Exploring the Antimicrobial Potential of Diffusible Signal Factor Against Ca. Liberibacter spp.

Abstract

Huanglongbing (HLB) is the main disease affecting sweet orange production worldwide, leading to significant losses in the sector. It is predominantly caused by the fastidious gram-negative ±-proteobacteria Candidatus Liberibacter asiaticus (CLas) and transmitted by the psyllid, Diaphorina citri. Notably, diffusible signal factor (DSF), a molecule essential to the quorum-sensing (QS) systems of various bacteria, presents promise for HLB control. Extensive research in other pathosystems has revealed DSF's potential to attenuate bacterial virulence, rendering them more susceptible to antimicrobial compounds. However, this property was never explored using DSF from X. fastidiosa. Previous results of our research group have shown citrus transgenic plants overexpressing DSF from X. fastidiosa exhibit increased tolerance to HLB. Thus, since Ca. Liberibacter asiaticus (Clas), causal agent of HLB, lacks the QS bacterial communication system, our results, combined with previous studies with DSF in other pathosystems, raise the hypothesis that DSF may be acting in two ways: first, by activating the plants' immune system, and second, by acting as an antimicrobial molecule, resulting in increased disease tolerance. The first hypothesis is already being investigated by our group (FAPESP 2023/05118-1), however, the potential action of DSF as an antimicrobial and/or antagonist to CLas growth is more challenging to study due to the fact that this bacterium cannot be cultured in vitro. Consequently, this project aims to explore the application of DSF derived from X. fastidiosa as both an antimicrobial agent and an enhancer for peptides and antibiotics, thereby increasing their efficacy. To this end, a series of in vitro experiments utilizing Liberibacter crescens species will be undertaken. Ultimately, this study aims to enhance our comprehension of DSF's function within the HLB pathosystem and its potential application as an antimicrobial agent.