A superexpressão de proteínas relacionadas com atividades fotoquímica e fotorespiratória induzida por silenciamento das APX citosólicas contribui para uma fotoinibição similar a de arroz não-transformado submetido à alta luz

Abstract

In tropical regions, where there is a high incidence of light, electrons can accumulate in the transport chain (PET) producing large quantities of H2O2 and other ROS, which might generate photodamage and photoinhibition. To survive to these challenges, plants have developed several mechanisms to mitigate the excess energy in photosystems, besides having an efficient machinery for removal of excess H2O2, which includes cytosolic APX (cAPX). However, double-silenced rice plants for cAPX (OsAPX1/2) do not show large differences in morpho-phenotype when compared to non-transformed (NT), although OsAPX1/2 presents induction of expression on several proteins related to photosynthesis. The physiological implications of this induction, as well as its consequences for OsAPX1/2 resistance against stresses of high light (HL), are still poorly known. Aiming to clarify the role of cAPx in photosynthesis, OsAPX1/2 plants were produced, subjected to 24 hours of HL (2,000 μmol m-2s-1) and studied for the expression and activity of proteins related to photosynthesis, photorespiration and redox homeostasis. The amount of several PET proteins (Lhcb1, PsbO, PsbP, PsbQ, PSAC, PC, FNR and FDX) and Chl and Pheo were increased in OsAPX1/2 in normal growth conditions, however without causing changes in the in vivo photochemistry activity parameters (Fv/Fm and ΔFm/Fm'). In contrast, expression of proteins associated with Calvin-Benson cycle (Rls, ativase RBC) and rubisco carboxylation activity (in vivo and in vitro) were not altered in mutants under normal growth conditions. In HL, the expression of proteins related to photosynthesis was strongly repressed in all genotypes, as well as gas exchange parameters and Fv/Fm, the latter being strong indication of photoinhibition. Moreover, proteins related to photorespiration showed increased expression/activity in response to HL in NT and maintenance of already high levels in OsAPX1/2. In OsAPX1/2 the expression and activity of chloroplastic Cu/Zn-SOD showed a similar response exhibited by photorespiration-related proteins, although the activity of thylakoid APX has been greatly reduced, meaning deficiency in water-water cycle. Taken together, these data demonstrate that induction of expression of proteins related PET in OsAPX1/2 plants may represent a compensatory mechanism for maintaining the photosynthetic activity levels similar to NT. Moreover, in HL, it is possible that the increased expression of photorespiration-related proteins in OsAPX1/2 acts as alternative electron sink, compensating the deficiency in the water-water cycle from these plants.