ABSTRACT
ABSTRACT Infection of Nicotiana obtusifolia plant introduction (PI) #555573 by the downy mildew pathogen Peronospora tabacina resulted in a compatible interaction, in which P. tabacina penetrated and freely colonized host leaf tissue. This interaction became incompatible 5 to 6 days later, with the appearance of necrotic lesions (NLs) and inhibition of pathogen growth and subsequent sporulation. NL development depended upon the presence of P. tabacina in host tissue, was not due to the effects of other microbes, and occurred co-incident in time with the pathogen's ability to produce asexual sporangia on a susceptible N. obtusifolia genotype. Inhibition of the necrotic response by CoCl(2) (a calcium channel blocker) and pathogen-induced transcription of a defense-related gene (PR-1a) suggested that necrosis was due to hypersensitive cell death in the host. In contrast, N. obtusifolia PI#555543 did not exhibit hypersensitivity upon infection by P. tabacina, but rather developed characteristic symptoms of tobacco blue mold disease: chlorotic lesions accompanied by abundant pathogen sporulation. Disease reactions scored on PI#555573 x PI#555543 F(2) progeny inoculated with P. tabacina sporangia indicated that the resistance phenotype was due to the action of a single gene from N. obtusifolia PI#555573, which we have named Rpt1. To date, Rpt1 is the only gene known to confer a hypersensitive response (HR) to P. tabacina infection in any species of Nicotiana. A survey of wild N. obtusifolia revealed that the HR to P. tabacina was expressed in the progeny of 7 of 21 (33%) plants collected in southern Arizona, but not in the progeny of plants originating from Death Valley National Park in California and the Big Bend National Park in west Texas.
ABSTRACT
ABSTRACT Peronospora tabacina is an obligate plant pathogen that causes downy mildew disease on several species of Nicotiana, including N. tabacum (tobacco). The primary objective of this study was to use gnotobiotic associations to describe interactions between the pathogen and roots of either N. tabacum (cv. KY14) or N. repanda. We found that the pathogen was capable of moving systemically from shoots to roots of both host species and emerged from the root tissues as hyphae. We also demonstrated that root-associated hyphae were infectious on roots of nearby plants and resulted in new systemic infections. Following overnight darkness, sporulation of the pathogen was observed on infected roots exposed to air on both host species. We also found that within 2 months in culture, structures resembling resting stages of Peronospora tabacina were produced on hyphae emerging from roots of N. repanda but not N. tabacum. These findings appear relevant to both the epidemiology of the disease and to future studies of this and other downy mildew pathosystems.
ABSTRACT
ABSTRACT Long-term cocultures of the tobacco blue mold pathogen, Peronospora tabacina, with Nicotiana tabacum and N. repanda callus were derived from infected host plant tissue. In this apparently contaminant-free system, sporulation occurred under similar conditions as in intact plants. Sporangia were collected from cocultures and used to complete Koch's postulates. The cocultures were grown under two light regimes. One consisted of 23 h of light followed by 1 h of darkness and the second comprised total darkness. Sporulation occurred frequently in the 23 h light-grown cocultures but resulted in production of abnormal sporangiophores and sporangia. Production of normal sporangiophores and sporangia was achieved by transferring light-grown cocultures to overnight darkness and resulted in necrosis of the callus. Cocultures of Peronospora tabacina with either host species, grown in total darkness, frequently sporulated with minimal necrosis over the course of 1 year. Thus, cocultures should prove useful as a source of Peronospora tabacina over extended periods of time at low risk of pathogen release, for studying the physiology of Peronospora tabacina- Nicotiana interactions, maintaining Peronospora tabacina lines for genetic studies, and providing a reliable source of axenic inoculum for research.