Abiotic stresses affect plants in different ways and are causes of reduction in crop productivity. In order to increase crop productivity it becomes necessary to evolve efficient low-cost technologies for abiotic stress management. Soil microorganisms, surviving in the soil under extreme conditions, have shown great properties, which, if exploited can serve agriculture for increasing and maintaining crop productivity. While it is well established that beneficial soil microorganisms can promote growth and increase productivity through mechanisms such as nutrient mobilization, hormone secretion and disease suppression, it is also becoming increasingly clear that their effects may be more far-reaching. Several studies have reported that soil microorganisms may have mechanisms for alleviation of abiotic stresses in plants such as water and temperature stress, salinity, heavy metals etc. Some of these include tolerance to salinity, drought (Azospirillum sp., Pseudomonas syringae, P. fluorescens, Bacillus sp.) and nutrient deficiency (Bacillus polymyxa, Pseudomonas alacaligenes). Other than bacteria, salinity- and drought-tolerant isolates of Trichoderma harzianum and the effect of other strains of Trichoderma in amelioration of such abiotic stresses have also been reported. Arbuscular mycorrhizal fungi (Glomus mosseae, G. etunicatum, G. intraradices, G. fasciculatum, G. macrocarpum, G. coronatum etc.) help in alleviating abiotic stresses in different crops by enhancing nutrient uptake (phosphorus, nitrogen, magnesium and calcium), biochemical (accumulation of proline, betaines, polyamines, carbohydrates and antioxidants), physiological, molecular and ultra-structural changes. In the present chapter, we attempt an overview of current knowledge on how plant-rhizobacteria, plant-Trichoderma as well as plant-mycorrhiza interactions help in alleviating abiotic stress conditions in different crop systems, which can be used for sustainable agriculture.
