Evidence that γ-aminobutyric acid is a major nitrogen source during Cladosporium fulvum infection of tomato

Abstract

The growth of the biotrophic pathogen Cladosporium fulvum within the tomato (Lycopersicon esculentum Mill.) leaf is restricted to the intercellular space. Previous studies from this laboratory have demonstrated that gamma-aminobutyric acid (GABA) accumulates to millimolar concentrations in the apoplast during a compatible interaction. We decided to further investigate the role of GABA during infection. A gene encoding a required enzyme for GABA metabolism, GABA transaminase (Gat1), was cloned and sequenced from C. fulvum. The predicted protein sequence of Gat1 had high homology to other fungal GABA transaminases, particularly from Aspergillus nidulans. In vitro expression experiments revealed Gat1 to be strongly expressed during fungal growth on both GABA and glutamate whereas nearly no expression was evident during nitrogen starvation conditions. Expression of Gat1 was also apparent during infection, suggesting for the first time that C. fulvum actively metabolises GABA during infection. This indicates that the fungus may be utilising the GABA in the apoplast as a nutrient source. Further analysis revealed that the expression of tomato glutamate decarboxylase, the enzyme responsible for GABA synthesis, appeared appreciably higher during a compatible interaction than in the incompatible interaction. These findings imply that the infecting fungus may alter the physiology of the tomato leaf with the result that a source of nitrogen is supplied.

Comparative study of Trichoderma gene expression in interactions with tomato plants using high-density oligonucleotide microarrays

“…The GABA carbon is converted into succinic semialdehyde and then to succinate in reactions catalysed by these mitochondrial enzymes. An induction of both enzymes has been observed previously in Cladosporium fulvum-tomato and Pseudomonas-Arabidopsis interactions (Solomon & Oliver, 2002;Park et al, 2010), and in the symbiosis of Rhizobium leguminosarum with the pea host plant (Prell et al, 2002). The authors suggested that during the interactions the micro-organisms could alter the physiology of the plant, resulting in an enhanced production of GABA, which in turn plays a signalling role in the induction of the fungal enzymes responsible for its degradation.…”

Targeted Enhancement of Glutamate-to-γ-Aminobutyrate Conversion in Arabidopsis Seeds Affects Carbon-Nitrogen Balance and Storage Reserves in a Development-Dependent Manner

“…Values are means 6 SD from six determinations per time point. stress events (Breitkreuz et al, 1999;Solomon and Oliver, 2002;Beuvé et al, 2004;Bouché and Fromm, 2004). However, only recently has direct evidence, integrating metabolite and transcript profiling and stable-isotope labeling, emerged for the metabolic significance of the GABA shunt (Studart-Guimarães et al, 2007).…”

A Conserved Mechanism of GABA Binding and Antagonism Is Revealed by Structure-Function Analysis of the Periplasmic Binding Protein Atu2422 in Agrobacterium tumefaciens

“…In the ameba Dictyostelium, GABA binds to the protein GrlE, which controls terminal steps of sporulation (4). In plants, GABA accumulates in response to abiotic and biotic stresses (6), including wind, injury, and infection by fungi (7) and symbiotic and pathogenic bacteria, such as Rhizobium leguminosarum (8) and Agrobacterium tumefaciens (9). In A. tumefaciens, GABA acts as a signal that triggers gene expression (10,11).…”