Summary Arbuscular mycorrhizal (AM) fungi gain access to nutrient patches outside the rhizosphere by producing an extensive network of fine hyphae. Here, we focused on establishing the mechanism by which AM fungal hyphae reach discrete organic patches with a cohort of functional bacteria transported in a biofilm on their surface. We investigated the mechanisms and impact of the translocation of phosphate solubilising bacteria (PSB) along AM fungal hyphae in bespoke microcosms. An in vitro culture experiment was also conducted to determine the direct impact of hyphal exudates of AM fungi upon the growth of PSB. The extraradical hyphae of AM fungi can transport PSB to organic phosphorus (P) patches and enhance organic P mineralisation both under in vitro culture and soil conditions. Bacteria move in a thick water film formed around fungal hyphae. However, the bacteria cannot be transferred to the organic P patch without an energy source in the form of hyphal exudates. Our results could be harnessed to better manage plant–microbe interactions and improve the ability of biological inocula involving AM fungi and bacteria to enhance the sustainability of agricultural crops in P limited conditions.

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The role of the seed coat in adaptation of dimorphic seeds of the euhalophyte Suaeda salsa to salinity was investigated during germination and early seedling growth. Black and brown seeds were treated with chloroform for 1 min before the extract was used to analyze waxes and the seeds to investigate the protective role of the seed coat under saline conditions. Waxes in black seed coats were more abundant than those in brown seed coats. Salinity (500 mM NaCl) increased the concentration of Na + and decreased the concentration of K + in both black and brown seeds regardless of chloroform treatment. Chloroform treatment alone (in the absence of NaCl) had no effect on the concentration of Na + or K + in black or brown seeds and in the presence of 500 mM NaCl had no effect on the concentration of Na + or K + in brown seeds. However, chloroform treatment increased Na + and decreased K + in black seeds with 500 mM NaCl. A change of MDA (malondialdehyde) concentration in black and brown seeds treated with or without chloroform was similar to the change of Na + concentration. High salinity (1500 mM NaCl) pretreatment for 40 days had a less adverse effect on germination of black seeds compared with brown seeds after they were transferred to fresh water regardless of chloroform treatment. Similar results were found for seedling emergence. In conclusion, a black seed coat may be more protective than a brown seed coat, probably by shielding the embryo from ion toxicity, because of its higher content of waxes. Thus black seeds can better maintain seed viability than brown seeds for extended periods under hypersaline conditions.

Salinity and nitrogen are two important environmental factors that affect the distribution of halophytes in their natural saline habitats. Seeds of the euhalophyte Suaeda salsa L. were harvested from plants that had been treated with 1 or 500 mM NaCl combined with 0.5 or 5 mM NO3 − -N (nitrate) for 115 days in a glasshouse. Germination was evaluated under different concentrations of NaCl and nitrate. Plants exposed to high salinity (500 mM) and low nitrate (0.5 mM) tended to produce heavy seeds. Either high salinity (500 mM) or high nitrate (5 mM) increased the brown/black seed ratio. The concentrations of Na + , K + , and Cl − were higher in brown than in black seeds, and NO3 − concentrations were higher in black than in brown seeds, regardless of NaCl and nitrate treatments during plant culture. Regardless of NaCl and nitrate concentrations during germination, seeds from plants grown with 0.5 mM nitrate generally germinated more rapidly than seeds from plants grown with 5 mM nitrate, and the difference was greater for black than for brown seeds. Exogenous nitrate during germination enhanced the germination of brown seeds less than that of black seeds. Producing more brown seeds and heavy black or brown seeds appears to be an adaptation of S. suaeda to saline environments. Producing more black seeds, which tend to remain dormant, should reduce competition for nitrogen and appears to be an adaptation to nitrogen-limited environments. In conclusion, nitrate provided exogenously or by mother plants to black seeds may act as a signal molecule that enhances the germination of black S. suaeda seeds.

Arbuscular mycorrhizal (AM) fungi form tight symbiotic relationships with the majority of terrestrial plants and play critical roles in plant P acquisition, adding a further dimension of complexity. The plant-AM fungus-bacterium system is considered a continuum, with the bacteria colonizing not only the plant roots, but also the associated mycorrhizal hyphal network, known as the hyphosphere microbiome. Plant roots are usually colonized by different AM fungal species which form an independent phosphorus uptake pathway from the root pathway, i.e., the mycorrhizal pathway.