April 23, 2013

The Scientist: Prof. Susheng Gan Studies Plant Longevity

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For vegetable lovers, having a big bowl of salad for a summer dinner comes with a potential risk. If not completely consumed in time, the romaine lettuce will wither and turn yellow in just a few hours. According to Prof. Susheng Gan, horticulture, it is possible to extend the freshness of leaves through the discovery of the genes that trigger tissue aging.

The terminal phase in a leaf’s life, known as life senescence, is a genetically programmed active degeneration process during the life of the plant. Plant senescence occurs when there are not enough resources for the plant to continue growth, so the plant decides to end its leaves’ life cycles to reduce consumption of nutrition and support necessary growing tissues.

Such a “suicidal” process is more complicated than it sounds. The purpose of senescence is future regeneration. Leaf senescence is like a recycling process. The proteins are degraded and the amino acids are stored for the new leaves in the next growing season.

“If you look carefully at the leaves in the fall, you will notice that the vascular tissue on each leaf is the last part to die,” Gan said. “Without the veins, materials cannot be transported during senescence.”

During the mobilization of leaf senescence, about 10 percent of plants’ genes are activated, each with different functions. One of Gan’s main projects is to figure out, at the molecular level, how senescence genes are regulated.

“If the whole senescence process is a drama,” Gan said. “What we are doing is to find the director of the drama, the master regulator gene, from whom we learn quickly how the drama works.”

Learning about the mechanism behind senescence helps scientists control the master regulator gene. From there, they may be able to remove its function, disable the subset genes, and delay the senescence process.

“This is like the army,” Gan said. “When the commander is removed, his soldiers don’t know how to fight.”

Gan has already found one of the master regulators in leaf senescence – called AtNAP. It is a protein that controls the flow of a group of genes. After AtNAP is removed, the leaf longevity of rockcress, the model plant used in Gan’s research, increased up to 50 percent.

Gan and his lab translated this discovery to other plants. The same gene-removal strategy was applied to the soybean plant, which is genetically similar to rockcress.

The result was dramatic – without the group of counter genes, not only was leaf longevity of the soybean plants extended, but the senescence of its underground root nodules was also delayed. This allowed the root nodules to fix more nitrogen into the ground.

“This is very important because the fixed nitrogen supports plant growth,” Gan said. “Meanwhile, it fertilizes the soil, forming sustainable agriculture.”

According to Gan, his lab aims to find more master regulator genes to prolong leaf longevity.

“The reason we especially focus on leaf longevity, is that the leafs’ function is like manufacturing – everything in a plant made from the leaf,” Gan said. “Delaying leaf senescence boosts the yield and biomass accumulation.”

Gan’s discovery also applies to food security and food preservation. For the vegetable lovers, Gan’s research provides a possible solution to wilting lettuce.

“As we know, old people are more vulnerable to pathogens than the youth,” Gan said. “The leaf is like us. As the leaf started to turn yellow, it became more vulnerable to pathogens. The post-harvest pathogens produce toxins that harms humans.”

Once leaf senescence is under control, harvested vegetables can be preserved fresh for longer without deterioration or generation of post-harvest toxins.

In addition to research, Gan shares his knowledge of senescence with students in Horticulture 4250: Postharvest Biology of Horticultural Crops, in the spring semester.

Original Author: Yidan Xu

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