SOS for Plant Stress: The role of Singlet Oxygen Signalling in chloroplast development.

Abstract

Plant seedling de-etiolation occurs when seedlings are transferred from darkness into the light and is characterised by shortening of the hypocotyl, cotyledon expansion, chlorophyll accumulation and assembly of the photosynthetic apparatus. During this process the plastid transitions from a pro-plastid or etioplast into a chloroplast. As most plastid-localised proteins are encoded by the nucleus, communication is required between the chloroplast and the nucleus to regulate this process. Communication from the chloroplast to the nucleus is achieved via retrograde signals, although the identity of these signals is largely unknown. One proposed retrograde signal is singlet oxygen (1O2) which has been shown to inhibit nuclear gene expression. Current methodologies have identified two potential sources of 1O2 that promote this signal, one is in the reaction centre of the grana core and is induced by over-excitation of the photosynthetic apparatus. Another is localised at the grana margin and stimulated by an over-accumulation of tetrapyrroles. This study has investigated the role of 1O2 as an inhibitory retrograde signal when chloroplast development is perturbed following tetrapyrrole accumulation in a far-red (FR) light-dependent manner to induce a 1O2 burst.
Transfer of dark (D)-grown etiolated wild-type (WT) seedlings into FR light induces the synthesis of the tetrapyrrole protochlorophyllide (Pchlide) in an unbound state, and upon transfer into white light (WL) a 1O2 burst is initiated which results in an inhibition of photosynthesis and tetrapyrrole biosynthesis-related gene expression. A number of 1O2 signalling pathway components have been identified following alternative methods of 1O2 generation at both the grana core and grana margin, however whether these components belonged to the 1O2 signalling pathway initiated in seedlings following a FR light-induced 1O2 burst was unknown. This work has tested a wide range of these components under FR light-induced 1O2-producing conditions and found that signalling components identified in systems that generate 1O2 in a tetrapyrrole-dependent manner at the grana margin are part of the signalling pathway, whereas signalling components identified in systems that generate a 1O2 burst in the reaction centre of the grana core are not.

This study also undertook work to characterise unknown components of the 1O2 signalling pathway initiated following a FR light-induced 1O2 burst. A previous screen isolated the novel 1O2 signalling mutant safe after far-red7 (saf7) which was later identified as a translocation of the outer chloroplast membrane132 (toc132) allele. The TOC complex is responsible for the import of nuclear-encoded chloroplast-localised proteins from the cytosol into the intermembrane space. Following a FR light-induced 1O2 burst both saf7 and toc132-2 displayed a maintenance of photosynthesis and tetrapyrrole biosynthesis-related gene expression, and crucially showed no reduction in Pchlide accumulation compared to WT. When tested the additional protein import mutants plastid protein import1 (ppi1) which is a toc33 allele, and translocation of the inner chloroplast membrane40 (tic40) also showed a maintenance of photosynthesis and tetrapyrrole biosynthesis gene expression, however these mutants displayed a low Pchlide phenotype, and therefore it is unknown if these protein import components are involved in 1O2 signalling. Two 1O2 signalling components known to be involved in 1O2 signalling following a FR light-induced 1O2 burst are the EXECUTER (EX) genes EX1 and EX2. To identify whether TOC132 belonged to the same 1O2 signalling pathway as the EX genes a triple toc132-2 ex1 ex2 mutant was generated and subjected to a FR screen. The toc132-2 ex1 ex2 mutant showed an additive maintenance of photosynthesis and tetrapyrrole biosynthesis gene expression compared to the toc132-2 or ex1 ex2 mutants, however the toc132-2 ex1 ex2 mutant also displayed a low Pchlide phenotype. It could therefore not be determined whether TOC132 and the EX genes are part of the same 1O2 signalling pathway.
Collectively this work has identified a clear distinction between 1O2 signalling pathways induced following over-excitation of the photosynthetic apparatus at the grana core, and by the misregulation of tetrapyrrole accumulation at the grana margin. The identification of Toc132 as a novel component of the 1O2 signalling pathway initiated following a FR light-induced 1O2 burst has also implicated the protein import machinery in 1O2 signalling. Furthermore, observations of a low Pchlide accumulation phenotype in the toc132-2 ex1 ex2 mutant may also suggest that the EX and Toc132 signalling components play a role in the function or regulation of the tetrapyrrole biosynthesis pathway.

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ORCID for Matthew Terry: orcid.org/0000-0001-5002-2708

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