Piriformospora indica enhances plant growth by transferring phosphate

Piriformospora indica is an endophytic fungus that colonized monocot as well as dicot. P. indica has been termed as plant probiotic because of its plant growth promoting activity and its role in enhancement of the tolerance of the host plants against abiotic and biotic stresses. In our recent study, we have characterized a high affinity phosphate transporter (PiPT) and by using RNAi approach, we have demonstrated the involvement of PiPT in P transfer to the host plant. When knockdown strains of PiPT-P. indica was colonized with the host plant, it resulted in the impaired growth of the host plants. Here we have analyzed and discussed whether the growth promoting activity of P. indica is its intrinsic property or it is dependent on P availability. Our data explain the correlation between the availability of P and growth-promoting activity of P. indica.Key words: Piriformospora indica, phosphate transport, plant growth promotionPhosphorous (P) is one of the most essential mineral nutrients for plant growth and development. In the soil P is present mainly in the form of sparingly soluble complexes that are not directly accessible to plants. Thus, it is the nutrient that limits crop production throughout the world.¹ Plants have therefore evolved a range of strategies to increase the availability of soil P, which include both morphological and biochemical changes at the soil-root interface. For example, increased root growth and branching, proliferation of root hairs, and release of root exudates can increase plant access to inorganic phosphate (Pi) from otherwise poorly available sources.^2,3 Plant root possess two distinct modes of phosphate uptake, direct uptake by its own transporters and indirect uptake through mycorrhizal associations. In plants several high affinity P transporters specifically associated with the uptake of Pi from soil solution. Expression of these transporters is induced in response to P deficiency and enables Pi to be effectively taken up against the large concentration gradient that occurs between the soil solution and internal plant tissues.⁴ However, in arbuscular mycorrhizal associations (indirect uptake), plants acquire Pi from the extensive network of fine extra radical hyphae of fungus, that extend beyond root depletion zones to mine new regions of the soil.⁵ In the case of arbuscular mycorrhizal fungi (AMF), including Glomus versiforme and G. intraradices, the regulation of phosphate transporters that are expressed, typically upregulated under P deficiency but their role in P transfer to the host plant have not been characterized.^5,6P. indica was reported to be involved in high salt tolerance, disease resistance and strong growth-promoting activities leading to enhancement of host plant yield.^7–9 Recently, we have shown the role of PiPT in the P transport to the host plant.¹⁰ Here we discuss the performance of P. indica (grown under P-rich and -deprived conditions and colonized with the host plant) and its involvement in the P transportation to, and the growth of the host plant.