Induction of Plant Tolerance to Semi-arid Environments by Beneficial Soil Microorganisms – A Review

Abstract

Currently arid or semi-arid land areas are increasing worldwide due to global warming and the soil is becoming saline because of the use of intensive irrigation in the crop fields. Consequently, the proportion of plants living under water shortage conditions is increasing. This phenomenon is limiting seriously crop production in such areas. In many cases, the fields are being abandoned and become uncultivable again in a period of time due to erosion. Although plants have their own mechanisms to cope with drought stress, they become more tolerant to drought when associated with different soil microorganisms. Among these soil microorganisms, the most abundant and effective are rhizobia, plant growth promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF). Rhizobia fix atmospheric nitrogen and transport this fixed nitrogen to the legume host plant. PGPR promote plant growth by means of several mechanisms. They are present in almost all ecosystems across the world. AMF were vital to plants when starting to colonize dry land surface, and hence for improving plant mineral nutrition, especially uptake of phosphorous, among other factors. Here we detail the most recent advances about how these microorganisms enhance plant drought tolerance at physiological and molecular levels, including decreased oxidative stress, improved water status or regulation of aquaporins. It has been found that legume plants inoculated with rhizobia grow faster under drought conditions than non-inoculated ones. However, how rhizobial symbiosis affects root water transport has not been addressed yet. At the same time, it seems that there is a correlation between drought tolerance in rhizobial bacteria and rhizobia-induced plant drought tolerance, at least in terms of reducing plant oxidative stress. Under drought conditions, PGPR regulate the levels of stress-related hormones, i.e. abscisic acid and ethylene. The regulation of these hormones could be the cause of an enhancement of plant drought tolerance mediated by PGPR. However, a more detailed molecular approach is still needed to fully understand this process. Arbuscular mycorrhizal symbiosis improves almost every physiological parameter of the host plant under drought stress, i.e. water status, leaf transpiration, photosynthesis or root water uptake. However, the molecular basis for this improvement is far to be understood. At the same time, AMF in combination with rhizobia or PGPR usually have an accumulative beneficial effect on plant drought tolerance, although this depends on the specific pair of strains inoculated. Therefore, although there are many studies in order to understand at the physiological level how beneficial soil microorganisms induce plant drought tolerance, there is still a lack of knowledge about the molecular basis behind this improvement.