Extrachloroplastic PP7L Functions in Chloroplast Development and Abiotic Stress Tolerance

Abstract

Chloroplast biogenesis is indispensable for proper plant development and environmental acclimation. In a screen for mutants affected in photosynthesis, we identified the protein phosphatase7-like (pp7l) mutant, which displayed delayed chloroplast development in cotyledons and young leaves. PP7L, PP7, and PP7-long constitute a subfamily of phosphoprotein phosphatases. PP7 is thought to transduce a blue-light signal perceived by crys and phy a that induces expression of SIGMA FACTOR5 (SIG5). We observed that, like PP7, PP7L was predominantly localized to the nucleus in Arabidopsis (Arabidopsis thaliana), and the pp7l phenotype was similar to that of the sig6 mutant. However, SIG6 expression was unaltered in pp7l mutants. Instead, loss of PP7L compromised translation and ribosomal RNA (rRNA) maturation in chloroplasts, pointing to a distinct mechanism influencing chloroplast development. Promoters of genes deregulated in pp7l-1 were enriched in PHYTOCHROME-INTERACTING FACTOR (PIF)-binding motifs and the transcriptome of pp7l-1 resembled those of pif and CONSTITUTIVE PHOTOMORPHOGENESIS1 (COP1) signalosome complex (csn) mutants. However, pif and csn mutants, as well as cop1, cryptochromes (cry)1 cry2, and phytochromes (phy)A phyB mutants, do not share the pp7l photosynthesis phenotype. PhyB protein levels were elevated in pp7l mutants, but phyB overexpression plants did not resemble pp7l. These results indicate that PP7L operates through a different pathway and that the control of greening and photosystem biogenesis can be separated. The lack of PP7L increased susceptibility to salt and high-light stress, whereas PP7L overexpression conferred resistance to highlight stress. Strikingly, PP7L was specifically recruited to Brassicales for the regulation of chloroplast development. This study adds another player involved in chloroplast biogenesis.

The plant mobile domain proteins MAIN and MAIL1 interact with the phosphatase PP7L to regulate gene expression and silence transposable elements in Arabidopsis thaliana

“…PP7L is a putative metallo-phosphatase that is related to the phosphatase PP7, and it was recently identified as a nuclear protein required for photosynthesis [22, 27]. Because PP7L interacts with MAIN and MAIL1, and the pp7l-2 null mutant displays abnormal developmental phenotype reminiscent of the two pmd mutant plants (Figs 2 and S3A), we decided to reanalyze available pp7l RNA-seq data [27]. Like in the two pmd mutants, several TEs were upregulated and hundreds of genes were misregulated in pp7l (S3B and S3C Fig, and S2 Table).…”

“…Recently, PP7L was involved in the biogenesis of chloroplasts and plant response upon abiotic stress [27]. The pp7l mutant shows dramatic developmental phenotype, which correlates with misregulation of gene expression and upregulation of TEs (S3A-C Fig) [27].…”

“…Recently, PP7L was involved in the biogenesis of chloroplasts and plant response upon abiotic stress [27]. The pp7l mutant shows dramatic developmental phenotype, which correlates with misregulation of gene expression and upregulation of TEs (S3A-C Fig) [27]. Using IPMS and co-IP experiments, we first showed that MAIN and MAIL1 interact together, which was already suggested based on transcriptomic analyses of main mail1 double mutant plants (Fig 2A and B) [17].…”

“…The drm1-2 (SALK_031705), drm2-2 (SALK_150863) cmt3-11 (SALK_148381) ddc triple mutant line was previously described [28]. The main-2 (GK-728H05), mail1-1 (GK-840E05) and pp7l-2 (SALK_003071) were previously described [17-19, 27], and obtained from The Nottingham Arabidopsis Stock Centre. T-DNA insertions were confirmed by PCR-based genotyping.…”

“…PP7L was recently identified as a nuclear protein involved in chloroplast development and abiotic stress tolerance [27]. The pp7l mutant plants showed photosynthetic defects and strong developmental phenotype associated with misregulation of several genes [27].…”