Plant productivity determinants beyond minerals, water and light: Piriformospora indica - a revolutionary plant growth promoting fungus.

Biotic factors, along with the more obvious abiotic factors, determine and greatly influence the producti vity and health of the plants. Arbuscular mycorrhizal fungi, AMF, the root-interacting predominant microbiota play an indispensable role in upgrading plant growth vigour and survival. However, the nonavailability of the axenic culture is a great bottleneck for the fundamental studies and their biotechnological applications. Piriformospora indica is a newly described axenically cultivable phytopromotional endosymbiont, which mimics the capabilities of AMF. The fungus having a broad host spectrum shows pronounced growth-promotional effects. It mobilizes the insoluble phosphates and translocates the phosphorus to the host in an energy-dependent process. As a biological hardening agent of micropropagat ed plants, it renders more than ninety per cent survival rate for laboratory to field transferred plantlets. The succes sful isolation of regenerative protoplasts of P. indica opens up important possibility of improving symbiosis by transgenic manipulation of fungal component in a symbiosis-specific manner. The immobilization of the fungus stabilizes the infective capacity of the fungus and promises its use as a viable inoculum for biotec hnological applications and long-distance transportation. The axenic cultivability of P. indica on economically viable synthetic media makes it suitable for mass scale inoculum production for application in agro -forestry and horticulture. In sum up, solar energy, water and soil nutrients are undoubtedly essential for plant productivity but the interaction with useful and friendly microbes also exert a tremendous impact. IT is a general belief that plants, because they are aut otrophs, can carry out all the functions of life with the availability of the so -called abiotic factors such as solar energy, moisture and mineral nutrients. However, what is not generally realized is that the plants, as all living o rganisms, also interact with the biotic factors, and their underground root system is under the direct influence of a diverse group of micro-organisms 1 . The mycorrhizal fungi and PGPRs (plant growth promoting rhizobacteria) being mutualistic symbionts, control, in many ways, the plant health 2 . More than 90% of the terrestrial plants (angi osperms, gymnosperms, pteridophytes, bryophytes and some algae) are colonized by mycorrhizal fungi. Arbusc ular mycorrhizal fungi (AMF), which belong to the order Glomales (Zygomycota), are an integral part of the living plant system in a great majority of terrestrial plants. They penetrate into the root cortex of the host and differentiate into special intracel lular structures called arbuscules. These fungi play a pivotal role in plant health, especially in stressed soils which are not normally fertilized and are dependent on rain -fed irrigation 3