Calcium regulates the mycophagous ability of Burkholderia gladioli strain NGJ1 in a type III secretion system-dependent manner

Abstract

Background: A rice associated bacterium Burkholderia gladioli strain NGJ1 demonstrates mycophagy, a phenomenon wherein bacteria feed on fungi. Previously, we have reported that NGJ1 utilizes type III secretion system (T3SS) to deliver a prophage tail-like protein (Bg_9562) into fungal cells to establish mycophagy.

Results: In this study, we report that calcium ion concentration influences the mycophagous ability of NGJ1 on Rhizoctonia solani, an important fungal pathogen. The calcium limiting condition promotes mycophagy while high calcium environment prevents it. The expression of various T3SS apparatus encoding genes of NGJ1 was induced and secretion of several potential T3SS effector proteins (including Bg_9562) into extracellular milieu was triggered under calcium limiting condition. Using LC-MS/MS proteome analysis, we identified several calcium regulated T3SS effector proteins of NGJ1. The expression of genes encoding some of these effector proteins was upregulated during mycophagous interaction of NGJ1 with R. solani. Further, mutation of one of these genes (endo-β-1, 3- glucanase) rendered the mutant NGJ1 bacterium defective in mycophagy while complementation with full length copy of the gene restored its mycophagous activity.

Conclusion: Our study provides evidence that low calcium environment triggers secretion of various T3SS effectors proteins into the extracellular milieu and suggests the importance of cocktail of these proteins in promoting mycophagy.

Keywords: 3- glucanase; Bacterial-fungal interaction; Effectors; Endo-β-1; Mycophagy; T3SS.

Fig. 1

Low calcium condition promotes mycophagous activity of NGJ1. a Mycophagous activity of NGJ1 on R. solani on PDA plates with or without EGTA/CaCl₂ supplementation. On EGTA supplemented plates, NGJ1 grew over the mycelial mass in less than 4 days while on PDA plates (without supplementation), the bacterium took 7–9 days to cover the fungal biomass. Supplementation of CaCl₂ completely inhibited mycophagous ability and prevented bacterial growth over fungal biomass. b The abundance of NGJ1 on R. solani mycelia on confrontation plates supplemented with or without EGTA/CaCl₂. The bacterial abundance was significantly more on EGTA containing PDA plates than that on PDA only or CaCl₂ supplemented plates. c The NGJ1 growth pattern in PDB with or without EGTA/CaCl₂ supplementation. The growth was similar under tested conditions. Similar results were obtained in at least three independent biological experiments and only representative photographs are shown. Asterisk * indicates significant difference at P < 0.05 (estimated using one-way ANOVA). Graphs show mean values ± standard deviation

Fig. 2

Expression dynamics of T3SS apparatus encoding genes of NGJ1. a RT-PCR based expression analysis of T3SS apparatus encoding genes of NGJ1 during mycophagous interaction with R. solani in PDB. The differential gene expression was estimated during mycophagy with respect to the expression observed during bacterial growth in absence of fungi. b RT-PCR based expression analysis of T3SS apparatus encoding genes of NGJ1 in PDB with or without EGTA/CaCl₂ supplementation. The differential gene expression was calculated in presence of EGTA/CaCl₂ with respect to that observed in absence of EGTA/CaCl_2. The 16S rRNA gene of NGJ1 was used as endogenous control in RT-PCR analysis. The experiments were independently repeated three times with a minimum three technical replicates. Asterisks * indicate significant difference at P < 0.05 (estimated using one-way ANOVA). Graphs show mean values ± standard deviation

