Genomics and transcriptomics of plant beneficial Serratia spp
Abstract: Plant growth stimulation and antagonism against phytopathogens by rhizobacteria are widely recognised phenomena, but the variation in the underlying causal mechanisms between and within different bacterial taxa is still poorly understood. Some bacteria of the genus Serratia are known to be associated with plant roots, and have potential as possible biocontrol agents in agriculture. This thesis provides the detailed genomic structure of three Serratia plymuthica and one S. proteamaculans isolates with their ability to promote oilseed rape plant growth and to inhibit its fungal pathogen, Rhizoctonia solani AG 2-1. Differences in the composition of these genomes were reflected in different phenotypes associated with antagonism and plant growth stimulation observed in different experiments. Their genomes contain the basic genetic traits required for survival in the rhizosphere, root colonisation, antibiosis, induced systemic resistance and production of phytohormones and other factors involved in plant growth promotion. Genetically S. proteamaculans is highly diverged from S. plymuthica isolates and was a better plant growth promoter. The three S. plymuthica genomes are almost identical, however significant variation was observed in plant growth promotion and antagonism by these three isolates. A possible explanation could be due to mutation in one or more putative genes. Genome-wide expression profiling of S. plymuthica and S. proteamaculans revealed that they responded to R. solani by activating genes associated with antibiosis, competition and defence mechanisms. Antibiosis seems to be a major mode of action in S. plymuthica AS13 since genes for pyrrolnitrin biosynthesis and transporters were highly up-regulated while in S. proteamaculans S4, combination of competition, defense mechanism and antibiosis play a role in antagonism. The results suggest that there is much variation in bacterial antagonism at the species level.
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