From Leguminosae/Gramineae Intercropping Systems to See Benefits of Intercropping on Iron Nutrition

Impact of various intercropping modes on soil quality, microbial communities, yield and quality of Platycodon grandiflorum (Jacq.) A. DC

INTRODUCTION

Intercropping has the function of promoting plant growth, improving yield and quality. Platycodon grandiflorus (P. grandiflorus) is a traditional Chinese medicinal herb; continuous cropping obstacles significantly inhibit its yield and quality. However, few study have established about P. grandiflorus interaction of various crops. This study provides a theoretical foundation to explore the most effective intercropping method, enhance soil utilization efficiency, and increase the yield and quality of P. grandiflorus. We conducted field experiment, P. grandiflorus monoculture (JG-JG), P. grandiflorus and Achyranthes bidentata intercropping (JG-NX), P. grandiflorus and Saposhnikovia divaricata intercropping (JG-FF), P. grandiflorus and Glehnia littoralis (JG-SS) intercropping. Additionally, we included three main intercropping crops with P. grandiflorus, Zea mays (JG-YM), Setaria italica (JG-GZ), and Glycine max (JG-DD). The soil physicochemical properties, enzyme activity, soil microorganisms, the yield and secondary metabolite content in the roots of P. grandiflorus were determined. The results showed that intercropping significantly increased the yield and quality of P. grandiflorus, and significantly reduced the incidence rate of root rot. The intercropping system enhances the physical and chemical properties of soil, soil enzyme activity, and soil microbial diversity. JG-SS intercropping significantly increased the abundance of bacteria and fungi, stimulated soil microbial communities, promoted plant growth, significantly increased yield and content of platycodin D, enhanced the complexity of microbial co-occurrence networks. This study could provide a sustainable planting system for the cultivation of P. grandiflorus, particularly the system JG-SS.

CLINICAL TRIAL NUMBER

Not applicable.

"Iron chlorosis paradox" in calcicoles and calcifuges from European dry grasslands: A meta-analysis

Iron (Fe) deficiency-related chlorosis (lime chlorosis) and the "paradox of iron chlorosis" in calcicole and calcifuge plants are unresolved problems. We performed a meta-analysis of our previous studies to determine whether calcicoles are more resistant to lime chlorosis than calcifuges are, whether lime chlorosis is Fe chlorosis per se and what is the source of the "paradox of iron chlorosis" in grassland plants. We found that both calcicoles and calcifuges are susceptible to chlorosis when grown on alkaline soil. In general, lime chlorosis does not result from Fe deficiency but is likely associated with manganese (Mn). Additionally, our analyses revealed that Ecological Indicator Values for Europe for soil reaction (R EIVEs) are not random to experimentally determined calcicole/calcifuge behavior but are not straightforward. Ultimately, neither the experimentally determined calcicole index nor the R EIVEs explained the microelemental nutrition of grassland plants in general (except for Zn acquisition), which highlights the need for species ecology-oriented studies to understand these phytocenoses.

Microbiome convergence enables siderophore-secreting-rhizobacteria to improve iron nutrition and yield of peanut intercropped with maize

Intercropping has the potential to improve plant nutrition as well as crop yield. However, the exact mechanism promoting improved nutrient acquisition and the role the rhizosphere microbiome may play in this process remains poorly understood. Here, we use a peanut/maize intercropping system to investigate the role of root-associated microbiota in iron nutrition in these crops, combining microbiome profiling, strain and substance isolation and functional validation. We find that intercropping increases iron nutrition in peanut but not in maize plants and that the microbiota composition changes and converges between the two plants tested in intercropping experiments. We identify a Pseudomonas secreted siderophore, pyoverdine, that improves iron nutrition in glasshouse and field experiments. Our results suggest that the presence of siderophore-secreting Pseudomonas in peanut and maize intercropped plays an important role in iron nutrition. These findings could be used to envision future intercropping practices aiming to improve plant nutrition.

