A role for auxin redistribution in the responses of the root system architecture to phosphate starvation in Arabidopsis

A microRNA528-ZmLac3 module regulates low phosphate tolerance in maize

MicroRNAs are known to play a crucial role in plant development and physiology and become a target for investigating the regulatory mechanism underlying plant low phosphate tolerance. ZmmiR528 has been shown to display significantly different expression levels between wild-type and low Pi-tolerant maize mutants. However, its functional role in maize low Pi tolerance remains unknown. In the present study, we studied the role and underlying molecular mechanism of miR528 in maize with low Pi tolerance. Overexpression of ZmmiR528 in maize resulted in impaired root growth, reduced Pi uptake capacity and compromised resistance to Pi deficiency. By contrast, transgenic maize plants suppressing ZmmiR528 expression showed enhanced low Pi tolerance. Furthermore, ZmLac3 and ZmLac5 which encode laccase were identified and verified as targets of ZmmiR528. ZmLac3 transgenic plants were subsequently generated and were also found to play key roles in regulating maize root growth, Pi uptake and low Pi tolerance. Furthermore, auxin transport was found to be potentially involved in ZmLac3-mediated root growth. Moreover, we conducted genetic complementary analysis through the hybridization of ZmmiR528 and ZmLac3 transgenic plants and found a favorable combination with breeding potential, namely anti-miR528:ZmLac3OE hybrid maize, which exhibited significantly increased low Pi tolerance and markedly alleviated yield loss caused by low Pi stress. Our study has thus identified a ZmmiR528-ZmLac3 module regulating auxin transport and hence root growth, thereby determining Pi uptake and ultimately low Pi tolerance, providing an effective approach for low Pi tolerance improvement through manipulating the expression of miRNA and its target in maize.

Exploring Phosphate Solubilizing Bacterial Communities in Rhizospheres of Native and Exotic Forage Grasses in Alkaline-Sodic Soils of the Flooding Pampa

The flooding pampa is one of the most important cattle-raising regions in Argentina. In this region, natural pastures are dominated by low-productivity native grass species, which are the main feed for livestock. In this context, previous studies in the region with the subtropical exotic grass Panicum coloratum highlight it as a promising species to improve pasture productivity. Cultivable phosphate solubilizing bacteria (PSB) communities associated to native (Sporobolus indicus) and exotic (Panicum coloratum) forage grasses adapted to alkaline-sodic soils of the flooding pampa were analyzed. PSB represented 2-14% of cultivable rhizobacteria and Box-PCR fingerprinting revealed a high genetic diversity in both rhizospheres. Taxonomic identification by MALDI-TOF showed that PSB populations of P. coloratum and S. indicus rhizospheres are dominated by the phylum Proteobacteria (92,51% and 96,60% respectively) and to a lesser extent (< 10%), by the phyla Actinobacteria and Firmicutes. At the genus level, both PSB populations were dominated by Enterobacter and Pseudomonas. Siderophore production, nitrogen fixation, and indoleacetic acid production were detected in a variety of PSB genera of both plant species. A higher proportion of siderophore and IAA producers were associated to P. coloratum than S. indicus, probably reflecting a greater dependence of the exotic species on rhizospheric microorganisms to satisfy its nutritional requirements in the soils of the flooding pampa. This work provides a novel knowledge about functional groups of bacteria associated to plants given that there are no previous reports dedicated to the characterization of PSB rhizosphere communities of S indicus and P coloratum. Finally, it should be noted that the collection obtained in this study can be useful for the development of bioinputs that allow reducing the use of chemical fertilizers, providing sustainability to pasture production systems for livestock.

