Plant root exudates serve pivotal roles in supporting plant development and interactions with the physicochemical and biological factors in the rhizosphere. Under stress conditions, root exudation is ...involved in enhancing plant resource-use efficiency and facilitating the crosstalk between plant and soil microbes to ameliorate stress. Although there are a large number of root exudates that remain to be characterized, recent technological advancements have allowed for the function of many exudate compounds to be elucidated. In this review, we discuss current knowledge about the key root exudates that modulate plant resource-use efficiency under various abiotic stresses including drought, aluminum toxicity, phosphorus, nitrogen, and iron deficiency. The role that key root exudates play in shaping microbial communities in the rhizosphere under stress conditions is also an important consideration addressed in this review.
Root exudation plays important roles in plant interactions with soil chemical factors and belowground microorganisms that impact abiotic stresses.Carboxylate efflux is a key mechanism for plant phosphorus acquisition and tolerance to aluminum.Phenolics are utilized by dicots and non-graminaceous monocots while phytosiderophore exudation is used by grasses to mobilize iron for root uptake.Secondary metabolites such as flavonoids and strigolactones facilitate plant association with mycorrhizal fungi under drought and phosphorus-deficient conditions.Under nitrogen-limited conditions root exudates facilitate plant associations with nitrogen-fixing microbes and may reduce losses of soil nitrogen through inhibition of the bacteria involved in nitrification and denitrification.
Plants often grow in soils that contain very low concentrations of the macronutrients nitrogen, phosphorus, potassium, and sulfur. To adapt and grow in nutrient-deprived environments plants must ...sense changes in external and internal mineral nutrient concentrations and adjust growth to match resource availability. The sensing and signal transduction networks that control plant responses to nutrient deprivation are not well characterized for nitrogen, potassium, and sulfur deprivation. One branch of the signal transduction cascade related to phosphorus-deprivation response has been defined through the identification of a transcription factor that is regulated by sumoylation. Two different microRNAs play roles in regulating gene expression under phosphorus and sulfur deprivation. Reactive oxygen species increase rapidly after mineral nutrient deprivation and may be one upstream mediator of nutrient signaling. A number of molecular analyses suggest that both short-term and longer-term responses will be important in understanding the progression of signaling events when the external, then internal, supplies of nutrients become depleted.
•Root architecture and function are important for yield and stress tolerance.•Root function has been modified using genetic engineering to improve crop stress tolerance and yield.•New methods are ...being developed to more rapidly phenotype roots in situ and ex situ.•Improving methods to phenotype roots poses the greatest future challenge.
Roots play an essential role in the acquisition of water and minerals from soils. Measuring crop root architecture and assaying for changes in function can be challenging, but examples have emerged showing that modifications to roots result in higher yield and increased stress tolerance. In this review, we focus mainly on the molecular genetic advances that have been made in altering root system architecture and function in crop plants, as well as phenotyping methods. The future for the modification of crop plant roots looks promising based on recent advances, but there are also important challenges ahead.
Potassium is an essential macronutrient that is involved in regulating turgor, in driving plant growth, and in modulating enzyme activation. The changes in root morphology, root function, as well as ...cellular and molecular responses to low potassium conditions have been studied in the model plant Arabidopsis and in other plant species. In Arabidopsis ethylene plays a key role in roots in the transduction of the low potassium signal, which results in altered root function and growth. The first clues regarding the role of ethylene were detected through transcriptional profiling experiments showing changes in the expression of genes related to ethylene biosynthesis. Later it was shown that ethylene plays a foundational early role in the many responses observed in Arabidopsis. One of the most striking findings is the link between ethylene and reactive oxygen species (ROS) production, which is part of the signal transduction pathway in K(+) deprived plants. This mini-review will summarize what is known about the role ethylene plays in response to low potassium in Arabidopsis and other plant species.
Plants respond to low-nutrient conditions through metabolic and morphology changes that increase their ability to survive and grow. The transcription factor RAP2.11 was identified as a component in ...the response to low potassium through regulation of the high-affinity K＋ uptake transporter AtHAK5 and other components of the low- potassium signal transduction pathway. RAP2.11 was identified through the activation tagging of Arabidopsis lines that contained a luciferase marker driven by the AtHAK5 promoter that is normally only induced by low potassium. This factor bound to a GCC-box of the AtHAK5 promoter in vitro and in vivo. Transcript profiling revealed that a large number of genes were up-regulated in roots by RAP2.11 overexpression. Many regulated genes were identified to be in functional cate- gories that are important in Iow-K＋ signaling. These categories included ethylene signaling, reactive oxygen species pro- duction, and calcium signaling. Promoter regions of the up-regulated genes were enriched in the GCCGGC motif also contained in the AtHAK5 promoter. These results suggest that RAP2.11 regulates AtHAK5 expression under Iow-K＋ con- ditions and also contributes to a coordinated response to low-potassium conditions through the regulation of other genes in the Iow-K＋ signaling cascade.
ABSTRACT
Soil pH is a major factor shaping bulk soil microbial communities. However, it is unclear whether the belowground microbial habitats shaped by plants (e.g. rhizosphere and root endosphere) ...are also affected by soil pH. We investigated this question by comparing the microbial communities associated with plants growing in neutral and strongly alkaline soils in the Sandhills, which is the largest sand dune complex in the northern hemisphere. Bulk soil, rhizosphere and root endosphere DNA were extracted from multiple plant species and analyzed using 16S rRNA amplicon sequencing. Results showed that rhizosphere, root endosphere and bulk soil microbiomes were different in the contrasting soil pH ranges. The strongest impact of plant species on the belowground microbiomes was in alkaline soils, suggesting a greater selective effect under alkali stress. Evaluation of soil chemical components showed that in addition to soil pH, cation exchange capacity also had a strong impact on shaping bulk soil microbial communities. This study extends our knowledge regarding the importance of pH to microbial ecology showing that root endosphere and rhizosphere microbial communities were also influenced by this soil component, and highlights the important role that plants play particularly in shaping the belowground microbiomes in alkaline soils.
