Influence of short-term exposure to high light on photosynthesis and proteins involved in photo-protective processes in tomato leaves

Plants in natural environments have to cope with fluctuations in light conditions on different timescales, ranging from seconds to days. Toward this, plants employ both adaptive and protective mechanisms. Here, we studied the response of low-light-grown tomato plants to short-term high light (HL) exposure followed by a recovery period using large-scale proteomic profiling, photosyntheticand biochemical measurements. We found that following a 6-h HL treatment, plants exhibited photosystem II photoinhibition, a tendency toward a reduced electron transport and induction of non-photochemical quenching (NPQ). The proteomic data showed that key enzymes of the Mg-branch of the tetrapyrrole pathway, leading to chlorophyll biosynthesis, were significantly down-regulated by HL. Nonetheless, no changes in chlorophyll content or in the levels of most photosynthetic proteins, including all identified photosystem and light-harvesting antenna proteins, were detected. Notably, we observed that following a recovery period (24 h), plants were characterized by higher electron transport and an increased NPQ capacity, indicative of an acclimation that resulted in better photosynthetic performance. Our proteomic analysis also revealed prominent up-regulation of several enzymes of the flavonoid pathway, as well as of enzymatic components of other plant antioxidant systems. In accord with these, we found significant increases in the content of flavonoids and ascorbate, and in the activity of ascorbate peroxidase following the HL and recovery. Together, the results highlight the key alterations involved in photo-protection and detoxification of reactive oxygen species which take part in the short-term response to HL.