Protein ‘decides’ if plant acquires phosphorus

Researchers have discovered a protein that can have both a negative and a positive regulatory effect on phosphorus use

Researchers at La Trobe University in Victoria, Australia, have uncovered a protein that can sense phosphorus levels and alter gene regulation to either turn off or turn on phosphorus acquisition.

Phosphorus is essential to maintaining plant health and one of the three main nutrients most commonly found in fertilizers, the other two being nitrogen and potassium.

Using thale cress (Arabidopsis thaliana), the team provided the shoots with phosphorus fertilizer and then observed the behaviour of the newly found protein named SPX4. The observations showed that the protein can have both a negative and a positive regulatory effect on phosphorus take-up and plant growth.

“We were specifically interested in the SPX4 protein,” said Ricarda Jost, senior research scientist with the university’s ARC Centre of Excellence in Plant Energy Biology. “Other members of the (protein) family had been characterized before, but what set SPX4 apart was that its abundance changed with phosphate availability.”

They found that the protein could sense when the plant had taken in enough phosphorus and influenced the roots to stop taking up the nutrient. But if the protein shuts down phosphorus uptake too early, it can limit plant growth. However, a beneficial aspect of the protein is that it can activate the processes of crop development and initiate flowering and seed production. The expectation is that a greater understanding of how the protein works could lead to a more precise identification of the genes it regulates and therefore the potential to enhance its beneficial effects while potentially switching off the negative responses.

“For reasons that are not entirely clear, plants have evolved to shut down phosphate uptake whenever there is ample phosphorus fertilizer around,” said Jost. “This is the exact opposite response to the presence of nitrogen, i.e., plants increase their uptake capacity when nitrogen fertilizer is applied. (Shutting down phosphate uptake) is the main reason why crops only use about 30 percent of the phosphorus fertilizer that farmers put on their fields. SPX4 is one of the reasons why phosphate transporters are not active under these conditions.

He said their research, and that of collaborators in China working with rice, has demonstrated that removing SPX4 is not the answer

Plants that lack a functional SPX4 protein have higher phosphate uptake capacity and accumulate more phosphorus in leaves but they grow very slowly and produce less biomass.

“There must be a positive regulatory effect that SPX4 has on plant development. We need to find out what this effect is and how we can disentangle it from its negative effect. The ultimate goal is to remove the negative and retain the positive in SPX4.”

She said that Australia’s soils are generally poor in phosphorus levels. But some native plants have adapted and their cluster roots “mine” phosphorus. At the same time, these plants are also highly sensitive to too much phosphorus.

The research team is looking into the key regulators they believe are affected by SPX4, those that influence plant development and flowering time.

“This is the key question,” Jost said. “We currently do not understand why SPX4 exists. Clearly, there must be evolutionary advantages. Australian (flowering plant family) Proteaceae lack negative regulators of phosphorus uptake, but clearly are very sensitive to higher doses of phosphorus. Research by us and others would suggest that SPX4 plays an important role in integrating information about the availability of different nutrients, such as phosphorus, nitrogen, iron and sulfur. This could be a response to changing environments, in particular soil age, and changes in temperature and precipitation.”

Jost said that plants can store large amounts of nutrients for later use, and as long as dry spells do not last for too long, they should be able to maintain growth and seed production.

“(Our next research stage) is understanding how SPX4 helps to integrate information about plant nutrient status and how we can alter its function to boost phosphorus uptake.”

They will be looking at ways for breeders to apply their research knowledge in the development of future crops.

With climate change and widespread droughts in Australia, farmers need to be as efficient as possible with fertilizer inputs.

“In our no-till cropping systems, phosphorus gets stratified in the top layers of soil,” said Dr. James Hunt, a La Trobe agronomist. “When this layer gets dry, crops cannot access these reserves and enter what we a call a phosphorus drought.”

Jost said that nutrient deficiencies occur when it is too cold, too hot, or too dry. Better understanding of the role and function of the newly discovered protein may lead to more efficient update of phosphorus for plant growth and yield while reducing fertilizer waste.

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