Basel L. E., Zukowski A. T., Cleland R. E.
Botany Department, University of Washington, Seattle, Washington 98195.
Plant Physiol. 1994 Feb;104(2):691-697. doi: 10.1104/pp.104.2.691.
The phytotoxin fusicoccin (FC), after binding to a plasma membrane-localized receptor, causes higher plant cells to excrete protons. Ligand-binding analysis has been used to show that the plasma membrane of mung bean (Vigna radiata L.) hypocotyls contains both high-affinity (HA) and low-affinity (LA) binding sites for FC. The effect of tissue maturation on these sites was determined on isolated membrane vesicles from the meristematic region (hook) and the elongation zone and from mature 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 maturet issues. FC-induced proton excretion correlates with the HA density, not the LA density. When sections isolated from each region were incubated with FC prior to isolation of membranes, there was 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- fold increase in binding sites in all regions, accompanied by a slight decline in dissociation constant. The increase in binding sites, but not the apparent conversion of LA to HA, was partly blocked by cycloheximide. These data suggest that FC alters FC-binding protein activity in two ways: first, by causing an increase in affinity for FC of preexisting 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.
植物毒素壳梭孢菌素（FC）与质膜定位受体结合后，会使高等植物细胞分泌质子。配体结合分析已用于表明绿豆（Vigna radiata L.）下胚轴的质膜含有FC的高亲和力（HA）和低亲和力（LA）结合位点。通过从分生组织区域（弯钩）、伸长区以及成熟下胚轴组织分离的膜泡，测定了组织成熟对这些位点的影响。在分生组织区域，HA:LA比例为1:20。随着下胚轴组织成熟，HA的位点密度增加，而LA密度没有变化，因此在成熟组织中HA:LA比例增加到1:2。FC诱导的质子排泄与HA密度相关，而非LA密度。当从每个区域分离的切片在分离膜之前与FC一起孵育时，在所有区域的最初90分钟内，LA位点明显转化为HA位点。在接下来的1至3小时内，所有区域的结合位点进一步增加2.5至3倍，同时解离常数略有下降。结合位点的增加，但不是LA到HA的明显转化，部分被放线菌酮阻断。这些数据表明，FC以两种方式改变FC结合蛋白活性：第一，通过使预先存在的LA受体对FC的亲和力增加；第二，通过诱导额外的FC受体的合成。植物中其配体对植物毒素受体的这种明显上调作用以前尚未见报道。
