transcript levels of citrate synthesis, transport, storage, and utilization related genes were compared between leaves and fruit JS of Citrus reticulata cv. 'Huagan No. 2', C. grandis cv. 'Hirado Buntan', and C. sinensis cv. 'Anliu'. Results indicated that the citrate content in fruit JS was significantly higher than in leaves of each cultivar. Only the relative mRNA levels of a P-type proton pump gene, CsPH8, was significantly lower in leaves than in fruit JS of three citrus cultivars, while other genes related to citrate biosynthesis, transport, storage, and utilization were highly expressed in leaves as compared to fruit JS. Furthermore, CsPH8 transient and stable transformation in leaves indicated that the change in citrate content is highly consistent with the change of CsPH8 transcript levels. Taken together, our results strongly suggest that the low accumulation of citrate in citrus leaves is mainly due to the low expression level of CsPH8; additionally, the high level of expression of citrate-utilizing genes would prevent citrate accumulation in the leaf organ.

Comparative Metabolites and Citrate-Degrading Enzymes Activities in Citrus Fruits Reveal the Role of Balance between ACL and Cyt-ACO in Metabolite Conversions.

Citric acid metabolism is considered to be the central cellular process of metabolite conversions. ATP-citrate lyase (ACL) and cytosolic aconitase (cyt-ACO) are the two citrate-degrading enzymes that decide the carbon flux towards different metabolite biosynthesis pathways. However, the correlation of their activities with metabolite concentrations in citrus fruits is still unclear. Here, the concentrations of soluble sugars, organic acids, acetyl-CoA, flavonoids, carotenoids, and γ-aminobutyric acid, as well as the activities of ACL, cyt-ACO, acetyl-CoA C-acetyltransferase, and acetyl-CoA carboxylase, were compared among the fruits of six citrus cultivars during fruit development and ripening. The results showed that the correlation between citrate concentration and cyt-ACO or ACL activity varied greatly among cultivars, while the activities of cyt-ACO and ACL had a significantly negative correlation (r = -0.4431). Moreover, ACL overexpression and RNA interference in the Citrus callus indicated that increasing and decreasing the ACL activity could reduce and induce cyt-ACO activity, respectively. In addition, significant correlation was only observed between the ACL activity and the concentration of acetyl-CoA (r = 0.4333). Taken together, the present study suggested that ACL and cyt-ACO synergistically control the citrate fate for the biosynthesis of other metabolites, but they are not the key determinants for the accumulation of citrate, as well as other metabolites in citrus fruits.