Scientists Show How Plant Pathogen Dampens Host Immune Response

(Left to Right) Caroline Roper, associate professor of plant pathology, Jeannette Rapicavoli, Claudia Castro

The bacterial pathogen Xylella fastidiosa (Xf) delays the immune response of the plants it infects, allowing it to go undetected for long enough to cause disease. A team led by Caroline Roper, an associate professor of plant pathology, has published a paper in Nature Communications describing how this happens and how it might be prevented.

Xf is the major threat to viticulture in California, the U.S. and worldwide. It causes a lethal disease called Pierce’s disease and affects wine, raisin and table grapes, which are a multibillion dollar industry in California alone. The pathogen can also affect many other hosts including citrus, olives and almonds.

Lipopolysaccharides (LPS), which are found in the cell walls of many bacteria, trigger strong immune responses in plants and animals. To subvert this response, many pathogenic bacteria have evolved ways to weaken LPS-triggered immune responses. Roper’s group found that a long terminal polysaccharide chain (known as the O-antigen) present in Xf LPS enables it to delay recognition by grapevine hosts. They also showed that pretreating grapevine with LPS that lacked the O-antigen enabled it to better resist Xf infection.

“We are working on understanding the molecular basis of the pre-treatment and its effects to determine if we can leverage this information for disease control,” Roper said.

The title of the paper is “Lipopolysaccharide O-antigen delays plant innateimmune recognition of Xylella fastidiosa.” Jeannette Rapicavoli, who earned her Ph.D. in Roper’s lab at UCR, was first author on the paper. Other authors are Roper and Claudia Castro from UCR; and Barbara Blanco-Ulate, Rosa Figueroa-Balderas, Abraham Morales-Cruz and Dario Cantu from UC Davis; and Artur Muszyński, Parastoo Azadi and Justyna Dobruchowska from the University of Georgia.

Sarah Nightingale

Professor of Biochemistry report in the Journal of Biological Chemistry

Richard Debus, professor of biochemistry

During photosynthesis, green plants and some other organisms, such as cyanobacteria, use sunlight to synthesize carbohydratefrom carbon dioxide and water. To liberate molecular oxygen from water, electrons and protons first need to be extracted – a thermodynamically and kinetically difficult feat achieved by an inorganic (Mn4CaO5) cluster serving as the catalytic center in “Photosystem II” (PSII), a light-driven nano-machine that green plants and cyanobacteria possess.

Richard Debus, a professor of biochemistry at UCR, and colleagues report in the Journal of Biological Chemistry their new insight into the mechanism of water-splitting in PSII.

Water access and proton egress, they explain, are controlled by networks of hydrogen bonds that are arranged as “channels” of amino acid residues.  A chloride ion located near the inorganic cluster and an important component of one of these channels is associated more tightly with PSII in cyanobacteria than with PSII in plants, even though PSII from cyanobacteria and plants are nearly identical in their function and composition.

In their paper, Debus and colleagues go on to explain the difference in the chloride binding properties of cyanobacterial and plant PSII.

“The only statistically significant amino acid difference between cyanobacterial and plant PSII near the inorganic cluster is residue 87 of the D1 protein, one of two proteins that comprise the core of the photosystem,” Debus said.  “In cyanobacteria, this residue is asparagine.  In plants, this residue is alanine.  We found that changing D1-87 from asparagine to alanine in a cyanobacterium causes the association of chloride to PSII to diminish, mimicking the association of chloride to PSII in plants and showing that the networks of hydrogen bonds near the cluster can be altered slightly by the amino acid change.”

Identifying amino acid residues like asparagine and alanine in water access/proton egress channels improves scientists’ understanding of the dynamic mechanism of water oxidation by the inorganic cluster and provides insight into the design of new generations of synthetic catalysts that convert sunlight into useful forms of storable energy.

Debus’s laboratory was joined in the research by the laboratory of Gary W. Brudvig of Yale University.

Iqbal Pittalwala

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