The Contribution of the Use of Azospirillum sp. in Sustainable Agriculture: Learnings from the Laboratory to the Field

Abstract

Azospirillum sp. is one of the best-studied genus of plant growth-promoting rhizobacteria at present. These bacteria are able to colonize hundreds of plant species and significantly improve their growth, development, and productivity under field conditions. Different microbial abilities have been described to explain the plant growth regulation by Azospirillum sp.; clearly, a single mechanism is mostly not responsible for the full effect observed on inoculated plants, so the bacterial mode of action is currently better explained as an additive and selective effect. The most studied mechanism proposed for Azospirillum sp. to explain plant growth promotion of inoculated plants is the ability to produce several phytohormones and other related molecules with the capacity to promote plant growth or enhance the plant response to an environmental stressing condition. One of the most important achievements obtained from the research is the utilization of azospirilla commercial inoculants in approximately 3.5 million ha mainly cultivated with cereal crops in South America. Recently published reports of Azospirillum spp. inoculation of dryland crops showed a mean grain response of 10 % with greater benefits in winter (14.0 %) than in summer cereals (9.5 %) or legumes (6.6 %). In general, the increase of crop production could be obtained 70 % of the time, explained in part not only due to the complex interaction between the modes of action of Azospirillum sp. and plants but also by the multiplicity of abiotic stress conditions that the microbes help to mitigate. Azospirilla behavior leads to the theory of multiple mechanisms acting in sequential or cumulative patterns. Also, part of the variability of the plant response could be related to the different methods of inoculation (farmer or industrial seed treatments, in-furrow, foliar, or soil-sprayed applications) as well as the interaction with crop management practices related with the occurrence of abiotic limitations for crop growth (i.e., irrigation, fertilization, genotypes, combination with other beneficial microbes, etc.). But, under strong stressful growing conditions (i.e., severe droughts, major nutrients limitations, etc.), these responses are barely observed. Azospirillum sp. inoculation promotes corn (Zea mays L.) productivity, and this response may be related with the increase in the root development that increases the soil volume that the plant uses to explore for nutrients and water acquisition. The crop responses to the inoculation are greater in plant attributes defined during early growth than in those from late reproductive crop development stages. The combined inoculation of legumes with rhizobia and azospirilla, among other beneficial soil microorganisms, could over-improve the performance of the plants, compared with a single inoculation, due to the complementary biological processes of both microbes. Although the contribution of the co-inoculation with rhizobia and azospirilla on the productivity of diverse legume crops and pastures is promising, the available information under large production conditions is still limited.