Burkholderia glumae: next major pathogen of rice?

Abstract

Burkholderia glumae causes bacterial panicle blight of rice, which is an increasingly important disease problem in global rice production. Toxoflavin and lipase are known to be major virulence factors of this pathogen, and their production is dependent on the TofI/TofR quorum-sensing system, which is mediated by N-octanoyl homoserine lactone. Flagellar biogenesis and a type III secretion system are also required for full virulence of B. glumae. Bacterial panicle blight is thought to be caused by seed-borne B. glumae; however, its disease cycle is not fully understood. In spite of its economic importance, neither effective control measures for bacterial panicle blight nor rice varieties showing complete resistance to the disease are currently available. A better understanding of the molecular mechanisms underlying B. glumae virulence and of the rice defence mechanisms against the pathogen would lead to the development of better methods of disease control for bacterial panicle blight.

Taxonomy: Bacteria; Proteobacteria; Betaproteobacteria; Burkholderiales; Burkholderiaceae; Burkholderia.

Microbiological properties: Gram-negative, capsulated, motile, lophotrichous flagella, pectolytic.

Disease symptoms: Aborted seed, empty grains as a result of failure of grain filling, brown spots on panicles, seedling rot.

Disease control: Seed sterilization, planting partially resistant lines (no completely resistant line is available). KNOWN VIRULENCE FACTORS: Toxoflavin, lipase, type III effectors.

Figure 1

Symptoms and virulence factors produced by Burkholderia glumae: (A, B) typical symptoms of bacterial panicle blight on rice panicles; (C) transmission electron micrograph of B. glumae cells and flagella; (D) toxin (toxoflavin) production by B. glumae indicated by the yellow pigment on a King's B agar plate; (E) lipase activity indicated by the opaque and iridescent halo around a B. glumae colony on a Luria–Bertani (LB) agar plate containing 0.2% Tween‐20; (F) pectinase activity indicated by the pitting zone around a B. glumae colony on a pectate semi‐solid agar plate (Kelemu and Collmer, 1993); (G) hypersensitive reaction (HR) on a tobacco leaf infiltrated with approximately 10⁸ colony‐forming units (cfu)/mL of B. glumae; (H) swimming (SWI) and swarming (SWA) motilities indicated by the round and wavy bacterial zones, respectively, around a B. glumae colony on an LB with 0.3% agar plate.