Amino acid residues required for the activity of avrD alleles

Comparison of avrD alleles from Pseudomonas syringae pv. glycinea

Avirulence gene D alleles resided on indigenous plasmids in races 0, 2, 3, 4, 5, and 6 of Pseudomonas syringae pv. glycinea (Psg), but the allele in race 1 appeared to be chromosomal. These were all nonfunctional avirulence genes because they neither induced the avirulence phenotype on Rpg4 soybean cultivars nor directed the production of syringolide elicitors when expressed in Escherichia coli cells. The predicted proteins encoded by the seven Psg avrD genes were very similar to that of a functional class II allele from P. syringae pv. phaseolicola G50 race 2, but contained mutations collectively affecting only nine amino acid positions. Despite these relatively small amino acid differences and the location of avrD from each isolate on a 5.6-kb HindIII restriction fragment, the flanking regions varied considerably among the Psg isolates. The presence of avrD alleles with few alterations but different locational contexts in all tested Psg races argues that they provide an important selected function in the bacteria but have been modified to escape defense surveillance in Rpg4 soybean plants.

Differential effect of site-directed mutations in pelC on pectate lyase activity, plant tissue maceration, and elicitor activity

Oligonucleotide site-directed mutations were introduced into the pelC gene of Erwinia chrysanthemi EC16 that directed single or double amino acid changes affecting disulfide linkages, calcium binding, catalysis, and protein folding. Subsequent characterization of the purified PelC mutant proteins demonstrated that pectinolytic function involves amino acids located near the calcium binding site rather than those surrounding an invariant vWiDH sequence. Wild-type PelC and the tested mutant proteins generally macerated plant tissue in proportion to their specific pectinolytic activity in vitro. However, some mutants gave higher maceration activity in plant tissue and elicited greater production of the phytoalexin, glyceollin, in soybean cotyledons than predicted by their in vitro pectinolytic activity. Most notable in this regard were three different mutations at lysine 172 with greatly reduced pectinolytic activity but as much elicitor activity as the wild-type protein. PelE macerated plant tissue 10 times more efficiently than PelC, as observed previously, but surprisingly showed equal activity in the elicitor assay. The results indicate that factors other than pectinolytic activity per se are involved in plant tissue maceration and elicitor activity.

Bacterial avirulence genes

Although more than 30 bacterial avirulence genes have been cloned and characterized, the function of the gene products in the elictitation of resistance is unknown in all cases but one. The product of avrD from Pseudomonas syringae pv. glycinea likely functions indirectly to elicit resistance in soybean, that is, evidence suggests the gene product is an enzyme involved in elicitor production. In most if not all cases, bacterial avirulence gene function is dependent on interactions with the hypersensitive response and pathogenicity (hrp) genes. Many hrp genes are similar to genes involved in delivery of pathogenicity factors in mammalian bacterial pathogens. Thus, analogies between mammalian and plant pathogens may provide needed clues to elucidate how virulence gene products control induction of resistance.