Date Published: May 4, 2018
Publisher: Springer Berlin Heidelberg
Author(s): Josiane Fukami, Paula Cerezini, Mariangela Hungria.
The genus Azospirillum comprises plant-growth-promoting bacteria (PGPB), which have been broadly studied. The benefits to plants by inoculation with Azospirillum have been primarily attributed to its capacity to fix atmospheric nitrogen, but also to its capacity to synthesize phytohormones, in particular indole-3-acetic acid. Recently, an increasing number of studies has attributed an important role of Azospirillum in conferring to plants tolerance of abiotic and biotic stresses, which may be mediated by phytohormones acting as signaling molecules. Tolerance of biotic stresses is controlled by mechanisms of induced systemic resistance, mediated by increased levels of phytohormones in the jasmonic acid/ethylene pathway, independent of salicylic acid (SA), whereas in the systemic acquired resistance—a mechanism previously studied with phytopathogens—it is controlled by intermediate levels of SA. Both mechanisms are related to the NPR1 protein, acting as a co-activator in the induction of defense genes. Azospirillum can also promote plant growth by mechanisms of tolerance of abiotic stresses, named as induced systemic tolerance, mediated by antioxidants, osmotic adjustment, production of phytohormones, and defense strategies such as the expression of pathogenesis-related genes. The study of the mechanisms triggered by Azospirillum in plants can help in the search for more-sustainable agricultural practices and possibly reveal the use of PGPB as a major strategy to mitigate the effects of biotic and abiotic stresses on agricultural productivity.
Projections of population increases, especially in developing countries, as well as of life expectancy worldwide, imply greater needs for food and feed (FAO 2009). To achieve higher productivity, agriculture is being intensified, mainly with monocultures highly dependent on increased chemical inputs, including pesticides and fertilizers (McArthur and McCord 2017; Roser and Ritchie 2017). However, to ensure long-term food production, we must develop sustainable agricultural practices, based on conservationist practices, to achieve economic returns for farmers, but with stability in long-term production and minimal adverse impact on the environment (Sá et al. 2017). In this context, the use of microbial inoculants plays a key role, and we may say that we are starting a “microgreen revolution.”
Abiotic and biotic stresses result in oxidative damage to plants due to an increase in reactive oxygen species (ROS), representing an initial mechanism of plant response to the attack of pathogens (Finkel 2000; León and Montesano 2013), and of defense against abiotic stresses (Heidari and Golpayegani 2012; Wang et al. 2012).
Plants are commonly exposed to several environmental stresses such as high and low temperatures, drought, salinity, alkalinity, UV-rays (Sharma et al. 2012); estimates are that about 30% of the global crop production is lost as a result of abiotic stresses (Goswami et al. 2016), and PGPB can play a strategic role in reducing these losses, by activating several physiological and biochemical tolerance mechanisms in plants (Yang et al. 2009; Kim et al. 2012; Sarma et al. 2012), named induced systemic tolerance (IST). The mechanisms related to IST include antioxidant defense (Heidari and Golpayegani 2012; Wang et al. 2012), osmotic adjustment (Sarma and Saikia 2014), production of phytohormones such as indole-3-acetic-acid (IAA) (Spaepen and Vanderleyden 2015), defense strategies such as the expression of PR-genes (Kim et al. 2014), and the induction of heat-shock proteins (HSP) (Lim and Kim 2013).
Azospirillum is currently one of the most broadly studied and commercially employed PGPB. Previous studies with Azospirillum emphasize its capacity of fixing atmospheric N2, followed by benefits in promoting plant growth via synthesis of phytohormones. More recently, it has been shown that the benefits should be extended to the capacity of some Azospirillum strains to protect plants from biotic stresses, triggering ISR defense mechanisms, and from abiotic stresses, through IST. Figure 1 summarizes the mechanisms discussed in this review of tolerance of abiotic and biotic stresses promoted by inoculation of Azospirillum in plants, encompassing detoxification of oxidative stress, ISR and IST. The mechanisms that PGPB use to cope with biotic and abiotic stresses vary with the plant species and cultivar and with the bacterial species and strains, and also depend on the phytopathogen and the intensity of the abiotic stress. Further studies to elucidate the mechanisms of action of PGPB—as well as of the response of plants to stresses—are of fundamental importance for understanding the potential and increasing the use of PGPB as an important and sustainable strategy to mitigate the effects of biotic and abiotic stresses in agriculture.