Strategies for adaptation to high light in plants

Plants absorb light energy for photosynthesis via photosystem complexes in their chloroplasts.However,excess light can damage the photosystems and decrease photosynthetic output,thereby inhibiting plant growth and development.Plants have developed a series of light acclimation strategies that allow them to withstand high light.In the first line of defense against excess light,leaves and chloroplasts move away from the light and the plant accumulates compounds that filter and reflect the light.In the second line of defense,known as photoprotection,plants dissipate excess light energy through non-photochemical quenching,cyclic electron transport,photorespiration,and scavenging of excess reactive oxygen species.In the third line of defense,which occurs after photodamage,plants initiate a cycle of photosystem(mainly photosystem II)repair.In addition to being the site of photosynthesis,chloroplasts sense stress,especially light stress,and transduce the stress signal to the nucleus,where it modulates the expression of genes involved in the stress response.In this review,we discuss current progress in our understanding of the strategies and mechanisms employed by plants to withstand high light at the whole-plant,cellular,physiological,and molecular levels across the three lines of defense.

Arahidopsis CHLOROPHYLLASE 1 protects young leaves from long-term photodamage by facilitating FtsH-mediated D1 degradation in photosystemⅡrepair

The proteolytic degradation of the photodamaged D1 core subunit during the photosystemⅡ(PSⅡ)repair cycle is well understood,but chlorophyll turnover during D1 degradation remains unclear.Here,we report that Arabidopsis thaliana CHLOROPHYLLASE 1(CLH1)plays important roles in the PSII repair process.The abundance of CLH1 and CLH2 peaks in young leaves and is induced by high-light exposure.Seedlings of clh1 single and clh1-1/2-2 double mutants display increased photoinhibition after long-term high-light exposure,whereas seedlings overexpressing CLH1 have enhanced light tolerance compared with the wild type.CLH1 is localized in the developing chloroplasts of young leaves and associates with the PSⅡ-dismantling complexes RCC1 and RC47,with a preference for the latter upon exposure to high light.Furthermore,degradation of damaged D1 protein is retarded in young clh1-1/2-2 leaves after 18-h highlight exposure but is rescued by the addition of recombinant CLH1 in vitro.Moreover,overexpression of CLH1 in a variegated mutant(var2~2)that lacks thylakoid protease FtsH2,with which CLH1 interacts,suppresses the variegation and restores D1 degradation.A var2-2 clh1-1/2-2triple mutant shows more severe variegation and seedling death.Taken together,these results establish CLH1 as a long-sought chlorophyll dephytylation enzyme that is involved in PSⅡrepair and functions in long-term adaptation of young leaves to high-light exposure by facilitating FtsH-mediated D1 degradation.

Plastid ribosomal protein LPE2 is involved in photosynthesis and the response to C/N balance in Arabidopsis thaliana^oo

The balance between cellular carbon (C) and nitrogen (N) must be tightly coordinated to sustain op-timal growth and development in plants. In chloroplasts, photosynthesis converts inorganic C to organic C, which is important for maintenance of C content in plant cells. However, little is known about the role of chloroplasts in C/N balance. Here, we identified a nuclear-encoded pro-tein LOW PHOTOSYNTHETIC EFFICIENCY2 (LPE2) that it is required for photosynthesis and C/N balance in Arabidopsis. LPE2 is specifically localized in the chlor-oplast. Both loss-of-function mutants, lpe2-1 and lpe2-2, showed lower photosynthetic activity, characterized by slower electron transport and lower PSII quantum yield than the wild type. Notably, LPE2 is predicted to encode the plastid ribosomal protein S21 (RPS21). Deficiency of LPE2 significantly perturbed the thylakoid membrane composition and plastid protein accumulation, although the transcription of plastid genes is not affected ob-viously. More interestingly, transcriptome analysis in-dicated that the loss of LPE2 altered the expression of C and N response related genes in nucleus, which is con-firmed by quantitative real-time-polymerase chain re-action. Moreover, deficiency of LPE2 suppressed the re-sponse of C/N balance in physiological level. Taken together, our findings suggest that LPE2 plays dual roles in photosynthesis and the response to C/N balance.

Quantitative proteomics reveals redox-based functional regulation of photosynthesis under fluctuating light in plants

Photosynthesis involves a series of redox reactions and is the major source of reactive oxygen species in plant cells.Fluctuating light(FL) levels,which occur commonly in natural environments,affect photosynthesis;however,little is known about the specific effects of FL on the redox regulation of photosynthesis.Here,we performed global quantitative mapping of the Arabidopsis thaliana cysteine thiol redox proteome under constant light and FL conditions.We identified8857 redox-switched thiols in 4350 proteins,and1501 proteins that are differentially modified depending on light conditions.Notably,proteins related to photosynthesis,especially photosystem I(PSI),are operational thiol-switching hotspots.Exposure of wild-type A.thaliana to FL resulted in decreased PSI abundance,stability,and activity.Interestingly,in response to PSI photodamage,more of the PSI assembly factor PSA3 dynamically switches to the reduced state.Furthermore,the Cys199 and Cys200 sites in PSA3 are necessary for its full function.Moreover,thioredoxin m(Trx m) proteins play roles in redox switching of PSA3,and are required for PSI activity and photosynthesis.This study thus reveals a mechanism for redox-based regulation of PSI under FL,and provides insight into the dynamic acclimation of photosynthesis in a changing environment.