Hypersensitivity of an Arabidopsis sugar signaling mutant toward exogenousproline application

In transgenic Arabidopsis a patatin class I promoter from potato is regulat ed by sugars and proline (Pro), thus integrating signals derived from carbo n and nitrogen metabolism. In both cases a signaling cascade involving prot ein phosphatases is involved in induction. Other endogenous genes are also regulated by both Pro and carbohydrates. Chalcone synthase (CHS) gene expre ssion is induced by both, whereas the Pro biosynthetic Delta(1)-pyrroline-5 -carboxylate synthetase (P5CS) is induced by high Suc concentrations but re pressed by Pro, and Pro dehydrogenase (ProDH) is inversely regulated. The m utant rsr1-1, impaired in sugar dependent induction of the patatin promoter , is hypersensitive to low levels of external Pro and develops autofluoresc ence and necroses. Toxicity of Pro can be ameliorated by salt stress and ex ogenously supplied metabolizable carbohydrates. The rsr1-1 mutant shows a r educed response regarding sugar induction of CHS and P5CS expression. ProDH expression is de-repressed in the mutant but still down-regulated by sugar . Pro toxicity seems to be mediated by the degradation intermediate Delta(1 )-pyrroline-5-carboxylate. Induction of the patatin promoter by carbohydrat es and Pro, together with the Pro hypersensitivity of the mutant rsr1-1, de monstrate a new link between carbon/nitrogen arid stress responses.