3.11 Fld presence ameliorated the increase of soluble carbohydrates and amino acid levels in leaves of drought-exposed plants
When plants were grown in the absence of stress, starch and the soluble sugars glucose, fructose and sucrose accumulated to higher levels inStpfld 252 leaves compared to WT siblings (Figure 8a). Water limitation, in turn, led to a major decline of starch in both WT andStpfld 252 leaves, with concomitant increases in soluble sugars (Figure 8a). Unlike carbohydrates, most amino acids accumulated to similar levels in leaves from the two genotypes grown under normal conditions, with the conspicuous exception of Pro and its related metabolite pyrroline-5-carboxylate (P5C) whose levels were significantly higher in the transformant (Supplementary Figure S7). Glu and Asp also showed moderately increased contents in Stpfld 252 leaves (Supplementary Figure S7).
With few exceptions (Met, Ala, Glu, GABA and ACC), amino acid contents were up-regulated by water deprivation in WT leaves. These drought-dependent increases were completely prevented by chloroplast Fld for 11 proteinogenic amino acids and ameliorated in three more (Pro, P5C and Thr), whereas Ser and Gly build-up was not affected by the flavoprotein (Figure 8b; Supplementary Figure S7). As expected, drought caused a major increase in the stress marker proline and its precursor P5C, which can act as compatible osmolytes and antioxidants (Szabados & Savouré 2010). The lower levels observed in Stpfld 252 leaves (Figure 8b; Supplementary Figure S7) suggest that these plants suffered less stress than their WT counterparts.