How do Xanthomonas bacteria cause bacterial blight disease?

The Xoo bacteria enter the rice plant through wounds or guttation fluid. Inside the plant host, the bacteria induce leakage of sugar from the rice plant to the bacteria. While the bacteria thrive on the sugar, the rice plants starve and eventually will wilt and die.

Rice SWEETs transporters export sugar

Sugar transporters are essential for moving glucose between cells, i.e. from our intestines to the blood stream or from a plant’s phloem to seeds. In both animals and plants, a particular family of sugar transporters, named SWEETs, exports sugar from the inside of cells into the extracellular lumen.

SWEET transporters span the plasma membrane with seven helices to form a single pore that transports sugar across the membrane. SWEET-encoding genes are widespread in diverse organisms, ranging from the worm Caenorhabditis elegans to humans, mice and plants.

Rice (Oryza sativa) encodes 21 SWEET genes and SWEET transporters are involved in various developmental processes. For example, rice OsSWEET11 contributes to pollen viability. In pollen, sugar is stored in long-chained starch molecules. Without SWEET11, the pollen starch content is reduced, which causes male sterility of the rice plants.

Xanthomonas bacteria feed on exported sugar

When rice is infected by the bacterium Xanthomonas oryzae pv. oryzae (Xoo), SWEET production increases in leaves. The more SWEETs are produced, the more sugar exporters are present on the cell surface and huge amount of sugar leaks into the extracellular space, where the Xoo bacteria reside. The bacteria feed on the sugar, multiply and cause blight disease.

The Xoo bacteria are picky about their sugar source. What they want is sucrose. Of the 21 rice SWEET genes, only five encode sucrose transporters and three of these five genes (OsSWEET11, OsSWEET13 and OsSWEET14) can support Xoo growth.

Xanthomonas bacteria switch on sucrose transporter production with the help of TAL effectors to feed on sugar that is exported from the plant cell.

Xoo secretes TAL effectors to activate SWEET genes

Accessing the host’s sugar pot is key to the multiplication and survival of the Xoo bacteria. To do so, Xoo bacteria secrete small effector molecules directly into the rice cell with the help of a needle-like structure; the type III secretion system. These so-called Transcription Activator Like (TAL) effectors can bind to specific sites in the promoters of the SWEET genes.

Promoters are DNA sequences that control the activity of a gene in a manner similar to an electric light switch. They can turn a gene ON or OFF or, more often, fine-tune its activity -analogously to a dimmer switch. TAL effectors specifically bind to effector binding elements (EBEs) in the SWEET gene promoters to activate SWEET production and thereby sugar efflux to the bacteria.

Rice plants are resistant to bacterial blight when they prevent TAL effector binding

At heart, the Xoo TAL effectors are like keys that open a backdoor to the sugar storage of the rice plant. Some rice cultivars have evolved a mechanism to close this backdoor again. For example, rice varieties with the recessive xa13 mutation have a mutation in the EBE of the SWEET11 promoter that prevents binding of the PthXo1 TAL effector. The TAL key does not fit any more; SWEET11 is not activated, the bacteria starve, and the xa13 rice plants are resistant to bacterial blight.

The recessive xa13 gene is successfully bred in elite rice varieties, particularly in India.

Our strategy

To make rice plants durable resistant to bacterial blight, we close the backdoor that the Xoo bacteria have discovered and exploited – the SWEET activation in leaves. By preventing the binding of the bacterial TAL effectors to the EBEs in the SWEET promoters, we can make rice varieties resistant against bacterial blight.

The devil is in the details. Each Xoo strains has its own set of TAL effectors, its own set of keys to open the SWEET backdoor, if you like. Sometimes, when one key does not work, they will switch to another key and induce production of an alternative SWEET transporter. To achieve broad-spectrum resistance against diverse Xoo strains, we need to know the TAL effectors that a given Xoo strain uses to activate the different rice SWEET transporters. Therefore, our SWEET-RESISTANCE KIT comprises diagnostic tools for pathogen surveillance as well as resistant rice lines for customized resistance gene employment.

Principal Investigator

Prof. Dr. Wolf B. Frommer

Project Coordinator

Dr. Sarah M. Schmidt

Phone +49 221 5062 388
Responsible for the content: E-MailProf. Dr. Wolf B. Frommer