Genes expressed by the biological control bacterium Pseudomonas protegens Pf‐5 on seed surfaces under the control of the global regulators GacA and RpoS

Abstract

Gene expression profiles of the biological control strain Pseudomonas protegens Pf-5 inhabiting pea seed surfaces were revealed using a whole-genome oligonucleotide microarray. We identified genes expressed by Pf-5 under the control of two global regulators (GacA and RpoS) known to influence biological control and secondary metabolism. Transcript levels of 897 genes, including many with unknown functions as well as those for biofilm formation, cyclic diguanylate (c-di-GMP) signalling, iron homeostasis and secondary metabolism, were influenced by one or both regulators, providing evidence for expression of these genes by Pf-5 on seed surfaces. Comparison of the GacA and RpoS transcriptomes defined for Pf-5 grown on seed versus in broth culture overlapped, but most genes were regulated by GacA or RpoS under only one condition, likely due to differing levels of expression in the two conditions. We quantified secondary metabolites produced by Pf-5 and gacA and rpoS mutants on seed and in culture, and found that production profiles corresponded generally with biosynthetic gene expression profiles. Future studies evaluating biological control mechanisms can now focus on genes expressed by Pf-5 on seed surfaces, the habitat where the bacterium interacts with seed-infecting pathogens to suppress seedling diseases.

Secondary Metabolism and Interspecific Competition Affect Accumulation of Spontaneous Mutants in the GacS-GacA Regulatory System in Pseudomonas protegens

“…The wild-type subpopulation may dominate in other conditions, such as in the presence of competitors when GacS-GacA is needed to optimize fitness. Given the known role of GacS-GacA in controlling diverse cellular traits, including motility, biofilm formation, and secondary metabolism (15,16,20), spontaneous mutation of gacS or gacA provides a mechanism to regulate bacterial adaption at both cellular and population levels.…”

“…Accordingly, different GacS-GacA-regulated phenotypes are likely to influence the relative fitness of Gac Ϫ mutants versus wild type in different conditions. For example, the motility of strain Pf-5 is influenced by the GacS-GacA system (16), and the predominance of Gac Ϫ mutants at the advancing edge of swarming colonies of P. protegens Pf-5 has been attributed to their hyperflagellation (17). Biofilm formation is also influenced by the GacS-GacA system (16), and Driscoll et al (23) reported evidence for a mutualism between Gac Ϫ mutant and wild-type cells of P. chlororaphis 30-84 in laboratory biofilms.…”

“…In the presence of an unknown signal(s), the sensor kinase GacS autophosphorylates and transfers a phosphate group to the response regulator GacA, which governs a complex signal transduction pathway involving regulatory RNAs and translational repression (14). The GacS-GacA system controls the expression of hundreds of genes, including those required for the production of secondary metabolites and extracellular enzymes (15)(16)(17)(18)(19). Accordingly, mutation of gacA and/or gacS has pleiotropic effects on the bacterial cell, influencing motility, iron acquisition, biofilm formation, aspects of primary metabolism, and many other phenotypes (15,16,20).…”

“…The GacS-GacA system controls the expression of hundreds of genes, including those required for the production of secondary metabolites and extracellular enzymes (15)(16)(17)(18)(19). Accordingly, mutation of gacA and/or gacS has pleiotropic effects on the bacterial cell, influencing motility, iron acquisition, biofilm formation, aspects of primary metabolism, and many other phenotypes (15,16,20). Given the broad influence of the GacS-GacA system on bacterial physiology, it is striking that spontaneous gacS and gacA mutants (i.e., Gac Ϫ mutants) emerge and accumulate in cultures of Pseudomonas spp.…”

Sustainable Agriculture—Enhancing Environmental Benefits, Food Nutritional Quality and Building Crop Resilience to Abiotic and Biotic Stresses

“…For example, A. aculeatus Asp-4 up-regulated numerous genes and proteins involved in mitigation of environmental stress and dissolution of sclerotial compounds during colonization and degradation of sclerotia of S. sclerotiorum [81]. Omic studies of other biological control interactions revealed equally complex gene and/or protein expression profiles by biological control agents during interactions with pathogens or the environment [82][83][84]. In turn, soils and plant surfaces, where biological control agents are expected to effect disease control, are highly heterogeneous regarding compounds and conditions that impact expression of genes important to biological control; ultimately impacting expression of these genes and reliability and efficacy of disease control (reviewed in [80]).…”