Abstract
A plant growing under natural conditions is always associated with a substantial, diverse, and well-orchestrated community of microbes—the phytomicrobiome. The phytomicrobiome genome is larger and more fluid than that of the plant. The microbes of the phytomicrobiome assist the plant in nutrient uptake, pathogen control, stress management, and overall growth and development. At least some of this is facilitated by the production of signal compounds, both plant-to-microbe and microbe back to the plant. This is best characterized in the legume nitrogen fixing and mycorrhizal symbioses. More recently lipo-chitooligosaccharide (LCO) and thuricin 17, two microbe-to-plant signals, have been shown to regulate stress responses in a wide range of plant species. While thuricin 17 production is constitutive, LCO signals are only produced in response to a signal from the plant. We discuss how some signal compounds will only be discovered when root-associated microbes are exposed to appropriate plant-to-microbe signals (positive regulation), and this might only happen under specific conditions, such as abiotic stress, while others may only be produced in the absence of a particular plant-to-microbe signal molecule (negative regulation). Some phytomicrobiome members only elicit effects in a specific crop species (specialists), while other phytomicrobiome members elicit effects in a wide range of crop species (generalists). We propose that some specialists could exhibit generalist activity when exposed to signals from the correct plant species. The use of microbe-to-plant signals can enhance crop stress tolerance and could result in more climate change resilient agricultural systems.
Highlights
Plants in nature are always in relationships (Raina et al, 2018) with a microbial community; some members of the soil microbial community assist plant growth and development (Prithiviraj et al, 2003; Smith et al, 2015a,b)
When adaptation to environmental stressors is needed, the plant: (1) alters its own gene expression and resulting physiology, and (2) adjusts the diversity, composition, and activity of its phytomicrobiome (Smith et al, 2015b; Gopal and Gupta, 2016). The latter allows for very short-term adjustments, including evolution of the phytomicrobiome; the plant genome evolves much more slowly (Mueller and Sachs, 2015)
Ascophyllum nodosum appears to be a fusion of a macroalga and a fungus (Deckert and Garbary, 2005)
Summary
Plants in nature are always in relationships (Raina et al, 2018) with a microbial community (the phytomicrobiome); some members of the soil microbial community assist plant growth and development (Prithiviraj et al, 2003; Smith et al, 2015a,b). Phytomicrobiome Signaling in Crop Production genome comprises the hologenome or the pan-genome (the host plus the microbial metagenome) (Berendsen et al, 2012; Guerrero et al, 2013; Turner et al, 2013; Bordenstein and Theis, 2015). It seems that evolution of more complex eukaryotic cells (Phylum Lokiarchaeota—Turner et al, 2013; Spang et al, 2015) from simpler prokaryotes, allowed development of the holobiont (Embley and Martin, 2006; Douglas, 2014; Koonin and Yutin, 2014; Graham et al, 2018). Rhizomicrobiome members can stimulate root growth and so improve plant water and nutrient uptake
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