Soybean type-B response regulator GmRR1 mediates phosphorus uptake and yield by modifying root architecture

Phosphorus (P) plays a pivotal role in plant growth and development. Low P stress can greatly hamper plant growth. Here, we identified a QTL (named QPH-9-1), which is associated with P efficiency across multiple environments through linkage analysis and genome-wide association study. Furthermore, we successfully cloned the underlying soybean (Glycine max) gene GmRR1 (a soybean type-B Response Regulator 1) that encodes a type-B response regulator protein. Knockout of GmRR1 resulted in a substantial increase in plant height, biomass, P uptake efficiency, and yield-related traits due to the modification of root structure. In contrast, overexpression of GmRR1 in plants resulted in a decrease in these phenotypes. Further analysis revealed that knockout of GmRR1 substantially increased the levels of auxin and ethylene in roots, thereby promoting root hair formation and growth by promoting the formation of root hair primordium and lengthening the root apical meristem. Yeast two-hybrid, bimolecular fluorescence complementation, and dual-luciferase assays demonstrated an interaction between GmRR1 and Histidine-containing Phosphotransmitter protein 1. Expression analysis suggested that these proteins coparticipated in response to low P stress. Analysis of genomic sequences showed that GmRR1 underwent a selection during soybean domestication. Taken together, this study provides further insights into how plants respond to low P stress by modifying root architecture through phytohormone pathways.

Potassium transporter KUP9 regulates plant response to K+ deficiency and affects carbohydrate allocation in A.thaliana

Due to the essential roles of K+ in plants, its up to 10% share in plant dry matter, and its mostly low availability in soil, effective potassium management poses a significant challenge for the plant. To enable efficient uptake and allocation of K+, numerous transporters and channels have evolved. During the last two decades, efforts have been made to characterise these transport proteins in Arabidopsis thaliana using knock-out mutants. Several KT/HAK/KUP transporters have been assigned specific functions. In this work, we contribute to an understanding of the role of AtKUP9 in plant adaptation to low K+ availability. We found that in vitro, atkup9 has reduced lateral root growth under low-K conditions, and root apical meristem proliferation is reduced in lateral roots compared with the primary root. We also documented AtKUP9 expression in both roots and shoots and showed that AtKUP9 expression is modulated during plant ontogeny and as a result of K+ deprivation. Altered carbohydrate allocation was also documented in atkup9. Mutants exported more soluble saccharides from leaves under K+ rich conditions and, under K+ deficiency, atkup9 accumulated more soluble saccharides in the shoots. A possible role of AtKUP9 in these processes is discussed.

Genetic regulation of lateral root development

Lateral roots (LRs) are an important part of plant root systems. In dicots, for example, after plants adapted from aquatic to terrestrial environments, filamentous pseudorhizae evolved to allow nutrient absorption. A typical plant root system comprises a primary root, LRs, root hairs, and a root cap. Classical plant roots exhibit geotropism (the tendency to grow downward into the ground) and can synthesize plant hormones and other essential substances. Root vascular bundles and complex spatial structures enable plants to absorb water and nutrients to meet their nutrient quotas and grow. The primary root carries out most functions during early growth stages but is later overtaken by LRs, underscoring the importance of LR development water and mineral uptake and the soil fixation capacity of the root. LR development is modulated by endogenous plant hormones and external environmental factors, and its underlying mechanisms have been dissected in great detail in Arabidopsis, thanks to its simple root anatomy and the ease of obtaining mutants. This review comprehensively and systematically summarizes past research (largely in Arabidopsis) on LR basic structure, development stages, and molecular mechanisms regulated by different factors, as well as future prospects in LR research, to provide broad background knowledge for root researchers.

Arabidopsis PDE1 confers phosphate-deficiency tolerance in primary root growth

KEY MESSAGE

PDE1 acts as a mediator of primary root growth in response to Pi deficiency. Phosphorus is commonly considered as a limiting nutrient for plant growth, which is mainly due to the immobility and uneven distribution of phosphate (Pi) in soils so that available Pi is not adequate in the rhizosphere. Although various mediators have been identified in Pi sensing and response, more details need to be uncovered in plant Pi-deficiency tolerance. Here, we isolated a mutant, termed pde1 (phosphate-deficiency sensitive 1), showing the hypersensitive Pi-deficiency-induced growth inhibition of primary roots. PDE1 encodes a hydroxyphenylpyruvate reductase with rare activity in vitro and repressed by Pi deficiency. Histochemical analysis displayed that Pi-deprived pde1 accumulated more Fe and reactive oxygen species (ROS) in primary roots than the wild type (WT). Addition of ferrozine, a Fe2+ chelator, or a ROS scavenger (e.g., thiourea and potassium iodide), alleviated the sensitivity of Pi-deficiency in pde1 primary roots. By contrast, pde1 showed reduced cotyledon expansion rate with treatment of H2O2 compared to WT. Taken together, these results suggested that PDE1 is responsible for regulating primary root growth in response to Pi deficiency, which is associated with ROS.