Intercropping system modulated soil-microbe interactions that enhanced the growth and quality of flue-cured tobacco by improving rhizospheric soil nutrients, microbial structure, and enzymatic activities

As the promotive/complementary mechanism of the microbe-soil-tobacco (Nicotiana tabacum L.) interaction remains unclear and the contribution of this triple interaction to tobacco growth is not predictable, the effects of intercropping on soil nutrients, enzymatic activity, microbial community composition, plant growth, and plant quality were studied, and the regulatory mechanism of intercropping on plant productivity and soil microenvironment (fertility and microorganisms) were evaluated. The results showed that the soil organic matter (OM), available nitrogen (AN), available phosphorus (AP), available potassium (AK), the urease activity (UE) and sucrase activity (SC), the diversity, abundance, and total and unique operational taxonomic units (OTUs) of bacteria and fungi as well as plant biomass in T1 (intercropping onion), T2 (intercropping endive), and T3 (intercropping lettuce) treatments were significantly higher than those of the controls (monocropping tobacco). Although the dominant bacteria and fungi at the phylum level were the same for each treatment, LEfSe analysis showed that significant differences in community structure composition and the distribution proportion of each dominant community were different. Proteobacteria, Acidobacteria, and Firmicutes of bacteria and Ascomycota and Basidiomycetes of fungi in T1, T2, and T3 treatments were higher than those of the controls. Redundancy analysis (RDA) suggested a close relation between soil characteristic parameters and microbial taxa. The correlation analysis between the soil characteristic parameters and the plant showed that the plant biomass was closely related to soil characteristic parameters. In conclusion, the flue-cured tobacco intercropping not only increased plant biomass and improved chemical quality but also significantly increased rhizospheric soil nutrient and enzymatic activities, optimizing the microbial community composition and diversity of rhizosphere soil. The current study highlighted the importance of microbe-soil-tobacco interactions in maintaining plant productivity and provided the potential fertilization practices in flue-cured tobacco production to maintain ecological sustainability.

Do agronomic approaches aligned to regenerative agriculture improve the micronutrient concentrations of edible portions of crops? A scoping review of evidence

Regenerative Agriculture (RA) is used to describe nature-based agronomic approaches that aim to build soil health and crop resilience, minimize negative environmental outcomes, and improve farmer livelihoods. A benefit that is increasingly attributed to crops grown under RA practices is improved nutritional content. However, we do not know the extent to which RA influences crop nutritional quality and under what management approaches and context, can such effects be realized. A scoping review of recent literature (Web of Science, 2000-2021) was carried out to assess the evidence that RA approaches improve crop micronutrient quality. Papers included combinations of agronomic approaches that could be defined as Regenerative: "Organic Inputs" including composts and manures, cover crops, crop rotations, crop residues and biochars; "Reduced Tillage", "Intercropping", "Biostimulants" e.g. arbuscular mycorrhizal fungi; plant growth promoting bacteria, and "Irrigation", typically deficit-irrigation and alternate wetting and drying. The crop types reviewed were predetermined covering common sources of food and included: Tomato (Solanum lycopersicum L.), Wheat (Triticum aestivum L.), Rice (Oryza sativa L.), Maize (Zea mays L.), Pulses (Fabaceae), Alliums (Allium spp.), and "other" crop types (30 types). This scoping review supports a potential role for RA approaches in increasing the concentrations of micronutrients in the edible portions of several crop types under specific practices, although this was context specific. For example, rice grown under increased organic inputs showed significant increases in grain zinc (Zn) concentration in 15 out of 16 studies. The vitamin C concentration of tomato fruit increased in ~50% of studies when plants were grown under increased organic inputs, and in 76% of studies when plants were grown under deficit irrigation. Overall, the magnitude and reproducibility of the effects of RA practices on most crop nutritional profiles were difficult to assess due to the diversity of RA approaches, geographical conditions, and the limited number of studies for most crops in each of these categories. Future research with appropriate designs, improved on-farm surveillance and nutritional diagnostics are needed for better understanding the potential role of RA in improving the quality of food, human nutrition, and health.

