Fruit Cell Wall Proteins Help Fungus Turn Tomatoes From Ripe to Rotten

Using tomatoes as a research plant, scientists at the University of California, Davis, have discovered that two plant enzymes that occur in the plant's cell walls cooperate with each other to make ripe fruit more susceptible to a disease-causing fungus.

"Identifying the role that these cell-wall enzymes play in making the harvested fruit more vulnerable to disease is important for designing strategies for limiting fruit losses to disease during storage, handling and distribution," said Ann Powell, a plant scientist who led the research team on this project.

The study findings are reported in the Jan. 22 issue of the Proceedings of the National Academy of Sciences.

One of the hallmarks of plant cells is their tough exterior cell wall. As the fruit ripens, the cell walls break down, and the fruit becomes softer and more flavorful. At the same time, the fruit also becomes more susceptible to diseases caused by fungi and bacteria.

Researchers have known for some time that two enzymes, known as polygalacturonase and expansin, contribute individually to the ripening-related breakdown of the cell walls. (Enzymes are proteins that trigger and control chemical reactions.) The UC Davis research team wondered if these two cell wall enzymes might also be responsible for the increased disease-susceptibility of ripening fruit. These diseases are responsible for substantial losses of high-quality harvested fruit during storage, shipping, marketing and consumer handling.

To test this notion, they selected two genetically modified tomato plant varieties. One of the varieties had been altered so that it did not produce polygalacturonase, and the other did not produce expansin. Powell had crossed these two varieties, resulting in a variety that produced neither of these enzymes.

The researchers, including plant biology graduate student Dario Cantu and postdoctoral fellow Ariel Vicente, inoculated tomatoes from each of the genetically modified varieties, as well as the variety resulting from the cross, with Botrytis cinerea, a common fungus that causes rotting on many fruits and vegetables. Tomatoes from a control group, whose enzyme production had not been altered, were also inoculated with the fungus.

The research team found that tomatoes from the plants that were genetically modified to suppress production of only one of the cell-wall enzymes were not any less susceptible to the fungus. However, when both of the enzymes were lacking in the crossed variety, the cell walls of the fruit did not readily break down, and the fruit was dramatically less susceptible to infection by the Botrytis cinerea fungus.

"It appears that these two enzymes work cooperatively in a way that breaks down the cell wall and leaves the fruit more vulnerable to pathogens like the gray mold caused by this fungus," Powell said.

"Interestingly, this process occurs at a stage in the plant's development that allows both the plant and the pathogen to successfully reproduce," she noted. "This convenient and mutually beneficial timing may well have resulted from the co-evolution of the fruit and its respective pathogens."

Collaborating on this study with Powell, Cantu and Vicente were professional researcher Molly Dewey, formerly of UC Davis and Oxford University; researcher Carl Greve and plant science professors John Labavitch and Alan Bennett.

Source: UC Davis

Related stories:

Using novel tool, researchers dig through cell 'trash' and find treasure
A person's trash can reveal valuable information, as detectives, historians and identity thieves well know. Likewise, a cell's "trash" may yield certain treasures, University of Delaware researchers have found.
Biochemists manipulate fruit flavor enzymes
Would you like a lemony watermelon? How about a strawberry-flavored banana? Biochemists at The University of Texas Medical School at Houston say the day may be coming when scientists will be able to fine tune enzymes responsible for flavors in fruits and vegetables. In addition, it could lead to environmentally-friendly pest control.
Consortium develops new method to manipulate genetic material
A multi-institutional team of researchers, including scientists at the University of Minnesota Medical School, have developed a powerful tool for genomic research and medicine. The robust method will allow researchers to generate synthetic enzymes that can target and manipulate DNA sequences for inactivation or repair.
The 21st century tomato
When tomatoes ripen in our gardens, we watch them turn gradually from hard, green globules to brightly colored, aromatic, and tasty fruits. This familiar and seemingly commonplace transformation masks a seething mass of components interacting in a well-regulated albeit highly complex manner. For generations, agriculturalists and scientists have bred tomatoes for size, shape, texture, flavor, shelf-life, and nutrient composition, more or less, one trait at a time. With the advent of molecular biology, mutagenesis and genetic transformation could produce tomatoes that were more easily harvested or transported or turned into tomato paste. Frequently, however, optimizing for one trait led to deterioration in another. For example, improving flavor could have a negative effect on yield.
First draft of transgenic papaya genome yields many fruits
A broad collaboration of research institutions in the U.S. and China has produced a first draft of the papaya genome. This draft, which spells out more than 90 percent of the plant’s gene coding sequence, sheds new light on the evolution of flowering plants. And because it involves a genetically modified plant, the newly sequenced papaya genome offers the most detailed picture yet of the genetic changes that make the plant resistant to the papaya ringspot virus. The findings appear as the cover article in the journal Nature.
Plant geneticists find veritas in vino
Viticulture, the growing of grapes (Vitis vinifera) chiefly to make wine, is an ancient form of agriculture, evidence of which has been found from the Neolithic and Early Bronze Ages. We have a detailed understanding of how nurture affects the qualities of a grape harvest leading to the concept of terroir (the range of local influences that carry over into a wine).
Researchers successfully simulate photosynthesis and design a better leaf
University of Illinois researchers have built a better plant, one that produces more leaves and fruit without needing extra fertilizer. The researchers accomplished the feat using a computer model that mimics the process of evolution. Theirs is the first model to simulate every step of the photosynthetic process.
Genes involved in coffee quality have been identified
To maintain their incomes, growers are increasingly banking on producing quality coffee. However, improving coffee beverage quality means knowing more about the biological processes - flowering, fruit ripening, etc - that determine end product characteristics.

News discussion:

General Science news