Nutritional metabolite accumulation and transcription of selected biosynthesis genes in response to drought stress in leaf amaranth (Amaranthus sp.)

Abstract

Amaranth is a hardy leafy vegetable, with most of its species able to grow wildly even under harsh environmental conditions. It is an important vegetable among many African communities, including those in East Africa. However, whether and how water stress affects its nutritional profile is still unclear. The objective of this study was therefore to determine the effect of water deficit stress on accumulation of selected nutrients, metabolites and on transcription of specific biosynthesis genes in leaf amaranth. Ten accessions of amaranth were grown in a greenhouse at temperatures between 25 °C to 27 °C. Half of the plants were subjected to drought stress conditions from the third week after transplanting, the other half were watered normally as a control for three days. Putative genes involved in the biosynthetic pathways of oxalates, vitamin C and carotenoids were identified and their expression levels were compared under well-watered and water deficit stress conditions in the amaranth plants. Nutritional analysis of the leaves was also done for oxalates, vitamin C, carotenoids, flavonoids, phenolic acids, calcium, iron and zinc. There was no significant $\left(P\le 0.05\right)$change in calcium, iron and zinc content due to the stress, while antioxidant biochemical components were significantly $\left(P\le 0.05\right)$ increased. Correlation analysis showed low correlation coefficients between the expression of most genes and the concentration of respective biochemical components. However, the GDP-L-galactose phosphorylase (GGP/VTC2) gene was up-regulated in all the accessions and showed a significant association with vitamin C content. There was also a significant correlation between the lycopene beta cyclase (LCY) gene with total carotenoids. All genes of the same pathways showed significant correlations. Significant partial correlations were also evident in gene-gene pair of the same pathway vs associated components. In conclusion, water deficit stress increased the accumulation of carotenoids, flavonoids and phenolic acids, as well as vitamin C. It also resulted in changes in gene expression patterns. While a single gene in a pathway did not strongly affect a particular component, changes in vitamin C levels were associated with expression levels of GGP/VTC2 gene.