Rhizosphere soil properties of waxy sorghum under different row ratio configurations in waxy sorghum-soybean intercropping systems

To overcome the continuous planting obstacle and promote the sustainable production of waxy sorghum, a two-years field experiment was performed to determine the responses of waxy sorghum rhizosphere soil properties to different row ratio configurations in waxy sorghum-soybean intercropping systems. The treatments included five row ratio configurations, which were two rows of waxy sorghum intercropped with one row of soybean (2W1S), two rows of waxy sorghum intercropped with two rows of soybean (2W2S), three rows of waxy sorghum intercropped with one row of soybean (3W1S), three rows of waxy sorghum intercropped with two rows of soybean (3W2S), and three rows of waxy sorghum intercropped with three rows of soybean (3W3S), and sole cropping waxy sorghum (SW) was used as control. The nutrients, enzyme activities, and microbes of waxy sorghum rhizosphere soil were investigated at the jointing, anthesis, and maturity stages. Results showed that rhizosphere soil properties of waxy sorghum were significantly affected by row ratio configurations of waxy sorghum intercropped soybean. Among all treatments, the performances of rhizosphere soil nutrients contents, enzymes activities, and microbes contents were 2W1S > 3W1S > 3W2S > 3W3S > 2W2S > SW. Compared to SW treatment, the 2W1S treatment increased the organic matter, total N, total P, total K, gram-negative bacteria phospholipid fatty acids (PLFAs), and gram-positive bacteria PLFAs contents and catalase, polyphenol oxidase, and urease activities by 20.86%-25.67%, 34.33%-70.05%, 23.98%-33.83%, 44.12%-81.86%, 74.87%-194.32%, 81.59-136.59%, 91.44%-114.07%, 85.35%-146.91%, and 36.32%-63.94%, respectively. Likewise, the available N, available P, available K, total PLFAs, fungus PLFAs, actinomycetes PLFAs, and bacteria PLFAs contents under the 2W1S treatment were 1.53-2.41, 1.32-1.89, 1.82-2.05, 1.96-2.91, 3.59-4.44, 9.11-12.56, and 1.81-2.71 times than those of SW treatment, respectively. Further, the determining factors of soil microbes were total K, catalase, and polyphenol oxidase for total microbes, bacteria, and gram-negative bacteria, total P and available K for fungus, available N, available K, and polyphenol oxidase for actinomycetes, and total K and polyphenol oxidase for gram-positive bacteria. In conclusion, the 2W1S treatment was the optimal row ratio configuration of waxy sorghum intercropped with soybean, which can improve the rhizosphere soil quality and promote the sustainable production of waxy sorghum.

Research progress on iron absorption, transport, and molecular regulation strategy in plants

Iron is a trace element essential for normal plant life activities and is involved in various metabolic pathways such as chlorophyll synthesis, photosynthesis, and respiration. Although iron is highly abundant in the earth's crust, the amount that can be absorbed and utilized by plants is very low. Therefore, plants have developed a series of systems for absorption, transport, and utilization in the course of long-term evolution. This review focuses on the findings of current studies of the Fe²⁺ absorption mechanism I, Fe³⁺ chelate absorption mechanism II and plant-microbial interaction iron absorption mechanism, particularly effective measures for artificially regulating plant iron absorption and transportation to promote plant growth and development. According to the available literature, the beneficial effects of using microbial fertilizers as iron fertilizers are promising but further evidence of the interaction mechanism between microorganisms and plants is required.

Different changes of bacterial diversity and soil metabolites in tea plants-legume intercropping systems

As an essential agroforestry, intercropping legumes can improve the physical, chemical, and biological fertility of the soil in tea plantations. However, the effects of intercropping different legume species on soil properties, bacterial communities, and metabolites remain elusive. In this study, the 0-20 cm and 20-40 cm soils of three planting patterns (T1: tea plants/mung bean intercropping, T2: tea plants/adzuki bean intercropping, T3: tea plants/mung bean and adzuki bean intercropping) were sampled to explore the diversity of the bacterial community and soil metabolites. The findings showed that, as compared to monocropping, intercropping systems had greater concentrations of organic matter (OM) and dissolved organic carbon (DOC). Notably, pH values were significantly lower, and soil nutrients increased in intercropping systems compared with monoculture in 20-40 cm soils, especially in T3. In addition, intercropping resulted in an increased relative abundance of Proteobacteria but a decreased relative abundance of Actinobacteria. 4-methyl-Tetradecane, acetamide, and diethyl carbamic acid were key metabolites mediating the root-microbe interactions, especially in tea plants/adzuki intercropping and tea plants/mung bean, adzuki bean mixed intercropping soils. Co-occurrence network analysis showed that arabinofuranose, abundant in tea plants and adzuki bean intercropping soils, showed the most remarkable correlation with the soil bacterial taxa. Our findings demonstrate that intercropping with adzuki beans is better at enhancing the diversity of soil bacteria and soil metabolites and is more weed-suppressing than other tea plants/legume intercropping systems.

