Understanding the genetics behind senescence — the process of aging — should help make crops more productive
Senescence is the condition of aging in plants and animals. One of the best examples is the change in the colour of leaves in autumn as they die and fall off trees. The energy saved from the leaves is stored in the trunk and branches for next spring’s growth.
While the process of senescence is widely known, the genetics behind it has never been clearly understood.
At Clemson University in South Carolina, scientists have made a breakthrough in deciphering the molecular, physiological and biochemical background to the senescence in corn that has been bred to stay green longer and produce greater yields.
“Breeders have known and bred for stay-green trait for several decades,” said Rajan Sekhon, plant geneticist and assistant professor in the university’s department of genetics and biochemistry.
“However, knowledge of the genetics behind this trait and, in particular, identity of the genes that control senescence, has been very limited.”
Stay-green plants live up to their name. They stay green longer, produce greater yields and are more resistant to environmental challenges such as drought and heat.
Sekhon said that the delay of senescence has been successfully achieved in cereal crops, including corn, rice, sorghum, and wheat. However, from the point of view of yield and quality, while the stay-green trait is useful for cereals, it is undesirable in beans and oilseed crops.
“This is because cereal seeds primarily store carbon (in the form of carbohydrates) while beans and oilseeds are rich in nitrogen (in the form of proteins and oil),” he said. “One of the major evolutionary roles of senescence is to dismantle proteins in the leaves by a process called proteolysis, recover nitrogen, and transport that extra nitrogen to developing seeds, where this nitrogen is again used to make proteins and other products.
“It is important to note that the most abundant protein on earth, called RuBisCO, is found in the plant leaves. Therefore, while excess carbohydrates made by stay-green cereals result in higher total yield, stay-green beans and oilseeds may actually have lower protein content and, therefore, poor quality produce.”
To study the stay-green trait in corn, his team used two separate approaches to identify candidate genes. In the first approach, they used 400 diverse corn varieties (inbred lines) each genetically distinct from the other based on their DNA. They then compared the observable characteristics among them.
“This approach provided a list of 64 genes in the genome that likely control senescence,” said Sekhon. “In the second approach, we compared the expression of all (about 43,000) maize (corn) genes in a normal maize senescing and a stay-green maize variety and identified some 10,000 genes that express differently in these two varieties. We then took these 10,000 genes and asked how many of these genes act in a co-ordinated manner.”
He said that the process gave them a network of 600 genes that co-ordinated to control senescence. They combined the results from both approaches and found a core set of genes common to both approaches. That core set became the target of further studies which, in turn, narrowed the field to 14 candidate genes and, ultimately, two genes that were studied in detail.
The trigger for senescence is sugar but there is a complex relationship in the roles of sugar in the leaves of corn that drives the onset of aging.
“The research community is aware of the role of sugars in senescence,” said Sekhon. “A lesser understood aspect, however, is the role of the complex interplay of sugar source and sugar sink in controlling senescence. Sugar source refers to leaves that make the sugars in the first place via photosynthesis (the source of sugar) and sugar sink is the plant organ that ultimately stores these sugars. This source-sink interaction and its role in senescence is the big question my laboratory is trying to address.”
The research showed that, in non-pollinated plants, excess sugars accumulate in the leaves and trigger natural senescence.
Sekhon said they then investigated if the extra sugars could be transported to vegetative, non-grain organs of the plants, such as the stems, once the seed development was complete. The non-grain plant parts, called stover, could then be used for animal feed or for making biofuels.
“This will be a win-win outcome as redirecting sugars to other organs will delay senescence and lead to more total sugars in the plant.”
The implications of the study are important for food security.
“Our study provides target genes that can be exploited to divert extra sugars. Such a diversion would help make the plant stay-green and also result in more net carbon yield. This approach will also be applicable to other cereal crops.”
The research paper was published in the journal The Plant Cell.