Gene Renders Resistance to Plants in Freezing, Dry, and Salty Conditions

Our population now is the highest it has ever been, thanks to huge advancements in healthcare. There are more mouths to feed now than ever before. But  the amount of land and resources available on earth for farming and cultivation is limited – and despite our immense progress, millions of people across the globe still go to bed hungry. Even worse, climate change is further straining our environment and threatening our food supply. In 2011 alone, tens of thousands of people – not to mention livestock – perished in the East African famine caused by crop failure due to a devastating drought.

What if we can grow crops that are resistant to extreme cold and dry weather? Sound like a good way to solve the hunger crises in poor nations? Well it might just be possible. Researchers have discovered a gene from a grass that when inserted into other plants and overexpressed, results in dramatic improvements in their survival under stressful environmental conditions. What’s more, their growth rate and seed yield is also boosted under non-stressful conditions.

Researchers studied a grass, Sporobolus stapfianus that can withstand intense drought by drying up the inside of their cells. When water becomes readily available, they are able to quickly rehydrate their cells – within 24 hours. How does the grass manage such a feat? Upon close examination of the plant’s genes, researchers pinpointed a particular gene called SDG8i that was expressed in high levels under dry conditions. The gene SDG8i encodes an enzyme – a gycosyltranferase (UGT) – responsible for transferring carbohydrate groups to other acceptor molecules, and may affect plant growth hormones.

The researchers added a highly-expressible promoter to this gene, which allows the gene to be expressed constantly rather than only in dry or stressful conditions; they then introduced this engineered gene into Arabidopsis thaliana plants using bacteria that can transfer the gene directly into the plant’s genome. Subsequently, they compared the SDG8i transgenic plants’ growth with the wild-type in long (16 hours light) and short-day (8 hours light) scenarios and under stressful conditions such as in cold, dry, and salty environments.

In normal conditions, the transgenic SDG8i plants showed superior growth characteristics compared with wild-type plants: they had more leaves, larger leaves, higher shoot biomass, branching, and seed yield. These gains were more prominent in the short-day plants. The main stem of the transgenic plants grew 20 percent faster compared with the wild type. Even the roots of the SDG8i plants grew longer and had a greater biomass than the wild-type.

journal.pone.0080035.g001_cropped

A comparison of SDG8i transgenic (T) and wild-type (WT) plants.
Plants were grown at 21°C in (A) Long-day (16 hours light), and (B) Short-day (8 hours light)
Image: Islam S, Griffiths CA, Blomstedt CK, Le T-N, Gaff DF, et al. (2013)

Coming to salt tolerance, SDG8i plant roots grew up to 7-fold higher than the wild-type roots in a high salt media concentration.

When the plants were exposed for an hour to bitterly freezing temperatures of -12° Celsius, more than two-thirds of transgenic SDG8i plants survived upon return to room temperature compared with only a third of the wild-type plants.

In the drought test, researchers deprived pre-flowering plants of water for three weeks, after which they were re-watered and monitored for survival after a day. They found that after 13 days none of the wild-type plants survived whereas, to their amazement, all of the transgenic plants managed to persist for 6 more days, until day 19, when they too succumbed to severe dehydration. The leaves of the wild-type plants showed signs of drought-induced senescence (aging). In stark contrast, transgenic plant leaves were healthy, large, and green even after 13 days of enduring the dry spell. Further tests also showed that the SDG8i plant leaves could retain their green color longer than the wild-type plants when in the dark.

Wild-type (WT) plant and SDG8i transgenic plant (T) after 13 days without water  Image:

Wild-type (WT) plant and SDG8i transgenic plant (T) after 13 days without water
Image: Islam S, Griffiths CA, Blomstedt CK, Le T-N, Gaff DF, et al. (2013)

How does the SDG8i-encoded enzyme initiate these changes? The researchers hypothesized that the enzyme UGT may affect the activity of plant hormones such as the senescence-promoting hormone abscisic acid. To investigate they measured the chlorophyll (a green pigment) content in both types of plant leaves, after incubating them in light, both in the presence and absence of abscisic acid. It turned out that in the presence of the hormone, the SDG8i leaves retained significantly higher levels of chlorophyll for longer compared with the wild-type leaves. They concluded that SDG8i expression reduces senescence induced by abscisic acid and it does so by acting further down the pathway than abscisic acid does.

Finally, the researchers wanted to find which growth or stress hormone is targeted by the SDG8i enzyme UGT. They examined UGT activity in the leaves of another SDG8i plant against many known plant growth and stress hormones and substrates. They learned that UGT was highly active in adding sugar groups to a strigolactone-like compound.

Strigolactones are a newly identified class of plant hormones that affect shoot branching and radiate chemical signals from the roots that trigger nearby parasitic plant seeds to germinate, possibly allowing root colonization by beneficial parasites. Interestingly, the SDG8i plants significantly reduced the germination of Orobanche seeds, a harmful parasitic weed that robs its host of nutrients, compared with the wild-type plants.

Scientists envision crops overexpressing SDG8i as a promising means to buffer effects of climate change and increase world food supplies to feed our ever-growing population. These transgenic crops may not only be more tolerant in stressful conditions, but also produce higher yields in non-stressful conditions. Although more research needs to be done, someday transgenic crops may help poor countries to cope in adverse environmental conditions – and famines may finally be a scourge of the past.

Reference

Islam S, Griffiths CA, Blomstedt CK, Le T-N, Gaff DF, et al. (2013) Increased Biomass, Seed Yield and Stress Tolerance Is Conferred in Arabidopsis by a Novel Enzyme from the Resurrection Grass Sporobolus stapfianus That Glycosylates the Strigolactone Analogue GR24. PLoS ONE 8(11): e80035. doi:10.1371/journal.pone.0080035

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