Researchers from Zhejiang University College of Life Sciences uncover how we may improve crop production by improving phosphate uptake in soil.
Found in rock formations and ocean sediments as phosphate salts, phosphorus is released into the environment through weathering and is subsequently taken up by plants. It is an essential macronutrient for plant growth that is found in minute amounts in soil and so, limits plant development. As a result, phosphate fertilisers are often used in agricultural production and yet, only 20 to 30 per cent of phosphate is taken up by crops while the rest is lost or becomes insoluble. For this reason, there is a need to improve the efficiency of phosphate uptake by plants to safeguard the sustainable development of agriculture.
A study led by Prof. Zheng Shao-Jian from the Zhejiang University College of Life Sciences has revealed the cause of organic acid secretion by low phosphorus levels and the process behind phosphorus and nitrogen coordination in plants, thereby providing new knowledge in the use of insoluble phosphorus in soil.
Under phosphorus-deficient conditions, plants have adaptive strategies to boost phosphate uptake. Such strategies include activating the expression of phosphate transporter genes, changed root system architecture, and increasing the secretion of phosphatases and organic acids.
Previous studies have shown that in low phosphate conditions, a transcription factor STOP1 (or Sensitive to Proton Rhizotoxicity 1) directly activates the expression of proteins MATE (Multidrug and Toxic Compound Extrusion) and ALMT1 (Aluminium-Activated Malate Transporter 1), effectively regulating citrate and malate secretion in plants, which mediates the production of hydroxyl radicals that inhibit the growth of primary roots. However, much remains to be known about the upstream signalling events of low phosphate-induced secretion of organic acids and the increased accumulation of STOP1 protein. Other studies have also found that when ammonium was removed, primary root growth was insensitive to phosphate deficiency, suggesting that ammonium might be involved in the repression of primary root growth in low phosphate conditions. Despite this, molecular interpretations for these observations are still lacking.
Prof. Zheng and colleagues set out to determine if ammonium affects plant root growth in low phosphate conditions. When exposed to light, low phosphate inhibited root growth of plants grown in a medium containing ammonium. Alternatively, when the roots were shielded from light, low phosphate did not repress root growth. However, when the researchers planted the seedlings in a medium without ammonium, low phosphate did not inhibit root growth, even when exposed to light. Further analyses into ammonium found that ammonium uptake by ammonium transporters participates in root growth inhibition in low phosphate conditions.
Looking deeper, the uptake of ammonium was found to acidify the rhizosphere, which enhances STOP1 protein accumulation. The build-up of STOP1 protein, in turn, stimulates the secretion of organic acids, which will dissolve phosphate from insoluble iron or calcium phosphates. Although ammonium uptake improves phosphate acquisition, the over-application of ammonium-based fertilisers can lead to ammonium toxicity and excess soil acidification. The team provided evidence that upon exposure to high ammonium levels, STOP1 protein enhances the expression of CIPK23 protein kinase, which has been known to repress ammonium transporters and effectively inhibit ammonium uptake.
From the study, Prof. Zheng and his team have identified a STOP1-centred regulatory network that links external ammonium with efficient phosphate uptake from insoluble phosphate sources. “These findings provide a framework towards strategies to improve crop production by enhancing the utilisation of non-bioavailable nutrients in soil,” said Zheng.
Source: Tian et al. (2021). A Transcription Factor STOP1-Centered Pathway Coordinates Ammonium and Phosphate Acquisition in Arabidopsis. Molecular Plant.