Abstract
Argon-ion beam is an effective mutagen capable of inducing a variety of mutation types. In this study, an argon ion-induced pale green mutant of Arabidopsis thaliana was isolated and characterized. The mutant, designated Ar50-33-pg1, exhibited moderate defects of growth and greening and exhibited rapid chlorosis in photosynthetic tissues. Fluorescence microscopy confirmed that mesophyll chloroplasts underwent substantial shrinkage during the chlorotic process. Genetic and whole-genome resequencing analyses revealed that Ar50-33-pg1 contained a large 940 kb deletion in chromosome V that encompassed more than 100 annotated genes, including 41 protein-coding genes such as TYRAAt1/TyrA1, EGY1, and MBD12. One of the deleted genes, EGY1, for a thylakoid membrane-localized metalloprotease, was the major contributory gene responsible for the pale mutant phenotype. Both an egy1 mutant and F1 progeny of an Ar50-33-pg1 × egy1 cross-exhibited chlorotic phenotypes similar to those of Ar50-33-pg1. Furthermore, ultrastructural analysis of mesophyll cells revealed that Ar50-33-pg1 and egy1 initially developed wild type-like chloroplasts, but these were rapidly disassembled, resulting in thylakoid disorganization and fragmentation, as well as plastoglobule accumulation, as terminal phenotypes. Together, these data support the utility of heavy-ion mutagenesis for plant genetic analysis and highlight the importance of EGY1 in the structural maintenance of grana in mesophyll chloroplasts.
Highlights
Chloroplasts belong to a class of double-envelope membrane-bound organelles—the plastids
The extent of chloroplast biogenesis and degeneration in leaves is reflected in leaf chlorophyll levels, which can be used as an indicator of leaf developmental stage and physiological state to understand chloroplast differentiation, proliferation, and degeneration
Our research has investigated the effects of nuclide species, linear energy transfer (LET) values, dose amount, and target plant conditions on the structure of DNA mutations in targeted A. thaliana tissues [25,26,27,28,29,30,31]
Summary
Chloroplasts belong to a class of double-envelope membrane-bound organelles—the plastids. Compared with the cell nucleus and mitochondria, plastids undergo flexible morphological alterations that depend on the plant tissues and cells in which they reside In this context, chloroplast development in photosynthetic tissues is one of the most remarkable features of plastid differentiation. During leaf development of the model plant Arabidopsis thaliana, proplastids, a primitive plastid form (~1 μm in diameter) with rudimentary internal membranes in meristematic tissues, actively proliferate and expand several hundred-fold, a process that accompanies massive development of thylakoids. This results in the formation of typically 6 μm (usually 3–10 μm in diameter) chloroplasts in mature mesophyll cells [2,3]. The extent of chloroplast biogenesis and degeneration in leaves is reflected in leaf chlorophyll levels, which can be used as an indicator of leaf developmental stage and physiological state to understand chloroplast differentiation, proliferation, and degeneration
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