Constitutive and inducible ER bodies of Arabidopsis thaliana accumulate distinct beta-glucosidases.

The endoplasmic reticulum (ER) body is an ER-related organelle that accumulates high levels of PYK10, a beta-glucosidase with an ER retention signal. Constitutive ER bodies are present in the epidermal cells of cotyledons, hypocotyls and roots of young Arabidopsis seedlings, but absent in rosette leaves. When leaves are wounded, ER bodies are induced around the wounding site of the leaves (inducible ER bodies). To clarify the functional differences between these two ER bodies, we compared constitutive ER bodies with inducible ER bodies in wounded cotyledons of Arabidopsis seedlings. We found that the number of ER bodies increased both in cotyledons wounded directly (locally wounded cotyledons) and in unwounded cotyledons exposed to the systemic wound response (systemically wounded cotyledons). Quantitative reverse transcription-PCR and immunoblot analyses revealed that BGLU18, encoding another beta-glucosidase with an ER retention signal, was induced at the site of wounding, whereas PYK10 was not. Immunocytochemical analysis showed that BGLU18 protein was exclusively localized in ER bodies formed directly at the wounding site on cotyledons. ER bodies were not induced in locally and systemically wounded cotyledons of the bglu18 knock-out mutant. These results indicate that constitutive and inducible ER bodies accumulate different sets of beta -glucosidases and may have distinct functions in defense responses.