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How can a Bacterium Cause One of the Major Problems for Rice Farmers?
The bacteria of the family Xanthomonas oryzae pathovar oryzae (Xoo) belong to plant pathogens that cause the bacterial blight disesase, one of the major problems of smallholder farmers in most of the rice growing counties. Once the Xoo bacterium infects the rice it forces the plants to secrete sugar which is consumed by the bacterium, resulting in its increased reproduction. This in turn, results in wilting, desiccation of leaves and subsequently the death of the plants. The decrease in rice yield therefore, has a massive impact on the smallholder’s economic stability, and on regional nutrition, consequently.
SWEET Transporters as important Regulators for Sugar Transport
The transport of sugar is an essential procedure in eukaryotic organisms to garantuee the propper supply with nutritions for every single cell. The SWEET family consist of conserved transmembrane sugar transporters that have been shown to typically export sugar from the inside of cells into the extracellular lumen. Therefore, they are important e.g. in humans for sugar release from our intestines to the blood stream but also in plants for the transport from the leaves to the seeds. How conserved the SWEET genes are can be seen best if we compare their occurance in other organisms. In phylogenetics we can find them in humans, animals, plants, protozoans, and even in bacteria.
In rice (Oryza sativa), we can find more than 20 genes that encode for members of the SWEET family which have different roles in its developemental process and the supply of nutrients.
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.
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 into elite rice varieties, particularly in India.
To make rice plants permanant resistant to bacterial blight, we close the backdoor that the Xoo bacteria have discovered and exploited – Excessive SWEET activation. By prevention of bacterial TAL effector binding to 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.