Stress-Responsive Alternative Sigma Factor SigB Plays a Positive Role in the Antifungal Proficiency of Bacillus subtilis

Abstract

DifferentBacillusspecies with PGPR (plant growth-promoting rhizobacterium) activity produce potent biofungicides and stimulate plant defense responses against phytopathogenic fungi. However, very little is known about how these PGPRs recognize phytopathogens and exhibit the antifungal response. Here, we report the antagonistic interaction betweenBacillus subtilisand the phytopathogenic fungusFusarium verticillioides. We demonstrate that this bacterial-fungal interaction triggers the induction of the SigB transcription factor, the master regulator ofB. subtilisstress adaptation. Dual-growth experiments performed with live or dead mycelia or culture supernatants ofF. verticillioidesshowed that SigB was activated and required for the biocontrol of fungal growth. Mutations in the different regulatory pathways of SigB activation in the isogenic background revealed that only the energy-related RsbP-dependent arm of SigB activation was responsible for specific fungal detection and triggering the antagonistic response. The activation of SigB increased the expression of the operon responsible for the production of the antimicrobial cyclic lipopeptide surfactin (thesrfAoperon). SigB-deficientB. subtiliscultures produced decreased amounts of surfactin, andB. subtiliscultures defective in surfactin production (ΔsrfA) were unable to control the growth ofF. verticillioides.In vivoexperiments of seed germination efficiency and early plant growth inhibition in the presence ofF. verticillioidesconfirmed the physiological importance of SigB activity for plant bioprotection.IMPORTANCEBiological control using beneficial bacteria (PGPRs) represents an attractive and environment-friendly alternative to pesticides for controlling plant diseases. Different PGPRBacillusspecies produce potent biofungicides and stimulate plant defense responses against phytopathogenic fungi. However, very little is known about how PGPRs recognize phytopathogens and process the antifungal response. Here, we report howB. subtilistriggers the induction of the stress-responsive sigma B transcription factor and the synthesis of the lipopeptide surfactin to fight the phytopathogen. Our findings show the participation of the stress-responsive regulon of PGPRBacillusin the detection and biocontrol of a phytopathogenic fungus of agronomic impact.

The Stress-Responsive Alternative Sigma Factor SigB of Bacillus subtilis and Its Relatives: An Old Friend With New Functions

“…observations (Nadezhdin et al, 2020) agree with the results of Bartolini et al (2019b), showing that only B. subtilis strains proficient in surfactin production and RsbPQ-dependent SigB activation formed beneficial biofilms at the roots of maize plants to protect them from the fungi assault. Overall these results (Bartolini et al, 2019b;Nadezhdin et al, 2020) point to the importance of the energy stress route of SigB activation in the environmentally friendly biofilm established at the plane of B. subtilis-plant interaction against phytopathogenic fungi of agronomic impact (Figure 6).…”

“…Table 1 shows an updated list of the alternative sigma factors present in B. subtilis and their functions. Within this group, the ones involved in spore formation (SigH, SigF, SigE, SigG, and SigK) ( Nicholson et al, 1989 ; Sun et al, 1991 ; Predich et al, 1992 ; Roels et al, 1992 ; Zheng et al, 1992 ; Piggot and Hilbert, 2004 ; Aguilar et al, 2007 ; Kovács, 2016 ) and stress adaption (SigB and SigH) ( Boylan et al, 1991 ; Predich et al, 1992 ; Méndez et al, 2004 ; Bartolini et al, 2019a , b ; Nadezhdin et al, 2020 ) are the best characterized.…”

Bacillus Responses to Plant-Associated Fungal and Bacterial Communities

“…These observations strongly suggest that specific soluble signals, emitted by fungal pathogens, could be perceived by bacilli which in turn modulate BSMs synthesis. As observed by Bartolini et al (2019), cells of the Bacillus colony, physically close to the fungal culture, responded to signals by over-expressing genes coding for transcription factors involved in CLPs synthesis regulation. In contrast, colony cells positioned on the opposite side of the fungi did not react to the fungus (Bartolini et al, 2019).…”

“…As observed by Bartolini et al (2019), cells of the Bacillus colony, physically close to the fungal culture, responded to signals by over-expressing genes coding for transcription factors involved in CLPs synthesis regulation. In contrast, colony cells positioned on the opposite side of the fungi did not react to the fungus (Bartolini et al, 2019). This phenomenon indicates that the specific fungal metabolite diffuses on a short distance and has an influence on closely located Bacillus cells.…”

“…Fungal triggers may affect both types of regulatory systems involved in BSMs production. For instance, upon sensing F. verticillioides, the global stress-related regulator SigB is activated in B. subtilis NCIB3610 which in return enhances surfactin production (Bartolini et al, 2019). In interaction with F. culmorum under biofilm-conducive conditions, B. subtilis Bs12 down-regulates the expression of the sinR gene known as a repressor of biofilm formation which also negatively regulates surfactin production (Kearns et al, 2005;Khezri et al, 2016;Zhi et al, 2017).…”