Biocontrol potential of an endophytic Serratia sp. G3 and its mode of action

Abstract

Biological control is being considered as an alternative or a supplemental way of reducing the use of chemicals in agriculture. An endophytic strain G3 with potential as a biocontrol agent was isolated from the stems of Triticum aestivum L. It was classified by 16S rDNA sequencing as a member of Serratia. Strain G3 displayed a broad spectrum of antifungal activity in vitro against a number of phytopathogens such as Botrytis cinerea, Cryphonectria parasitica, Rhizoctonia cerealis and Valsa sordida. Molecular mechanisms involved in biocontrol by Serratia sp. G3 was investigated for its potential application to plant health management. The results showed that G3 produces an array of antimicrobial exoproducts, including chitinase, protease, antibiotic pyrrolnitrin, and siderophores for iron competition. Moreover, it also produced the plant growth hormone indole-3-acetic acid, suggesting that multiple mechanisms and their synergistic effects may be involved in biocontrol of plant diseases. Additionally, strain G3 can produce at least ten N-acyl homoserine lactones (AHLs) signal molecules for cell to cell communication, including unsubstituted, 3-oxo, and 3-hydroxy at the C3 position through liquid chromatography-tandem mass spectrometry (LC-MS/MS), which is different from the previously reported Serraia species. For the first time, N-3-oxo-heptanoyl-homoserine lactone, one of the main molecules was reported in the genus Serratia. The role of AHL-dependent quorum sensing system in the interactions between the endophytic strain G3 and host plants and its potential application in improving biocontrol efficacy will be further explored.

Cloning and preliminary identification of SptR, a LuxR-like regulator from Serratia plymuthica

“…To determine the impact of SptR on production of exoenzymes and secondary metabolite, firstly proteolytic activities were assayed on skimmed milk agar plates by seeding 2 µl of bacterial overnight cultures onto the plate and monitoring the appearance of haloes around the developing bacterial colonies after 48 h at 28°C as previously described (Liu et al, 2010). Furthermore, overnight cultures of bacteria in LB broth with appropriate antibiotics were diluted (OD600 0.02) into 2 ml of LB with 200 μg/ml of tryptophan.…”

“…To evaluate the effects of SptR in the suppression of pathogenic fungi, strain G3 and the ∆sptR mutant were tested for in vitro antagonistic activity against gray mould Botrytis cinerea as previously described (Liu et al, 2010). Briefly, overnight culture of strain G3 and its derivatives in LB at 28°C were spotted (2 μl) in a line 3 cm away from the centre of the PDA plate, respectively; 5 mm diameter agar disks from an actively growing fungal culture were seeded at the centre of PDA plates (60 mm diameter).…”

“…Extracellular enzymes and secondary metabolites are important biocontrol determinants of strain G3 (Liu et al, 2010). To investigate whether sptR plays a role in production of exoenzymes and secondary metabolites, we compared the proteolytic activity and IAA levels of the ∆sptR mutant with the wild type G3.…”

“…Serratia plymuthica G3 is an endophytic strain isolated from the stems of Triticum aestivum L. with a broad spectrum of antifungal activities and can produce an array of antimicrobial exoproducts such as chitinase, protease, antibiotic pyrrolnitrin, and siderophores. In addition it also produced the plant hormone indole-3acetic acid (IAA) (Liu et al, 2010). Previous studies have characterized two QS systems (splIR and spsIR) in biocontol strains of S. plymuthica and shown to control biocontrol-related traits (Liu et al, 2007;Liu et al, 2011).…”

Role of the RNA-binding protein Hfq in Serratia plymuthica

“…Plant-associated Serratia plymuthica belongs to the group of gamma-proteobacterial species of Gramnegative bacteria as a biocontrol agent (BCA) of mainly fungal pathogens, as well as entomopathogens and weeds. Multiple modes of action and their synergistic effects have been recognized for plant growth promotion and biocontrol (12,13). Previous studies have shown that multiple global regulators including GacS/GacA homologues, GrrA/GrrS, the stationary-phase sigma factor RpoS, and Nacylhomoserine lactone (AHL)-mediated QS system are involved in control of biocontrol activities in S. plymuthica strains IC1270 and HRO-C48 (14)(15)(16)(17).…”