Characterizing the interplay between photosystem II subunit S (PsbS) and chloroplast-derived signalling in response to high light.
Light is a vital resource to plants, providing energy to drive photosynthesis and fuel growth. However, there can be too much of a good thing, which in the case of light transforms it from a vital resource to a potent stress factor. Absorption of too much light energy at the photosystem level increases the probability of charge recombination, chlorophyll triplet formation, production of oxygen radicals and ultimately bleaching. To avoid excessive light stress, non-photochemical quenching (NPQ) is induced in the antenna complex of photosystem II in response to high light. NPQ works like a biochemical safety valve, opening up thermal dissipation routes to downregulate the efficiency of light-harvesting and avoid excessive charge separation. The importance of NPQ for fitness and productivity has been repeatedly demonstrated, with mutants impaired in NPQ typically showing decreased fecundity under field conditions. Efficient NPQ relies on a range of biophysical and biochemical components, which are neatly intertwined across different timescales to create an adaptive, hysteretic system, with the chloroplast thylakoid lumen pH as one of the central regulators.
The major form of NPQ in higher plants is so-called energy-dependent quenching (qE). Although the precise quenching mechanism is still unclear, qE engagement seems to rely strongly on the presence of photosystem II subunit S (PsbS). Ever since its discovery in a chlorophyll fluorescence mutant screen1, PsbS has been something of an enigma. Its role in NPQ induction seems to be that of a chloroplast thylakoid lumen pH sensor, where upon acidification of the thylakoid lumen, glutamate residues on two lumen-exposed loops become protonated. This is hypothesized to give rise to a conformational change, which facilitates qE engagement. The strong effect of PsbS on qE and corresponding photoprotection levels can impact canopy carbon gain in a species-specific manner2-4. However, more recent work has implicated a role for PsbS in several other aspects of plant performance, including herbivore preference5, water use efficiency3 and excess-light inducible memory6. These additional roles all rely on chloroplast-derived signals to be modified by PsbS, but the nature of these signals is unclear, as well as how PsbS might be able to affect them.
This project aims to characterize which chloroplast-derived signals are affected by PsbS. Using a combination of existing and newly generated mutant lines within the project, the work will focus both on short-distance intracellular and cell-to-cell signalling, as well as long-distance signalling, whereby local signal perception may elicit responses distant from the perception site, up to the organismal level. The proposed work is expected to have important ramifications for understanding of plant responses to high light, as well as for designing NPQ bioengineering strategies to improve crop performance.
Enhanced four-year postgraduate studentships starting in October 2020, will once again be awarded by the Gatsby Charitable Foundation. The nominated supervisor will select a candidate who will then compete at interview, with Sainsbury Undergraduate students, for one of up to four Sainsbury PhD Studentships. Interviews will be held in London on 6th March 2020. It would be expected that the studentship holder spend six months during their 3rd or 4th year at another university/institute to gain additional experience. Please note that students cannot apply to their home institution.
1. Li XP, et al. 2000 A pigment-binding protein essential for regulation of photosynthetic light harvesting. Nature 403 (6768): 391-5.
2. Kromdijk et al. 2016 Improving photosynthetic efficiency and crop productivity by accelerating recovery from photoprotection. Science 354 (6314): 857-861.
3. Glowacka et al. 2018 Photosystem II ssubunit S overexpression increases the efficiency of water use in a field-grown crop. Nature communications 9 (1): 868.
4. Hubbart et al. 2018 Enhanced thylakoid photoprotection can increase yield and canopy radiation use efficiency in rice. Communications Biology 1: 22.
5. Kűlheim et al. 2002 Rapid r
egulation of light harvesting and plant fitness in the field. Science 297 (5578): 91-93.
6. Górecka et al. 2019 Photosystem II 22kDa protein level a prerequisite for excess light-inducible memory, cross-tolerance to UV-C, and regulation of electrical signalling. Plant Cell Environment doi:10.1111/pce.13686.