Localization of a cytokinin-binding protein CBP57/S-adenosyl-l-homocysteine hydrolase in a tobacco root

Previously we identified the 57 kDa subunit (CBP57) of a cytokinin-binding protein complex in tobacco as S-adenosyl-L-homocysteine (SAH) hydrolase, which is known as a regulatory enzyme for methyl transfer reactions. We investigated the expression of the enzyme in tobacco roots and cultured cells. In cultured cells, CBP57/SAH hydrolase was constitutively expressed independently of cell proliferation. Immunohistochemistry indicated that the enzyme is distributed in the cortex, stele and the root cap. The highest expression was observed in the boundary between elongation and specialization zones of a root tip. These results suggest that CBP57/SAH hydrolase may be involved in differentiation rather than cell proliferation. The physiological function of CBP57/SAH hydrolase is discussed.