Intragenic recombination of a single plant pathogen gene provides a mechanism for the evolution of new host specificities

Abstract

Gene pthA is required for virulence of Xanthomonas citri on citrus plants and has pleiotropic pathogenicity and avirulence functions when transferred to many different xanthomonads. DNA sequencing revealed that pthA belongs to a family of Xanthomonas avirulence/pathogenicity genes characterized by nearly identical 102-bp tandem repeats in the central region. By inserting an nptI-sac cartridge into the tandemly repeated region of pthA as a selective marker, intragenic recombination among homologous repeats was observed in both Xanthomonas spp. and Escherichia coli. Intragenic recombination within pthA created new genes with novel host specificities and altered pathogenicity and/or avirulence phenotypes. Many pthA recombinants gained or lost avirulence function in pathogenicity assays on bean, citrus, and cotton cultivars. Although the ability to induce cell division (hyperplastic cankers) on citrus could be lost, this ability was not acquired on cotton or bean plants. Intragenic recombination therefore provides a genetic mechanism for the generation of multiple, different, and gratuitous avirulence genes from a single, required, host-specific pathogenicity gene.In gene-for-gene interactions between plants and microbial plant pathogens, host plant resistance results from the genetic recognition of resistance (R) genes in the plant and avirulence (avr) genes in the pathogen (10,17). It is unknown why microbial plant pathogens carry genes that function to limit virulence, since the majority of these genes are apparently dispensable (11). Numerous studies of pathogen avr genes have revealed spontaneous loss-of-function mutations (15,20). Not surprisingly, these mutations can involve gene deletion (8) or transposon insertion (16). Mutational analyses also reveal spontaneous gain-of-function mutations that generate new avirulence specificities and new pathogen races at approximately the same frequency as loss-of-function mutations (28). Although presumably an avr gene may be reactivated by reverse mutation, there are no reports of natural mechanisms whereby new avirulence specificities, pathogen races, or avr genes are generated.Xanthomonas citri is the causal agent of citrus canker disease, which occurs worldwide and is subject to eradication and quarantine regulations in many countries. A pathogenicity gene, pthA, is essential for the pathogen to cause hyperplastic canker symptoms on citrus (29). Furthermore, when transferred to other xanthomonads, pthA confers the ability to induce hyperplastic cankers on citrus and a hypersensitive response (HR) on other hosts (30). Therefore, pthA exhibits pleiotropic pathogenicity and avirulence functions. Southern hybridization, restriction analysis, and partial DNA sequencing have shown that pthA belongs to a major avr gene family widespread in the genus Xanthomonas (30). Members of this gene family include avrBs3 and avrBs3-2 of Xanthomonas campestris pv. vesicatoria (2, 3); avrB4, avrb6, avrb7, avrBIn, avrB101, and avrB102 of X. campestris pv. malvacearum (5, 8); and avrXa10 ...

Reiterated repeat region variability in the ciliary adhesin gene of Mycoplasma hyopneumoniae

“…Intragenic recombination amongst tandem repeats in a single gene in Xanthomonas campestris pv. maluacearum provide a mechanism for the evolution of new host specificities (Yang & Gabriel, 1995). Surface proteins displaying reiterated repeats are increasingly being reported in mycoplasmas (Proft et al, 1995;Zheng et al, 1995;Zhang & Wise 1996) and phylogenetically related Grampositive bacteria (Wren, 1991 ;Wastfelt et al, 1996).…”

Diversity and Plasticity of the Intracellular Plant Pathogen and Insect Symbiont “Candidatus Liberibacter asiaticus” as Revealed by Hypervariable Prophage Genes with Intragenic Tandem Repeats

“…pthA contains 17.5 copies of a nearly identical tandem repeat unit that encodes 34 amino acids and is responsible for pathogenicity and host specificity (50). It undergoes intragenic recombination resulting in rapid adaptations to new hosts (9,52), while the expression of pthA within plant cells elicits division, enlargement, and cell death, mimicking canker (14). Genes that contain coding tandem repeats often encode proteins with diverse functions, and the variable numbers of tandem repeats affect the pathogenicity or antigenicity in several human/animal pathogens, such as Treponema denticola and Streptococcus agalactiae (23,35,36).…”

Genome Sequence of Xanthomonas oryzae pv. oryzae Suggests Contribution of Large Numbers of Effector Genes and Insertion Sequences to Its Race Diversity

“…To our knowledge, there are no reports of natural mechanisms of how new avr genes or race differentiation are generated. Mutational analyses of avrBs3/ pth genes revealed that the order and type of repeats in the central domain were important for race-specificity or pathogen fitness 3,38,39 . In general, repetitive regions of DNA are known to be active sites for homologous recombination 28 .…”