Exploring root system architecture and its importance in solanaceous vegetables: A review

Root system architecture (RSA) is an essential characteristic for efficient water and nutrient absorption in terrestrial plants; its plasticity enables plants to respond to different soil environments. Better understanding of root plasticity is important in developing stress-tolerant crops. Non-invasive techniques that can measure roots in soils nondestructively, such as X-ray computed tomography (CT), are useful to evaluate RSA plasticity. However, although RSA plasticity can be measured by tracking individual root growth, only a few methods are available for tracking individual roots from time-series three-dimensional (3D) images. We developed a semi-automatic workflow that tracks individual root growth by vectorizing RSA from time-series 3D images via two major steps. The first step involves 3D alignment of the time-series RSA images by iterative closest point registration with point clouds generated by high-intensity particles in potted soils. This alignment ensures that the time-series RSA images overlap. The second step consists of backward prediction of vectorization, which is based on the phenomenon that the root length of the RSA vector at the earlier time point is shorter than that at the last time point. In other words, when CT scanning is performed at time point A and again at time point B for the same pot, the CT data and RSA vectors at time points A and B will almost overlap, but not where the roots have grown. We assumed that given a manually created RSA vector at the last time point of the time series, all RSA vectors except those at the last time point could be automatically predicted by referring to the corresponding RSA images. Using 21 time-series CT volumes of a potted plant of upland rice (Oryza sativa), this workflow revealed that the root elongation speed increased with age. Compared with a workflow that does not use backward prediction, the workflow with backward prediction reduced the manual labor time by 95%. We developed a workflow to efficiently generate time-series RSA vectors from time-series X-ray CT volumes. We named this workflow 'RSAtrace4D' and are confident that it can be applied to the time-series analysis of RSA development and plasticity.

Malcolm Bennett

Root system architecture plays an important role in plant adaptation to drought stress. The root system architecture (RSA) consists of several structural features, which includes number and length of main and lateral roots along with the density and length of root hairs. These features exhibit plasticity under water-limited environments and could be critical to developing crops with efficient root systems for adaptation under drought. Recent advances in the omics approaches have significantly improved our understanding of the regulatory mechanisms of RSA remodeling under drought and the identification of genes and other regulatory elements. Plant response to drought stress at physiological, morphological, biochemical, and molecular levels in root cells is regulated by various phytohormones and their crosstalk. Stress-induced reactive oxygen species play a significant role in regulating root growth and development under drought stress. Several transcription factors responsible for the regulation of RSA under drought have proven to be beneficial for developing drought tolerant crops. Molecular breeding programs for developing drought-tolerant crops have been greatly benefitted by the availability of quantitative trait loci (QTLs) associated with the RSA regulation. In the present review, we have discussed the role of various QTLs, signaling components, transcription factors, microRNAs and crosstalk among various phytohormones in shaping RSA and present future research directions to better understand various factors involved in RSA remodeling for adaptation to drought stress. We believe that the information provided herein may be helpful in devising strategies to develop crops with better RSA for efficient uptake and utilization of water and nutrients under drought conditions.

Potato plants are often exposed to soil drought, which results in crop losses, because potato root system is shallow. The aim of our study was to examine the relationship between root system architecture and drought tolerance. We measured rooting depth, total length, surface area, average diameter, and dry weight of the root system every 20 cm of soil depth. We examined four potato cultivars, two that were more tolerant to drought (Gwiazda and Tajfun), and two that were more sensitive (Oberon and Cekin). Our results showed different responses by the tested cultivars to drought. The root dry mass decreased in response to drought and this decrease was smaller in the more drought-tolerant cultivars. In response to drought, the more drought-tolerant cultivars developed elongated roots, but the roots of the less-tolerant cultivars remained the same length. The decrease in root diameter was also smaller in the more drought-tolerant cultivars.

The root system architecture (RSA) determines plant growth and yield. The characterization of optimal RSA and discovery of genetic loci or candidate genes that control root traits are therefore important research goals. However, the hidden nature of the root system makes it difficult to perform nondestructive, rapid analyses of RSA. In this study, we developed an automated, nondestructive, high-throughput root phenotyping platform (Root-HTP) and a corresponding data processing pipeline for efficient, large-scale characterization of wheat (Triticum aestivum L.) RSA. This system is capable of tracking root growth dynamics and RSA variation across all wheat developmental stages. In situ phenotyping using Root-HTP extracted 47 RSA traits, including 33 novel traits in wheat and 23 novel traits in other crops. We used root trait data from the phenotyping system and yield trait data to conduct a genome-wide association study (GWAS) of 155 wheat accessions, which identified 2,650 SNPs and 233 quantitative trait loci (QTLs) associated with aspects of root architecture. The candidate gene TaMYB93 was detected in a QTL for root tortuosity, and EMS mutants confirmed its effect on RSA in wheat. We explored the relationship between root- and yield-related traits and identified 20 root-related QTLs that were also associated with yield traits. Furthermore, we have built a predictive model for wheat yield based on 18 RSA traits and propose a parsimonious RSA ideotype associated with high yields. The data generated from this study provide insight into the genetic architecture of wheat RSA and support for RSA ideotype-based wheat breeding and yield prediction.