Daylily intercropping: Effects on soil nutrients, enzyme activities, and microbial community structure

The daylily (Hemerocallis citrina Baroni)/other crop intercropping system can be a specific and efficient cropping pattern in a horticultural field. Intercropping systems contribute to the optimization of land use, fostering sustainable and efficient agriculture. In the present study, high-throughput sequencing was employed to explore the diversity in the root-soil microbial community in the intercropping of four daylily intercropping systems [watermelon (Citrullus lanatus)/daylily (WD), cabbage (Brassica pekinensis)/daylily (CD), kale (Brassica oleracea)/daylily (KD), watermelon/cabbage/kale/daylily (MI)], and determine the physicochemical traits and enzymatic activities of the soil. The results revealed that the contents of available potassium (2.03%-35.71%), available phosphorus (3.85%-62.56%), available nitrogen (12.90%-39.52%), and organic matter (19.08%-34.53%), and the urease (9.89%-31.02%) and sucrase (23.63%-50.60%) activities, and daylily yield (7.43%- 30.46%) in different intercropping soil systems were significantly higher compared to those in the daylily monocropping systems (CK). The bacterial Shannon index increased significantly in the CD and KD compared to the CK. In addition, the fungi Shannon index was also increased significantly in the MI, while the Shannon indices of the other intercropping modes were not significantly altered. Different intercropping systems also caused dramatic architectural and compositional alterations in the soil microbial community. A prominently higher relative richness of Bacteroidetes was noted in MI compared to that in CK, while Acidobacteria in WD and CD and Chloroflexi in WD were pronouncedly less abundant compared to those in CK. Furthermore, the association between soil bacteria taxa and soil characteristic parameters was stronger than that between fungi and soil. In conclusion, the present study demonstrated that the intercropping of daylily with other crops could significantly improve the nutrient levels of the soil and optimize the soil bacterial microflora composition and diversity.

The diversity of iron acquisition strategies of calcifuge plant species from dry acidic grasslands

Although the calcifuge plant species existing in dry acidic grasslands are believed to be prone to iron (Fe)-dependent limitations, little is known about their susceptibility and reaction to pH-dependent Fe starvation. Therefore, the present study examines the effects of contrasting soils (acidic Podzol vs alkaline Rendzina) and Fe supplementation (Fe-HBED) on alkaline substratum (5 and 25 μmol Fe-HBED kg^-1 soil). Five calcifuge dicotyledonous plant species (Alyssum montanum L., Antennaria dioica (L.) Gaertn., Hypochaeris radicata L., Jasione montana L. and Potentilla arenaria Borkh.) were tested in a pot experiment under field conditions. Chlorosis, chlorophyll content, growth and chlorophyll a fluorescence were measured. The elemental composition (contents of Ca, Mg, Fe, Mn, Zn and Cu) of the roots and shoots were analyzed, as well as their specialized metabolites. Two studied species (A. dioica d and J. montana) were susceptible to pH-dependent chlorosis, and this deficiency was successfully diminished by the application of Fe-HBED. Almost all the studied species (except A. montanum) preferred the acidic soil. Fe-HBED treatments were not sufficient for supporting the growth of H. radicata and J. montana in alkaline soil to the same degree as in acidic soil, which suggests additional non-Fe-dependent limitations. Both Fe starvation and Fe over-supplementation caused species-specific changes in chlorophyll a fluorescence. The disturbed Fe acquisition in the alkaline soil was not the sole source of the observed limitations, as the chlorosis-susceptible species demonstrated a complex interaction between Fe, Mn and Zn. The species resistant to lime chlorosis contained greater amounts of specialized metabolites than the susceptible plants. Our findings do not support hypothesis that all calcifuges are susceptible to Fe-dependent chlorosis: calcifuge plant species from dry acidic grasslands appear to have diverse Fe requirements and acquisition strategies.

