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The Rice Cytosolic Pentatricopeptide Repeat Protein Osppr2-1 Regulates Osglk1 to Control Tapetal Plastid Development and Programmed Cell Death

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The Rice Cytosolic Pentatricopeptide Repeat Protein Osppr2-1 Regulates Osglk1 to Control Tapetal Plastid Development and Programmed Cell Death The rice cytosolic pentatricopeptide repeat protein OsPPR2-1 regulates OsGLK1 to control tapetal plastid development and programmed cell death Shaoyan Zheng State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, College of Life Sciences, South China Agricultural University Jingfang Dong Guangdong Key Laboratory of New Technology in Rice Breeding, Rice Research Institute, Guangdong Academy of Agricultural Sciences Jingqin Lu State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, College of Life Sciences, South China Agricultural University Jing Li State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, College of Life Sciences, South China Agricultural University Dagang Jiang State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, College of Life Sciences, South China Agricultural University Simiao Ye State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, College of Life Sciences, South China Agricultural University Wenli Bu State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, College of Life Sciences, South China Agricultural University Zhenlan Liu State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, College of Life Sciences, South China Agricultural University Hai Zhou State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, College of Life Sciences, South China Agricultural University Chuxiong Zhuang ( [email protected] ) State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, College of Life Sciences, South China Agricultural University Article Keywords: OsPPR2-1, tapetum, PCD, plastid, OsGLK1 Posted Date: January 29th, 2021 DOI: https://doi.org/10.21203/rs.3.rs-131663/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License 1 The rice cytosolic pentatricopeptide repeat protein OsPPR2-1 2 regulates OsGLK1 to control tapetal plastid development and 3 programmed cell death 4 5 Shaoyan Zheng1,3,4, Jingfang Dong2,4, Jingqin Lu1,3,4, Jing Li1,3, Dagang Jiang1,3, Simiao 6 Ye1,3, Wenli Bu1,3, Zhenlan Liu1,3, Hai Zhou1,3, Chuxiong Zhuang1,3,* 7 8 1State Key Laboratory for Conservation and Utilization of Subtropical Agro-Bioresources, 9 South China Agricultural University, Guangzhou 510642, China 10 2Guangdong Key Laboratory of New Technology in Rice Breeding, Rice Research Institute, 11 Guangdong Academy of Agricultural Sciences, Guangzhou 510640, China 12 3Guangdong Laboratory for Lingnan Modern Agriculture, Guangzhou 510642, China 13 4These authors contributed equally to this article. 14 15 *Correspondence: Chuxiong Zhuang ([email protected]) 16 17 18 19 ABSTRACT 20 Most pentatricopeptide repeat (PPR) proteins localize to plastids or mitochondria, where 21 they participate in RNA metabolism and post-transcriptionally regulate organelle gene 22 expression. However, whether PPR proteins regulate the expression of nucleus-encoded 23 genes remains unclear. Here, we uncovered a function for the rice (Oryza. sativa L.) PPR 24 protein OsPPR2-1 (Os02g0110400) in pollen development and showed that, in contrast to 25 most other PPR proteins, OPPR2-1 resides in the cytoplasm. Downregulating OsPPR2-1 26 expression led to abnormal plastid development in tapetal cells, prolonged programmed 27 cell death (PCD), prolonged tapetum degradation, and significantly reduced pollen fertility. 28 Transcriptome analysis revealed that the expression of OsGOLDEN-LIKE 1 (OsGLK1), 29 encoding a transcription factor that regulates plastid development and maintenance, was 30 significantly higher in plants with downregulated OsPPR2-1 expression compared to the 31 wild type. Moreover, OsPPR2-1 bound to the OsGLK1 mRNA in RNA 32 immunoprecipitation and RNA-electrophoretic mobility shift assays. An in vitro cleavage 33 assay showed that OsPPR2-1 could degrade the OsGLK1 mRNA. Notably, knockdown of 34 OsGLK1 partially restored pollen fertility in OsPPR2-1-knockdown plants and OsGLK1- 35 overexpressing plants showed abnormal tapetum and plastid development, similar to the 36 OsPPR2-1-knockdown plants. Together, our findings demonstrate that OsPPR2-1 37 regulates OsGLK1 expression, thereby controlling plastid development and PCD in the 38 tapetum. 39 40 Key words: OsPPR2-1, tapetum, PCD, plastid, OsGLK1 41 42 43 44 INTRODUCTION 45 Pentatricopeptide repeat (PPR) proteins are ubiquitous in eukaryotes but are rarely 46 found in prokaryotes1. More than 600 PPR genes have been identified in Arabidopsis 47 thaliana, accounting for approximately 2% of the total number of Arabidopsis genes. In 48 addition, 435 and 441 non-redundant PPR genes have been identified in japonica and 49 indica rice (Oryza sativa), respectively2-6. 