Sep. 6, 2019
During the summer monsoon season this year, many regions of India saw spells of floods and droughts. In a country that predominantly depends on rain for irrigation, loss of crops due to such disruptive weather continues to be a source of distress to farmers. With about half of the country’s land under cultivation being rain-fed, approaches to make crops tolerant to the vagaries of weather are necessary.
A large portion of a plant’s weight is from water, which is maintained by drawing water from the ground. During droughts, increasing evaporation takes away water from the ground and leaves behind the salt, thereby increasing salt concentration in the soil. When the groundwater is saltier than the plant, the flow of water reverses direction, and the parched earth sucks the plant dry. In a recent study, researchers from J.C. Bose Institute, University of Calcutta and Bethune College, Kolkata, and Louisiana State University in the US, have shown that, by modifying particular genes, rice plants that would normally die of dehydration can be kept alive through periods of acute salinity in their water supply.
Modifying or improving plants through selective breeding is as old as civilisation. Studies show that rice, in particular, has been bred for 10,000 years to make them sturdier and produce more yield. Traditional breeding methods, which involve selecting the best plants at each generation, make it a long process. However, with modern gene-editing techniques, scientists can get quicker results by modifying specific genes that influence yield or adaptation of the plant to its environment.
To home in on the particular genes, scientists need detailed knowledge of the metabolism of the plant. A compound called inositol plays a crucial role in cellular metabolism, and thus in the growth of the plant. Furthermore, compounds derived from inositol are involved in various functions of the plant cell including communication inside the cell and between the cell and its environment.
Any disturbance to the balance of inositol, in the cell, damages the plant. One such factor that can perturb this balance is high salinity. When salt levels are high, the production of inositol shuts down in the cells. As a result, the plant is unable to produce food or perform photosynthesis, and withers away and dies. Many plants, including most rice varieties, are susceptible to this. However, some rice varieties like Porteresia coarctata can survive for about ten days under high salt concentrations.
"Earlier studies have suggested that the mechanism by which Porteresia achieves its salt-tolerance could be transferred to other rice species," say the researchers.
Porteresia has two genes that are responsible for the production of inositol in its cells under high salinity. The first, PcIN01, is involved in producing an enzyme that generates the most common form of inositol called myoinositol. The second gene, PcIMT1, codes for a protein that produces a derivative of inositol, called pinitol. In the current study, the researchers transferred the two genes into 20-day old saplings of IR64 indica, a rice variety that produces high yield but is not salt-tolerant.
The researchers compared the genetically-modified plants with the unmodified IR64s and those with extra copies of the gene that usually produces inositol. They found that the plants with the salt-tolerant genes could withstand high salt concentrations of up to 18 grams of salt per litre for ten days. That is equivalent to about half the salinity of seawater. The modified plants resumed healthy growth when regular water supply was restored. On the other hand, the unmodified plants withered and died.
The researchers found that the genetically modified plants had longer (by 300-400%) and better-branched roots at all salt concentration levels than the unmodified plants. They also better retained their chlorophyll content through the ten days of salt-exposure—retaining about 50% while the unmodified plants lose almost 90%. Besides, the fourth and later generations of the modified plants continued to produce seeds of comparable quantity and weight as unmodified plants grown under normal conditions.
Interestingly, plants with only the PcINO1 gene added outperformed those with only the PcIMT1 gene, and those with both genes, when exposed to salinity. The former also had marginally better yields in the fourth and later generations. These observations suggest that a complex web of interactions centred around inositol is involved in the salt-tolerance and recovery of the modified plants.
Modifying the genome of a plant, especially that of a food crop, can be risky as there may be unforeseen interactions between the genes in the plant or of the plant with its environment. Hence, genetically modified crops are extensively tested before they are introduced for consumption.
"Both the genes considered in the study are non-allergenic," say the researchers, explaining that neither gene is involved in the production of compounds known to cause diseases.
As extreme weather occurrences becoming more common, successfully modifying crops to withstand drought and high salinity could become essential in preventing crop-failures and averting famines.