Abstract
Author SummaryPlants are able to adapt photosynthesis to changes in light levels by adjusting the activities of their two photosystems, the structures responsible for light energy capture. During a process called state transitions, a part of the photosynthetic complex responsible for light harvesting (the photosynthetic antennae) becomes reversibly phosphorylated and migrates between the photosystems to redistribute light-derived energy. The protein kinase responsible for phosphorylating photosynthetic antenna proteins was identified recently. However, despite extensive biochemical efforts to isolate the enzyme that catalyzes the corresponding dephosphorylation reaction, the identity of this protein phosphatase has remained unknown. In this study, we identified and characterized the thylakoid-associated phosphatase TAP38. We first demonstrate by spectroscopic measurements that the redistribution of excitation energy between photosystems that are characteristic of state transitions do not take place in plants without a functional TAP38 protein. We then show that the phosphorylation of photosynthetic antenna proteins is markedly increased in plants without TAP38, but decreased in plants that express more TAP38 protein than wild-type plants. This, together with the observation that addition of recombinant TAP38 decreases the level of antenna protein phosphorylation in an in vitro assay, suggests that TAP38 directly acts on the photosynthetic antenna proteins as the critical phosphatase regulating state transitions. Moreover, in plants without TAP38, photosynthetic electron flow is enhanced, resulting in more rapid growth under constant low-light regimes, thus providing the first instance of a mutant plant with improved photosynthesis.
Highlights
Owing to their sessile life style, plants have to cope with environmental changes in their habitats, such as fluctuations in the incident light
LHCII phosphorylation and state transitions have been extensively studied in the green alga Chlamydomonas reinhardtii and the flowering plant Arabidopsis thaliana [2,4,5,6]
We first demonstrate by spectroscopic measurements that the redistribution of excitation energy between photosystems that are characteristic of state transitions do not take place in plants without a functional TAP38 protein
Summary
Owing to their sessile life style, plants have to cope with environmental changes in their habitats, such as fluctuations in the incident light. Changes in light quantity or quality (i.e., spectral composition) result in imbalanced excitation of the two photosystems and decrease the efficiency of the photosynthetic light reactions Plants can counteract such excitation imbalances within minutes by a mechanism called state transitions, which depends on the reversible association of the mobile pool of major light-harvesting (LHCII) proteins with photosystem II (state 1) or photosystem I (PSI) (state 2) (reviewed in [1,2,3,4,5]). The accumulation of phosphorylated LHCII (pLHCII), stimulated in low white light, or by light of wavelengths exciting PSII (red light), causes association of pLHCII with PSI (state 2), directing additional excitation energy to PSI. When Arabidopsis state transition mutants are perturbed in linear electron flow, effects on plant performance and growth rate become evident [14], indicating that in flowering plants, state transitions are physiologically relevant
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