This family consists of Rhizobium NolX and Xanthomonas HrpF proteins. The interaction between the plant pathogen Xanthomonas campestris pv. vesicatoria and its host plants is controlled by hrp genes (hypersensitive reaction and pathogenicity), which encode a type III protein secretion system. Among type III-secreted proteins are avirulence proteins, effectors involved in the induction of plant defence reactions. HrpF is dispensable for protein secretion but required for AvrBs3 recognition in planta, is thought to function as a translocator of effector proteins into the host cell [1]. NolX, a soybean cultivar specificity protein, is secreted by a type III secretion system (TTSS) and shows homology to HrpF of the plant pathogen Xanthomonas campestris pv. vesicatoria. It is not known whether NolX functions at the bacterium-plant interface or acts inside the host cell. NolX is expressed in planta only during the early stages of nodule development [2].

1: HrpB2 and HrpF from Xanthomonas are type III-secreted proteins and essential for pathogenicity and recognition by the host plant. Rossier O, Van den Ackerveken G, Bonas U; Mol Microbiol 2000;38:828-838. PMID:11115117

2: NolX of Sinorhizobium fredii USDA257, a type III-secreted protein involved in host range determination, Iis localized in the infection threads of cowpea (Vigna unguiculata [L.] Walp) and soybean (Glycine max [L.] Merr.) nodules. Krishnan HB; J Bacteriol 2002;184:831-839. PMID:11790754