Phenotypic plasticity of the maize root system in response to heterogeneous nitrogen availability

Mineral nutrients are distributed in a non-uniform manner in the soil. Plasticity in root responses to the availability of mineral nutrients is believed to be important for optimizing nutrient acquisition. The response of root architecture to heterogeneous nutrient availability has been documented in various plant species, and the molecular mechanisms coordinating these responses have been investigated particularly in Arabidopsis, a model dicotyledonous plant. Recently, progress has been made in describing the phenotypic plasticity of root architecture in maize, a monocotyledonous crop. This article reviews aspects of phenotypic plasticity of maize root system architecture, with special emphasis on describing (1) the development of its complex root system; (2) phenotypic responses in root system architecture to heterogeneous N availability; (3) the importance of phenotypic plasticity for N acquisition; (4) different regulation of root growth and nutrients uptake by shoot; and (5) root traits in maize breeding. This knowledge will inform breeding strategies for root traits enabling more efficient acquisition of soil resources and synchronizing crop growth demand, root resource acquisition and fertilizer application during crop growing season, thereby maximizing crop yields and nutrient-use efficiency and minimizing environmental pollution.     

Root system architecture determines fitness in an Arabidopsis mutant in competition for immobile phosphate ions but not for nitrate ions

Plant root systems often have complex branching patterns. Models indicate that a complex architecture is only required for the acquisition of immobile resources, such as phosphate; mobile ions, notably nitrate, can be effectively taken up by very restricted root systems. We have tested this prediction using the axr4 mutation of Arabidopsis thaliana, the principal phenotypic effect of which is to reduce the number of lateral roots. Arabidopsis thaliana is not a host for mycorrhizal fungi and so acquires all its nutrients through the root system. In both a pot experiment and a field experiment conducted under natural conditions for A. thaliana, we found that only phosphate, and not nitrate, affected the fitness of the mutant relative to the isogenic wild-type line, Columbia. These results confirm model predictions and have implications both for the evolution of complex root systems and for the design of efficient root systems for crops.     

Effects of different cultivation types on native and alien weed species richness and diversity in Moravia (Czech Republic)

Changes in weed species richness and beta-diversity are partly attributable to different types and intensity of disturbance and partly to broad-scale variation in environmental conditions. We compiled a data set of 434 vegetation plots of weed vegetation in root crop and cereal fields in Moravia (eastern Czech Republic) to compare the effects of environmental conditions and different disturbance regimes on species richness and beta-diversity. To detect changes in species richness, we related the variation in species richness to individual environmental conditions. To assess differences in beta-diversity between the vegetation of cereal and root crop fields, we used Whittaker's measure of beta-diversity. The relative importance of each environmental variable for the variation in species composition was evaluated using canonical correspondence analysis. All analyses were done for all vascular plant species and separately for native species, archaeophytes and neophytes. A comparison of weed vegetation of root crops and cereals showed a distinct dichotomy between these two types of weed vegetation. There was no significant difference in total species richness and native species richness; however, cereal fields were richer in archaeophytes and root crop fields were richer in neophytes. The beta-diversity of weed vegetation was higher in root crops. Environmental factors explained a significant part of the variability in richness of both natives and aliens. The richness of native species increased and beta-diversity decreased with increasing precipitation. The opposite relationship was found for archaeophytes, in both cereals and root crops. These results confirmed the importance of climatic factors and management practices for changes in weed species composition. They also showed a distinct pattern of species richness and beta-diversity of native and alien weed species.     

Comparative genetics of flowering time

Analysis of genes controlling flowering time (heading date) contributes to our understanding of fundamental principles of plant development and is of practical importance because of the effects of flowering time on plant adaptation and crop yield. This review discusses the extent to which plants may share common genetic mechanisms for the control of flowering time and the implications of such conservation for gene isolation from the major cereal crops. Gene isolation may exploit the small genome of rice in map-based approaches, utilizing the conservation of gene order that is revealed when common DNA markers are mapped in different species. Alternatively, mechanisms may be conserved within plants as a whole, in which case genes cloned from the model dicot Arabidopsis thaliana provide an alternative route.     

Trichome morphogenesis in Arabidopsis

Trichomes (plant hairs) in Arabidopsis thaliana are large non-secreting epidermal cells with a characteristic three-dimensional architecture. Because trichomes are easily accessible to a combination of genetic, cell biological and molecular methods they have become an ideal model system to study various aspects of plant cell morphogenesis. In this review we will summarize recent progress in the understanding of trichome morphogenesis.     

