A comparison of lateral root patterning among dicot and monocot plants

Magnetic iron-based nanoparticles biogeochemical behavior in soil-plant system: A critical review

Increasing attention is being given to magnetic iron-based nanoparticles (MINPs) because of their potential environmental benefits. Owing to the earth abundance and high utilization of MINPs, as well as the significant functions of Fe in sustainable agriculture and environmental remediation, an understanding of the environmental fate of MINPs is indispensable. However, there are still knowledge gaps regarding the largely unknown environmental behaviors and fate of MINPs in soil-plant system. Thus, this review summarizes recent literature on the biogeochemical behavior (uptake, transportation, and transformation) of MINPs in soil and plants. The different possible uptake (e.g., foliar and root adsorption) and translocation (e.g., xylem, phloem, symplastic/apoplastic pathway, and endocytosis) pathways are discussed. Furthermore, drivers of MINPs uptake and transportation (e.g., soil characteristics, fertilizer treatments, copresence of inorganic and organic anions, meteorological conditions, and cell wall pores) in both soil and plant environments are summarized. This review also details the physical, chemical, and biological transformations of MINPs in soil-plant system. More importantly, a metadata analysis from the existing literature was employed to investigate the distinction between MINPs and other engineering nanoparticles biogeochemical behavior. In the future, more attention should be given to understanding the behavior of MINPs in soil-plant system and improving the capabilities of predictive models. This review thus highlights the main knowledge gaps regarding MINPs behavior and fate to provide guidance for their safe application in agrochemicals, crop production, and soil health.

Lateral root branching: evolutionary innovations and mechanistic divergence in land plants

The root system architecture in plants is a result of multiple evolutionary innovations over time in response to changing environmental cues. Dichotomy and endogenous lateral branching in the roots evolved in lycophytes lineage but extant seed plants use lateral branching instead. This has led to the development of complex and adaptive root systems, with lateral roots playing a key role in this process exhibiting conserved and divergent features in different plant species. The study of lateral root branching in diverse plant species can shed light on the orderly yet distinct nature of postembryonic organogenesis in plants. This insight provides an overview of the diversity in lateral root (LR) development in various plant species during the evolution of root system in plants.

Arbuscular mycorrhizal symbiosis enhances tomato lateral root formation by modulating CEP2 peptide expression

Plant lateral root (LR) growth usually is stimulated by arbuscular mycorrhizal (AM) symbiosis. However, the molecular mechanism is still unclear. We used gene expression analysis, peptide treatment and virus-induced gene alteration assays to demonstrate that C-terminally encoded peptide (CEP2) expression in tomato was downregulated during AM symbiosis to mitigate its negative effect on LR formation through an auxin-related pathway. We showed that enhanced LR density and downregulated CEP2 expression were observed during mycorrhizal symbiosis. Synthetic CEP2 peptide treatment reduced LR density and impaired the expression of genes involved in indole-3-butyric acid (IBA, the precursor of IAA) to IAA conversion, auxin polar transport and the LR-related signaling pathway; however, application of IBA or synthetic auxin 1-naphthaleneacetic acid (NAA) to the roots may rescue both defective LR formation and reduced gene expression. CEP receptor 1 (CEPR1) might be the receptor of CEP2 because its knockdown plants did not respond to CEP2 treatment. Most importantly, the LR density of CEP2 overexpression or knockdown plants could not be further increased by AM inoculation, suggesting that CEP2 was critical for AM-induced LR formation. These results indicated that AM symbiosis may regulate root development by modulating CEP2, which affects the auxin-related pathway.

