The role of nutrient availability in regulating root architecture

The ability of plants to respond appropriately to nutrient availability is of fundamental importance for their adaptation to the environment. Nutrients such as nitrate, phosphate, sulfate and iron act as signals that can be perceived. These signals trigger molecular mechanisms that modify cell division and cell differentiation processes within the root and have a profound impact on root system architecture. Important developmental processes, such as root-hair formation, primary root growth and lateral root formation, are particularly sensitive to changes in the internal and external concentration of nutrients. The responses of root architecture to nutrients can be modified by plant growth regulators, such as auxins, cytokinins and ethylene, suggesting that the nutritional control of root development may be mediated by changes in hormone synthesis, transport or sensitivity. Recent information points to the existence of nutrient-specific signal transduction pathways that interpret the external and internal concentrations of nutrients to modify root development. Progress in this field has led to the cloning of regulatory genes that play pivotal roles in nutrient-induced changes to root development.     

Nitrate and glutamate as environmental cues for behavioural responses in plant roots

As roots explore the soil, they encounter a complex and fluctuating environment in which the different edaphic resources (water and nutrients) are heterogeneously distributed in space and time. Many plant species are able to respond to this heterogeneity by modifying their root system development, such that they colonize the most resource-rich patches of soil. The complexities of these responses, and their dependence on the implied ability to perceive and integrate multiple external signals, would seem to amply justify the term 'behaviour'. This review will consider the types of behaviour that are elicited in roots of Arabidopsis thaliana by exposure to variations in the external concentrations and distribution of two different N compounds, nitrate and glutamate. Molecular genetic studies have revealed an intricate N regulatory network at the root tip that is responsible for orchestrating changes in root growth rate and root architecture to accommodate variations in the extrinsic and intrinsic supply of N. The review will discuss what is known of the genetic basis for these responses and speculate on their physiological and ecological significance.     

The role of long-distance signalling in plant responses to nitrate and other nutrients.

The phenotypic plasticity that plants display in response to changes in their nutrient supply requires the operation of both short- and long-range signalling pathways. Long-distance signals arising in the root can provide the shoot with an early warning of fluctuations in external nutrient concentrations, while signals in the reverse direction are needed to ensure that root physiology and development are integrated with the nutritional demands of the shoot. In this review, the focus is on recent advances in the understanding of these long-distance signalling pathways with an emphasis on nitrate nutrition, and a personal view of the key issues for future research is put forward.     

Nitrogen regulation of root branching

BACKGROUND: Many plant species can modify their root architecture to enable them to forage for heterogeneously distributed nutrients in the soil. The foraging response normally involves increased proliferation of lateral roots within nutrient-rich soil patches, but much remains to be understood about the signalling mechanisms that enable roots to sense variations in the external concentrations of different mineral nutrients and to modify their patterns of growth and development accordingly. SCOPE: In this review we consider different aspects of the way in which the nitrogen supply can modify root branching, focusing on Arabidopsis thaliana. Our current understanding of the mechanism of nitrate stimulation of lateral root growth and the role of the ANR1 gene are summarized. In addition, evidence supporting the possible role of auxin in regulating the systemic inhibition of early lateral root development by high rates of nitrate supply is presented. Finally, we examine recent evidence that an amino acid, L-glutamate, can act as an external signal to elicit complex changes in root growth and development. CONCLUSIONS: It is clear that plants have evolved sophisticated pathways for sensing and responding to changes in different components of the external nitrogen supply as well as their own internal nitrogen status. We speculate on the possibility that the effects elicited by external L-glutamate represent a novel form of foraging response that could potentially enhance a plant's ability to compete with its neighbours and micro-organisms for localized sources of organic nitrogen.     

Long-distance stress and developmental signals associated with abscisic acid signaling in environmental responses

Flowering plants consist of highly differentiated organs, including roots, leaves, shoots and flowers, which have specific roles: root system for water and nutrient uptake, leaves for photosynthesis and gas exchange and reproductive organs for seed production. The communication between organs through the vascular system, by which water, nutrient and signaling molecules are transported, is essential for coordinated growth and development of the whole plant, particularly under adverse conditions. Here, we highlight recent progress in understanding how signaling pathways of plant hormones are associated with long-distance stress and developmental signals, with particular focus on environmental stress responses. In addition to the root-to-shoot peptide signal that induces abscisic acid accumulation in leaves under drought stress conditions, we summarize the diverse stress-responsive peptide signals reported to date to play a role in environmental responses.     

The ups and downs of signalling between root and shoot

It is becoming increasingly apparent that the long-distance signalling associated with many developmental processes is complex and that novel hormone-like signals may play substantial roles. The past decades have seen several substances (e.g. brassinosteroids, systemin and other polypeptides, mevalonic and jasmonic acids, polyamines, oligosaccharides, flavonoids, and quinones) vie for a place among the classical plant hormones (e.g. Spaink, 1996). Recent microinjection and grafting studies have also shown that RNA may act as a long-distance signal (Jorgensen et al ., 1998; Xoconostle-Cázares et al ., 1999). In this issue, Hannah et al . describe long-distance signalling and the regulation of root–shoot partitioning in dwarf lethal or dosage-dependent lethal ( DL ) mutants of common bean (Shii et al ., 1980, 1981), and present evidence indicating that substances in addition to classical plant hormones (e.g. cytokinins) may be involved.
As in the report by Hannah et al ., much of the evidence for roles of unidentified long-distance signals in the control of plant development is indirect. The possibility that a small number of long-distance signals might control a multitude of developmental processes arises through the potential for differences in tissue sensitivity, fluctuations in hormone levels and differences in the nature of responses of different tissues to the same hormone. Consequently, particular hormones may influence numerous processes seemingly simultaneously, yet independently. Even so, long-distance signalling is involved in processes as diverse as root–shoot balance, senescence, branching, flowering, nodulation, stress responses and nutrient uptake. Through comparison of even a few different developmental processes, progress can be made to reveal the true complexity of plant development. Using this approach it is also clear that many unknown signals may be involved.     

