Chloroplast redox metabolism and npq regulation in arabidopsis and rice exposed to excessive light

Abstract

Plants are sessile organisms, which evolved from aquatic environment to land. This crucial change in habitat opened a new word of resources, but also generated numerous adaptive challenges. Among these defies, sustain photosynthesis under an excessive light environment consist in one of the most important adverse factors to be overcame by plants. Excessive light is defined as the light absorbed by plants, which exceeds their photosynthetic capacity. Under such conditions, the over-accumulation of oxygen reactive species (ROS) could lead to impairment on photosystem two reaction centers (RCII), generating photoinhibition and affecting negatively crop productivity. In this sense, the development of photoprotective mechanisms was essential for plants surviving. Among such mechanisms, non-photochemical quenching (NPQ) and antioxidative metabolism have been intensively studied. However, the real importance of such photoprotective mechanisms and the possible integrative processes which connect NPQ and redox metabolism still not totally understood. The present study addressed to comprehend the integrative mechanisms underlying NPQ and antioxidative processes in two different plants species: one classic plant model (Arabidopsis thaliana) and one important crop model (Oryza sativa). In order to reach such objectives, first was employed new fluorimetric-based methodologies related to photoinhibition quantification, involving photochemical quenching measured in the dark subsequently illumination (qPd) and the fraction of NPQ formed which was related to photoprotection (pNPQ). The use of this methodology was well succeed with A.Thaliana plants under an important physiological problem: light tolerance during plant ontogenesis. The importance of ROS accumulation during less tolerant developmental phases (juvenile and senescent) and chlorophyll concentrations were discussed. Subsequently, the mechanisms underlying pNPQ formation were investigated in the same model, which revealed the importance of combined adjustments in PsbS protein expression, delta pH formation and zeaxanthin/violaxanthin pigment contents for phototolerance during A. thaliana ontogenesis. The importance of total xanthophyll pool in pNPQ generation was evaluated. Finally, the current study attempted to employ the methodology involving qPd in a crop model (rice) knockdown for thylakoidal ascorbate peroxidase isoform (APX8 proteins; apx8 = transformed plants). The data revealed new importance for APX8, which were more sensitive to light, specially under moderate growth light (400 µmol m-2 s-1) compared with non-transformed (NT). Transformed apx8 plants accumulated less chlorophyll, over-accumulated H2O2 and presented impairment on thermal component of NPQ (qE) formation. Under high light intensities growth conditions (4 days), apx8 plants activated compensatory mechanisms which were probably effective as photoprotective mechanism.