Root developmental adaptation to phosphate starvation: better safe than sorry

Phosphorus is a crucial component of major organic molecules such as nucleic acids, ATP and membrane phospholipids. It is present in soils in the form of inorganic phosphate (Pi), which has low availability and poor mobility. To cope with Pi limitations, plants have evolved complex adaptive responses that include morphological and physiological modifications. This review describes how the model plant Arabidopsis thaliana adapts its root system architecture to phosphate deficiency through inhibition of primary root growth, increase in lateral root formation and growth and production of root hairs, which all promote topsoil foraging. A better understanding of plant adaptation to low phosphate will open the way to increased phosphorus use efficiency by crops. Such an improvement is needed in order to adjust how we manage limited phosphorus stocks and to reduce the disastrous environmental effects of phosphate fertilizers overuse.     

Genetic modification of plant architecture and variety improvement in rice

The structure of the aerial part of a plant, referred to as plant architecture, is subject to strict genetic control, and grain production in cereal crops is governed by an array of agronomic traits. Rice is one of the most important cereal crops and is also a model plant for molecular biological research. Recently, significant progress has been made in isolating and collecting rice mutants that exhibit altered plant architecture. In this article we summarize the recent progress in understanding the basic patterning mechanisms involved in the regulation of tillering (branching) pattern, stem structure and leaf arrangement in rice plants. We discuss the relationship between the genetic modification of plant architecture and the improvement of pivotal agronomic traits in rice.     

Statistical modeling of nitrogen-dependent modulation of root system architecture in Arabidopsis thaliana

Plant root development is strongly affected by nutrient availability. Despite the importance of structure and function of roots in nutrient acquisition,statistical modeling approaches to evaluate dynamic and temporal modulations of root system architecture in response to nutrient availability have remained as widely open and exploratory areas in root biology. In this study,we developed a statistical modeling approach to investigate modulations of root system architecture in response to nitrogen availability. Mathematical models were designed for quantitative assessment of root growth and root branching phenotypes and their dynamic relationships based on hierarchical con figuration of primary and lateral roots formulating the fishbone-shaped root system architecture in Arabidopsis thaliana. Time-series datasets reporting dynamic changes in root developmental traits on different nitrate or ammonium concentrations were generated for statistical analyses. Regression analyses unraveled key parameters associated with:(i) inhibition of primary root growth under nitrogen limitation or on ammonium;(ii) rapid progression of lateral root emergence in response to ammonium; and(iii) inhibition of lateral root elongation in the presence of excess nitrate or ammonium. This study provides a statistical framework for interpreting dynamic modulation of root system architecture,supported by metaanalysis of datasets displaying morphological responses of roots to diverse nitrogen supplies.     

Turfgrasses as model assay systems for high-throughput <Emphasis Type="Italic">in planta</Emphasis> screening of beneficial endophytes isolated from cereal crops

Cereal crops including maize (Zea mays L.) are inhabited by non-disease causing microbes known as endophytes that can promote plant growth, aid in host nutrient acquisition and promote host pathogen resistance. Screening endophytes for beneficial traits in planta using large, slow-growing cereals is challenging, thus a rapid but relevant in planta system is needed. Here, we propose that turfgrasses can be used as high-throughput assay systems for screening cereal microbes for beneficial nutrient traits. Turfgrasses are genetic relatives of cereals, but small with fast growth rates; they can be grown in test tubes under sterile conditions on defined media. Five turfgrass genotypes were evaluated for traits ideal for assaying endophytes with nutrient acquisition traits. Based on these criteria, annual ryegrass (Lolium multiflorum) was selected as a high-throughput assay system. Annual ryegrass was then used to test a collection of maize endophytes for their ability to promote plant biomass in the absence of nitrogen. Out of 75 bacterial endophytes tested, one strain (an Enterobacter sp) consistently promoted root and shoot biomass. We discuss the potential of annual ryegrass as a model assay system to test cereal endophytes for acquisition of various nutrients, changes in root/shoot architecture as well as anti-pathogen traits.     

