The microtubule cytoskeleton does not integrate auxin transport and gravitropism in maize roots

The Cholodny-Went hypothesis of gravitropism suggests that the gravirespons e is controlled by the distribution of auxin, Howe, er, the mechanism of au xin transport during the graviresponse of roots is still unresolved. To det ermine whether the microtubule (MT) cytoskeleton is participating in auxin transport, the cytoskeleton was examined and the movement of H-3-IAA measur ed in intact acid excised taxol, oryzalin, and naphthylphthalamic acid (NPA )-treated roots of Zen mays cv. Merit. Taxol and oryzalin did not inhibit t he graviresponse of roots but the auxin transport inhibitor NPA greatly inh ibited both auxin transport and graviresponse, NPA had no effect on MT orga nization in vertical roots, but caused MT reorientation in horizontally pla ced roots. Regardless of treatment, the organization of MTs in intact roots differed from that in root segments. The MT inhibitors, taxol and oryzalin had opposite effects on the MTs, namely, depolymerization (oryzalin) and s tabilization and thickening (taxol), but both treatments caused smelling of the roots. The data indicate that the MT cytoskeleton does not directly in terfere with auxin transport or auxin-mediated growth responses in maize ro ots.