A meta-QTL analysis highlights genomic hotspots associated with phosphorus use efficiency in rice (Oryza sativa L.)

Phosphorus use efficiency (PUE) is a complex trait, governed by many minor quantitative trait loci (QTLs) with small effects. Advances in molecular marker technology have led to the identification of QTLs underlying PUE. However, their practical use in breeding programs remains challenging due to the unstable effects in different genetic backgrounds and environments, interaction with soil status, and linkage drag. Here, we compiled PUE QTL information from 16 independent studies. A total of 192 QTLs were subjected to meta-QTL (MQTL) analysis and were projected into a high-density SNP consensus map. A total of 60 MQTLs, with significantly reduced number of initial QTLs and confidence intervals (CI), were identified across the rice genome. Candidate gene (CG) mining was carried out for the 38 MQTLs supported by multiple QTLs from at least two independent studies. Genes related to amino and organic acid transport and auxin response were found to be abundant in the MQTLs linked to PUE. CGs were cross validated using a root transcriptome database (RiceXPro) and haplotype analysis. This led to the identification of the eight CGs (OsARF8, OsSPX-MFS3, OsRING141, OsMIOX, HsfC2b, OsFER2, OsWRKY64, and OsYUCCA11) modulating PUE. Potential donors for superior PUE CG haplotypes were identified through haplotype analysis. The distribution of superior haplotypes varied among subspecies being mostly found in indica but were largely scarce in japonica. Our study offers an insight on the complex genetic networks that modulate PUE in rice. The MQTLs, CGs, and superior CG haplotypes identified in our study are useful in the combination of beneficial alleles for PUE in rice.

Phosphorus Plays Key Roles in Regulating Plants' Physiological Responses to Abiotic Stresses

Phosphorus (P), an essential macronutrient, plays a pivotal role in the growth and development of plants. However, the limited availability of phosphorus in soil presents significant challenges for crop productivity, especially when plants are subjected to abiotic stresses such as drought, salinity and extreme temperatures. Unraveling the intricate mechanisms through which phosphorus participates in the physiological responses of plants to abiotic stresses is essential to ensure the sustainability of agricultural production systems. This review aims to analyze the influence of phosphorus supply on various aspects of plant growth and plant development under hostile environmental conditions, with a special emphasis on stomatal development and operation. Furthermore, we discuss recently discovered genes associated with P-dependent stress regulation and evaluate the feasibility of implementing P-based agricultural practices to mitigate the adverse effects of abiotic stress. Our objective is to provide molecular and physiological insights into the role of P in regulating plants' tolerance to abiotic stresses, underscoring the significance of efficient P use strategies for agricultural sustainability. The potential benefits and limitations of P-based strategies and future research directions are also discussed.

A Review: Systemic Signaling in the Regulation of Plant Responses to Low N, P and Fe

Plant signal transduction occurs in response to nutrient element deficiency in plant vascular tissue. Recent works have shown that the vascular tissue is a central regulator in plant growth and development by transporting both essential nutritional and long-distance signaling molecules between different parts of the plant's tissues. Split-root and grafting studies have deciphered the importance of plants' shoots in receiving root-derived nutrient starvation signals from the roots. This review assesses recent studies about vascular tissue, integrating local and systemic long-distance signal transduction and the physiological regulation center. A substantial number of studies have shown that the vascular tissue is a key component of root-derived signal transduction networks and is a regulative center involved in plant elementary nutritional deficiency, including nitrogen (N), phosphate (P), and iron (Fe).