ESCRTing in cereals: still a long way to go

The multivesicular body(MVB) sorting pathway provides a mechanism for the delivery of cargo destined for degradation to the vacuole or lysosome. The endosomal sorting complex required for transport(ESCRT) is essential for the MVB sorting pathway by driving the cargo sorting to its destination. Many efforts in plant research have identified the ESCRT machinery and functionally characterised the first plant ESCRT proteins. However, most studies have been performed in the model plant Arabidopsis thaliana that is genetically and physiologically different to crops. Cereal crops are important for animal feed and human nutrition and have further been utilized as promising candidates for recombinant protein production. In this review, I summarize the role of plant ESCRT components in cereals that are involved in efficient adaptation to environmental stress and grain development. A special focus is on barley(Hordeum vulgare L.) ESCRT proteins, where recent studies show their quantitative mapping during grain development, e.g. associating HvSNF7.1 with protein trafficking to protein bodies(PBs) in starchy endosperm. Thus, it is indispensable to identify the molecular key-players within the endomembrane system including ESCRT proteins to optimize and possibly enhance tolerance to environmental stress, grain yield and recombinant protein production in cereal grains.     

New insights to lateral rooting: Differential responses to heterogeneous nitrogen availability among maize root types

Historical domestication and the "Green revolution" have both contributed to the evolution of modern, high-performance crops. Together with increased irrigation and application of chemical fertilizers, these efforts have generated sufficient food for the growing global population. Root architecture, and in particular root branching, plays an important role in the acquisition of water and nutrients, plant performance, and crop yield. Better understanding of root growth and responses to the belowground environment could contribute to overcoming the challenges faced by agriculture today. Manipulating the abilities of crop root systems to explore and exploit the soil environment could enable plants to make the most of soil resources, increase stress tolerance and improve grain yields, while simultaneously reducing environmental degradation. In this article it is noted that the control of root branching, and the responses of root architecture to nitrate availability, differ between root types and between plant species. Since the control of root branching depends upon both plant species and root type, further work is urgently required to determine the appropriate genes to manipulate to improve resource acquisition by specific crops.     

Root plasticity under abiotic stress

Abiotic stresses increasingly threaten existing ecological and agricultural systems across the globe. Plant roots perceive these stresses in the soil and adapt their architecture accordingly. This review provides insights into recent discoveries showing the importance of root system architecture (RSA) and plasticity for the survival and development of plants under heat, cold, drought, salt, and flooding stress. In addition, we review the molecular regulation and hormonal pathways involved in controlling RSA plasticity, main root growth, branching and lateral root growth, root hair development, and formation of adventitious roots. Several stresses affect root anatomy by causing aerenchyma formation, lignin and suberin deposition, and Casparian strip modulation. Roots can also actively grow toward favorable soil conditions and avoid environments detrimental to their development. Recent advances in understanding the cellular mechanisms behind these different root tropisms are discussed. Understanding root plasticity will be instrumental for the development of crops that are resilient in the face of abiotic stress.

Recent discoveries show the importance of root system architecture plasticity for the survival and growth of plants under several abiotic stresses.     

Biological nitrogen fixation in mixed legume-cereal cropping systems

Cereal/legume intercropping increases dry matter production and grain yield more than their monocultures. When fertilizer N is limited, biological nitrogen fixation (BNF) is the major source of N in legume-cereal mixed cropping systems. The soil N use patterns of component crops depend on the N source and legume species. Nitrogen transfer from legume to cereal increases the cropping system's yield and efficiency of N use. The use of nitrate-tolerant legumes, whose BNF is thought to be little affected by application of combined N, may increase the quantity of N available for the cereal component. The distance between the cereal and legume root systems is important because N is transferred through the intermingling of root systems. Consequently, the most effective planting distance varies with type of legume and cereal. Mutual shading by component crops, especially the taller cereals, reduces BNF and yield of the associated legume. Light interception by the legume can be improved by selecting a suitable plant type and architecture. Planting pattern and population at which maximum yield is achieved also vary among component species and environments. Crops can be mixed in different proportions from additive to replacement or substitution mixtures. At an ideal population ratio a semi-additive mixture may produce higher gross returns.     

