PIS1, A NEGATIVE REGULATOR OF THE ACTION OF AUXIN TRANSPORT INHIBITORS IN ARABIDOPSIS-THALIANA

In order to clarify the mechanism underlying the polar auxin transport system, the pis1 mutant in Arabidopsis thaliana that is hypersensitiv e to N-1-naphthylphthalamic acid (NPA), an auxin transport inhibitor w as isolated and characterized. Whereas the pis I mutant is normally se nsitive to phytohormones, auxins, cytokinin and ethylene precursor, th is mutant is hypersensitive to NPA over the broad spectrum of its effe cts such as growth of seedlings, root elongation, root gravitropism, r oot phototropism and root curling. This result indicates that the pis? mutant is specifically affected in the polar auxin transport system. This result also defines a genetic factor controlling both gravitropis m and phototropism, and strongly indicates the involvement of auxin tr ansport during both tropic responses. NPA, 2,3,5-triiodobenzoic acid ( TIBA) and 9-hydroxyfIuorene-9-carboxylic acid (HFCA) represent differe nt classes of auxin transport inhibitors. The pis1 mutation conferred hypersensitivity to both NPA and TIBA but not to HFCA. These results s how the genetic separation of the actions of NPA/TIBA and of HFCA. The PISI gene product might be specifically involved in the response path way of NPA/TIBA, leading to interference with auxin-efflux carriers, a nd might act as a negative regulator of the action of NPA/TIBA.