Comparative proteome analysis of phosphorus-responsive genotypes reveals the proteins differentially expressed under phosphorous starvation stress in rice

Phosphorus (P)-deficiency is one of the major nutrient constraints for global rice production. P-deficiency tolerance in rice involves complex regulatory mechanisms. To gain insights into the proteins involved in phosphorus acquisition and use efficiency in rice, proteome analysis of a high-yielding rice cultivar Pusa-44 and its near-isogenic line (NIL)-23 harboring a major phosphorous uptake (Pup1) QTL, grown under control and P-starvation stress, was performed. Comparative proteome profiling of shoot and root tissues from the plants grown hydroponically with P (16 ppm, +P) or without P (0 ppm, -P) resulted in the identification of 681 and 567 differentially expressed proteins (DEPs) in shoot of Pusa-44 and NIL-23, respectively. Similarly, 66 and 93 DEPs were identified in root of Pusa-44 and NIL-23, respectively. These P-starvation responsive DEPs were annotated to be involved in metabolic processes like photosynthesis, starch-, sucrose-, energy-metabolism, transcription factors (mainly ARF, ZFP, HD-ZIP, MYB), and phytohormone signaling. Comparative analysis of the expression patterns observed by proteome analysis with that reported at the transcriptome level indicated the Pup1 QTL-mediated post-transcriptional regulation plays an important role under -P stress. Thus, the present study describes the molecular aspect of the regulatory functions of Pup1 QTL under P-starvation stress in rice, which might help develop an efficient rice cultivar with enhanced P acquisition and assimilation for better performance in P-deficient soil.

Comparative transcriptome analysis reveals the phosphate starvation alleviation mechanism of phosphate accumulating Pseudomonas putida in Arabidopsis thaliana

Phosphate starvation is one of the major factors limiting plant productivity globally. Soil microflora with an inherent trait of phosphate accumulation directly influences soil phosphorus level by regulating its labile form in soil solution. However, the detailed mechanism involved during their interaction with plants under phosphate deficient conditions is still unexplored. Hence, to dissect these complex gene regulatory networks, transcriptome analysis of A. thaliana roots grown under phosphate starved conditions in presence of phosphate accumulating bacteria (Pseudomonas putida; RAR) was performed. Plants grown under phosphate starved conditions showed upregulation of phosphate starvation responsive genes associated with cell biogenesis, stress, photosynthesis, senescence, and cellular transport. Inoculation of RAR upregulated genes linked to defense, cell wall remodeling, and hormone metabolism in stressed plants. Gene ontology analysis indicated the induction of S-glycoside, glucosinolate, and glycosinolate metabolic processes in RAR inoculated plants under phosphate stressed conditions. Further, protein-protein interaction analysis revealed upregulation of root development, cation transport, anion transport, sulfur compound metabolic process, secondary metabolic process, cellular amino metabolic process, and response to salicylic acid in RAR inoculated plants under phosphate starved conditions. These results indicate the potential role of phosphate accumulating bacteria in alleviating phosphate starvation in plants by involving multiple pathways.

Phenotypes and Molecular Mechanisms Underlying the Root Response to Phosphate Deprivation in Plants

Phosphorus (P) is an essential macronutrient for plant growth. The roots are the main organ for nutrient and water absorption in plants, and they adapt to low-P soils by altering their architecture for enhancing absorption of inorganic phosphate (Pi). This review summarizes the physiological and molecular mechanisms underlying the developmental responses of roots to Pi starvation, including the primary root, lateral root, root hair, and root growth angle, in the dicot model plant Arabidopsis thaliana and the monocot model plant rice (Oryza sativa). The importance of different root traits and genes for breeding P-efficient roots in rice varieties for Pi-deficient soils are also discussed, which we hope will benefit the genetic improvement of Pi uptake, Pi-use efficiency, and crop yields.

ATL8, a RING E3 ligase, modulates root growth and phosphate homeostasis in Arabidopsis

