Plant mechanism of an adaptive stress response homologous to bacterial stringent response

All living organisms possess adaptive responses to environmental stresses that are essential to ensuring cell survival. One of them is the stringent response, initially discovered forty years ago in the gram-negative model organism E. coli. Recently plant homologues to the bacterial relA/spoT genes were identified (RSH genes--RelA/SpoT Homologues). Also the products of rsh proteins activity--(p)ppGpp were identified in the chloroplasts of plant cells. Levels of ppGpp increased markedly when plants were subjected to some biotic and abiotic stresses. Elevation of ppGpp levels was elicited also by treatment with plant hormones. What is more--in vitro, chloroplast RNA polymerase activity was inhibited in the presence of ppGpp. It is supposed that plant stringent response is a conserve stress-response pathway possibly operating via regulation of chloroplast gene expression and, thus, the regulation of plastid metabolism.