Tissue Localization of the Glycine Betaine Biosynthetic Enzymes in Barley Leaves

Abstract

Barley (Hordeum vulgare L.) plants accumulate glycine betaine (GB), a major compatible solute, in response to salt stress. In barley, GB is produced by a two-step oxidation of choline in a cooperative way in the cytosol and peroxisomes. In this study, we investigated the localization of two GB biosynthetic enzymes, choline monooxygenase (CMO) and betaine aldehyde dehydrogenase (BADH), in the tissues of barley plants (cv. Haruna-nijyo) grown under normal and saline conditions. Three-week-old barley plants grown hydroponically were treated with a hydroponic culture solution containing 200 mM NaCl for 72 h. Salt treatment resulted in increased expression of CMO and BADH proteins mainly in the leaves of barley but not in the roots. The expression of CMO protein was increased by the presence of NaCl in younger leaves but decreased in older leaves. The tissue localization of CMO and BADH proteins was analyzed by immunofluorescent labeling method using their primary antibodies and a fluorescein-conjugated secondary antibody. CMO and BADH proteins were constitutively co-localized in mesophyll and bundle sheath cells under both normal and saline conditions. A possible physiological function of GB in the salt tolerance of barley plants is discussed.