Host-pathogen interaction of the tepary bean, Phaseolus acutifolius with the common bean blight pathogen, Xanthomonas campestris pv. phaseoli

Little is known about host specialization in Xanthomonas campestris pv. phaseoli (Xcp). Variations in virulence on cultivars of Phaseolus vulgaris are well known. While high levels of resistance in P. acutifolius, a natural host of Xcp, has been shown, its use as a differential for the pathogen was unknown. Thus, this is the first report of host specialization in Xcp into pathogenic races by using phenotypic reactions in P. acutifolius. The identification of Xcp races on tepary differentials is important for the identification of virulence genes in the pathogen and resistant genes in the host and for the study of pathogen distribution in specific geographical localities. Seven pathogenic races were determined by qualitative analysis (hypersensitivity (HR) and water-soaking (WS)) on detached pods and cotyledons. Two bacterial phenotypes were found on compatible interactions, strains able to induce water-soaking (WS) plus callus (CA+) and strains able to induce only WS (CA$-$). Wild Xcp strains representing the CA+ and CA$-$ and also a non-pathogenic mutant (CA$-$) were studied for the production of hormones in vitro and their relationship to symptom induction in planta. The phytohormones indole-3-acetic acid (IAA) and isopentenyl adenosine (iPA) were detected in Xcp filtrates. It was found that CA$-$ produce more IAA than CA+. In contrast, the cytokinin iPA was detected greater by CA+ than CA$-$. Studies using IAA deficient (IAA$-$) and high producer (IAA++) mutants showed no difference in the ability to induce callus. Also bacterial filtrates containing IAA did not induce callus. Production of IAA by the bacterium could be involved during the host-pathogen interaction as a cytokinin activator but the ratio of the hormones may influence the phenotypic expression. Polypeptide differences were detected during the early stages of pathogenesis. The detection of a 24 kD 3 polypeptide was characteristic of the compatible line but not of the incompatible lines during a period of 6 hours after inoculation. This finding may contrast to reports in which the incompatible systems are faster in their response to infection than compatible systems when measured several days after inoculation