4.2 Al tolerance in XZ29 is associated with glycolysis and phosphate homeostasis
Phosphate is a vital substrate in glycolysis and P deficiency leads to a sharp decrease in some sugar phosphates including Glu-6-P and Fru-6-P (Huang et al., 2008). Al-tolerant XZ29 showed higher level of the sugar phosphates than XZ9 under Al stress (Figure 5), thus facilitating relatively less inhibition of glycolysis pathway for normal root growth. Positive correlations between P-containing glycolytic intermediates and P concentration, and rapid recovery of these metabolites after P addition, suggests that the decrease of P and Pi concentrations during Al stress, especially in XZ9, caused the reduction of P-containing glycolytic intermediates (Figures 4, 5, 9). The higher activity of glycolysis in XZ29 could be associated with higher cytoplasmic Pi level. Why does XZ29 maintain higher cytoplasmic Pi level? Vacuolar Pi serves as a pool of Pi storage in plants (Pratt et al., 2009). SPX-MFS2 and SPX-MFS3 mediate Pi unloading from the vacuole into the cytosol (Wang et al., 2012; Wang et al., 2015). The up-regulation of HvSPX-MFS2and HvSPX-MFS3 in XZ29 may mediate Pi transport from the vacuole to the cytosol, to supply Pi for glycolysis (Figures 6, 9).