MODULATION OF FUSICOCCIN-BINDING PROTEIN-ACTIVITY IN MUNG BEAN (VIGNA-RADIATA L) HYPOCOTYLS BY TISSUE MATURATION AND BY FUSICOCCIN

The phytotoxin fusicoccin (FC), after binding to a plasma membrane-loc alized receptor, causes higher plant cells to excrete protons. Ligand- binding analysis has been used to show that the plasma membrane of mun g bean (Vigna radiata L.) hypocotyls contains both high-affinity (HA) and tow-affinity (LA) binding sites for FC. The effect of tissue matur ation on these sites was determined on isolated membrane vesicles from the meristematic region (hook) and the elongation zone and from matur e hypocotyl tissues. In the meristematic region the HA:LA ratio was 1: 20. As hypocotyl tissues matured, the site density of HA increased and there was no change in LA density, so that the HA:LA ratio increased to 1:2 in mature tissues. PC-induced proton excretion correlates with the HA density, not the LA density. When sections isolated from each r egion were incubated with FC prior to isolation of membranes, there wa s an apparent conversion of LA to HA sites during the first 90 min in all regions. During the next 1 to 3 h there was a further 2.5- to 3-fo ld increase in binding sites in all regions, accompanied by a slight d ecline in dissociation constant. The increase in binding sites, but no t the apparent conversion of LA to HA, was partly blocked by cyclohexi mide. These data suggest that FC alters FC-binding protein activity in two ways: first, by causing an increase in affinity for FC of preexis ting LA receptors, and second by inducing the synthesis of additional FC receptors. This apparent up-regulation of a phytotoxin receptor by its ligand in plants has not previously been reported.