Pseudomonas syringae pv. tomato DC3000 Uses Constitutive and Apoplast-Induced Nutrient Assimilation Pathways to Catabolize Nutrients That Are Abundant in the Tomato Apoplast

The plant apoplast is the intercellular space that surrounds plant cells, in which metabolic and physiological processes relating to cell wall biosynthesis, nutrient transport, and stress responses occur. The apoplast is also the primary site of infection for hemibiotrophic pathogens such as P. syringae, which obtain nutrients directly from apoplastic fluid. We have used apoplastic fluid extracted from healthy tomato leaves as a growth medium for Pseudomonas spp. in order to investigate the role of apoplastic nutrients in plant colonization by Pseudomonas syringae. We have confirmed that apoplast extracts mimic some of the environmental and nutritional conditions that bacteria encounter during apoplast colonization by demonstrating that expression of the plant-induced type III protein secretion pathway is upregulated during bacterial growth in apoplast extracts. We used a modified phenoarray technique to show that apoplast-adapted P. syringae pv. tomato DC3000 expresses nutrient utilization pathways that allow it to use sugars, organic acids, and amino acids that are highly abundant in the tomato apoplast. Comparative analyses of the nutrient utilization profiles of the genome-sequenced strains P. syringae pv. tomato DC3000, P. syringae pv. syringae B728a, P. syringae pv. phaseolicola 1448A, and the unsequenced strain P. syringae pv. tabaci 11528 with nine other genome-sequenced strains of Pseudomonas provide further evidence that P. syringae strains are adapted to use nutrients that are abundant in the leaf apoplast. Interestingly, P. syringae pv. phaseolicola 1448A lacks many of the nutrient utilization abilities that are present in three other P. syringae strains tested, which can be directly linked to differences in the P. syringae pv. phaseolicola 1448A genome.