Mechanisms underlying cereal/legume intercropping as nature-based biofortification: A review

The deficiencies of micronutrients known as hidden hunger are severely affecting more than one-half of the world’s population, which is highly related to low bioavailability of micronutrients, poor quality diets, and consumption of cereal-based foods in developing countries. Although numerous experiments proved biofortification as a paramount approach for improving hidden hunger around the world, its effectiveness is highly related to various soil factors, climate conditions, and the adoption rates of biofortified crops. Furthermore, agronomic biofortification may result in the sedimentation of heavy metals in the soil that pose another detrimental effect on plants and human health. In response to these challenges, several studies suggested intercropping as one of the feasible, eco-friendly, low-cost, and short-term approaches for improving the nutritional quality and yield of crops sustainable way. Besides, it is the cornerstone of climate-smart agriculture and the holistic solution for the most vulnerable area to solve malnutrition that disturbs human healthy catastrophically. Nevertheless, there is meager information on mechanisms and processes related to soil-plant interspecific interactions that lead to an increment of nutrients bioavailability to tackle the crisis of micronutrient deficiency in a nature-based solution. In this regard, this review tempted to (1) explore mechanisms and processes that can favor the bioavailability of Zn, Fe, P, etc. in soil and edible parts of crops, (2) synthesize available information on the benefits and synergic role of the intercropping system in food and nutritional security, and (3) outline the bottlenecks influencing the effectiveness of biofortification for promoting sustainable agriculture in sub-Saharan Africa (SSA). Based on this review SSA countries are malnourished due to limited access to diverse diets, supplementation, and commercially fortified food; hence, I suggest integrated research by agronomists, plant nutritionists, and agroecologist to intensify and utilize intercropping systems as biofortification sustainably alleviating micronutrient deficiencies.

Graphical Abstract

Melon/cowpea intercropping pattern influenced the N and C soil cycling and the abundance of soil rare bacterial taxa

The high use of pesticides, herbicides, and unsustainable farming practices resulted in losses of soil quality. Sustainable farming practices such as intercropping could be a good alternative to traditional monocrop, especially using legumes such as cowpea (Vigna unguiculata L. Walp). In this study, different melon and cowpea intercropping patterns (melon mixed with cowpea in the same row (MC1); alternating one melon row and one cowpea row (MC2); alternating two melon rows and one cowpea row (MC3)) were assayed to study the intercropping effect on soil bacterial community through 16S rRNA region in a 3-year experiment. The results indicated that intercropping showed high content of total organic carbon, total nitrogen and ammonium, melon yield, and bacterial diversity as well as higher levels of beneficial soil microorganisms such a Pseudomonas, Aeromicrobium, Niastella, or Sphingomonas which can promote plant growth and plant defense against pathogens. Furthermore, intercropping showed a higher rare taxa diversity in two (MC1 and MC2) out of the three intercropping systems. In addition, N-cycling genes such as nirB, nosZ, and amoA were more abundant in MC1 and MC2 whereas the narG predicted gene was far more abundant in the intercropping systems than in the monocrop at the end of the 3-year experiment. This research fills a gap in knowledge about the importance of soil bacteria in an intercropping melon/cowpea pattern, showing the benefits to yield and soil quality with a decrease in N fertilization.

Shift in beneficial interactions during crop evolution

Plant domestication can be viewed as a form of co-evolved interspecific mutualism between humans and crops for the benefit of the two partners. Here, we ask how this plant-human mutualism has, in turn, impacted beneficial interactions within crop species, between crop species, and between crops and their associated microbial partners. We focus on beneficial interactions resulting from three main mechanisms that can be promoted by manipulating genetic diversity in agrosystems: niche partitioning, facilitation, and kin selection. We show that a combination of factors has impacted either directly or indirectly plant-plant interactions during domestication and breeding, with a trend toward reduced benefits arising from niche partitioning and facilitation. Such factors include marked decrease of molecular and functional diversity of crops and other organisms present in the agroecosystem, mass selection, and increased use of chemical inputs. For example, the latter has likely contributed to the relaxation of selection pressures on nutrient-mobilizing traits such as those associated to root exudation and plant nutrient exchanges via microbial partners. In contrast, we show that beneficial interactions arising from kin selection have likely been promoted since the advent of modern breeding. We highlight several issues that need further investigation such as whether crop phenotypic plasticity has evolved and could trigger beneficial interactions in crops, and whether human-mediated selection has impacted cooperation via kin recognition. Finally, we discuss how plant breeding and agricultural practices can help promoting beneficial interactions within and between species in the context of agroecology where the mobilization of diversity and complexity of crop interactions is viewed as a keystone of agroecosystem sustainability.