Root exudates as mediators of mineral acquisition in low-nutrient environments

Plant developmental processes are controlled by internal signals that depend on the adequate supply of mineral nutrients by soil to roots. Thus, the availability of nutrient elements can be a major constraint to plant growth in many environments of the world, especially the tropics where soils are extremely low in nutrients. Plants take up most mineral nutrients through the rhizosphere where micro-organisms interact with plant products in root exudates. Plant root exudates consist of a complex mixture of organic acid anions, phytosiderophores, sugars, vitamins, amino acids, purines, nucleosides, inorganic ions (e.g. HCO₃ ^–, OH^–, H⁺), gaseous molecules (CO₂, H₂), enzymes and root border cells which have major direct or indirect effects on the acquisition of mineral nutrients required for plant growth. Phenolics and aldonic acids exuded directly by roots of N₂-fixing legumes serve as major signals to Rhizobiaceae bacteria which form root nodules where N₂ is reduced to ammonia. Some of the same compounds affect development of mycorrhizal fungi that are crucial for phosphate uptake. Plants growing in low-nutrient environments also employ root exudates in ways other than as symbiotic signals to soil microbes involved in nutrient procurement. Extracellular enzymes release P from organic compounds, and several types of molecules increase iron availability through chelation. Organic acids from root exudates can solubilize unavailable soil Ca, Fe and Al phosphates. Plants growing on nitrate generally maintain electronic neutrality by releasing an excess of anions, including hydroxyl ions. Legumes, which can grow well without nitrate through the benefits of N₂ reduction in the root nodules, must release a net excess of protons. These protons can markedly lower rhizosphere pH and decrease the availability of some mineral nutrients as well as the effective functioning of some soil bacteria, such as the rhizobial bacteria themselves. Thus, environments which are naturally very acidic can pose a challenge to nutrient acquisition by plant roots, and threaten the survival of many beneficial microbes including the roots themselves. A few plants such as Rooibos tea (Aspalathus linearis L.) actively modify their rhizosphere pH by extruding OH^– and HCO₃ ^– to facilitate growth in low pH soils (pH 3 – 5). Our current understanding of how plants use root exudates to modify rhizosphere pH and the potential benefits associated with such processes are assessed in this review.     

植物养分捕获策略随成土年龄的变化及生态学意义

自然成土过程中土壤养分的变化与植被原生演替常同时发生。随成土年龄变化的植物养分捕获策略(NASs)对植物竞争能力和演替过程具有重要影响。该文将植物NASs划分为细根、微生物、特殊根系、食虫和寄生策略等5个类型; 发现植物NASs的多样性随成土年龄的增加呈哑铃型变化模式; 特殊根系策略对植物捕获养分的作用在成土中期最小、后期最大, 细根和微生物策略的作用随成土年龄的增加逐渐降低; 分析了成土过程中NASs对植物种间关系影响的变化, 发现NASs对成土早期植物的促进作用和中期的竞争关系具有重要影响, 而成土后期多样和互补的NASs对植被群落的稳定共存及多样性的形成具有影响; 提出应进一步探究成土过程中土壤养分与植物NASs变化之间的定量关系, 开展更多研究以阐明NASs对植被原生演替、物种多样性形成和成土过程的贡献与机理。     

Nutrient demand and fungal access to resources control the carbon allocation to the symbiotic partners in tripartite interactions of Medicago truncatula

Legumes form tripartite interactions with arbuscular mycorrhizal fungi and rhizobia, and both root symbionts exchange nutrients against carbon from their host. The carbon costs of these interactions are substantial, but our current understanding of how the host controls its carbon allocation to individual root symbionts is limited. We examined nutrient uptake and carbon allocation in tripartite interactions of Medicago truncatula under different nutrient supply conditions, and when the fungal partner had access to nitrogen, and followed the gene expression of several plant transporters of the Sucrose Uptake Transporter (SUT) and Sugars Will Eventually be Exported Transporter (SWEET) family. Tripartite interactions led to synergistic growth responses and stimulated the phosphate and nitrogen uptake of the plant. Plant nutrient demand but also fungal access to nutrients played an important role for the carbon transport to different root symbionts, and the plant allocated more carbon to rhizobia under nitrogen demand, but more carbon to the fungal partner when nitrogen was available. These changes in carbon allocation were consistent with changes in the SUT and SWEET expression. Our study provides important insights into how the host plant controls its carbon allocation under different nutrient supply conditions and changes its carbon allocation to different root symbionts to maximize its symbiotic benefits.