TIME-COURSE AND AUXIN SENSITIVITY OF CORTICAL MICROTUBULE REORIENTATION IN MAIZE ROOTS

The kinetics of MT reorientation in primary roots of Zea mays cv. Meri t, were examined 15, 30, 45, and 60 min after horizontal positioning. Confocal microscopy of longitudinal tissue sections showed no change i n MT orientation 15 and 30 min after horizontal placement. However, af ter 45 and 60 min, MTs of the outer 4-5 cortical cell layers along the lower side were reoriented. In order to test whether MT reorientation during graviresponse is caused by an auxin gradient, we examined the organization of MTs in roots that were incubated for 1 h in solutions containing 10(-9) to 10(-6) M IAA. IAA treatment at 10(-8) M or less s howed no major or consistent changes but 10(-7) M IAA resulted in MT r eorientation in the cortex. The auxin effect does not appear to be aci d-induced since benzoic acid (10(-5) M) did not cause MT reorientation . The region closest to the maturation zone was most sensitive to IAA. The data indicate that early stages of gravity induced curvature occu r in the absence of MT reorientation but sustained curvature leads to reoriented MTs in the outer cortex. Growth inhibition along the lower side of graviresponding roots appears to result from asymmetric distri bution of auxin following gravistimulation.