Soil pH affects root-associated microbiomes.
Much effort has been placed on developing microbial inoculants to replace or supplement fertilizers to improve crop productivity and environmental sustainability. However, many studies ignore the ...dynamics of plant‐microbe interactions and the genotypic specificity of the host plant on the outcome of microbial inoculation. Thus, it is important to study temporal plant responses to inoculation in multiple genotypes within a single species. With the implementation of high‐throughput phenotyping, the dynamics of biomass and nitrogen (N) accumulation of four sorghum genotypes with contrasting N‐use efficiency were monitored upon the inoculation with synthetic microbial communities (SynComs) under high and low‐N. Five SynComs comprising bacteria isolated from field grown sorghum were designed based on the overall phylar composition of bacteria and the enriched host compartment determined from a field‐based culture independent study of the sorghum microbiome. We demonstrated that the growth response of sorghum to SynCom inoculation is genotype‐specific and dependent on plant N status. The sorghum genotypes that were N‐use inefficient were more susceptible to the colonization from a diverse set of inoculated bacteria as compared to the N‐use efficient lines especially under low‐N. By integrating high‐throughput phenotyping with sequencing data, our findings highlight the roles of host genotype and plant nutritional status in determining colonization by bacterial synthetic communities.
Through the integration of high‐throughput phenotyping, this study elucidated key interactions between nitrogen availability, synthetic bacterial community colonization, and host genotype.
Potassium ( K+) is an essential nutrient required by plants in large quantities, but changes in soil concentrations may limit K+acquisition by roots. It is not known how plant root cells sense or ...signal the changes that occur after the onset of K+deficiency. Changes in the kinetics of Rb+uptake in Arabidopsis roots occur within 6 h after K+deprivation. Reactive oxygen species (ROS) and ethylene increased when the plants were deprived of K+. ROS accumulated in a discrete region of roots that has been shown to be active in K+uptake and translocation. Suppression of an NADPH oxidase in Arabidopsis (rhd2), which is involved in ROS production, prevented the up-regulation of genes that are normally induced by K+deficiency, but the induction of high-affinity K+transport activity was unchanged. Application of H2O2restored the expression of genes induced by K+deficiency in rhd2 and was also sufficient to induce high-affinity K+transport activity in roots grown under K+-sufficient conditions. ROS production is an early root response to K+deficiency that modulates gene expression and physiological changes in the kinetics of K+uptake.
This study investigated the differences in microbial community abundance, composition, and diversity throughout the depth profiles in soils collected from corn and soybean fields in Iowa (United ...States) using 16S rRNA amplicon sequencing. The results revealed decreased richness and diversity in microbial communities at increasing soil depth. Soil microbial community composition differed due to crop type only in the top 60 cm and due to location only in the top 90 cm. While the relative abundance of most phyla decreased in deep soils, the relative abundance of the phylum Proteobacteria increased and dominated agricultural soils below the depth of 90 cm. Although soil depth was the most important factor shaping microbial communities, edaphic factors, including soil organic matter, soil bulk density, and the length of time that deep soils were saturated with water, were all significant factors explaining the variation in soil microbial community composition. Soil organic matter showed the highest correlation with the exponential decrease in bacterial abundance with depth. A greater understanding of how soil depth influences the diversity and composition of soil microbial communities is vital for guiding sampling approaches in agricultural soils where plant roots extend beyond the upper soil profile. In the long term, a greater knowledge of the influence of depth on microbial communities should contribute to new strategies that enhance the sustainability of soil, which is a precious resource for food security. IMPORTANCE Determining how microbial properties change across different soils and within the soil depth profile will be potentially beneficial to understanding the long-term processes that are involved in the health of agricultural ecosystems. Most literature on soil microbes has been restricted to the easily accessible surface soils. However, deep soils are important in soil formation, carbon sequestration, and providing nutrients and water for plants. In the most productive agricultural systems in the United States where soybean and corn are grown, crop plant roots extend into the deeper regions of soils (>100 cm), but little is known about the taxonomic diversity or the factors that shape deep-soil microbial communities. The findings reported here highlight the importance of soil depth in shaping microbial communities, provide new information about edaphic factors that influence the deep-soil communities, and reveal more detailed information on taxa that exist in deep agricultural soils.
Reactive oxygen species （ROS） play an important role in root responses to potassium deprivation by regulating the expression of the high-affinity K＋ transporter gene AtHAK5 and other genes. ...Activation-tagged lines of Arabidopsis plants containing the AtHAK5 promoter driving luciferase were screened for bioluminescence under potassium- sufficient conditions. A member of the type Ill peroxidase family, RCI3, was isolated and when it was overexpressed by the activation tag, this led to the enhanced expression of luciferase and the endogenous AtHAKS. RCI3 was found to be up- regulated upon potassium deprivation. Plants overexpressing RCI3 （RCI3-ox） showed more ROS production and AtHAK5 expression whereas the ROS production and AtHAK5 expression were reduced in rci3-1 under K＋-deprived conditions. These results suggested that RCI3 is involved in the production of ROS under potassium deprivation and that RCI3- mediated ROS production affects the regulation of AtHAK5 expression. This peroxidase appears to be another component of the low-potassium signal transduction pathway in Arabidopsis roots.
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