Burkholderia glumae, a pathogen causing bacterial panicle blight in rice, must maintain an acidic environment following infection to turn on expression of its virulence factors. A new study identifies a membrane protein needed for secretion of an acid that helps maintain this acidic environment.
The increasing world’s population is putting immense pressure on agriculture to fulfil the world’s food demands. Rice is an important cash crop, and it is consumed by half the world’s population. Bacterial panicle blight (BPB) is one of the major diseases of rice and is caused by Burkholderia glumae, a hardy Gram-negative bacterium. This disease can be found in 60% of rice fields and is capable of reducing rice production by 75% in severely infected fields. It is forecast to worsen due to climate change. While some rice diseases, such as rice blast and sheath blight, can be controlled with chemicals and pesticides, no chemical treatment has been approved for BPB.
Quorum sensing (QS) is a type of communication using small, secreted molecules allowing bacteria to “count” their local population. Bacterial pathogens often regulate virulence gene expression using QS. Indeed, B. glumae regulates production of several virulence factors in this manner, essentially waiting until the bacterial population is high enough before “launching” and attack. The most important of these is a molecule called toxoflavin which directly damages the plant.
In response to bacterial infection, plants, using their own immune responses, secrete their own molecules which raise the pH, alkalizing the bacteria’s environment, a compartment called the apoplast. Alkalinization inhibits the ability of B. glumae to grow and it also destroys the bacteria’s QS molecules as they are unstable at alkaline pH. B. glumae produces oxalic acid in a QS-dependent manner to maintain an acidic environment and avoid alkaline toxicity (Figure 1). A new study identified a DedA family protein, named DbcA, needed for secretion of oxalic acid.
The DedA superfamily is a highly conserved membrane protein family found in all species, even humans. The functions of these proteins are only beginning to be understood, but they seem to play roles in transport of different types of molecules across membranes.
Mutation of a B. glumae dedA family gene (encoding a protein called DbcA), or alteration of the proton motive force by addition of sodium bicarbonate (baking soda), results in a near total loss of secretion of oxalic acid, leading to alkalinization of the growth medium, and loss of QS, eliminating the ability of the bacterium to express its virulence genes (Figure 3). This also causes loss of virulence to the rice plant (Figure 2). Bicarbonate is inexpensive and has been used for many years as a nontoxic household item and is an additive to foods and dental products. This work establishes a link between the DedA family and QS of a plant pathogen and supports the possible use of sodium bicarbonate as a nontoxic chemical intervention for BPB.
Asif Iqbal, George Nwokocha, Vijay Tiwari, Inderjit K. Barphagha, Anne Grove, Jong Hyun Ham and William T. Doerrler published this study in Molecular Plant Pathology:
TITLE IMAGE: Rice growing in the field and a close-up image showing the seeds at the tip of plant. Both plants are displaying rice with bacterial panicle blight symptoms. All images used with permission of the author.