3D reconstruction and dynamic modeling of root architecture in situ and its application to crop phosphorus research

Root architecture plays important roles in plant water and nutrient acquisition. However, accurate modeling of the root system that provides a realistic representation of roots in the soil is limited by a lack of appropriate tools for the non‐destructive and precise measurement of the root system architecture in situ. Here we describe a root growth system in which the roots grow in a solid gel matrix that was used to reconstruct 3D root architecture in situ and dynamically simulate its changes under various nutrient conditions with a high degree of precision. A 3D laser scanner combined with a transparent gel‐based growth system was used to capture 3D images of roots. The root system skeleton was extracted using a skeleton extraction method based on the Hough transformation, and mesh modeling using Ball‐B spline was employed. We successfully used this system to reconstruct rice and soybean root architectures and determine their changes under various phosphorus (P) supply conditions. Our results showed that the 3D root architecture parameters that were dynamically calculated based on the skeletonization and simulation of root systems were significantly correlated with the biomass and P content of rice and soybean based on both the simulation system and previous reports. Therefore, this approach provides a novel technique for the study of crop root growth and its adaptive changes to various environmental conditions.     

赤霉素代谢调控与绿色革命

赤霉素作为重要的植物激素,参与了植物诸多发育过程的调控.一些涉及赤霉素生物合成和信号传导途径的重要调控基因对作物的株型、产量和品质能够产生积极的影响,已在农业生产中得到广泛应用.其中,Rht-1和sd-1等位基因由于分别赋予了小麦和水稻半矮化的特性,从而促成了20世纪后半叶的"绿色革命".本文回顾了与"绿色革命"相关的...     

植物与低磷环境研究进展——诱导、适应与对策

自从20世纪70年代人们发现适应低磷土壤的作物根际磷的有效性明显增加的现象之后。植物与低磷环境的研究便引起了人们的重视。植物如何适应低磷环境和如何有效利用土壤磷素资源的问题已成为国内外当前的研究热点之一。研究表明,低磷条件下,植物根系形态结构会发生适应性变化,根冠间的物质分配会向根部倾斜使根冠比增加;植物根际酸度变化、有机酸分泌和磷酸酶释放有利于活化和利用土壤中的磷素资源;不同种类或品种的植物具有不同的磷营养效率基因型,具有不同亲和力的磷转运体,也具有不同的磷活化机制。人类对植物适应低磷机制的研究还将继续,揭示植物对低磷环境的响应对策和发掘植物有效利用磷素资源的潜力,在经济上和环保上均有非常现实的意义。     

MicroRNA expression profiles in response to drought stress in Sorghum bicolor

The regulatory role of small non-coding RNAs that are 20–24 nucleotides in length has become the foremost area of research for biologists. A major class of small RNAs represented by the microRNAs (miRNAs), has been implicated in various aspects of plant development including leaf pattering, meristem function, root patterning etc. Recent findings support that miRNAs are regulated by drought and other abiotic stresses in various plant species. In this study, were report the expression profiling of 8 known abiotic stress deregulated miRNAs in 11 elite sorghum genotypes, under watered and drought conditions. Significant deregulation was observed with miR396, miR393, miR397-5p, miR166, miR167 and miR168. Among these, the expression levels of sbi-miR396 and sbi-miR398 were the highest in all the genotypes. The expression of sbi-miR396 was maximum in the grain sorghum HSD3226 under well-watered conditions and the profile shifted towards HSD3221 under drought stress. Forage accessions, N98 and Atlas, showed an opposite behavior in expression patterns of miR397-5p in drought physiologies. Such dynamic expression patterns could be indicative of prevailing drought tolerant mechanisms present in these sorghum accessions. This data provides insights into sorghum miRNAs which may have potential use in improving drought tolerance in sorghum and other cereal crops.     

An auxin transport-based model of root branching in Arabidopsis thaliana

Root architecture is a crucial part of plant adaptation to soil heterogeneity and is mainly controlled by root branching. The process of root system development can be divided into two successive steps: lateral root initiation and lateral root development/emergence which are controlled by different fluxes of the plant hormone auxin. While shoot architecture appears to be highly regular, following rules such as the phyllotactical spiral, root architecture appears more chaotic. We used stochastic modeling to extract hidden rules regulating root branching in Arabidopsis thaliana. These rules were used to build an integrative mechanistic model of root ramification based on auxin. This model was experimentally tested using plants with modified rhythm of lateral root initiation or mutants perturbed in auxin transport. Our analysis revealed that lateral root initiation and lateral root development/emergence are interacting with each other to create a global balance between the respective ratio of initiation and emergence. A mechanistic model based on auxin fluxes successfully predicted this property and the phenotype alteration of auxin transport mutants or plants with modified rhythms of lateral root initiation. This suggests that root branching is controlled by mechanisms of lateral inhibition due to a competition between initiation and development/emergence for auxin.