Ubiquitination-mediated post-translational modification of proteins is a pivotal regulatory mechanism involved in the growth and development of the plant. The Arabidopsis Tóxicos en Levadura (ATL) family is a group of RING-type ubiquitin ligases (E3) and ATL8 is a membrane-localized protein. Here, a reverse genetics approach was used to elucidate the role of ATL8 in phosphate (Pi) homeostasis. Deficiencies of Pi and sucrose (Suc) enhanced the relative expression level of ATL8 in different tissues of the wild-type (Wt). The relative expression level of ATL8 was attenuated and augmented in the mutant (atl8) and overexpression lines (Oe1 and Oe2), respectively. There were significant reductions in different root traits, root hairs, root to shoot ratio, and total Pi content in atl8 compared with the Wt under different Pi regimes. On the contrary, Oe1 and Oe2 lines exhibited enhancement in some of these traits. Noticeably, anthocyanin content was significantly reduced in Oe1 and Oe2 compared with the Wt and atl8 under P- condition. Abscisic acid (ABA) treatment led to an increase in the primary root length of atl8 compared with the Wt, suggesting a cross-talk between ABA and ATL8 on root growth. Furthermore, the relative expression levels of the genes involved in the maintenance of Pi homeostasis (WRKY75, RNS1, E3L, and ACP5) were differentially modulated in atl8, Oe1, and Oe2 compared with the Wt under different Pi regimes. The results revealed the pivotal role of ATL8 in mediating morphophysiological and molecular adaptive responses to Pi deficiency.

The Root Hair Development of Pectin Polygalacturonase PGX2 Activation Tagging Line in Response to Phosphate Deficiency

Pectin, cellulose, and hemicellulose constitute the primary cell wall in eudicots and function in multiple developmental processes in plants. Root hairs are outgrowths of specialized epidermal cells that absorb water and nutrients from the soil. Cell wall architecture influences root hair development, but how cell wall remodeling might enable enhanced root hair formation in response to phosphate (P) deficiency remains relatively unclear. Here, we found that POLYGALACTURONASE INVOLVED IN EXPANSION 2 (PGX2) functions in conditional root hair development. Under low P conditions, a PGX2 activation tagged line (PGX2^AT ) displays bubble-like root hairs and abnormal callose deposition and superoxide accumulation in roots. We found that the polar localization and trafficking of PIN2 are altered in PGX2^AT roots in response to P deficiency. We also found that actin filaments were less compact but more stable in PGX2^AT root hair cells and that actin filament skewness in PGX2^AT root hairs was recovered by treatment with 1-N-naphthylphthalamic acid (NPA), an auxin transport inhibitor. These results demonstrate that activation tagging of PGX2 affects cell wall remodeling, auxin signaling, and actin microfilament orientation, which may cooperatively regulate root hair development in response to P starvation.

Jasmonic acid is involved in root cell wall phosphorus remobilization through the nitric oxide dependent pathway in rice

Jasmonic acid (JA) is involved in phosphorus (P) stress in plants, but its underlying molecular mechanisms are still elusive. In this study, we found root endogenous JA content in rice increased under P deficiency (-P), suggesting that JA might participate in P homeostasis in plants. This hypothesis was further confirmed through the addition of exogenous JA (+JA), as this could increase both the root and shoot soluble P content through regulating root cell wall P reutilization. In addition, -P+JA treatment significantly induced the expression of P transporter gene OsPT2, together with increased xylem P content, implying that JA is also important for P translocation from the root to the shoot in P-deficient rice. Furthermore, the accumulation of the molecular signal nitric oxide (NO) was enhanced under -P+JA treatment when compared with -P treatment alone, while the addition of c-PTIO, a scavenger of NO, could reverse the P-deficient phenotype alleviated by JA. Taken together, our results reveal a JA-NO-cell wall P reutilization pathway under P deficiency in rice.

The functions of phospholipases and their hydrolysis products in plant growth, development and stress responses

Cell membranes are the initial site of stimulus perception from environment and phospholipids are the basic and important components of cell membranes. Phospholipases hydrolyze membrane lipids to generate various cellular mediators. These phospholipase-derived products, such as diacylglycerol, phosphatidic acid, inositol phosphates, lysophopsholipids, and free fatty acids, act as second messengers, playing vital roles in signal transduction during plant growth, development, and stress responses. This review focuses on the structure, substrate specificities, reaction requirements, and acting mechanism of several phospholipase families. It will discuss their functional significance in plant growth, development, and stress responses. In addition, it will highlight some critical knowledge gaps in the action mechanism, metabolic and signaling roles of these phospholipases and their products in the context of plant growth, development and stress responses.

