Arabidopsis EGY1 Is Critical for Chloroplast Development in Leaf Epidermal Guard Cells.

Alvin Sanjaya,Hiroki Ishikawa,Shiho Sato,Shun Sasaki,Mao Suzuki,Sumie Ohbu,Kengo Kanamaru,Makoto Asano,Ryohsuke Muramatsu,Yuki Fujii,Yusuke Kazama,Ryuuichi D Itoh,Tomoko Abe,Makoto T Fujiwara

doi:10.3390/plants10061254

Abstract

In Arabidopsis thaliana, the Ethylene-dependent Gravitropism-deficient and Yellow-green 1 (EGY1) gene encodes a thylakoid membrane-localized protease involved in chloroplast development in leaf mesophyll cells. Recently, EGY1 was also found to be crucial for the maintenance of grana in mesophyll chloroplasts. To further explore the function of EGY1 in leaf tissues, we examined the phenotype of chloroplasts in the leaf epidermal guard cells and pavement cells of two 40Ar17+ irradiation-derived mutants, Ar50-33-pg1 and egy1-4. Fluorescence microscopy revealed that fully expanded leaves of both egy1 mutants showed severe chlorophyll deficiency in both epidermal cell types. Guard cells in the egy1 mutant exhibited permanent defects in chloroplast formation during leaf expansion. Labeling of plastids with CaMV35S or Protodermal Factor1 (PDF1) promoter-driven stroma-targeted fluorescent proteins revealed that egy1 guard cells contained the normal number of plastids, but with moderately reduced size, compared with wild-type guard cells. Transmission electron microscopy further revealed that the development of thylakoids was impaired in the plastids of egy1 mutant guard mother cells, guard cells, and pavement cells. Collectively, these observations demonstrate that EGY1 is involved in chloroplast formation in the leaf epidermis and is particularly critical for chloroplast differentiation in guard cells.

Highlights

Chloroplasts are semi-autonomous double-membrane-bound organelles containing their own DNA and ribosomes [1,2]
We isolated an argon ion (40Ar17+)-irradiated pale green Arabidopsis mutant, Ar50-33-pg1 [49], which carried a single large deletion (940,000 bp) on chromosome 5, encompassing over 40 protein-coding genes, including Ethylene-dependent Gravitropism-deficient and Yellow-green 1 (EGY1). We showed that both Ar50-33-pg1 and another egy1 mutant, egy1-4 [50], exhibited preferential degeneration of mesophyll chloroplast grana, along with progression of leaf chlorosis [49], suggesting that EGY1 is critical for the maintenance of chloroplasts in leaf mesophyll cells
Encodes a critical factor involved in the biogenesis of epidermal chloroplasts—in particular the differentiation of guard cell chloroplasts— mutations in EGY1 permitted the development of mesophyll chloroplasts in mutant leaves [49]

Summary

Introduction

Chloroplasts are semi-autonomous double-membrane-bound organelles containing their own DNA and ribosomes [1,2]. Among the structurally and functionally divergent plastid family members, chloroplasts contain unique, flattened, sac-like thylakoids that carry out oxygenic photosynthesis and release O2 gas into the atmosphere, while converting atmospheric CO2 into carbohydrates to support plant growth and development [3]. Chloroplasts are generally distributed in mesophyll, bundle sheath, and epidermal guard cells. Chloroplasts are present in epidermal pavement cells in many species [2,4,5]. Mesophyll chloroplasts are physiologically important, accessible for biochemical and physiological analyses, and have been the primary subject of chloroplast development-related research to date [6,7,8]. Non-mesophyll chloroplasts have rarely been the target of chloroplast-focused studies

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Results

Conclusion