Many recent studies demonstrate the potential as plant biostimulants of microbial consortia, rhizobacteria, and rhizofungi, that function as an agricultural probiotics (de Vries and Wallenstein, 2017; Wallenstein, 2017; Kong et al., 2018). The present work describes an example of two prospective mic Contact online >>
Many recent studies demonstrate the potential as plant biostimulants of microbial consortia, rhizobacteria, and rhizofungi, that function as an agricultural probiotics (de Vries and Wallenstein, 2017; Wallenstein, 2017; Kong et al., 2018). The present work describes an example of two prospective microbes and their qualities as consortium
In this review, we summarize some of the best-known mechanisms of plant probiotic bacteria to improve plant growth and develop a more sustainable agriculture. Keywords: plant growth promotion, biofertilizer, sustainable agriculture, beneficial bacteria. 1. Plant Probiotic Bacteria: Why Are They Necessary?
Probiotic farming, sometimes referred to as biointensive agriculture, combines various organic farming techniques that focus on making soil healthier. It introduces beneficial microorganisms into the growing environment.
Probiotics are defined as beneficial microbes, conferring health benefits to the host whether it is a plant or an animal. In this sense, in the last years the exploitation of probiotic microorganisms for the plant wellness and growth has largely caught on, so much so that nowadays, bioinoculants based on plant growth promoting (PGP
Strategies for applying endophytes as probiotics in plant in-vitro system. Plant growing under natural habitat is the potential source of promising endophytes that can be used for in-vitro system of the specific host plant.
Citation: Woo SL and Pepe O (2018) Microbial Consortia: Promising Probiotics as Plant Biostimulants for Sustainable Agriculture. Front. Plant Sci. 9:1801. doi: 10.3389/fpls.2018.01801
*Correspondence: Sheridan L. Woo, d29vQHVuaW5hLml0 Olimpia Pepe, b2xpcGVwZUB1bmluYS5pdA==
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Received 2017 Apr 27; Accepted 2017 Jun 12; Collection date 2017.
This is an open access article distributed under the terms of the Creative Commons Attribution License ()
According to the Population Division of the United Nations, the world human population is estimated to reach around 9.5 billion people in 2050, and this fact will be accompanied by significant shifts in diets in developing countries, including more intake of animal origin calories, which should be satisfied by more intensive animal agriculture that will also demand more food consumption in a short period in time [2],[3]. This situation inevitably implies a search for more efficient procedures to produce animal feed and food for humans, with higher yields and more resistance to biotic and abiotic stresses [4],[5], while protecting animal and human health and, at the same time, being friendly with the environment.
On the other hand, high-quality food demand is also increasing in both developed and developing countries [6].
In this sense, the application of microorganisms, especially bacteria, with plant growth promoting features, the so-called plant probiotics, may be a possible solution to increase crop production while avoiding the above mentioned problems related to the application of chemical fertilizers and pesticides and, moreover, allowing the obtention of better quality products [6],[7].
In this review, we summarize the main mechanisms of plant growth promotion presented by bacterial species, which were reported as able to improve crops yields and hence refer to updated studies that have evaluated the potential applications of a wide diversity of bacterial isolates in different plants, mostly with agronomical interest. Finally, we will discuss some aspects of the current application of those strains as biofertilizers and suggest future perspectives concerning the role of bacterial-based biofertilizers in sustainable agro-ecosystems.
Rhizobacteria can promote plant growth through a broad range of mechanisms, which can be grouped according to their mode of action in: (i) the synthesis of substances that can be assimilated directly by plants, (ii) the mobilization of nutrients, (iii) the induction of plant stress resistance, (iv) the prevention of plant diseases. PGPR presenting one or several of those plant growth-promoting traits are summarized in Table 1.
Some species belonging to the genera Gluconacetobacter, Azospirillum and Herbaspirillum are frequent sugarcane endophytes and act as nitrogen fixers contributing to this plant nutrition [23],[39],[40]. The genus Herbaspirillum has also been identified as a nitrogen fixing endophyte of several crops [40],[41],[42]. Last but not least, the genera Bacillus and Paenibacillus are free-living nitrogen fixers and have other PGP traits, which make them suitable candidates for application [94],[95].
Moreover, some of those free-living bacteria may enter roots of some crops, such as species of Azoarcus, Azospirillum and Burkholderia in rice roots, which increase the nitrogen concentration of this specific crop [21],[35],[96],[97] or nitrogen-fixing Azorhizobium strains in wheat plants [22]. Interestingly, some strains of the genera Rhizobium and Bradyrhizobium, which were found in association with rice and wheat roots, increase this nutrient concentration in those plant yields [98],[99],[100].
On the other hand, certain diazotrophic bacteria are able to establish truly mutualistic symbiosis within plant tissues, mainly through the formation of root nodules. These symbioses are found between rhizobia and legumes, rhizobia and Parasponia, Frankia and actinorhizal plants and cyanobacteria and cycads [15],[38],[44],[101],[102],[103].
Many bacterial endophytes are able to synthetize phytohormones, which are defined as organic molecules involved in several processes of the different stages of plant growth and development. The biosynthesis of these phytohormones by certain microorganisms might be involved in plant pathogenesis; however, a wide spectrum of beneficial bacteria are able to produce them and have them involved in plant growth and development as plant growth promotion traits [104],[105].
Amongst the phytohormone-producing PGP rhizobacteria, we will focus in the ones producing auxins, cytokinins, gibberellins and ethylene. Each one of these phytohormones are involved in key processes of plant development [106].
Auxin-producing Bacillus spp. have been reported to exert a positive effect in the development of several crops, such as Solanun tuberosum (potato) or Oryza sativa (rice) [31],[46],[47]. Moreover, members of the genus Bacillus were reported as cytokinin producers [47],[48],[49]. Bacillusmegaterium and also Azotobacterchroococcum strains were found to produce cytokinins and promote cucumber growth [45]. Liu et al. [48] reported that oriental thuja seedlings inoculated with cytokinin-producing Bacillussubtilis strains have better resistance to drought stress. Moreover, a gibberellins-producing strain of Bacilluscereus enhances the growth of red pepper plants [113].
The genus Paenibacillus was also reported as a good phytohormone producer. Bent et al. [53] have reported elevated root IAA level in lodgepole pine (Pinus contorta) plantlets inoculated with a strain of Paenibacilluspolymyxa. Moreover, other studies report the effects of the genus Paenibacillus as phytohormone producer for rice, barley and wheat plant crops [31],[54].
Enterobacter and related Enterobacteriales are also good phytohormone producers and have PGP effects in sugarcane, wheat, pepper and soybean, amongst others [33],[37],[50],[51],[114].
Interestingly, rhizobia are also described as phytohormone synthesizers. IAA-producing rhizobial strains improve the growth of several crops, such as Capsicumannuum (pepper), Solanum lycopersicum (tomato), Fragaria anannasa (strawberry), Dianthus caryophyllus (red carnation), Lactucasativa (lettuce) and Daucuscarota (carrot) [6],[57]–[63]. Moreover, Rhizobiumleguminosarum strains isolated from Delta Nile rice fields in rotation with clover were reported as producers of auxins and gibberellins, amongst other phytohormones [99],[100].
The genus Sphingomonas was also reported as phytohormone-producing bacteria; tomato plants inoculated with the gibberellin-producing Sphingomonas sp. LK11 strain showed a significant increment in several growth attributes [63]. Moreover, in a recent study, Asaf et al. [51] reported the positive effect and the production of phytohormones of Sphingomonas and Serratia, an enterobacteria, in soybean plant development.
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