Interaction between phosphate starvation signalling and hexokinase-independent sugar sensing in Arabidopsis leaves

Expression of selected genes in relation to phosphate (P-i) starvation and sugar sensing was studied in leaves of Arabidopsis. Excised leaf segments with different P status were supplied with combinations of P-i and sugars. Sugar-inducible genes, encoding beta-amylase (beta-AMY) and chalcone synthase (CHS), were also induced by P deficiency, and were more strongly regulated by sugars when leaf segments originated from P-starved plants. Furthermore, transcript levels of the P-starvation-inducible genes ACP5 (encoding an acid phosphatase), RNS1 (encoding a ribonuclease), and IPS1 (unknown function) increased in response to exogenously applied sugars. Supply of P-i to the leaf segments reversed both P-starvation-induced and sugar-induced gene expression. These interactions reveal a close relationship between P and sugar sensing. To differentiate between hexokinase-dependent and hexokinase-independent sugar sensing the effect of the glucose analogue 2-deoxyglucose and gene expression in the hexokinase-1 deficient mutant, gin2-1, were studied. Both beta-AMY and CHS were induced by supplying sucrose to excised leaves but not by 2-deoxyglucose, confirming that these genes are regulated by hexokinase-independent sugar sensing. In the gin2-1 mutant both beta-AMY and CHS responded clearly to P starvation excluding that hexokinase-1 mediates the response to P. Similarly, the P-responding genes, IPS1 and RNS1 were repressed by addition of P-i also in the gin2-1 mutant. In conclusion, several phosphate starvation-induced genes are also sugar-induced and hexokinase-independent sugar sensing in Arabidopsis is strongly intensified by phosphate starvation.