Immunogold localization of free abscisic acid in tomato root cells

Abstract

Ultrastructural localization of abscisic acid in tomato (Lycopersicon esculentum Mill. cv. Vedettos) seedling roots is determined with the indirect protein A–gold approach. Polyclonal antibodies recognizing specifically the free (+)cis, trans form of abscisic acid are used as the first step in the immunolocalization procedure. Evidence is presented that its distribution varies within the different tissues of the tomato root. In the root cap cells, free abscisic acid accumulates mainly in the apoplast, in the cytoplasmic vesicles, and in the amyloplasts, around starch grains. In columella and meristematic cells, it accumulates mainly at the junction area with root cap cells, in the wall, and in the mucigel layer outside the root. Abscisic acid accumulation in the wall, the middle lamella, and the mucigel layer of the root cap cells may play a role in the root response to environmental stimuli. The cytochemical labeling of polygalacturonic acids, as recognized by the Aplysia depilans gonad lectin complexed to colloidal gold, follows closely the immunolocalization of abscisic acid. The secretory process of the root mucilage and the translocation of abscisic acid may be related. The significance of the apparent relationship between abscisic acid accumulation and the secretion of polygalacturonic acids is not yet understood. A role for apoplastic abscisic acid in a root-to-shoot communication system is discussed. Key words: abscisic acid, root physiology, immunolocalization, tomato.