| Quorum sensing (QS) within the phyllosphere is affected by many biotic and abiotic elements. QS has been classically characterized as operative in assemblages of high cell density planktonic cells or within biofilms wherein signal of a sufficient concentration to trigger positive feedback loops is achieved. In these conditions, QS activation is defined by or limited to the large volumes of liquid into which signal diffuses away from cells. The plant pathogenic Pseudomonas syringae pv. syringae (Pss ) exhibits quorum sensing utilizing the production of the diffusible signal molecules called N-(acyt)-L-homoserine lactones (AHL). QS in Pss controls genes conferring extracellular polysaccharide production, motility, and factors contributing to virulence to bean only when cells reach a relatively high local concentration. QS deficient mutants are hypermotile and entered moist bean leaves more readily and incite formation of more lesions after topical application to bean leaves than the parental strain. We analyzed quorum sensing as affected by water availability in Pss in its natural habitat on leaf surfaces. Using confocal microscopy, we utilized the red fluorescent protein mRFP1 as an AHL-dependent reporter of autoinduction in strains harboring a GFP marker gene allowing us to account for the total epiphytic cell population of Pss. On leaves, it is clear that cells can exhibit QS behavior quickly and when found in small groups on dry leaves where AHL signal diffusion is apparently restricted. QS in Pss is affected by about 18% of culturable epiphytic bacteria via production of small diffusible molecules. About 7% of bacterial epiphytes produced the same AHL, often in amounts more than 20-fold higher, as Pss. While coinoculation of AHL-producing strains with Pss reduced the number of lesions when sprayed together on leaves compared with that of plants inoculated with Pss alone, increased lesion number occurred on bean coinoculated with Pss and QS-inhibiting strains. Premature induction of QS in Pss thus inhibits disease initiation and can be exploited for disease control. Mutants of QS-interfering strains deficient in this phenotype did not increase disease when coinoculated with Pss. Analysis of mutants in QS-interfering strains revealed interruptions in genes involved in iron-uptake, such as corA, mvaT, and mucA, suggesting iron plays an inductive role in QS. Supplementation of iron increases the production of AHLs, affects downstream quorum regulated traits, and reduced the number of lesions on Pss-inoculated bean. An additional mutagenesis screen of Pss in iron depleted conditions revealed genes, ptsP, sigX, and oqsR, involved in iron-uptake that regulate QS. These genes have provided a clear link between iron and QS regulation of virulence in Pss.. |