Role of Nodulation-Enhancing Rhizobacteria in the Promotion of Medicago sativa Development in Nutrient-Poor Soils

Legumes are usually used as cover crops to improve soil quality due to the biological nitrogen fixation that occurs due to the interaction of legumes and rhizobia. This symbiosis can be used to recover degraded soils using legumes as pioneer plants. In this work, we screened for bacteria that improve the legume–rhizobia interaction in nutrient-poor soils. Fourteen phosphate solubilizer-strains were isolated, showing at least three out of the five tested plant growth promoting properties. Furthermore, cellulase, protease, pectinase, and chitinase activities were detected in three of the isolated strains. Pseudomonas sp. L1, Chryseobacterium soli L2, and Priestia megaterium L3 were selected to inoculate seeds and plants of Medicago sativa using a nutrient-poor soil as substrate under greenhouse conditions. The effects of the three bacteria individually and in consortium showed more vigorous plants with increased numbers of nodules and a higher nitrogen content than non-inoculated plants. Moreover, bacterial inoculation increased plants’ antioxidant activities and improved their development in nutrient-poor soils, suggesting an important role in the stress mechanisms of plants. In conclusion, the selected strains are nodulation-enhancing rhizobacteria that improve leguminous plants growth and nodulation in nutrient-poor soils and could be used by sustainable agriculture to promote plants’ development in degraded soils.

Intercropping Walnut and Tea: Effects on Soil Nutrients, Enzyme Activity, and Microbial Communities

The practice of intercropping, which involves growing more than one crop simultaneously during the same growing season, is becoming more important for increasing soil quality, land-use efficiency, and subsequently crop productivity. The present study examined changes in soil physicochemical properties, enzymatic activity, and microbial community composition when walnut (Juglans spp.) was intercropped with tea (Camellia sinensis L.) plants in a forest and compared with a walnut and tea monocropping system. The results showed that walnut-tea intercropping improved the soil nutrient profile and enzymatic activity. The soil available nitrogen (AN), available phosphorus (AP), available potassium (AK), organic matter (OM) content, and sucrase activity were significantly boosted in intercropped walnut and tea than in monocropping forests. The interaction between crops further increased bacterial and fungal diversity when compared to monoculture tea forests. Proteobacteria, Bacteroidetes, Firmicutes, Chlamydiae, Rozellomycota, and Zoopagomycota were found in greater abundance in an intercropping pattern than in monoculture walnut and tea forest plantations. The walnut-tea intercropping system also markedly impacted the abundance of several bacterial and fungal operational taxonomic units (OTUs), which were previously shown to support nutrient cycling, prevent diseases, and ameliorate abiotic stress. The results of this study suggest that intercropping walnut with tea increased host fitness and growth by positively influencing soil microbial populations.

Reactions of two xeric-congeneric species of Centaurea (Asteraceae) to soils with different pH values and iron availability

Centaurea scabiosa L. and C. stoebe Tausch are known to co-exist naturally in two extremely different types of open dry habitats in the temperate zone, alkaline xerothermic grasslands and acidic dry grasslands. However, knowledge about their preferences to edaphic conditions, including soil acidity (pH), and iron (Fe) availability is scarce. Therefore, experimental comparison of soil requirements (acidic Podzol vs alkaline Rendzina) of these species was carried out. The study was designed as a pot experiment and conducted under field conditions. Fe availability was increased by application of Fe-HBED. Reactions of plants to edaphic conditions were determined using growth measurements, leaf morphometric measurements, chlorosis scoring, chlorophyll content and chlorophyll a fluorescence (OJIP) quantification as well as determination of element content (Ca, Mg, Fe, Mn, Zn and Cu). Growth and leaf morphometrical traits of the studied congeneric species were affected similarly by the soil type and differently by the chelate treatment. Increased availability of Fe in Rendzina contrasted the species, as treatment with 25 µmol Fe-HBED kg⁻¹ soil promoted growth only in C. stoebe. Both species turned out to be resistant to Fe-dependent chlorosis which was also reflected in only minor changes in chlorophyll a fluorescence parameters. Both species showed relatively low nutritional demands. Surprisingly, Fe-HBED did not stimulate Fe acquisition in the studied species, nor its translocation along the root:shoot axis. Furthermore, contrary to expectations, C. scabiosa took up less Fe from the acidic than alkaline soil. C. scabiosa not only absorbed more Ca and Zn but also translocated greater amounts of these elements to shoots than C. stoebe. Both species acquired more Mg on Podzol than on Rendzina which suggests adaptation allowing avoidance of aluminum (Al) toxicity on acidic soils. Overall, it seems that C. scabiosa prefers alkaline soils, whilst C. stoebe prefers acidic ones.

