Early changes in apoplast composition associated with defence and disease in interactions between Phaseolus vulgaris and the halo blight pathogen Pseudomonas syringae Pv. phaseolicola

Abstract

The apoplast is the arena in which endophytic pathogens such as Pseudomonas syringae grow and interact with plant cells. Using metabolomic and ion analysis techniques, this study shows how the composition of Phaseolus vulgaris leaf apoplastic fluid changes during the first six hours of compatible and incompatible interactions with two strains of P. syringae pv. phaseolicola (Pph) that differ in the presence of the genomic island PPHGI‐1. Leaf inoculation with the avirulent island‐carrying strain Pph 1302A elicited effector‐triggered immunity (ETI) and resulted in specific changes in apoplast composition, including increases in conductivity, pH, citrate, γ‐aminobutyrate (GABA) and K+, that are linked to the onset of plant defence responses. Other apoplastic changes, including increases in Ca2+, Fe2/3+ Mg2+, sucrose, β‐cyanoalanine and several amino acids, occurred to a relatively similar extent in interactions with both Pph 1302A and the virulent, island‐less strain Pph RJ3. Metabolic footprinting experiments established that Pph preferentially metabolizes malate, glucose and glutamate, but excludes certain other abundant apoplastic metabolites, including citrate and GABA, until preferred metabolites are depleted. These results demonstrate that Pph is well‐adapted to the leaf apoplast metabolic environment and that loss of PPHGI‐1 enables Pph to avoid changes in apoplast composition linked to plant defences.

Defining essential processes in plant pathogenesis with Pseudomonas syringae pv. tomato DC3000 disarmed polymutants and a subset of key type III effectors

“…g-Aminobutyric acid (GABA) represents another defenceassociated apoplastic molecule that could affect the expression of the T3SS during pathogenesis (Mirabella et al, 2007;O'Leary et al, 2016). The use of Arabidopsis and tobacco plants altered in GABA metabolism and Pst DC3000 mutants altered in GABA uptake and metabolism that collectively promote higher levels of GABA within bacteria suggest that GABA can also reduce T3E delivery (McCraw et al, 2016;Park et al, 2010).…”

Metabolomics of tomato xylem sap during bacterial wilt reveals Ralstonia solanacearum produces abundant putrescine, a metabolite that accelerates wilt disease

“…Xylem sap is nutrient poor (Fatima and Senthil-Kumar, 2015); most plant carbon is sequestered intracellularly and transported in the phloem. Metabolites are 10-to 100-fold less concentrated in xylem sap than in the phloem or leaf apoplast (Fatima and Senthil-Kumar, 2015;O'Leary et al, 2016). Xylem is also hypoxic, posing further metabolic constraints for R. solanacearum growth (Dalsing et al, 2015).…”

Role of the NAD(P)H quinone oxidoreductase NQR and the cytochrome b AIR12 in controlling superoxide generation at the plasma membrane

“…Reported pH values in the apoplast vary largely depending on the species and the developmental state of the tissue and the used method. For example, values between pH 6.1 in potato leaves (Oja et al 1999) and pH 4.8 in Phaeseolus vulgaris (O'Leary et al 2016) have been reported in the literature. Regarding these pH values, deprotonation of the quinol at the cytoplasmic side of the plasma membrane seems to be the most likely.…”