Introduction
Citrus huanglongbing (HLB), caused by
Candidatus
Liberibacter asiaticus (CLas), is a globally devastating plant disease characterized by severe disruption of host physiology. Although alterations in plant hormone homeostasis, including salicylic acid and abscisic acid (ABA), are hallmarks of HLB, the contribution of plant-associated microbiota to these hormonal changes remains poorly understood. Here, we investigated whether ABA depletion in HLB-affected citrus is associated with microbial metabolism.
Methods
ABA and its metabolites were quantified in HLB-affected citrus tissues by liquid chromatography–mass spectrometry. The capacity of CLas strain Ishi-1 and
Flavobacterium
spp. strains CGRL1 and CGRL2 to metabolize ABA was assessed using
in vitro
culture assays. The effects of exogenous ABA on bacterial growth were also evaluated.
Results
ABA concentrations were significantly decreased in symptomatic HLB-affected tissues, concomitant with the accumulation of the ABA catabolite dehydrovomifoliol. Furthermore, CLas strain Ishi-1 and
Flavobacterium
strains CGRL1 and CGRL2 degraded ABA and produced dehydrovomifoliol, whereas no such activity was detected in non-inoculated controls. Notably, exogenous ABA promoted bacterial growth, suggesting that ABA may serve as a metabolic substrate or growth-enhancing factor for these bacteria.
Discussion
These findings identify microbial ABA metabolism as a potential driver of hormone depletion in HLB-affected citrus. Our results further suggest that CLas and its associated bacteria may actively reshape host hormone homeostasis through ABA metabolism, providing a mechanistic link between pathogen-associated microbial communities and hormonal imbalance during HLB development.