Possible involvement of 65 kDa MAP in elongation growth of azuki bean epicotyls

Although regulation of the dynamics of plant microtubules (MTs) by microtub ule-associated proteins (MAPs) has been suggested, the mechanism has not ye t been elucidated. As one candidate, a MAP composed of a 65 kDa polypeptide (65 kDa MAP) has been isolated from tobacco cultured cells [Jiang and Sono be (1993), J. Cell Sci. 105: 891]. To investigate the physiological role of the 65 kDa MAP in situ, me analyzed the changes in content and colocalizat ion of this MAP with cortical MTs in relation to elongation growth, using a zuki bean epicotyls (Vigna angularis Ohwi et Ohashi). All apical, intermedi ate, and basal segments prepared from 6 d seedlings showed high growth acti vity. In 12 d seedlings, growth activity of intermediate and basal segments was low, although that of apical segments was high. The relationship betwe en the growth activity and the orientation of cortical MTs in the epidermal cells was analyzed. Cells could be classified into four types with respect to orientation of cortical MTs: transverse (T), oblique (O), longitudinal (L) to the vertical axis of cells, and random (R). In rapidly growing segme nts, three types of cells, T, O, L, were observed at similar ratios. In suc h segments, significant amounts of the 65 kDa MAP were expressed, and it co localized well with cortical MTs. In segments showing low growth activity, most of the cells showed oblique and longitudinal orientation of cortical M Ts. In such segments, the content of the 65 kDa MAP was low These results s uggested involvement of this 65 kDa MAP in regulation of the elongation gro wth of this epicotyl.