Exploiting inter-organismal chemical communication for improved inoculants

Abstract

[951][952][953][954][955][956][957][958][959][960][961][962][963][964][965][966]. The combination of rising fossil fuel prices and a need to reduce greenhouse gas emissions will lead to expanded use of crop inoculants (bio-fertilizers) both for increased production of biomass (for bio-fuels and soil C storage) and to reduce production of nitrous oxide, through increased reliance on biological nitrogen fixation. Over the last century inoculants have been improved through strain selection, improved carriers (including sterile carriers), and increased cell densities. During the last few decades our understanding of signalling between symbiotic bacteria and plants has expanded enormously, with the signalling between rhizobia and their legume hosts being the model system. Recent work has shown that adverse environmental conditions can inhibit this signalling and that addition of plant-to-microbe signals into inoculants can help overcome this. This is also true of addition of the microbe-to-plant signals that act as the return signals of this system; however, they have also been shown to cause a general and direct stimulation of plant growth that is not yet well understood. Finally, very recent work has shown that some of the plant growth-promoting rhizobacteria produce novel signal compounds that stimulate plant growth. This is a time of rapid increase in understanding with regard to plant-microbe signalling; the use of these signals in commercial inoculants offers a new wave in innovations for this industry at a time when there is great need.Key words: Inoculants, rhizobacteria, rhizobia, signalling Mabood, F., Gray, E. J., Lee, K. D., Supanjani, et Smith D. L. 2006. Exploitation de la médiation chimique entre les organismes pour créer de meilleurs inoculants. Can. J. Plant Sci. 86: 951-966. La hausse du prix des combustibles fossiles et la néces-sité de réduire les émissions de gaz à effet de serre déboucheront sur une utilisation accrue des inoculants en agriculture (engrais biologiques) pour la production d'une plus grande quantité de biomasse (pour les biocarburants et le stockage du carbone dans le sol) mais aussi pour la réduction des dégagements d'oxydes nitreux par un plus grand recours à des mécanismes biologiques pour fixer l'azote dans le sol. Au cours du siècle dernier, on a raffiné les inoculants par la sélection, amélioré les excipients (y compris les substrats stériles) et accru la population de cellules. Durant les quelque dernières décennies, nos connaissances sur les signaux que s'échangent microorganismes et végétaux en symbiose se sont considérablement élargies et le système de signalisation entre les rhizobiums et les légumineuses qui sont leur hôte est devenu un modèle du genre. Des recherches récentes indiquent que des conditions environnementales difficiles peuvent nuire à l'échange de signaux et qu'on peut surmonter ce problème en ajoutant des médiateurs plante-microorganisme à l'inoculant. La même remarque s'applique à l'addition de médiateurs microorganisme-plante qui véhiculent la répons...

Phytomicrobiome Coordination Signals Hold Potential for Climate Change-Resilient Agriculture

“…At a time when we are concerned about the environmental impacts of pesticides (Busby et al, 2017) and extensive fertilizer application, PGPR and microbe-to-plant signal molecules offer alternative strategies for increasing, or at least maintaining, crop yields with reduced pesticide and fertilizer inputs while developing more climate change-resilient agricultural systems. There is enormous potential in our ability to manipulate the phytomicrobiome and its signals, as our understanding of this very complex system grows (Mabood et al, 2006a;Schlaeppi and Bulgarelli, 2015;Smith et al, 2015b;Gopal and Gupta, 2016;Głodowska et al, 2017;Lamont et al, 2017;Wallenstein, 2017). In addition, LCO and thuricin 17 are effective at very low concentrations (LCOs: 10 −6 -10 −8 M, thuricin 17: 10 −9 -10 −11 M; Smith et al, 2015a,b;Subramanian and Smith, 2015) and are inexpensive to produce.…”

Signaling in the phytomicrobiome: breadth and potential

“…These include bacteria in the soil near plant roots, on the surface of plant root systems, in spaces between root cells or inside specialized cells of root nodules; they stimulate plant growth through a wide range of mechanisms ( Gray and Smith, 2005 ; Mabood et al, 2014 ), such as: (1) nutrient solubilization (particularly phosphorus – Boddey et al, 2003 ; Kennedy et al, 2004 ; Trabelsi and Mhamdi, 2013 ), (2) production of metal chelating siderophores, (3) nitrogen fixation ( Vessey, 2003 ; Bhattacharyya and Jha, 2012 ; Drogue et al, 2012 ), (4) production of phytohormones, (5) production of 1-aminocyclopropane-1-carboxylate deaminase, (6) production of volatile organic compounds, (7) induction of systemic resistance [induced systemic resistance (ISR) and systemic required resistance (SAR) – Jung et al, 2008b , 2011 ], and (8) suppression of disease through antibiosis ( Bhattacharyya and Jha, 2012 ; Spence et al, 2014 ). It has also been shown that “signal” compounds produced by bacteria in the phytomicrobiome stimulate plant growth ( Prithiviraj et al, 2003 ; Mabood et al, 2006a ; Lee et al, 2009 ), particularly in the presence of abiotic stress ( Wang et al, 2012 ; Subramanian, 2014 ; Prudent et al, 2015 ). In the broadest sense PGPR include legume-nodulating rhizobia.…”

“…That plants and microbes use signal compounds to communicate during establishment of beneficial plant-microbe interactions ( Desbrosses and Stougaard, 2011 ), is well-described for the legume-rhizobia nitrogen fixing symbiosis ( Oldroyd et al, 2010 ; Giles et al, 2011 ; Oldroyd, 2013 ), and somewhat elucidated for mycorrhizal associations ( Gough and Cullimore, 2011 ). In the legume-rhizobia relationship the plant releases flavonoid signals to rhizobia ( Hassan and Mathesius, 2012 ) or, in some cases, jasmonate signals ( Mabood et al, 2006a , b ; Mabood et al, 2014 ), followed by rhizobial production of lipo-chitooligosaccharides (LCOs) as return signals ( Oldroyd, 2013 ). The LCOs are bound by LysM receptors, which have kinase activity ( Antolin-Llovera et al, 2012 ), changing root hormone profile ( Zamioudis et al, 2013 ) and triggering development of root nodules.…”

Intertwined Existence: The Life of Plant Symbiotic Fungi in Agricultural Soils

“…Mineral fertilizers, while increasing yield, have contributed to environmental problems related to leakage of nutrients out of the soil to the environment, such as nitrous oxide emission to the atmosphere and eutrophication of water bodies. These inoculants may include bioactive molecules, sometimes called signal molecules, to improve symbiotic formation and function (Mabood et al 2006). These inoculants can maximize crop yields and quality while minimizing applications of chemical fertilizers and pesticides that can be harmful to people and the environment.…”