A Burkholderia endophyte of the ancient maize landrace Chapalote utilizes c-di-GMP-dependent and –independent signaling to suppress diverse plant fungal pathogen targets

Abstract

Chapalote is a maize (corn) landrace grown continuously by subsistence farmers in the Americas since 1000 BC, valued in part for its broad-spectrum pathogen resistance. Previously we showed that Chapalote possesses a bacterial endophyte, Burkholderia gladioli strain 3A12, which suppresses growth of Sclerotinia homoeocarpa, a fungal pathogen of a maize relative, used as a model system. Ten mutants that lost the anti-pathogen activities were identified, corresponding to five genes. However, S. homoeocarpa is not a known maize pathogen, hence the relevance of these anti-fungal mechanisms to its ancient host has not been clear. Here, the strain 3A12 mutants were tested against a known pathogen of maize and many crops, Rhizoctonia solani. Microscopy established that wild-type 3A12 swarms towards, and attaches onto, the pathogen, forming microcolonies, resulting in hyphal cleavage. Analysis of the mutants revealed that 3A12 uses common downstream gene products (e.g. fungicides) to suppress the growth of both S. homoeocarpa and R. solani, but apparently different upstream regulatory machinery, with the former, but not latter pathogen, requiring YajQ, a receptor for the secondary messenger c-di-GMP. We conclude that Burkholderia gladioli strain 3A12, an endophyte of an ancient maize employs both c-di-GMP-dependent and –independent signaling to target diverse fungal pathogens.

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