Plants, given a helping hand

I’ve been known to remind my students that just because testing demonstrates presence of e.g. ‘phosphate’ in a soil sample that doesn’t necessarily mean it’s in a form that can be accessed/utilised by plants. And phosphorus (P) is a good example to use because it is one of the essential nutrients that plants need for healthy growth and to complete their life cycle (and which, because of the knock-on effects upon productivity and yield, understandably has important implications in an agricultural, food production context), and is often in short supply (Jianbo Shen et al., Plant Physiology 156: 997-1005, 2011).

To supplement insufficient soil supplies of P, inorganic P compounds are frequently added. Unfortunately, not only can this lead to the undesirable consequence of eutrophication (Roberto Gaxiola et al., Chemosphere 84: 840–845, 2011, but supplies of the inorganic raw material have been predicted to be exhausted by 2050 (Carrol Vance et al., New Phytologist 157: 423–447, 2003).

It’s therefore not surprising that attention has turned to organic forms of P that may be present in the soil (and can represent >50% of total soil P – David Nash et al., Geoderma 221–222: 11-19, 2014) but which are largely unavailable because plants have limited ability to mobilise them – i.e. to release the phosphate groups that they could then absorb.

One such soil resident compound is phytate (inositol hexakisphosphate (IP6)), each molecule of which contains 6 phosphate groups. Although often laid down in seeds as a major store of P for the developing seedling (e.g. Williams, Plant Physiol. 45: 376-381, 1970; Victor Raboy, Plant Science 177: 281–296, 2009), mature plants have limited ability to access P from phytate in the soil (Alan Richardson et al., Plant utilization of inositol phosphates, pp. 242 -260 in Inositol Phosphates: Linking Agriculture and the Environment (eds BLTurner, AE Richardson and EJ Mullaney, 2007; 13).

Not too surprisingly therefore, GM approaches that exploit microbes’ capacity to utilise soil-borne phytate with phytase enzymes have been recommended to enable plants to overcome P deficiency (e.g. Bijender Singh & T Satyanarayana, Physiol Mol Biol Plants 17(2): 93–103, 2011). And that’s what Liya Valeeva et al. have done (Research Journal of Pharmaceutical, Biological and Chemical Sciences 6(4): 99-104, 2015 [PDF]).*

Using bacterial phytase genes they have created transgenic Arabidopsis plants that have strong expression of the phytase protein. However, that paper doesn’t present any data about the ability of the enzyme to utilise soil phytate and thereby boost availability of P to the plant which should help it overcome any environmental P-deficiency. Presumably that is still awaited (and is crucial to establish if this could be a real advance). But, and as the authors, conclude “bacterial phytase expression in plants can be an efficient way to potentially increase crop performance in conditions of inorganic phosphorus deficiency in the soil”. More on this story – hopefully! – anon. Still, one does wonder whether it might be more environmentally-sympathetic (and less controversial given the reaction that GM plants can generate!) to ‘encourage’ greater mobilisation of P from organic P sources in the soil by focus upon the mycorrhizal relationship that the majority of plants (probably found in 80 – 90% of plant species) have with fungi, as suggested by Bagyaraj et al. (Current Science 108: 1288 – 1293, 2015 [PDF]).

* Interestingly, this study supports earlier work in which Brassica napus (oilseed rape) was similarly transformed with microbial phytase genes (Yi Wang et al., PLoS ONE 8(4): e60801). Furthermore, those transgenic plants showed higher exuded phytase activity (when compared to wild-type controls), significantly improved P uptake and plant biomass, and seed yields increased by up to 60%. However, the web site shows 0 (zero) media coverage for that article; why? Is it that such advances – and in a proper crop! – are not deemed as newsworthy as those featuring the world’s favourite model plant Arabidopsis (e.g. Science Daily, 15 October 2015)? Discuss.