Microorganisms are essential for the development of plants. Crops yields are often limited by the availability of soil nutrients. The interphase between the roots of the plants and the soils is the rhizosphere, and it is there where the organic carbon flow from the roots feeds the microorganisms, and they in turn make available other nutrients to the plant.
Drought episodes are common in Mediterranean as well as other countries and regions of the globe. More than 50% of the land surface of the Earth is considered arid or semi-arid. Experimental results show that many microorganisms contribute to a better growth of plants in drought conditions. Some of the underlying mechanisms are being elucidated.
The ongoing climate change will increase the frequency and severity of the drought episodes in cotton (Gossypium hirsutum L.), soy (Glycine max L.), and corn (Zea mays L.), so it is expected an increase in the cultivated surfaces of these important crops 12.
This work will report some the beneficial interactions found between plants and microorganisms in drought conditions.
Pseudomonas spp. can survive under stress conditions through the production of an exopolysaccharide (EPS) that makes the bacterium more resilient to drought by an increase in the water retention and a regulation of the diffusion of the organic carbon sources 13. Additionally, the EPS increases the relative amount of soil that binds to the root. This soil plays a fundamental role in the acquisition of water and nutrients by the plant. Similar cases have been described in Rhizobium 1 and in Bacillus and Paenibacillus 16.
Hydric stress has been associated to the production of ethylene by the plants, and this volatile compound is responsible for the subsequent growth inhibition 11. Ethylene is considered a stress hormone in the plants, since it is produced under a number of both biotic and abiotic stress conditions 17. The precursor in the biosynthesis of ethylene is the compound aminocyclopropane-1-carboxilic acid (ACC). Any PGPB (Plant Growth Promoting Bacteria) that could express the enzyme ACC-deaminase would decrease the level of production of ethylene and stimulate the growth of the plant to which it associates. This effect has been observed when plants that had been inoculated with these types of bacteria were subjected to drought conditions 9, 10, 16, 19.
It has also been reported that strains of Azospirillum brasilense can augment the water content in the leaves through the production of the hormone ABA (abscisic acid)4,5. These Azospirillum strains present ACC-deaminase activity, and it correlates with an increase in deep water uptake by the plant 3, as well as a more efficient use of water in peas, and more resistance to drought in tomato and pepper 20.
Under drought conditions, some bacteria accumulate certain solutes as proline and sugars that confer protection to the microorganism. The accumulation of the compatible solutes as proline, glycine, betaine and trehalose increases the thermo-tolerance of the enzymes, decreases their thermal denaturation rate and helps in keeping the integrity of the cellular membrane 3,6,14,18.
Soil dryness can create physicochemical and biological conditions that are unfavorable for plants. Paenibacillus polymyxa confers tolerance to drought conditions by the induction of a sensitive gen, the one for the early response dehydration 15 (ERD15)) in Arabidopsis thaliana 15.
It has been found that Pseudomonas, Bacillus and Mycobacterium promote the growth of corn 7 and alleviate the adverse effect of water deficiency in wheat 8. Other authors describe some Azospirillum strains that promote the growth of wheat in water deficiency. These strains present a combination of characteristics as nitrogen fixation, auxins production, phosphate solubilization, ACC-deaminase activity, and siderophore production.
Finally, the field of research on the interaction of plant-microorganisms is in a state of great dynamism and certainly evolving fast. The next years should witness an increase in the application of microorganisms in the agriculture so to become more and more sustainable.