New hope for preventing black rot disease in vegetables

A study from Nagoya University may ultimately lay the groundwork for preventing black rot disease in vegetables. Black rot is a deadly disease that destroys plants in the crucifer family, including broccoli, cabbage, and cauliflower.

The research was focused on phytochromes, which are proteins found in plants, bacteria and fungi. Their molecular structure involves the existence of units, or modules, that can change position within the overall protein, thus changing the shape of the phytochrome molecule and altering it from an active to an inactive form. This happens in response to the molecule’s exposure to red light. Phytochromes are thus photosensitive and use light as a source of information to regulate the physiological processes of the organism (plant, bacteria or fungus).

The way in which phytochromes function to switch from active to inactive form has not been fully understood. One of the problems has been that scientists had not mapped out the complete structure of phytochrome molecules and so they did not understand how the structural changes could lead to different signals from the phytochrome molecule. Unfortunately, the fact that phytochrome molecules change shape when they are exposed to light makes detailed study even more complicated.

This is particularly relevant in the example of Xanthomonas campestris, a bacterium that causes black rot in may important vegetable crops. This bacterium is a significant pest, particularly of crops from the brassica family, which include cabbage, broccoli, kale and Brussels sprouts. Xanthomonas causes the most important disease of vegetable brassica crops worldwide and is therefore an economically important pathogen.  

Phytochrome molecules in Xanthomonas bacteria function as virulence regulators, by modulating key physiological processes when exposed to light, including xanthan production, biofilm formation, and infection capacity. Vegetable farmers and scientists would like to know how the phytochrome molecules function in these bacteria, so that they could potentially switch them to inactive form and, thereby, render the pathogen less destructive. 

A research team led by Professor Hernán Bonomi from the Fundación Instituto Leloir, Argentina, has now shed light on how light signals sensed by phytochromes are created and propagated. The team is a large international collaboration that includes researchers from Argentina, France, and Japan. Professor Leonard Chavas from Nagoya University provided expertise in synchrotron radiation and structural analysis. The work was published in the journal Science Advances.

The researchers worked on understanding how light signals, detected by the light-sensing module of a phytochrome molecule, are transmitted to an output (or “effector”) module that converts the light stimulus into a specific physiological signal.  This process involves changes in the structure of the phytochrome molecule that cause it to twist, bend and turn. Up until this study, no full-length phytochrome structure had been reported at atomic scale resolution for both the activated and the non-activated states. 

In their paper, the research team presents a complete characterization of the light-sensing module in phytochrome molecules of the Xanthomonas bacterium, both in its activated and inactivated state. Also, the light-induced shape changes of the modules making up the protein are described down to atomic scale resolution, highlighting for the first time in this family of photoreceptors, the remarkable structural rearrangements that take place. 

By combining these results with biochemical and computational studies, the scientists propose a new photoactivation model that explains the signaling mechanism, from the changes in chemical structure of the light-sensing module (that receives stimuli from red and “far red” light), to the complex ways in which the entire photochrome molecule is remodeled and takes on a new structure and shape.

The research results have implications in photobiology, as well as for understanding the mechanisms whereby bacterial pathogens cause disease in plants, particularly the disease of black rot in important crop vegetables.

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By Alison Bosman, Earth.com Staff Writer