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.
Understanding the mechanism of iron (Fe)‐deficiency responses is crucial for improving plant Fe bioavailability. Here, we found that the Arabidopsis Rho‐like GTPase 6 mutant (rop6) is less sensitive to Fe‐deficiency responses and has reduced levels of reactive oxygen species (ROS) compared to wild‐type (WT), while AtROP6‐overexpressing seedlings exhibit more sensitivity to Fe‐deficiency responses and has higher levels of ROS compared to WT. Moreover, treatment with H2O2 improves the sensitivity to Fe‐deficiency responses in rop6 mutants. By using the yeast two‐hybrid system, we further demonstrate the direct interaction between AtROP6 and Arabidopsis respiratory burst oxidase homolog D (AtRBOHD), which controls the generation of ROS. Overall, we suggest that AtROP6 is involved in AtRBOHD‐mediated ROS signaling to modulate Fe‐deficiency responses in Arabidopsis thaliana.
Nitrogen is critical for plant growth and development. With the increase of nitrogen fertilizer application, nitrogen use efficiency decreases, resulting in wasted resources. In apple (Malus domestica) rootstocks, the potential molecular mechanism for improving nitrogen uptake efficiency to alleviate low nitrogen stress remains unclear. We utilized multi-omics approaches to investigate the mechanism of nitrogen uptake in two apple rootstocks with different responses to nitrogen stress, Malus hupehensis and Malus sieversii. Under low nitrogen stress, Malus sieversii showed higher efficiency in nitrogen uptake. Multi-omics analysis revealed substantial differences in the expression of genes involved in flavonoid and lignin synthesis pathway between the two materials, which were related to the corresponding metabolites. We discovered that basic helix-loop-helix 130 (bHLH130) transcription factor was highly negatively associated with the flavonoid biosynthetic pathway. bHLH130 may directly bind to the chalcone synthase gene (CHS) promoter and inhibit its expression. Overexpressing CHS increased flavonoid accumulation and nitrogen uptake. Inhibiting bHLH130 increased flavonoid biosynthesis while decreasing lignin accumulation, thus improving nitrogen uptake efficiency. These findings revealed the molecular mechanism by which bHLH130 regulates flavonoid and lignin biosynthesis in apple rootstocks under low nitrogen stress.
Plants have developed complex mechanisms to adapt to the changing nitrate (NO3-) levels and can recruit microbes to boost nitrogen absorption. However, little is known about the relationship between functional genes and the rhizosphere microbiome in NO3- uptake of apple rootstocks. Here, we found that variation in MdNRT2.4 expression contributes to nitrate-uptake divergence between two apple rootstocks. Overexpression of MdNRT2.4 in apple seedlings significantly improved tolerance to low nitrogen via increasing net NO3- influx at root surface. But when inhibited the root plasma membrane H +-ATPase activity, which abolished NO3- uptake and led to NO3- release, suggesting that MdNRT2.4 encodes an H +-coupled nitrate transporter. Surprisingly, the nitrogen concentration of MdNRT2.4-overexpressing apple seedlings in unsterilized nitrogen-poor soil was higher than that in sterilized nitrogen-poor soil. Using 16S ribosomal RNA gene profiling to characterize rhizosphere microbiota, we found that MdNRT2.4-overexpressing apple seedlings recruited more bacterial taxa with nitrogen metabolic functions, especially Rhizobiaceae. We isolated a bacterial isolate AR11 from apple rhizosphere soil and identified it as Rhizobium. Inoculation with ARR11 improved apple seedling growth in nitrogen-poor soil compared with uninoculated seedlings. Together, our results highlight the interaction of host plant genes with the rhizosphere microbiota for host plant nutrient uptake.
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