This review article highlights a broader perspective of NPs and plant-root interaction by focusing on their beneficial anddeleterious impacts on root system architecture (RSA). The root performs a vital function by securing itself in the soil, absorbing and transporting water and nutrients to facilitate plant growth and productivity. In dicots, the architecture of the root system (RSA) is markedly shaped by the development of the primary root and its branches, showcasing considerable adaptability in response to changes in the environment. For promoting agriculture and combating global food hunger, the use of nanoparticles (NPs) may be an exciting option, for which it is essential to understand the behaviour of plants under NPs exposure. The nature of NPs and their physicochemical characteristics play a significant role in the positive/negative response of roots and shoots. Root morphological features, such as root length, root mass and root development features, may regulated positively/negatively by different types of NPs. In addition, application of NPs may also enhance nutrient transport and soil fertility by the promotion of soil microorganisms including plant growth-promoting rhizobacteria (PGPRs) and also soil enzymes. Interestingly the interaction of nanomaterials (NMs) with rhizospheric bacteria can enhance plant development and soil health. However, some studies also suggested that the increased use of several types of engineered nanoparticles (ENPs) may disrupt the equilibrium of the soil-root interface and unsafe morphogenesis by causing the browning of roots and suppressing the growth of root and soil microbes. Thus, this review article has sought to compile a broader perspective of NPs and plant-root interaction by focusing on their beneficial or deleterious impacts on RSA.

Understanding the development of roots exposed to contaminants and the potential of plant-associated bacteria for optimization of growth.

Root growth and development are of outstanding importance for the plant's ability to acquire water and nutrients from different soil horizons. To cope with fluctuating nutrient availabilities, plants integrate systemic signals pertaining to their nutritional status into developmental pathways that regulate the spatial arrangement of roots. Changes in the plant nutritional status and external nutrient supply modulate root system architecture (RSA) over time and determine the degree of root plasticity which is based on variations in the number, extension, placement, and growth direction of individual components of the root system. Roots also sense the local availability of some nutrients, thereby leading to nutrient-specific modifications in RSA, that result from the integration of systemic and local signals into the root developmental programme at specific steps. An in silico analysis of nutrient-responsive genes involved in root development showed that the majority of these specifically responded to the deficiency of individual nutrients while a minority responded to more than one nutrient deficiency. Such an analysis provides an interesting starting point for the identification of the molecular players underlying the sensing and transduction of the nutrient signals that mediate changes in the development and architecture of root systems.

Root system architecture (RSA) refers to the geometric features and topology of the root system. Ground-penetrating radar (GPR) is a possible method of RSA reconstruction. However, because the topology of the root system is not directly accessible by GPR, GPR-based reconstruction must be complemented by manual connection of root points, resulting in limited accuracy. In this study, we used both GPR and direct excavation to obtain 3D coordinates (XYZ coordinates) and diameters of moso bamboo rhizomes on an orthogonal grid. A score function for selecting the best-connected root points was developed using rhizome diameter, depth, extension angle, and measured line spacing, which was then used to recover the topology of discrete root points. Based on the recovered topology, the 3D RSA of the rhizomes was reconstructed using a smoothing function. Based on the excavation data, the reconstructed RSA was generally consistent with the measured RSA, with 78.13% of root points correctly connected. The reconstructed RSA based on GPR data thus provided a rough approximation of the measured RSA, with errors arising due to missing root points and rhizome displacement. The proposed algorithm for reconstructing 3D RSA further enriches the application of ground-penetrating radar to root detection.

Roots have a central role in plant resource capture and are the interface between the plant and the soil that affect multiple ecosystem processes. Field pennycress (Thlaspi arvense L.) is a diploid annual cover crop species that has potential utility for reducing soil erosion and nutrient losses; and has rich seeds (30-35% oil) amenable to biofuel production and as a protein animal feed. The objective of this research was to (1) precisely characterize root system architecture and development, (2) understand plastic responses of pennycress roots to nitrate nutrition, (3) and determine genotypic variance available in root development and nitrate plasticity. Using a root imaging and analysis pipeline, the 4D architecture of the pennycress root system was characterized under four nitrate regimes, ranging from zero to high nitrate concentrations. These measurements were taken at four time points (days 5, 9, 13, and 17 after sowing). Significant nitrate condition response and genotype interactions were identified for many root traits, with the greatest impact observed on lateral root traits. In trace nitrate conditions, a greater lateral root count, length, density, and a steeper lateral root angle was observed compared to high nitrate conditions. Additionally, genotype-by-nitrate condition interaction was observed for root width, width:depth ratio, mean lateral root length, and lateral root density. These findings illustrate root trait variance among pennycress accessions. These traits could serve as targets for breeding programs aimed at developing improved cover crops that are responsive to nitrate, leading to enhanced productivity, resilience, and ecosystem service.