Plant phosphate nutrition: sensing the stress

Phosphorus (P) is obtained by plants as phosphate (Pi) from the soil and low Pi levels affects plant growth and development. Adaptation to low Pi condition entails sensing internal and external Pi levels and translating those signals to molecular and morphophysiological changes in the plant. In this review, we present findings related to local and systemin Pi sensing with focus the molecular mechanisms behind root system architectural changes and the impact of hormones and epigenetic mechanisms affecting those changes. We also present some of the recent advances in the Pi sensing and signaling mechanisms focusing on inositol pyrophosphate InsP8 and its interaction with SPX domain proteins to regulate the activity of the central regulator of the Pi starvation response, PHR.

Modeling Auxin Signaling in Roots: Auxin Computations

Auxin signaling and patterning is an inherently complex process, involving polarized auxin transport, metabolism, and signaling, its effect on developmental zones, as well as growth rates, and the feedback between all these different aspects. This complexity has led to an important role for computational modeling in unraveling the multifactorial roles of auxin in plant developmental and adaptive processes. Here we discuss the basic ingredients of auxin signaling and patterning models for root development as well as a series of key modeling studies in this area. These modeling studies have helped elucidate how plants use auxin signaling to compute the size of their root meristem, the direction in which to grow, and when and where to form lateral roots. Importantly, these models highlight how auxin, through patterning of and collaborating with other factors, can fulfill all these roles simultaneously.

A phosphorus-limitation induced, functionally conserved DUF506 protein is a repressor of root hair elongation in plants

Root hairs (RHs) function in nutrient and water acquisition, root metabolite exudation, soil anchorage and plant-microbe interactions. Longer or more abundant RHs are potential breeding traits for developing crops that are more resource-use efficient and can improve soil health. While many genes are known to promote RH elongation, relatively little is known about genes and mechanisms that constrain RH growth. Here we demonstrate that a DOMAIN OF UNKNOWN FUNCTION 506 (DUF506) protein, AT3G25240, negatively regulates Arabidopsis thaliana RH growth. The AT3G25240 gene is strongly and specifically induced during phosphorus (P)-limitation. Mutants of this gene, which we call REPRESSOR OF EXCESSIVE ROOT HAIR ELONGATION 1 (RXR1), have much longer RHs, higher phosphate content and seedling biomass, while overexpression of the gene exhibits opposite phenotypes. Co-immunoprecipitation, pull-down and bimolecular fluorescence complementation (BiFC) analyses reveal that RXR1 physically interacts with a RabD2c GTPase in nucleus, and a rabd2c mutant phenocopies the rxr1 mutant. Furthermore, N-terminal variable region of RXR1 is crucial for inhibiting RH growth. Overexpression of a Brachypodium distachyon RXR1 homolog results in repression of RH elongation in Brachypodium. Taken together, our results reveal a novel DUF506-GTPase module with a prominent role in repression of plant RH elongation especially under P stress.

Mapping of the Quantitative Trait Loci and Candidate Genes Associated With Iron Efficiency in Maize

Iron (Fe) is a mineral micronutrient for plants, and Fe deficiency is a major abiotic stress in crop production because of its low solubility under aerobic and alkaline conditions. In this study, 18 maize inbred lines were used to preliminarily illustrate the physiological mechanism underlying Fe deficiency tolerance. Then biparental linkage analysis was performed to identify the quantitative trait loci (QTLs) and candidate genes associated with Fe deficiency tolerance using the recombinant inbred line (RIL) population derived from the most Fe-efficient (Ye478) and Fe-inefficient (Wu312) inbred lines. A total of 24 QTLs was identified under different Fe nutritional status in the Ye478 × Wu312 RIL population, explaining 6.1-26.6% of phenotypic variation, and ten candidate genes were identified. Plants have evolved two distinct mechanisms to solubilize and transport Fe to acclimate to Fe deficiency, including reduction-based strategy (strategy I) and chelation-based strategy (strategy II), and maize uses strategy II. However, not only genes involved in Fe homeostasis verified in strategy II plants (strategy II genes), which included ZmYS1, ZmYS3, and ZmTOM2, but also several genes associated with Fe homeostasis in strategy I plants (strategy I genes) were identified, including ZmFIT, ZmPYE, ZmILR3, ZmBTS, and ZmEIN2. Furthermore, strategy II gene ZmYS1 and strategy I gene ZmBTS were significantly upregulated in the Fe-deficient roots and shoots of maize inbred lines, and responded to Fe deficiency more in shoots than in roots. Under Fe deficiency, greater upregulations of ZmYS1 and ZmBTS were observed in Fe-efficient parent Ye478, not in Fe-inefficient parent Wu312. Beyond that, ZmEIN2 and ZmILR3, were found to be Fe deficiency-inducible in the shoots. These findings indicate that these candidate genes may be associated with Fe deficiency tolerance in maize. This study demonstrates the use of natural variation to identify important Fe deficiency-regulated genes and provides further insights for understanding the response to Fe deficiency stress in maize.

