The role of tetrapyrroles in plastid-to-nucleus retrograde signalling in <i>Arabidopsis thaliana</i>

Abstract

Since the majority of chloroplast proteins are encoded in the nucleus it is critical that the chloroplast can communicate with the nucleus to regulate their synthesis. Biogenic chloroplast-to-nucleus signalling pathways have been described at a response level through changes in nuclear gene expression, but the signalling components are essentially unknown. Various biogenic signals have been proposed including a recently hypothesized positive heme-related signal, a singlet oxygen (1O2)-mediated inhibitory signal resulting from mis-regulation of chlorophyll synthesis, and signals derived from chloroplast gene expression. Chloroplasts have also been implicated as a sensor of many environmental signals and must also convey this information to the nucleus. This project was undertaken to increase our understanding on mechanisms by which plastids signal to the nucleus during seedling development.Inhibition of photosynthetic gene expression after treatment with 2’2- dipyridyl (DP) and rescue of nuclear gene expression in the gun6-1D mutant following growth on norflurazon (NF), both support the model for plastid retrograde signalling with ferrochelatase 1 (FC1)-derived heme functioning as a positive plastid signal. One approach that has been taken was to identify new components of the chloroplast-tonucleus signalling pathways in the model plant Arabidopsis thaliana. While most of the treatments blocking chloroplast functions result in a down-regulation of nuclear gene expression, it was found that white light (WL)-grown seedlings of the chloroplast translation mutant prpl11-1 displayed elevated expression of some tetrapyrrole and photosynthesis-related genes (for example HEMA1, CP12-2). qPCR analyses showed that this phenotype was only partially reduced when chloroplast signalling was blocked by NF and lost when heme synthesis was blocked with DP. Physiological analyses showed prpl11 mutants have disturbed tetrapyrrole metabolism with reduced Pchlide, 5-aminolevulinic acid (ALA) synthesis and total heme levels and were unable to accumulate Pchlide after additional ALA feeding. This is partially supportive of a role for PRPL11 as a regulator redirecting tetrapyrrole distribution towards the heme branch. PRPL11 overexpressing plants were constructed to identify putative PRPL11-interacting proteins and preliminary evidence suggests an interaction of PRPL11 with Mg-chelatase. Mutant and overexpressor lines of GUN1 were also shown to affect tetrapyrrole metabolism and the elevated gene expression on NF and Lin seen for gun1 was lost after treatment with DP. This analysis revealed GUN1 is a negative regulator of tetrapyrrole synthesis and its retrograde signalling phenotype is tetrapyrrole dependent. The possible integration of tetrapyrrole synthesis and chloroplast protein synthesis in retrograde signalling to the nucleus is proposed.Feeding ALA to dark-grown seedlings resulted in 1O2 production in WL and blocked the induction of photosynthetic gene expression. Inhibition of these genes was also detected in flu, fc2-1 and fc2-2 mutants transferred from D to WL supporting the role of 1O2 in initiating an inhibitory retrograde signal. Genome-wide comparison of gene expression changes resulting from norflurazon (NF), a far red light pretreatment, and multiple abiotic stresses showed partial similarity between 1O2-dependent plastid signalling pathways and heat stress, and a strong overlap between drought stress and NF-induced transcriptomic changes. Further physiological analysis supports an interaction between these treatments. For example, exposure to heat shock blocked the induction of 1O2 marker genes and induction of selected drought and salt stress genes was inhibited when chloroplast signalling was blocked. Furthermore, this response was rescued in known retrograde signalling mutants