Not Just a Cycle: Three gab genes Enable the Non-cyclic Flux Toward Succinate Via GABA Shunt in ‘Candidatus Liberibacter asiaticus’-infected citrus
Although the mitochondria retain all required enzymes for an intact tricarboxylic acid (TCA) cycle, plants might shift the cyclic flux from the TCA cycle to an alternative non-cyclic pathway via γ-aminobutyric acid (GABA) shunt under specific physiological conditions. We hypothesize that several genes may ease this non-cyclic flux and contribute to the citrus response to the phytopathogenic bacterium ‘Candidatus Liberibacter asiaticus’, the causal agent of Huanglongbing in citrus. To test this hypothesis, we used multi-omics techniques (metabolomics, fluxomics, and transcriptomics) to investigate the potential role(s) of putative gab homologies from Valencia sweet orange (Citrus sinensis). Our findings showed that ‘Ca. L. asiaticus’ significantly increased the endogenous GABA and succinate content but decreased ketoglutarate in infected citrus plants. Citrus genome harbors three putative gab genes including amino-acid permease (aka GABA permease; CsgabP), GABA transaminase (CsgabT), and succinate-semialdehyde dehydrogenase (aka GABA dehydrogenase; CsgabD). The transcript levels of CsgabP, CsgabT, and CsgabD were upregulated in citrus leaves upon the infection with ‘Ca. L. asiaticus’ and after the exogenous application of GABA or deuterium-labeled GABA isotope (GABA-D6). Moreover, our finding showed that exogenously applied GABA is quickly converted to succinate and fed into the TCA cycle. Likewise, the fluxomics study showed that GABA-D6 is rapidly metabolized to succinate-D4. Our work proved that GABA shunt and three predicated gab genes from citrus, support the upstream non-cyclic flux toward succinate rather than an intact TCA cycle and contribute to citrus defense responses to ‘Ca. L. asiaticus’.