How Soybeans Offer Solutions for Nutrient-Stressed Agriculture

Phosphorus is likely to be a problem for farmers in the future. Overuse means that there could be a shortage of this vital nutrient. New research by Li and colleagues, published in the journal AoB PLANTS, investigates soybean (Glycine max) seedlings and how these seedlings respond to a lack of phosphorus. The team discovered that phosphorus shortage prompted intriguing changes in the soybean’s metabolism and gene expression. The research helps to unravel the hidden strategies plants deploy to survive under nutrient stress, potentially paving the way for hardier, more resilient crops in the future.

Soybeans are the second largest source of vegetable oil and a major source of animal feed. But it’s not just their nutritional punch that makes them special. These versatile plants can thrive in a range of environments, including harsh saline-alkaline soils, making them an excellent model for studying how plants resist adverse conditions.

However, like all plants, soybeans need certain nutrients to survive, and phosphorus is one of them. This vital nutrient is, unfortunately, hard to come by in many soils, often locked away due to the activities of microorganisms and its tendency to bond with metal ions. Farmers often respond by applying phosphorus-rich fertilisers, but this can lead to damaging environmental impacts like water eutrophication, an excessive growth of algae and depletion of oxygen in water bodies. 

Li and colleagues found that soybeans combat phosphorus deficiency through various adaptations, from changing their physical structures to modifying their metabolism. One fascinating survival strategy involves managing phosphorus in their cell membranes. Under phosphorus deficiency, plants degrade their phospholipids, the phosphorus-rich part of their cell membranes, to obtain phosphorus. They then replace these phospholipids with different lipids to maintain membrane stability.

They found this out by growing a bunch of soybean plants outdoors, carefully monitoring the temperature and humidity to ensure they were just right. Each plant had its own pot, filled with 2.5 kg of sand. Five weeks into their life, the researchers divided the pots into five groups, with one group being the “control” (it didn’t undergo any treatment). 

The other four groups were put on a diet. Instead of the usual full plate of nutrients, they received a solution with reduced phosphorus for different lengths of time, ranging from one to fifteen days. After the treatment, the scientists checked the growth of the plants. They washed off the sand, measured the roots, and recorded the weight of the plants, both when fresh and after drying in an oven and a vacuum dryer. 

To get even more detailed information, the researchers ground up the dried plants, treated them with a strong acid and measured the amount of various nutrient elements using a spectrometer. They also extracted a range of chemical compounds from the plants to study the plant’s metabolic response. 

Next, the team turned their attention to the plants’ genes. They extracted the plants’ RNA, a molecule that carries genetic information, to create libraries of genetic material. These libraries were then sequenced, or read, by a machine. This gives a detailed look at the plant’s genetic activity under phosphorus stress.

Finally, to confirm their results, the researchers randomly picked ten genes showing changes during the phosphorus shortage and tested them again with another method. All of this was done to see how the soybean plants adapted to the conditions they were placed in.

They found that under phosphorus deficiency stress, fresh and dry weight of roots and the number of root nodules started to decrease after two days, with greater reductions observed at 15 days. However, lighter roots didn’t mean shorter roots. Root length increased under P deficiency stress, with a rise of 30.3% observed after 15 days of stress.

The study found 61 metabolites affected by phosphorus deficiency, including sugars/polyols, amino acids, organic acids, fatty acids, and lipid substances. 

This new research opens up some intriguing pathways for understanding how plants, specifically soybeans, adapt to phosphorus scarcity. More than just an abstract biological puzzle, this study has significant practical implications. The genes identified, such as GmPS, GmPHT1, GmPAP, GmSPX and GmSQD, may act as critical points of intervention for enhancing phosphorus efficiency in crops. This could lead to improved crop varieties that can make better use of limited phosphorus supplies, which has broad implications for food security and sustainable agriculture.

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Li, M., Zhou, J., Liu, Q., Mao, L., Li, H., Li, S. and Guo, R. (2023) “Dynamic variation of nutrient absorption, metabolomic and transcriptomic indexes of soybean (Glycine max) seedlings under phosphorus deficiency,” AoB PLANTS, 15(2), p. lad014. Available at: https://doi.org/10.1093/aobpla/plad014.