Plant Cuttings

Reducing P uptake by plants

Some things are hard to digest, but is an excess of phosphorus really one of them?

Given the importance of phosphorus (P) to plants, and the fact that P is often present in insufficient amounts in the growing medium to sustain proper plant growth and development, you might wonder why anybody would want to restrict the ability of plants to take up this essential macronutrient. However, they do – in cereals anyway – and here’s why.

Phytic acid
Phytic Acid in 2D. Image: Harbinary / Wikipedia

Although P is also an essential macronutrient for humans, and must be derived from the diet, there are ‘good forms’ of P and ‘bad forms’. One such bad form of P is phytate, which is found in high amounts in the grains of cereals. Although such grains are important food sources, their phytate content cannot be digested by humans (likewise in the case of mankind’s non-ruminant domesticated animals such as poultry and pigs fed on cereal-based foodstuffs).

This non-digested P-source is excreted from the body and can contribute to eutrophication of waterways. Not only is that phenomenon detrimental to the environment it also represents a substantial loss of P from reserves in the soil, which consequently need to be replenished to sustain subsequent plant growth. This nutritional replenishment is usually by environmentally-expensive artificial fertilisers – which have their own financial, environmental and health-associated costs*. So, wouldn’t it be best all round if phytate content in cereal grains could be reduced? Arguably, yes. And this desirable state of affairs may be achievable, thanks to a discovery made by Naoki Yamaji et al.

Working with rice (Oryza sativa), they’ve identified SPDT (a phosphorus distribution transporter protein similar to  SULTRs a class of sulphate transporters in plants) that controls P allocation to the grain. The gene, SPDT, encodes a plasma-membrane-localised P transporter that is expressed in the xylem region of vascular bundles at the nodes . Knocking-out (i.e. preventing the gene from working) SPDT altered P distribution within the plant such that total P and phytate amounts in the brown de-husked rice were, respectively, 20% and 30% lower than normal (so, still providing good amounts of dietary P, but reducing the amount of excretable, eutrophication-causing P). Importantly, yield, seed germination and seedling vigour were unaffected by this P re-partitioning. Furthermore, P in the straw of the mutant plants, which would normally be returned to the soil post-harvest, was increased. And, reduction in grain phytate amount should increase bioavailability of zinc and iron, both essential nutrients for human health and well-being, whose absorption is impaired by phytate, from that dietary source.

This is great news for rice – the dietary staple for ‘more than half’ of the world’s population. And great news for the substantial fraction of humanity – and domesticated animals – for whose diet cereals such as wheat and maize are major staples (if this rice success can be translated to those crops). Sometimes, as in this item, less really is more!**

* An indication of the magnitude of this caryoptical P-depletion conundrum may be gleaned from the estimate that this removal of crop-containing P can account for 85% of the phosphorus fertilizers applied to the field each year (John Lott et al., Chapter 2 ‘A Global Estimate of Phytic Acid and Phosphorus in Crop Grains, Seeds, and Fruits’).

** However, there is evidence that dietary phytate can have human-healthpromoting properties, so maybe a balance has to be found – between being kind to the environment and to humankind. Who decides…?

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