As well as phytohormone production, microorganisms directly
enhance plant growth by making nutrients more available
in various ways. Ammonia (NH3) production by plantassociated
microorganisms is advantageous to plants because
NH3 provides available nitrogen that supports plant growth.
Microorganisms also assist in phosphorus uptake. Plants receive
phosphorus in the form of phosphate, which is sometimes
present in nature in an insoluble form that is
unusable by plants. Microorganisms that possess phosphate
solubilising capabilities provide phosphorus in a soluble
form, as required by plants. Similarly, the micronutrient zinc
needs to be converted from an insoluble to a soluble form,
and this is done by microbial activity. In the case of iron, siderophores
produced by microbes help in the accumulation of Fe
in Fe-deficient environments, and in its uptake by plants
(Vessey 2003). By producing siderophores that trap iron, plant
growth-promoting microorganisms support plant growth directly.
However, siderophores can also have a negative effect
on fungal pathogens through iron depletion (Calvente et al.
1999; Wang & Chi 2009). In this way, the growth of phytopathogens
is limited. Therefore, siderophore production by plant
growth-promoting microorganisms may also be regarded as
an indirect mechanism for plant growth enhancement.