Fig. 3

Calcium limiting condition enhances the secretion of Bg_9562 protein into extracellular milieu in a T3SS dependent manner. a Silver stained SDS-PAGE profile of proteins isolated from the culture supernatant of NGJ1 grown in PDB (lane 1), PDB + EGTA (lane 2) and PDB + CaCl₂ (lane 3). While the lane 4, 5 and 6, represents the protein isolated from culture supernatants of T3SS mutant strain of NGJ1 (NGJ12) grown in PDB, PDB + EGTA and PDB + CaCl₂, respectively. b Western blot analysis (using Bg_9562 specific polyclonal antibody) of proteins isolated from the culture supernatant of NGJ1 grown in PDB (lane 1), PDB + EGTA (lane 2), and PDB + CaCl₂ (Lane 3). Lane 4, 5 and 6 represent the protein isolated from the bacterial pellet grown in PDB, PDB + EGTA and PDB + CaCl₂, respectively. Lane 7 contains purified Bg_9562 protein obtained from the recombinant E. coli strain. c Western blot analysis of total protein isolated from the culture supernatant of wild type NGJ1 grown in absence (lane 1) or presence of EGTA (lane 2). While lane 3 and 4 represent proteins from the culture supernatant of NGJ12 grown in absence or presence of EGTA, respectively. Lane 5 contains purified Bg_9562 protein from the recombinant E. coli strain. M reflects pre-stained protein ladder

Fig. 4

Mycophagous behaviour and secretion profile of T3SS signal deleted variant of Bg_9562 in NGJ1 strains. a Mycophagous activity of NGJ1 and its variant strains on R. solani on PDA plates. The Bg_9562 mutant (NGJ101) and T3SS signal deleted variant Bg_9562 expressing strain (NGJ104) were mycophagy deficient while the wild type NGJ1 showed enhanced mycophagy in presence of EGTA. b Western blot analysis (using Bg_9562 specific polyclonal antibody) of proteins isolated from the culture supernatant and bacterial pellets of NGJ1 and NGJ104 grown in PDB broth with EGTA/CaCl₂ supplementation. Similar results were obtained in at least three independent biological experiments and only representative photographs are shown

Fig. 5

An endo-β-1, 3- glucanase is required for efficient mycophagous ability of NGJ1. The mycophagous behavior of (a) NGJ1, (b) NGJ105, the endo-β-1, 3- glucanase mutant strain and (c) NGJ106, the complementing strain on R. solani on PDA plates, at different time points. The treatment with 10⁹ cells/ml concentration of NGJ1 or NGJ106 completely prevented sclerotial germination. However, the sclerotia treated with 10⁵ cells/ml of NGJ1 or NGJ106 initially germinated but subsequently the bacteria grew over the fungal biomass. Notably, at both 10⁹ cells/ml as well as 10⁵ cells/ml concentration, the endo-β-1, 3- glucanase mutant strain (NGJ105) failed to prevent the R. solani sclerotia. Moreover, at high concentration (10⁹ cells/ml), limited NGJ105 growth was observed over fungal biomass. Similar results were obtained in at least three independent experiments

Fig. 6

Proposed model of calcium regulated mycophagous activity of B. gladioli strain NGJ1. During mycophagous interaction with R. solani, the T3SS of NGJ1 gets activated This facilitates the delivery of various effector molecules including Bg_9562 into fungal cells in a T3SS dependent manner, which in turn is required for mycophagy. The low calcium environment created by EGTA, deregulates the T3SS and triggers the secretion of various T3SS effectors of NGJ1 into the fungal cells as well as extracellular milieu. The presence of a cocktail of potent cell wall lytic enzymes/proteins as well as Bg_9562 protein into the extracellular milieu may facilitate efficient fungal cell lysis and leakage of fungal metabolites. NGJ1 may utilize these fungal metabolites as a nutrient source to promote its growth during mycophagy. On the other hand, presence of CaCl₂ prevents the induction of T3SS even in presence of fungi (during mycophagous interaction of NGJ1 with R. solani). Under this scenario, the T3SS effectors are neither delivered into the fungal cells nor secreted into the extracellular milieu. The bacterium is unable to lyse fungal membrane/release fungal metabolites and is compromised in foraging over fungal biomass