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

Abstract

In <i>Arabidopsis thaliana</i>, the <i>Ethylene-dependent Gravitropism-deficient and Yellow-green 1</i> (<i>EGY1</i>) gene encodes a thylakoid membrane-localized protease involved in chloroplast development in leaf mesophyll cells. Recently, <i>EGY1</i> was also found to be crucial for the maintenance of grana in mesophyll chloroplasts. To further explore the function of <i>EGY1</i> 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 <i>egy1-4</i>. Fluorescence microscopy revealed that fully expanded leaves of both <i>egy1</i> mutants showed severe chlorophyll deficiency in both epidermal cell types. Guard cells in the <i>egy1</i> mutant exhibited permanent defects in chloroplast formation during leaf expansion. Labeling of plastids with CaMV<i>35S</i> or <i>Protodermal Factor1</i> (<i>PDF</i><i>1</i>) promoter-driven stroma-targeted fluorescent proteins revealed that <i>egy1</i> 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 <i>egy1</i> mutant guard mother cells, guard cells, and pavement cells. Collectively, these observations demonstrate that <i>EGY1</i> is involved in chloroplast formation in the leaf epidermis and is particularly critical for chloroplast differentiation in guard cells.