Photosystem II Repair and Plant Immunity: Lessons Learned from Arabidopsis Mutant Lacking the THYLAKOID LUMEN PROTEIN 18.3

Chloroplasts play an important role in the cellular sensing of abiotic and biotic stress. Signals originating from photosynthetic light reactions, in the form of redox and pH changes, accumulation of reactive oxygen and electrophile species or stromal metabolites are of key importance in chloroplast retrograde signaling. These signals initiate plant acclimation responses to both abiotic and biotic stresses. To reveal the molecular responses activated by rapid fluctuations in growth light intensity, gene expression analysis was performed with Arabidopsis thaliana wild type and the tlp18.3 mutant plants, the latter showing a stunted growth phenotype under fluctuating light conditions (Biochem. J, 406, 415-425). Expression pattern of genes encoding components of the photosynthetic electron transfer chain did not differ between fluctuating and constant light conditions, neither in wild type nor in tlp18.3 plants, and the composition of the thylakoid membrane protein complexes likewise remained unchanged. Nevertheless, the fluctuating light conditions repressed in wild-type plants a broad spectrum of genes involved in immune responses, which likely resulted from shade-avoidance responses and their intermixing with hormonal signaling. On the contrary, in the tlp18.3 mutant plants there was an imperfect repression of defense-related transcripts upon growth under fluctuating light, possibly by signals originating from minor malfunction of the photosystem II (PSII) repair cycle, which directly or indirectly modulated the transcript abundances of genes related to light perception via phytochromes. Consequently, a strong allocation of resources to defense reactions in the tlp18.3 mutant plants presumably results in the stunted growth phenotype under fluctuating light.

Adaptive responses of plants to light stress: mechanisms of photoprotection and acclimation. A review

Plants depend on solar energy for growth via oxygenic photosynthesis. However, when light levels exceed the optimal range for photosynthesis, it causes abiotic stress and harms plant physiology. In response to excessive light, plants activate a series of signaling pathways starting from the chloroplast and affecting the entire plant, leading to stress-specific physiological changes. These signals prompt various physiological and biochemical adjustments aimed at counteracting the negative impacts of high light intensity, including photodamage and photoinhibition. Mechanisms to protect against light stress involve scavenging of chloroplastic reactive oxygen species (ROS), adjustments in chloroplast and stomatal positioning, and increased anthocyanin production to safeguard the photosynthetic machinery. Given that this machinery is a primary target for stress-induced damage, plants have evolved acclimation strategies like dissipating thermal energy via non-photochemical quenching (NPQ), repairing Photosystem II (PSII), and regulating the transcription of photosynthetic proteins. Fluctuating light presents a less severe but consistent stress, which has not been extensively studied. Nevertheless, current research indicates that state transitions and cyclic electron flow play crucial roles in helping plants adapt to varying light conditions. This review encapsulates the latest understanding of plant physiological and biochemical responses to both high light and low light stress.

Possible compensatory role among chloroplast proteases under excess-light stress condition

The reaction center protein D1 of photosystem II (PSII), known as a primary target of photodamage, is repaired efficiently by the PSII repair cycle, to cope with constant photooxidative damage. Recent studies of Arabidopsis show that the endo-type Deg protease and the exo-type FtsH proteases cooperatively degrade D1 in the PSII repair in vivo. It is particularly interesting that we observed upregulation of Clp and SppA proteases when FtsH was limited in the mutant lacking FtsH2. To examine how the complementary functions of chloroplastic proteases are commonly regulated, we undertook a high-light stress on wild-type Arabidopsis leaves. The result that wild type leaves also showed increased levels of these proteases upon exposure to excessively strong illumination not only revealed the importance of FtsH and Deg in the PSII repair, but also implied cooperation among chloroplastic proteases under chronic stress conditions.