REEVALUATION OF THE ROLE OF AUXIN-BINDING SITE-II

Binding of 1-naphthylacetic acid (1-NAA) was assayed in microsomal mem branes from Zea mays coleoptiles and from hypocotyls of Cucurbita pepo . Auxin binding site II was differentiated from site I binding by usin g phenylacetic acid (PAA) to saturate site I binding capacity. The amo unt of type-II binding sites, per gram original fresh weight, was 34 p mol with Zea and 6.4 pmol with Cucurbita. When maize membranes were se parated by dextran gradient centrifugation, auxin binding site II migr ated coincident with tonoplast marker enzymes. The physiologically act ive auxin 4-chloroindoleacetic acid (4-Cl-IAA) competed very poorly wi th 1-NAA binding to both site I and site II. This result suggests that sites I and II are not involved in the regulation of growth. When com paring isolated outer epidermis with intact coleoptile of Zea, similar amounts and ratios of site I and site II binding activities were obse rved.