Abstract
Chloroplasts are considered to be devoid of cysteine proteases. Using transgenic Arabidopsis lines expressing the rice cystatin, oryzacystatin I (OC-I), in the chloroplasts (PC lines) or cytosol (CYS lines), we explored the hypothesis that cysteine proteases regulate photosynthesis. The CYS and PC lines flowered later than the wild type (WT) and accumulated more biomass after flowering. In contrast to the PC rosettes, which accumulated more leaf chlorophyll and carotenoid pigments than the WT, the CYS lines had lower amounts of leaf pigments. High-light-dependent decreases in photosynthetic carbon assimilation and the abundance of the Rubisco large subunit protein, the D1 protein, and the phosphorylated form of D1 proteins were attenuated in the CYS lines and reversed in the PC lines relative to the WT. However, the transgenic lines had higher amounts of LHC, rbcs, pasbA, and pasbD transcripts than the WT, and also showed modified chloroplast to nucleus signalling. We conclude that cysteine proteases accelerate the reconfiguration of the chloroplast proteome after flowering and in response to high-light stress. Inhibition of cysteine proteases, such as AtCEP1, slows chloroplast protein degradation and stimulates photosynthetic gene expression and chloroplast to nucleus signalling, enhancing stress tolerance traits.
Highlights
While plant cysteine proteases fulfil many important functions in plants, they have never been shown to localize to chloroplasts, except in barley (Frank et al, 2019)
High-light-dependent decreases in photosynthetic carbon assimilation and the abundance of the Rubisco large subunit protein, the D1 protein, and the phosphorylated form of D1 proteins were attenuated in the CYS lines and reversed in the PC lines relative to the wild type (WT)
We compared shoot traits and photosynthetic properties in transgenic lines with oryzacystatin I (OC-I)-dependent inhibition of cysteine proteases targeted to the chloroplasts with those where OC-I was expressed with no intracellular targeting and the product of the transgene is presumed to be localized is the cytosol.We show that the growth and shoot phenotypes of transgenic lines expressing OC-I in the chloroplasts (PC lines) or in the cytosol (CYS lines) were modified in the Cysteine proteases regulate photosynthetic gene expression | 3443
Summary
While plant cysteine proteases fulfil many important functions in plants, they have never been shown to localize to chloroplasts, except in barley (Frank et al, 2019). SP1 and SP2 target the plastid protein import machinery [TOC (translocon of the outer membrane) proteins] for degradation in a novel proteolytic pathway, which is called chloroplast-associated protein degradation (CHLORAD). This pathway regulates the TOC machinery to facilitate reconfiguration of the plastid proteome, which is essential during stages of plant development that involve the interconversion of different plastid types, for example reconfiguration of plastid proteins when chloroplasts are converted into gerontoplasts during leaf senescence (Ling et al, 2012, 2019). CHLORAD has an additional role in abiotic stress responses (Ling et al, 2019)
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