Conditional Requirement for Chemotaxis in Induction of Rooty Tumors by Agrobacterium Tumefaciens on Roots of Pisum Sativum

The rhizosphere is a zone of intense microbial activity where the successful establishment of symbiotic or pathogenic organisms can have a dramatic impact on plant health. Little is known about production and regulation of plant factors that influence the rhizosphere environment. Components of exudates from roots can influence the gene expression and growth of soilborne microorganisms (for ex Stachel et al 1985, Peters et al 1986, reviewed in Curl & Truelove 1986), and can act as chemoattractants (for ex Ashby et al 1988, Caetano-Anolles et al 1988, Hawes et al 1988). The primary sources of such exudates in healthy plants of some species are secretory cells from the root cap (Oades 1978). Sloughed root cap (SRC) cells, which can be isolated nondestructively (Hawes and Wheeler 1982), and can be cultured (Hawes and Pueppke 1986), have been shown to survive for some time in the rhizosphere independently of the root (Vermeer and McCully 1982). Not unexpectedly, carbon-rich SRC cells can support the growth of bacteria in culture (Hawes and Pueppke 1989), and have been shown to act as nuclei for microbial colonies in soil. However, a series of studies in my laboratory suggest that the impact of these cell populations may go beyond the nonspecific stimulation of microbial growth. In studies with several fungal and bacterial pathogens, I have found that the cells from different plant species and genotypes exhibit dramatic selectivity in binding, chemotaxis, and susceptibility to infection (Goldberg et al 1989, Hawes 1983, 1989, Hawes & Pueppke 1985, 1986, 1989, Hawes et al 1989, Hawes & Wheeler 1982). I am using Agrobacterium tumefaciens as a model system to test the hypothesis that the expression of such properties in SRC cells may act to regulate microbial populations in the rhizosphere.