Smaller plants show promise for future food crops

At a time in history when we need to feed more people than ever, reducing the size of our food-producing plants may seem counter-intuitive.

But for many regions of the world, their productive agricultural lands will shrink as the climate warms. Other changes could include a shift in the amount and timing of rainfall.

This means agriculturalists want to develop crop varieties specifically adapted to grow in certain regions, explains Professor Prem Bhalla from the University of Melbourne’s Faculty of Veterinary and Agricultural Sciences.

“It’s all about ensuring food security,” Professor Bhalla says.

A successful crop yield is based on how a plant grows – including its size and ability to produce seed or fruit among other factors ­– so Professor Bhalla and her team have gone back to the genetic blueprint of plants to understand how these stages are controlled by the plant’s DNA.

Large plants do not necessarily mean high yield. In fact, the development of semi-dwarf varieties of our staple crops – like wheat and rice – have led to dramatic increase in yield.

Their latest study is focussed on soybeans, a major global crop, with increasing production in Australia. The beans themselves are harvested as a food source for humans and animals, and soybean oil is used for cooking and animal feed.

The soybeans we eat are the seeds that usually grow in groups of two or three inside a pod where the green, immature beans are known as edamame.

“We know from growers that large soybean plants are actually a problem during harvest,” Professor Bhalla says. They can grow up to an average of one to 1.5 metres tall and then bend or fall, making it hard to gain access to the beans for efficient harvesting.

Also, tall plants use most of their resources for excessive vegetative growth – that is, producing more leaves – that then reduces the energy available for seed yield. Compact plants can be grown more densely in the field, leading to potentially enhanced yield per acre of agricultural land.

Along with Professor Mohan Singh and graduate researcher Hina Arya, also from the University of Melbourne, they investigated soybean PIF4, a gene that interprets light and temperature signals from its environment, triggering plant growth.

“Because seed yield is based on how and when a plant flowers, our theory was that by affecting flowering time, the gene could have a knock-on effect on seed production which in this case is the soybean.”

The researchers used genetic tools to make PIF4 more active in the soybean plant, a technique known as over-expression, so that its function would become more obvious.

By choosing a commercially grown crop variety, known as Bragg, they hoped to provide valuable information to breeders.

Their study showed that overactivity of the gene resulted in a semi-dwarf soybean plant, with a reduced height of around 50 per cent. Despite their reduced size, the soybean plants with higher activity of the PIF4 gene still produced the same yield.

Other results showed that as well as a reduced height, the plants also had a smaller leaf area and a faster transition from flowering stage to full maturity and seed production.

“We think that by increasing the activity of the gene, the soybean grew smaller, meaning more energy could be transferred into seed production,” Professor Bhalla says.

“We were amazed that the effects were so widespread in the soybean plant.”

These dramatic effects suggest that – as predicted in laboratory studies in the Arabidopsis plant – PIF4 also acts in soybean as a regulator of many other genes, including those responsible for plant architecture and the production of growth hormones.

“Its effects on plant morphology and reproductive stages in soybean suggest the gene could be a target for soybean improvement programs to ensure food security,” says Professor Bhalla.

In what could be a helpful aid for breeding future crops, the study also found that when the PIF4 gene is overactive, the plant produced darker seed pods, making them easier to identify and useful as a ‘breeding marker’.

The next stage of their research is to understand if these plants also use less water and fertiliser to produce same yield, and how tolerant they could be to drier conditions.

According to Professor Bhalla, this new study paves the way to breed crops for specific growth conditions.

“And if the plants produce the same yield, we have the potential to gain more food from a smaller area of our increasingly precious cropland.”

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