Differential Inactivation of Seed Exudate Stimulation ofPythium ultimumSporangium Germination byEnterobacter cloacaeInfluences Biological Control Efficacy on Different Plant Species

Abstract

This study was initiated to understand whether differential biological control efficacy of Enterobacter cloacae on various plant species is due to differences in the ability of E. cloacae to inactivate the stimulatory activity of seed exudates to Pythium ultimum sporangium germination. In biological control assays, E. cloacae was effective in controlling Pythium damping-off when placed on the seeds of carrot, cotton, cucumber, lettuce, radish, tomato, and wheat but failed to protect corn and pea from damping-off. Seeds from plants such as corn and pea had high rates of exudation, whereas cotton and cucumber seeds had much lower rates of exudation. Patterns of seed exudation and the release of P. ultimum sporangium germination stimulants varied among the plants tested. Seed exudates of plants such as carrot, corn, lettuce, pea, radish, and wheat were generally more stimulatory to P. ultimum than were the exudates of cotton, cucumber, sunflower, and tomato. However, this was not directly related to the ability of E. cloacae to inactivate the stimulatory activity of the exudate and reduce P. ultimum sporangium germination. In the spermosphere, E. cloacae readily reduced the stimulatory activity of seed exudates from all plant species except corn and pea. Our data have shown that the inability of E. cloacae to protect corn and pea seeds from Pythium damping-off is directly related to its ability to inactivate the stimulatory activity of seed exudates. On all other plants tested, E. cloacae was effective in suppressing damping-off and inactivating the stimulatory activity of seed exudates.Pythium ultimum is a widespread and important oomycete plant pathogen causing seed decay, pre-and postemergence damping-off, and root rot in many plant species (8). The sporangium is a major survival structure in soil, even though oospores are produced (29,30). This is especially true for asexual strains of P. ultimum (10). Sporangia are exogenously dormant propagules that will germinate in response to stimulants from the seeds of host plants (14,15,17,19,28). This response occurs within the first few hours of seed germination, resulting in seed infection within the first 24 h after sowing (13). The molecules in seed exudates eliciting these rapid responses are largely unknown (14) but are believed to consist of long-chain unsaturated fatty acids (28).Enterobacter cloacae is a common plant-associated rhizobacterium effective in controlling Pythium diseases (3, 5-7, 9, 12, 13, 16, 22, 23, 31, 33, 34). Other biological control agents such as Pseudomonas and Bacillus species suppress Pythium diseases largely through the biosynthesis of antibiotics or other Pythium-inhibitory substances (2, 32, 34). However, no antibiotic production or parasitism has been found in E. cloacae, even though E. cloacae attaches to the hyphae of P. ultimumcolonizing seed surfaces (16). The control of Pythium dampingoff by E. cloacae can be attributed to its ability to reduce or eliminate responses of sporangia to germinating seeds by metabolizing long-chain...

Management of Plant Pathogens with Microorganisms

“…Typically, SAR is induced by pathogens while ISR is salicylic acidindependent and is induced by non-pathogenic bacteria [55]. SAR is mediated by a compound called salicylic acid which is frequently produced following pathogen infection that leads to expression of pathogenesis related (PR) proteins such as PR-1, PR-2, chitinases, and some peroxidases [47,49,77]. These PR proteins can cause lysis of invading cells, reinforcement of cell membranes to resist infections, or induce localized cell death.…”

“…Using this competition approach, control of soil-borne pathogens such as Fusarium and Pythium that infect through mycelial contact, has been achieved with greater success as compared to other pathogens that directly germinate on plant surfaces and infect through appressoria and infection pegs. Effective catabolism of nutrients has been identified as a mechanism contributing to the suppression of Pythium ultimum by Enterobacter cloacae [47,48]. Active motility and chemotactic response towards chemical attractants present in root exudates include organic acids, amino acids, and specific sugars govern arrival of biocontrol agent to the root surface [21,23,71].…”

Differential Interference with Pythium ultimum Sporangial Activation and Germination by Enterobacter cloacae in the Corn and Cucumber Spermospheres

“…Sporangia can transition from quiescence to active growth in as little as 1.5 h after exposure to a seed (7,19). Seed exudates from many plant species can induce these rapid responses (4,21). Although the specific exudate elicitors from most plants are unknown, the elicitors from cottonseed exudates have been identified as long-chain unsaturated fatty acids (LCUFA), comprised largely of oleic and linoleic acids (9,10,18).…”

“…Plant protection by E. cloacae is achieved largely by degradation of LCUFA in seed exudates (20), which eliminates the stimulation of P. ultimum sporangia (4,21). E. cloacae suppresses P. ultimum infections when applied as a coating onto seeds of plants such as carrot, cotton, cucumber, lettuce, radish, sunflower, tomato, and wheat.…”

“…Enterobacter cloacae is a common seed-associated bacterium that suppresses seed infections, protecting a number of plant species from P. ultimum-induced damping-off (3,4,8,12). Plant protection by E. cloacae is achieved largely by degradation of LCUFA in seed exudates (20), which eliminates the stimulation of P. ultimum sporangia (4,21).…”