We face major agricultural challenges that remain a threat for global food security. Soil microbes harbor enormous potentials to provide sustainable and economically favorable solutions that could introduce novel approaches to improve agricultural practices and, hence, crop productivity. In this review we give an overview regarding the current state-of-the-art of microbiome research by discussing new technologies and approaches. We also provide insights into fundamental microbiome research that aim to provide a deeper understanding of the dynamics within microbial communities, as well as their interactions with different plant hosts and the environment. We aim to connect all these approaches with potential applications and reflect how we can use microbial communities in modern agricultural systems to realize a more customized and sustainable use of valuable resources (e.g., soil).

This study analysed the contribution of each omega-3 desaturase to the cold response in soybean. Exposure to cold temperatures (5 °C) did not result in great modifications of the linolenic acid content in leaf membrane lipids. However, an increase in the GmFAD3A transcripts was observed both in plant leaves and soybean cells whereas no changes in GmFAD3B or GmFAD3C expression levels were detected. This increase was reversible and accompanied by the accumulation of an mRNA encoding a truncated form of GmFAD3A (GmFAD3A-T), which originated from alternative splicing of GmFAD3A in response to cold. When the expression of plastidial omega-3 desaturases was analysed, a transient accumulation of GmFAD7-2 mRNA was detected upon cold exposure in mature soybean trifoliate leaves while GmFAD7-1 transcripts remained unchanged. No modification of the GmFAD8-1 and GmFAD8-2 transcripts was observed. The functionality of GmFAD3A, GmFAD3B, GmFAD3C and GmFAD3A-T was examined by heterologous expression in yeast. No activity was detected with GmFAD3A-T, consistent with the absence of one of the His boxes necessary for desaturase activity. The linolenic acid content of Sacharomyces cerevisiae cells overexpressing GmFAD3A or GmFAD3B was higher when the cultures were incubated at cooler temperatures, suggesting that reticular desaturases of the GmFAD3 family, and more specifically GmFAD3A, may play a role in the cold response, even in leaves. The data point to a regulatory mechanism of omega-3 fatty acid desaturases in soybean affecting specific isoforms in both the plastid and the endoplasmic reticulum to maintain appropriate levels of linolenic acid under low temperature conditions.

Background: Despite the knowledge that the soil–plant–microbiome nexus is shaped by interactions amongst its members, very little is known about how individual symbioses regulate this shaping. Even less is known about how the agriculturally important symbiosis of nitrogen-fixing rhizobia with legumes is impacted according to soil type, yet this knowledge is crucial if we are to harness or improve it. We asked how the plant, soil and microbiome are modulated by symbiosis between the model legume Medicago truncatula and different strains of Sinorhizobium meliloti or Sinorhizobium medicae whose nitrogen-fixing efficiency varies, in three distinct soil types that differ in nutrient fertility, to examine the role of the soil environment upon the plant–microbe interaction during nodulation. Results: The outcome of symbiosis results in installment of a potentially beneficial microbiome that leads to increased nutrient uptake that is not simply proportional to soil nutrient abundance. A number of soil edaphic factors including Zn and Mo, and not just the classical N/P/K nutrients, group with microbial community changes, and alterations in the microbiome can be seen across different soil fertility types. Root endosphere emerged as the plant microhabitat more affected by this rhizobial efficiency-driven community reshaping, manifested by the accumulation of members of the phylum Actinobacteria. The plant in turn plays an active role in regulating its root community, including sanctioning low nitrogen efficiency rhizobial strains, leading to nodule senescence in particular plant–soil–rhizobia strain combinations. Conclusions: The microbiome–soil–rhizobial dynamic strongly influences plant nutrient uptake and growth, with the endosphere and rhizosphere shaped differentially according to plant–rhizobial interactions with strains that vary in nitrogen-fixing efficiency levels. These results open up the possibility to select inoculation partners best suited for plant, soil type and microbial community.

To overcome the difficulties to analyze membrane desaturases at the protein level, transgenic Arabidopsis plants expressing the plastidial AtFAD7 and AtFAD8 ω-3 desaturases fused to green fluorescent protein, under the control of their endogenous promoters, were generated and their tissue relative abundance was studied. Gene expression, glucuronidase promoter activity, immunoblot and confocal microscopy analyses indicated that AtFAD7 is the major ω-3 desaturase in leaves when compared to AtFAD8. This higher abundance of AtFAD7 was consistent with its higher promoter activity and could be related with its specificity for the abundant leaf galactolipids. AtFAD7 was also present in roots but at much lower level than leaves. AtFAD8 expression and protein abundance in leaves was consistent with its lower promoter activity, suggesting that transcriptional control modulates the abundance of both desaturases in leaves. AtFAD7 protein levels increased in response to wounding but not to jasmonate (JA), and decreased upon abscisic acid (ABA) treatment. Conversely, AtFAD8 protein levels increased upon cold or JA exposure and decreased at high temperatures, but did not respond to ABA or wounding. These results indicated specific and non-redundant roles for the plastidial ω-3 desaturases in defense, temperature stress or phytohormone mediated responses and a tight coordination of their activities between biotic and abiotic stress signaling pathways. Our data suggested that transcriptional regulation was crucial for this coordination. Finally, bimolecular fluorescence complementation analysis showed that both AtFAD7 and AtFAD8 interact with the AtFAD6 ω-6 desaturase in vivo, suggesting that quaternary complexes are involved in trienoic fatty acid production within the plastid membranes.

Legume-rhizobium symbiosis represents one of the most successfully co-evolved mutualisms. Within nodules, the bacterial cells undergo distinct metabolic and morphological changes and differentiate into nitrogen-fixing bacteroids. Legumes in the inverted repeat lacking clade (IRLC) employ an array of defensin-like small secreted peptides (SSPs), known as nodule-specific cysteine-rich (NCR) peptides, to regulate bacteroid differentiation and activity. While most NCRs exhibit bactericidal effects in vitro, studies confirm that inside nodules they target the bacterial cell cycle and other cellular pathways to control and extend rhizobial differentiation into an irreversible (or terminal) state where the host gains control over bacteroids. While NCRs are well established as positive regulators of effective symbiosis, more recent findings also suggest that NCRs affect partner compatibility. The extent of bacterial differentiation has been linked to species-specific size and complexity of the NCR gene family that varies even among closely related species, suggesting a more recent origin of NCRs followed by rapid expansion in certain species. NCRs have diversified functionally, as well as in their expression patterns and responsiveness, likely driving further functional specialisation. In this review, we evaluate the functions of NCR peptides and their role as a driving force underlying the outcome of rhizobial symbiosis, where the plant is able to determine the outcome of rhizobial interaction in a temporal and spatial manner.