Environmental events, such as drought, are predicted to increase in frequency and duration, and coupled with an expanding global population, improving cereal crop productivity and yield stability is crucial. A body of research suggests that roots may be fundamental to increasing crop yields, whereby optimised root systems could capture more water and nutrients with minimal metabolic costs. For efficient resource capture in most water-limited environments, a narrow and deep root architecture is likely advantageous. The central importance of the root system in plant productivity cannot be underestimated, yet the functional and genetic basis of root system architecture in cereal crops is relatively unknown.In this thesis, the genetics underpinning root system architecture in barley (Hordeum vulgare L.) were investigated and the value of roots as a drought adaptive trait in barley was examined. In addition, the genetic variation in delayed-foliar senescence and flowering time were also explored to investigate any shared genetic control between above- and below-ground drought adaptation traits. Through the characterisation of three diverse barley populations, key genes influencing root system architecture were identified. Preliminary evidence for shared genetic control between delayed foliar senescence and root architecture was observed through co-located quantitative trait loci (QTL), specifically the gibberellic acid biosynthesis gene, Hv20ox1. Preliminary associations between time to flowering and seminal root traits were confirmed through the identification of VERNALIZATION1 (VRN1) as a major gene influencing root architecture in barley. Whereby, VRN1 is a key regulator of flowering behaviour in cereal crops. The research described in this thesis provides novel insight into the genetic control of root system architecture and reveals a new role for the previously described pathway for regulation of flowering in modulating the largely unexplored genetic architecture of root development in barley. The knowledge generated from this research may be harnessed in barley breeding programs to assist in the development of robust cultivars better adapted to the increasingly variable future climate.

Under all environments, roots are important for plant anchorage and acquiring water and nutrients. However, there is a knowledge gap regarding how root architecture contributes to stress tolerance in a changing climate. Two closely related plant species, maize and sorghum, have distinct root system architectures and different levels of stress tolerance, making comparative analysis between these two species an ideal approach to resolve this knowledge gap. However, current research has focused on shared aspects of the root system that are advantageous under abiotic stress conditions rather than on differences. Here we summarize the current state of knowledge comparing the root system architecture relative to plant performance under water deficit, salt stress, and low phosphorus in maize and sorghum. Under water deficit, steeper root angles and deeper root systems are proposed to be advantageous for both species. In saline soils, a reduction in root length and root number has been described as advantageous, but this work is limited. Under low phosphorus, root systems that are shallow and wider are beneficial for topsoil foraging. Future work investigating the differences between these species will be critical for understanding the role of root system architecture in optimizing plant production for a changing global climate.

Studying the effects of small molecules on root system development in the context of a large-scale chemical genetic screen has previously been a technical challenge. The recent development of novel seedling growth devices ("Phytostrips"), used in combination with standard 96-well microtiter plates, has made it possible to perform detailed studies of changes in root morphology and root system architecture following the application of a library of chemical compounds. Phytostrips were originally designed to allow automated robotic capture of images of roots and shoots of the model species Arabidopsis thaliana, but can also be used for manual screens that are more laborious but do not require the investment in expensive robotics.Here we describe a protocol for the use of Phytostrips to perform chemical genetic screens that rely on clearly observable changes in root morphology or root system architecture. As an example, we describe the use of polyethylene glycol to impose an abiotic stress related to reduced water potential and the application of a chemical screen for small molecules that are able to rescue Arabidopsis root development from the disruptive effect of the polyethylene glycol treatment. The protocol we describe provides a template for the application of a multiplicity of other screens for compounds that can antagonize the effects of a range of abiotic stresses on root development.

The root system architecture (RSA) in monocotyledonous plants like rice is consists of primary roots, lateral roots, seminal/crown roots, and root hairs. The soil nutrients also influence many physiological processes via various root parameters like root length, root diameter and root angle for growth and development. The variation in root system architecture in rice is influenced by the intrinsic factors (phytohormones, transcription factors) and extrinsic factors (light, temperature and moisture) and their collective effect. The phytohormones such as; auxin, cytokinin, abscisic acid and ethylene, and their mutual effects play vital role for root architectural trait development. Many genes/QTLs were identified in rice which are strong role player for root development. But the biochemical signaling pathways are not completely understood. The modern molecular tools like genome editing, sequencing and multi-omics (transcriptomics and proteomics) approaches and multi-disciplinary system biology studies can provide a better solution for this issue. To improve the sustainable food grain production under extreme environment, it is important to understand the physiological and biochemical mechanism of root development. Moreover, it is imperative to establish a resilient root system in rice cultivation in order to mitigate the overuse of chemical fertilizers, enhance nutrient efficiency, and improve climate resilience of the plant.

Characterising shoot and root system trait variability and contribution to genotypic variability in juvenile cassava (Manihot esculenta Crantz) plants