Possible involvement of 65 kda MAP in elongation growth of azuki bean epicotyls.

Although regulation of the dynamics of plant microtubules (MTs) by microtubule-associated proteins (MAPs) has been suggested, the mechanism has not yet 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 Sonobe (1993), J. Cell Sci 105: 8911. To investigate the physiological role of the 65 kDa MAP in situ, we analyzed the changes in content and colocalization of this MAP with cortical MTs in relation to elongation growth, using azuki bean epicotyls (Vigna angularis Ohwi et Ohashi). All apical, intermediate, and basal segments prepared from 6 d seedlings showed high growth activity. In 12 d seedlings, growth activity of intermediate and basal segments was low, although that of apical segments was high. The relationship between 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 segments, three types of cells, T, O, L, were observed at similar ratios. In such segments, significant amounts of the 65 kDa MAP were expressed, and it colocalized well with cortical MTs. In segments showing low growth activity, most of the cells showed oblique and longitudinal orientation of cortical MTs. In such segments, the content of the 65 kDa MAP was low. These results suggested involvement of this 65 kDa MAP in regulation of the elongation growth of this epicotyl.