Plant-Microbe Interaction: Aboveground to Belowground, from the Good to the Bad

Soil health and fertility issues are constantly addressed in the agricultural industry. Through the continuous and prolonged use of chemical heavy agricultural systems, most agricultural lands have been impacted, resulting in plateaued or reduced productivity. As such, to invigorate the agricultural industry, we would have to resort to alternative practices that will restore soil health and fertility. Therefore, in recent decades, studies have been directed towards taking a Magellan voyage of the soil rhizosphere region, to identify the diversity, density, and microbial population structure of the soil, and predict possible ways to restore soil health. Microbes that inhabit this region possess niche functions, such as the stimulation or promotion of plant growth, disease suppression, management of toxicity, and the cycling and utilization of nutrients. Therefore, studies should be conducted to identify microbes or groups of organisms that have assigned niche functions. Based on the above, this article reviews the aboveground and below-ground microbiomes, their roles in plant immunity, physiological functions, and challenges and tools available in studying these organisms. The information collected over the years may contribute toward future applications, and in designing sustainable agriculture.

Root exudate signals in plant-plant interactions

Plant-to-plant signalling is a key mediator of interactions among plant species. Plants can perceive and respond to chemical cues emitted from their neighbours, altering survival and performance, impacting plant coexistence and community assembly. An increasing number of studies indicate root exudates as key players in plant-to-plant signalling. Root exudates mediate root detection and behaviour, kin recognition, flowering and production, driving inter- and intra-specific facilitation in cropping systems and mixed-species plantations. Altered interactions may be attributed to the signalling components within root exudates. Root ethylene, strigolactones, jasmonic acid, (-)-loliolide and allantoin are signalling chemicals that convey information on local conditions in plant-plant interactions. These root-secreted signalling chemicals appear ubiquitous in plants and trigger a series of belowground responses inter- and intra-specifically, involving molecular events in biosynthesis, secretion and action. The secretion of root signals, mainly mediated by ATP-binding cassette transporters, is critical. Root-secreted signalling chemicals and their molecular mechanisms are rapidly revealing a multitude of fascinating plant-plant interactions. However, many root signals, particularly species-specific signals and their underlying mechanisms, remain to be uncovered due to methodological limitations and root-soil interactions. A thorough understanding of root-secreted chemical signals and their mechanisms will offer many ecological implications and potential applications for sustainable agriculture.

Importance of the Rhizosphere Microbiota in Iron Biofortification of Plants

Increasing the iron content of plant products and iron assimilability represents a major issue for human nutrition and health. This is also a major challenge because iron is not readily available for plants in most cultivated soils despite its abundance in the Earth's crust. Iron biofortification is defined as the enhancement of the iron content in edible parts of plants. This biofortification aims to reach the objectives defined by world organizations for human nutrition and health while being environment friendly. A series of options has been proposed to enhance plant iron uptake and fight against hidden hunger, but they all show limitations. The present review addresses the potential of soil microorganisms to promote plant iron nutrition. Increasing knowledge on the plant microbiota and plant-microbe interactions related to the iron dynamics has highlighted a considerable contribution of microorganisms to plant iron uptake and homeostasis. The present overview of the state of the art sheds light on plant iron uptake and homeostasis, and on the contribution of plant-microorganism (plant-microbe and plant-plant-microbe) interactions to plant nutritition. It highlights the effects of microorganisms on the plant iron status and on the co-occurring mechanisms, and shows how this knowledge may be valued through genetic and agronomic approaches. We propose a change of paradigm based on a more holistic approach gathering plant and microbial traits mediating iron uptake. Then, we present the possible applications in plant breeding, based on plant traits mediating plant-microbe interactions involved in plant iron uptake and physiology.