The plastid-encoded RNA polymerase (PEP) plays an important role in the transcription machinery of mature chloroplasts, yet details of its function remain elusive in rice. In a recent study, researchers from the China National Rice Research Institute identified a novel PEP-associated protein (PAP), WLP2, based on its two allelic white leaf and panicle mutants, wlp2s and wlp2w.

The two mutants were albino lethal at high temperatures and showed decreased chlorophyll accumulation, abnormal chloroplast ultrastructure, and attenuated photosynthetic activity. Map-based cloning suggested that WLP2 encodes a putative pfkB-type carbohydrate kinase family protein, which is homologous to fructokinase-like 1 (AtFLN1) in Arabidopsis. WLP2 is mainly expressed in green tissues and its protein localizes in chloroplasts. RNA sequencing data suggests expression levels of PEP-encoded genes, chloroplast development genes and photosynthesis-related genes were compromised in wlp2 mutants, indicating that WLP2 is essential for normal chloroplast biogenesis. Moreover, WLP2 and its paralog OsFLN2 can physically interact with thioredoxin OsTRXz to form a TRX-FLN regulatory module, which not only regulates transcription of the PEP-encoded genes but also maintains the redox balance in chloroplasts under heat stress. Furthermore, the wlp2w mutant gene represents a potential advantage in enhancing seed purity and high-throughput breeding.

These results strongly indicate that WLP2 protects chloroplast development from heat stress via a TRX-FLN regulatory module in rice.

(A–C) Expression levels of PEP-dependent genes (A), NEP-dependent genes (B) and photosynthesis-associated genes (C) in 3-week-old seedlings of wild-type (WT), wlp2s, and wlp2w grown at 28 °C. (D) Heat map of gene expression in plants grown at 22 °C and 32 °C; the gene expression of WT plants represents a relative standard (Log2(RPKM)=0) at 32 °C. Data are shown as means±SD from three individual replicates.

Transcriptional changes of chloroplast-associated genes in wlp2 mutants

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Reference
Y. Lv, G. Shao, J. Qiu, P. Hu et al. (2017) White Leaf and Panicle 2, encoding a PEP-associated protein, is required for chloroplast biogenesis under heat stress in rice J. of Exp. Botany doi: 10.1093/jxb/erx332 [article]

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