The Hierarchy of Protoxylem Groupings in Primary Root and Their Plasticity to Nitrogen Addition in Three Tree Species

Protoxylem grouping (PG), a classification based on the number of protoxylem poles, is a crucial indicator related to other functional traits in fine roots, affecting growth and survival of individual root. However, within root system, less is known about the arrangement of PG. Moreover, the responses of PG to fertilization are still unclear. Here, we selected three common hardwood species in Northeast China, Juglans mandshurica, Fraxinus mandshurica, and Phellodendron amurense, conducted root pruning and nutrient addition. In this study, we analyzed the PG, morphology, and other anatomy traits of newly formed root branches. The results showed all root length, diameter, and stele, as well as hydraulic conductivity, were significantly positive related to the PG number, and the PG number generally decreased with ascending root developmental order; these patterns were independent of species and fertilization. Additionally, we also found the plasticity of PGs to environmental changes, in terms of the increased frequency of high PG roots after fertilization, significantly in J. mandshurica and F. mandshurica. Therefore, the heterogeneity, hierarchy, and plasticity of individual roots within root system may be widespread in woody plants, which is of great significance to deepen our understanding in root growth and development, as well as the belowground ecological process.

Ammonium transporters cooperatively regulate rice crown root formation responding to ammonium nitrogen

Crown roots (CRs) are major components of the rice root system. They form at the basal node of the shoot, and their development is greatly influenced by environmental factors. Ammonium nitrogen is known to impact plant root development through ammonium transporters (AMTs), but it remains unclear whether ammonium and AMTs play roles in rice CR formation. In this study, we revealed a significant role of ammonium, rather than nitrate, in regulating rice CR development. High ammonium supply increases CR formation but inhibits CR elongation. Genetic evidence showed that ammonium regulation of CR development relies on ammonium uptake mediated jointly by ammonium transporters OsAMT1;1, OsAMT1;2; OsAMT1;3, and OsAMT2;1, but not on root acidification which was the result of ammonium uptake. OsAMTs are also needed for glutamine-induced CR formation. Furthermore, we showed that polar auxin transport dependent on the PIN auxin efflux carriers acts downstream of ammonium uptake and assimilation to activate local auxin signaling at CR primordia, in turn promoting CR formation. Taken together, our results highlight a critical role for OsAMTs in cooperatively regulating CR formation through regulating auxin transport under nitrogen-rich conditions.

The Root Clock as a Signal Integrator System: Ensuring Balance for Survival

The root system is essential for the survival of terrestrial plants, plant development, and adaptation to changing environments. The development of the root system relies on post-embryonic organogenesis and more specifically on the formation and growth of lateral roots (LR). The spacing of LR along the main root is underpinned by a precise prepatterning mechanism called the Root Clock. In Arabidopsis, the primary output of this mechanism involves the generation of periodic gene expression oscillations in a zone close to the root tip called the Oscillation Zone (OZ). Because of these oscillations, pre-branch sites (PBS) are established in the positions from which LR will emerge, although the oscillations can also possibly regulate the root wavy pattern and growth. Furthermore, we show that the Root Clock is present in LR. In this review, we describe the recent advances unraveling the inner machinery of Root Clock as well as the new tools to track the Root Clock activity. Moreover, we discuss the basis of how Arabidopsis can balance the creation of a repetitive pattern while integrating both endogenous and exogenous signals to adapt to changing environmental conditions. These signals can work as entrainment signals, but in occasions they also affect the periodicity and amplitude of the oscillatory dynamics in gene expression. Finally, we identify similarities with the Segmentation Clock of vertebrates and postulate the existence of a determination front delimiting the end of the oscillations in gene expression and initiating LR organogenesis through the activation of PBS in an ARF7 dependent-manner.

Uncovering New Insights and Misconceptions on the Effectiveness of Phosphate Solubilizing Rhizobacteria in Plants: A Meta-Analysis

As the awareness on the ecological impact of chemical phosphate fertilizers grows, research turns to sustainable alternatives such as the implementation of phosphate solubilizing bacteria (PSB), which make largely immobile phosphorous reserves in soils available for uptake by plants. In this review, we introduce the mechanisms by which plants facilitate P-uptake and illustrate how PSB improve the bioavailability of this nutrient. Next, the effectiveness of PSB on increasing plant biomass and P-uptake is assessed using a meta-analysis approach. Our review demonstrates that improved P-uptake does not always translate in improved plant height and biomass. We show that the effect of PSB on plants does not provide an added benefit when using bacterial consortia compared to single strains. Moreover, the commonly reported species for P-solubilization, Bacillus spp. and Pseudomonas spp., are outperformed by the scarcely implemented Burkholderia spp. Despite the similar responses to PSB in monocots and eudicots, species responsiveness to PSB varies within both clades. Remarkably, the meta-analysis challenges the common belief that PSB are less effective under field conditions compared to greenhouse conditions. This review provides innovative insights and identifies key questions for future research on PSB to promote their implementation in agriculture.

