Apoplastic and chloroplastic redox signaling networks in plant stress responses.

SIGNIFICANCE: Interplay among apoplastic and chloroplastic redox signaling networks is emerging as a key mechanism in plant stress responses. RECENT ADVANCES: Recent research has revealed components involved in apoplastic and chloroplastic redox signaling. Also, the sequence of events from stress perception, activation of apoplastic reactive oxygen species (ROS) burst through NADPH oxidases, cytoplasmic and chloroplastic Ca(2+)-transients, and organellar redox signals to physiological responses is starting to emerge. Moreover, a functional overlap between light acclimation and plant immunity in photosynthetically active tissues has been demonstrated. CRITICAL ISSUES: Any deviations from the basal cellular redox balance may induce acclimation responses that continuously readjust cellular functions. However, diversion of resources to stress responses may lead to attenuation of growth, and exaggeration of defensive reactions may thus be detrimental to the plant. The ultimate outcome of acclimation responses must therefore be tightly controlled by the redox signaling networks between organellar and apoplastic signaling systems. FUTURE DIRECTIONS: Two major questions still remain to be solved: the sensory mechanism for ROS and the components involved in relaying the signals from the apoplast to the chloroplast. A comprehensive view of regulatory networks will facilitate the understanding on how environmental factors affect the production of phytonutrients and biomass in plants. Translation of such information from model plants to crop species will be at the cutting edge of research in the near future. These challenges give a frame for future studies on ROS and redox regulation of stress acclimation in photosynthetic organisms.