![]() ![]() Therefore, photosynthetic organisms have evolved a variety of mechanisms to minimize overexcitation of the photosystems. ROS are potentially harmful, with the ability to damage Fe–S proteins, oxidize amino acid residues, and generate further radicals and reactive electrophiles resulting in lipid peroxidation and DNA damage. The other source is singlet oxygen, which is formed by the interaction of oxygen with triplet-state chlorophyll in Photosystem II ( 2, 4, 5). For example, oxygen photoreduction, largely at Photosystem I, can result in superoxide and hydrogen peroxide. One source of damage is an increase in the production of reactive oxygen species (ROS). ![]() This can lead to damage, generate signals promoting repair, and also initiate responses allowing acclimation ( 1 – 3). Under high-light conditions, the capacity for light capture during photosynthesis can exceed use. We conclude that photosynthesis is not necessary for the local ROS response to high light but is in part mediated by NADPH oxidase activity. Despite the plethora of pathways able to generate ROS in response to excess light, NADPH oxidase–mediated accumulation of ROS in the rice bundle-sheath strand was detected in etiolated leaves lacking chlorophyll. ![]() ROS production in bundle-sheath strands was decreased in mutant alleles of the bundle-sheath strand preferential isoform of OsRBOHA and increased when it was overexpressed. However, transcripts encoding several isoforms of the superoxide/H 2O 2-producing enzyme NADPH oxidase were more abundant in bundle-sheath strands than mesophyll cells. Consistent with these findings, deep sequencing of messenger RNA (mRNA) isolated from mesophyll or bundle-sheath strands indicated balanced accumulation of transcripts encoding all major components of the photosynthetic apparatus. This response was not affected either by CO 2 supply or photorespiration. Using rice and the ROS probes diaminobenzidine and 2′,7′-dichlorodihydrofluorescein diacetate, we found that, after exposure to high light, ROS were produced more rapidly in bundle-sheath strands than mesophyll cells. In the case of excess light, although the initial local accumulation of ROS preferentially takes place in bundle-sheath strands, little is known about how this response takes place. In Arabidopsis thaliana, local stress such as excess heat or light initiates a systemic ROS wave in phloem and xylem cells dependent on NADPH oxidase/respiratory burst oxidase homolog (RBOH) proteins. When exposed to high light, plants produce reactive oxygen species (ROS). ![]()
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