Electrical impedance spectroscopy (EIS) in plant roots research: a review

Nondestructive testing of plant roots is a hot topic in recent years. The traditional measurement process is time-consuming and laborious, and it is impossible to analyze the state of plant roots without destroying the sample. Recent studies have shown that as an excellent nondestructive measurement method, although electrical impedance spectroscopy (EIS) has made great achievements in many botanical research fields such as plant morphology and stress resistance, there are still limitations. This review summarizes the application of EIS in plant root measurement. The experiment scheme, instrument and electrode, excitation frequency range, root electrical characteristics, equivalent circuit, and combination of EIS and artificial intelligence (AI) are discussed. Furthermore, the review suggests that future research should focus on miniaturization of measurement equipment, standardization of planting environment and intelligentization of root diagnosis, so as to better apply EIS technology to in situ root nondestructive measurement.

CropPAL for discovering divergence in protein subcellular location in crops to support strategies for molecular crop breeding

Agriculture faces increasing demand for yield, higher plant-derived protein content and diversity while facing pressure to achieve sustainability. Although the genomes of many of the important crops have been sequenced, the subcellular locations of most of the encoded proteins remain unknown or are only predicted. Protein subcellular location is crucial in determining protein function and accumulation patterns in plants, and is critical for targeted improvements in yield and resilience. Integrating location data from over 800 studies for 12 major crop species into the cropPAL2020 data collection showed that while >80% of proteins in most species are not localised by experimental data, combining species data or integrating predictions can help bridge gaps at similar accuracy. The collation and integration of over 61 505 experimental localisations and more than 6 million predictions showed that the relative sizes of the protein catalogues located in different subcellular compartments are comparable between crops and Arabidopsis. A comprehensive cross-species comparison showed that between 50% and 80% of the subcellulomes are conserved across species and that conservation only depends to some degree on the phylogenetic relationship of the species. Protein subcellular locations in major biosynthesis pathways are more often conserved than in metabolic pathways. Underlying this conservation is a clear potential for subcellular diversity in protein location between species by means of gene duplication and alternative splicing. Our cropPAL data set and search platform (https://crop-pal.org) provide a comprehensive subcellular proteomics resource to drive compartmentation-based approaches for improving yield, protein composition and resilience in future crop varieties.

Benefits of organic manure combined with biochar amendments to cotton root growth and yield under continuous cropping systems in Xinjiang, China

Organic manure and biochar amendments have been used in agriculture to improve soil fertility and enhance crop productivity. Plant roots play an important role in the functionality of individual plants, and although the addition of organic manure and biochar reportedly affect roots, it remains unclear how root morphology and physiology respond. We conducted a field experiment to test the hypothesis that organic manure combined with biochar amendment could also enhance the productivity of continuous cropping systems in Xinjiang cotton plantations. Different levels of organic manure and biochar were applied. Organic manure and biochar significantly affected root morphology and physiology by improving soil nutrients. In the absence of biochar, organic manure amendment increased Root TTC reducing capacity, glutamine synthetase and nitrate reductase activity. Furthermore, morphological and physiological parameters peaked with 6% organic manure combined with 1% biochar. A significant increase in root physiology was recognized with an increase in soil nutrient content at the bud stage and a negative relationship between root physiology and soil total K content at the harvesting stage. Thus, our results indicate that organic manure combined with biochar positively influenced cotton roots, and therefore should be used to improve root health in continuous cropping systems.

