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Pseudomonas isolates from tropical environments have been underexplored and may form an untapped reservoir of interesting secondary metabolites. In this study, we compared Pseudomonas and cyclic lipopeptide (CLP) diversity in the rhizosphere of a cocoyam root rot disease (CRRD) suppressive soil in Boteva, Cameroon with those from four conducive soils in Cameroon and Nigeria. Compared with other soils, Boteva andosols were characterized by high silt, organic matter, nitrogen and calcium. Besides, the cocoyam rhizosphere at Boteva was characterized by strains belonging mainly to the P. koreensis and P. putida (sub)groups, with representations in the P. fluorescens, P. chlororaphis, P. jessenii and P. asplenii (sub)groups. In contrast, P. putida isolates were prominent in conducive soils. Regarding CLP diversity, Boteva was characterized by strains producing 11 different CLP types with cocoyamide A producers, belonging to the P. koreensis group, being the most abundant. However, putisolvin III-V producers were the most dominant in the rhizosphere of conducive soils in both Cameroon and Nigeria. Furthermore, we elucidated the chemical structure of putisolvin derivatives-putisolvin III-V, and described its biosynthetic gene cluster. We show that high Pseudomonas and metabolic diversity may be driven by microbial competition, which likely contributes to soil suppressiveness to CRRD.

Root rot disease caused by Pythium myriotylum is the most devastating disease of the tropical tuber crop cocoyam (Xanthosoma sagittifolium) with production losses of up to 90%. This study was conducted in order to determine the role of phenazines (PCN), and two cyclic lipopeptides namely sessilins and orfamides, produced by Pseudomonas sp. CMR12a, in the suppression of the Cocoyam Root Rot Disease (CRRD). Previously generated biosynthetic mutants of CMR12a that were deficient in either one, two or all three metabolites were used in in vitro, microscopic and plant experiments. Using CMR12a and its biosynthetic mutants, plant experiments revealed that mutants, which produced sessilins, orfamides or phenazines alone or at least any two of the metabolites, could successfully suppress the cocoyam root rot disease caused by P. myriotylum albeit in varying capacities. In vitro assays using filter sterilized bacterial supernatants showed that besides the WT strain, all mutants except the null mutant, which makes none of the metabolites, could successfully inhibit P. myriotylum. Following contact between the mycelium of P. myriotylum and purified/semi-purified metabolites, microscopic analysis showed that a concentration of 1 nM and higher of crude sessilins resulted in cell damage including vacuolization and lysis whereas amounts of 25 nM PCN and higher could produce a similar phenotype including mycelial bloating. Treatments with orfamide at 10 nM and higher resulted in mycelial lysis. Interestingly, during plant experiments, the null mutant was able to give some level of disease suppression suggesting that CMR12a produces other compound(s) which could be antagonistic towards our target pathogen. In summary, phenazines, sessilins and orfamides, produced by Pseudomonas sp. CMR12a, can play independent and additive roles in the suppression of Pythium-mediated CRRD.

Pseudomonas isolates from tropical environments have been underexplored and may form an untapped reservoir of interesting secondary metabolites. In this study, we compared Pseudomonas and cyclic lipopeptide (CLP) diversity in the rhizosphere of a cocoyam root rot disease (CRRD) suppressive soil in Boteva, Cameroon with those from four conducive soils in Cameroon and Nigeria. Compared with other soils, Boteva andosols were characterized by high silt, organic matter, nitrogen and calcium. Besides, the cocoyam rhizosphere at Boteva was characterized by strains belonging mainly to the P. koreensis and P. putida (sub)groups, with representations in the P. fluorescens, P. chlororaphis, P. jessenii and P. asplenii (sub)groups. In contrast, P. putida isolates were prominent in conducive soils. Regarding CLP diversity, Boteva was characterized by strains producing 11 different CLP types with cocoyamide A producers, belonging to the P. koreensis group, being the most abundant. However, putisolvin III-V producers were the most dominant in the rhizosphere of conducive soils in both Cameroon and Nigeria. Furthermore, we elucidated the chemical structure of putisolvin derivatives-putisolvin III-V, and described its biosynthetic gene cluster. We show that high Pseudomonas and metabolic diversity may be driven by microbial competition, which likely contributes to soil suppressiveness to CRRD.