A Proteinase-Storing Body that Prepares for Cell Death or Stresses in the Epidermal Cells of Arabidopsis

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Abstract
Plants degrade cellular materials during senescence and under various stresses. We report that the precursors of two stress-inducible cysteine proteinases, RD21 and a vacuolar processing enzyme (VPE), are specifically accumulated in ~0.5 µm diameter × ~5 µm long bodies in Arabidopsis thaliana. Such bodies have previously been observed in Arabidopsis but their function was not known. They are surrounded with ribosomes and thus are assumed to be directly derived from the endoplasmic reticulum (ER). Therefore, we propose to call them the ER bodies. The ER bodies are observed specifically in the epidermal cells of healthy seedlings. These cells are easily wounded and stressed by the external environment. When the seedlings are stressed with a concentrated salt solution, leading to death of the epidermal cells, the ER bodies start to fuse with each other and with the vacuoles, thereby mediating the delivery of the precursors directly to the vacuoles. This regulated, direct pathway differs from the usual case in which proteinases are transported constitutively from the ER to the Golgi complex and then to vacuoles, with intervention of vesicle-transport machinery, such as a vacuolar-sorting receptor or a syntaxin of the SNARE family. Thus, the ER bodies appear to be a novel proteinase-storing system that assists in cell death under stressed conditions.