The dynamic nature and regulation of the root clock

Plants explore the soil by continuously expanding their root system, a process that depends on the production of lateral roots (LRs). Sites where LRs can be produced are specified in the primary root axis through a pre-patterning mechanism, determined by a biological clock that is coordinated by temporal signals and positional cues. This ‘root clock’ generates an oscillatory signal that is translated into a developmental cue to specify a set of founder cells for LR formation. In this Review, we summarize recent findings that shed light on the mechanisms underlying the oscillatory signal and discuss how a periodic signal contributes to the conversion of founder cells into LR primordia. We also provide an overview of the phases of the root clock that may be influenced by endogenous factors, such as the plant hormone auxin, and by exogenous environmental cues. Finally, we discuss additional aspects of the root-branching process that act independently of the root clock.

Cadmium stress suppresses lateral root formation by interfering with the root clock

A biological clock activated by oscillating signals, known as root clock, has been linked to lateral root (LR) formation and is essential for regular LR spacing along the primary root. However, it remains unclear how this internal mechanism is influenced by environmental factors known to affect the LR pattern. Here, we report that excessive cadmium (Cd) inhibits LR formation by disrupting the lateral root cap (LRC)-programmed cell death (PCD)-regulated root clock. Cd restricts the frequency of the oscillating signal rather than its amplitude. This could be attributed to the inhibition on meristematic activity by Cd, which resulted in decreased LRC cell number and LRC-PCD frequency. Genetic evidence further showed that LRC cell number is positively correlated with root resistance to Cd. Our study reveals root cap dynamics as a novel mechanism mediating root responses to Cd, providing insight into the signalling pathways of the root clock responding to environmental cues.

Plasticity of Lateral Root Branching in Maize

Extensively branched root systems can efficiently capture soil resources by increasing their absorbing surface in soil. Lateral roots are the roots formed from pericycle cells of other roots that can be of any type. As a consequence, lateral roots provide a higher surface to volume ratio and are important for water and nutrients acquisition. Discoveries from recent studies have started to shed light on how plant root systems respond to environmental changes in order to improve capture of soil resources. In this Mini Review, we will mainly focus on the spatial distribution of lateral roots of maize and their developmental plasticity in response to the availability of water and nutrients.

Adaptive minirhizotron for pepper roots observation and its installation based on root system architecture traits

BACKGROUND

Root is the principal part of plants to absorb water and nutrient, anchor the plant and affect yield and quality. Non-destructive detection of root traits is difficult to perform due to the hidden nature of the root. Therefore, improved methods to measure roots are necessary to support plant breeding, and optimization of cultivation and management. In this study, we present an adaptive minirhizotron along with installation patterns to focus on micro and local changes in multipoint of pepper roots.

RESULTS

The method is to improve minirhizotron by reducing its size to a microrhizotron (1.1 × 1.1 × 1.2 cm) and improving installation validity and rationality according to spatial distribution characteristics of Capsicum annuum root system. This adaptive minirhizotron could acquire root image in multipoint, and through image processing, root traits such as root length (including very fine roots or root hairs) and root width could be calculated. In order to install the microrhizotron reasonably and effectively, root system architecture (Capsicum annuum) was reconstructed using a three-dimensional caliper, and were quantified in circumferential distribution, vertical direction and root extension. The results showed that most lateral roots were constrained to 45° in horizontal direction to root initial position; Vertical angles were large, almost perpendicular to the root center line at initial position, and it became smaller when roots began to deepen. Root length density decreased with the increase of distance to plant center. According to Capsicum annuum root system traits, totally 8 installation methods were tested and verified to determine high probability of root interception. Horizontal angle 45° showed much higher interception probability than that of 90°. Vertical angle 45° has slightly higher root interception probability than that of 30°. Installation pattern horizontal angle 45° + radius 30 mm + vertical angle 45° showed the best performance in root interception with probability of 96.7%, followed by pattern horizontal angle 45° + radius 30 mm + vertical angle 30°. Comparison experiment showed that when root hair and very fine root were excluded, relative error was 12.1% between microrhizotron and soil sampling in root length, and 15.4% in root diameter. Microrhizotron was able to observe fine roots about 0.1 mm in diameter.

CONCLUSION

A new adaptive minirhizotron has been established for nondestructive observation on local and micro changes of roots in multipoint, and its application and installation patterns has been suggested according to root architecture traits. The microrhizotron can be used to study a wide range of research questions focused on quantitative trait locus analysis, root width changes, and root hair growth.