50 PPR proteins contain tandem repeats of 35-amino-acid motifs (PPR motifs), with 2– 51 26 PPR motifs per PPR protein7. PPR proteins are classified into the P-type and PLS-type 52 subfamilies based on the sequence characteristics of their PPR motifs. Almost half of PPR 53 family proteins are P-type subfamily members, most of which contain only typical PPR 54 motifs. P-type PPR proteins contain arrays of P motifs comprising the usual 35 amino acids, 55 whereas PLS-type subfamily members contain S (short, 31 amino acids) and L (long, 35– 56 36 amino acids) motifs as well as P motifs. Based on their C-terminal conserved domains, 57 PLS-type subfamily proteins can be divided into PLS, E, E+, and DYW subgroups2. P-type 58 PPR proteins usually function in RNA splicing, RNA stabilization, and activation of 59 translation8. Some PPR proteins have a small MutS-related domain (SMR) at the C- 60 terminus; this domain could be involved in protein–RNA interactions and PPR-SMR 61 proteins in plants have RNA endonuclease activity9. 62 The PPR protein family in rice is large, and most of these proteins localize to plastids 63 (chloroplasts), mitochondria, or the nucleus4,5. In rice, several chloroplast-targeted PPR 64 proteins function in leaf development; among them, WHITE STRIPE LEAF (WSL)10, 65 WSL411, and plastid TRANSCRIPTIONALLY ACTIVE CHROMOSOME PROTEIN 2 66 (OspTAC2)12 are essential for RNA splicing of chloroplast gene transcripts during leaf 67 development. In addition, some PPR members can restore male fertility or regulate plant 68 growth and development by regulating the mitochondrial RNA metabolism or participates 69 in mitochondrial RNA editing13-16. The nucleus-localized PPR protein OsNPPR3 functions 70 in starch biosynthesis and seed vigor by affecting the expression and splicing of nuclear 71 and mitochondrial genes17. The above studies mainly focused on the regulation of 72 mitochondrial- or chloroplast-encoded genes by PPR proteins located in mitochondria, 73 plastids, or the nucleus. Despite these advances, the roles of cytosolic PPR proteins remain 74 largely unclear, and how P-type PPR proteins are involved in regulating nuclear genes 75 remains to be elucidated. 76 Here, we determined the function of the PPR protein-encoding gene OsPPR2-1 in rice. 77 OsPPR2-1 encodes a P- type PPR protein with 16 PPR motifs that localizes to the cytosol 78 and functions in rice pollen development by regulating the expression of the nuclear gene 79 OsGOLDEN-LIKE 1 (OsGLK1). Our findings suggest that OsPPR2-1 regulates the 80 expression of OsGLK1 to control plastid development in the tapetum, thereby affecting 81 pollen development in rice. 82 83 RESULTS 84 OsPPR2-1 Is a Cytosol-Localized PPR Protein 85 In our previous research, we observed that the PPR protein-encoding gene OsPPR2- 86 1 (Os02g0110400, NM_001052184) is downregulated in thermo-sensitive male sterile 87 lines18. This reduced expression suggested that OsPPR2-1 may be related to pollen 88 development. To characterize the expression pattern of OsPPR2-1 in rice anthers, we 89 measured its transcript levels by quantitative real-time PCR (qRT-PCR). OsPPR2-1 was 90 expressed in various anther stages, with the highest transcript levels detected in stage 10 91 and stage 11 anthers (Fig. 1a). The cytological descriptions of the different stages of rice 92 anther development used in this study were based on those of Zhang et al19. To further 93 analyze the spatiotemporal expression patterns of OsPPR2-1 during anther development, 94 we performed RNA in situ hybridization. In situ signals were mainly observed in the tapetal 95 cells of anthers from stage 8 to stage 10, with the highest level detected in stage 10 anthers 96 (Fig. 1b). 97 To analyze the subcellular localization of OsPPR2-1, we fused the green fluorescent 98 protein (GFP) sequence to OsPPR2-1 and expressed the fusion protein (OsPPR2-1-GFP) 99 under the control of the CaMV 35S promoter. We transformed this construct into sheath 100 protoplasts of rice cv. Zhonghua11 (O. sativa L. ssp. japonica. cv. Zhonghua11, ZH11). 101 Diffuse GFP signals were detected in the cytoplasm, and the signals did not overlap with 102 mitochondria, chloroplasts, or nuclear signals (Fig. 1c). To validate these results, we 103 extracted mitochondrial and chloroplast proteins and carried out western blot experiments 104 with anti-OsPPR2-1 and different organelle-specific commercial antibodies. The 105 immunoblots showed a cytosolic localization for OsPPR2-1 (Fig. 1d). Together, these 106 observations support the conclusion that OsPPR2-1 is a PPR protein localized to the cytosol. 107 To examine the evolutionary
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