Mechano-sensitive orientation of cortical microtubules during gravitropism in azuki bean epicotyls

The orientation of cortical microtubules (cMT) during gravitropism was studied in epidermal cells of azuki epicotyls. The relative proportion of cells with longitudinal cMT increased in the upper epidermis, and those with transverse cMT increased in the lower epidermis. When epicotyls were kept straight during gravistimulation, no change in cMT orientation occurred in either the upper and lower epidermis. When epicotyls were forced to bend downward, cells with transverse cMT increased in the upper epidermis, and those with longitudinal cMT increased in the lower epidermis. When epicotyls were loaded with naphthylphthalamic acid, an inhibitor of auxin transport, both gravitropic bending and change in cMT orientation were inhibited. However, when a change in cMT orientation was induced by forced downward bending, cells with longitudinal cMT increased in the compressed (lower) side and those with transverse cMT increased in the extended (upper) side. It was suggested that cMT orientation was controlled by the bending of the epicotyl and not by a gravity signal per se. Loading with Gd3+, an inhibitor of the stretch-activated channel, did not inhibit gravitropic bending. However, it inhibited cMT reorientation induced by gravitropic bending and by forced bending. Involvement of the stretch-activated channel in mechano-sensitive orientation of cMT was suggested.