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.

Effects of Root Zone Warming on Maize Seedling Growth and Photosynthetic Characteristics Under Different Phosphorus Levels

Phosphorus content and root zone temperature are two major environmental factors affecting maize growth. Both low phosphorus and root zone high temperature stress significantly affect the growth of maize, but the comprehensive effects of phosphorus deficiency and root zone warming are less studied. This study aimed to explore the effects of phosphorus deficiency and root zone warming on the root absorption capacity, total phosphorus content, and photosynthetic fluorescence parameters of maize seedlings. The results showed that maize shoots and roots had different responses to root zone warming and phosphorus deficiency. Properly increasing the root zone temperature was beneficial to the growth of maize seedlings, but when the root zone temperature was too high, it significantly affected the root and shoot development of maize seedlings. The root zone warming had a more significant impact on the root system, while phosphorus deficiency had a greater impact on the shoots. Phosphorus content and root zone warming had a strong interaction. Under the comprehensive influence of normal phosphorus supply and medium temperature in the root zone, the growth of maize seedlings was the best. Under the combined effects of low phosphorus and high temperature in the root zone, the growth was the worst. Compared with the combination of normal phosphorus and root zone medium temperature treatment, the dry mass of the low-phosphorus root zone high temperature treatment was decreased by 55.80%. Under the condition of low-phosphorus too high root zone temperature reduced root vitality, plant phosphorus content, which in turn affected plant growth and light energy utilization efficiency. In the case of sufficient phosphate fertilizer supply, appropriately increasing the soil temperature in the root zone is beneficial to increase the absorption and utilization of phosphorus by plants and promote the growth and development of maize seedlings.

Diverse phosphate and auxin transport loci distinguish phosphate tolerant from sensitive Arabidopsis accessions

Phosphorus (P) is an essential element for plant growth often limiting agroecosystems. To identify genetic determinants of performance under variable phosphate (Pi) supply, we conducted genome-wide association studies on five highly predictive Pi starvation response traits in 200 Arabidopsis (Arabidopsis thaliana) accessions. Pi concentration in Pi-limited organs had the strongest, and primary root length had the weakest genetic component. Of 70 trait-associated candidate genes, 17 responded to Pi withdrawal. The PHOSPHATE TRANSPORTER1 gene cluster on chromosome 5 comprises PHT1;1, PHT1;2, and PHT1;3 with known impact on P status. A second locus featured uncharacterized endomembrane-associated auxin efflux carrier encoding PIN-LIKES7 (PILS7) which was more strongly suppressed in Pi-limited roots of Pi-starvation sensitive accessions. In the Col-0 background, Pi uptake and organ growth were impaired in both Pi-limited pht1;1 and two pils7 T-DNA insertion mutants, while Pi -limited pht1;2 had higher biomass and pht1;3 was indistinguishable from wild-type. Copy number variation at the PHT1 locus with loss of the PHT1;3 gene and smaller scale deletions in PHT1;1 and PHT1;2 predicted to alter both protein structure and function suggest diversification of PHT1 is a key driver for adaptation to P limitation. Haplogroup analysis revealed a phosphorylation site in the protein encoded by the PILS7 allele from stress-sensitive accessions as well as additional auxin-responsive elements in the promoter of the "stress tolerant" allele. The former allele's inability to complement the pils7-1 mutant in the Col-0 background implies the presence of a kinase signaling loop controlling PILS7 activity in accessions from P-rich environments, while survival in P-poor environments requires fine-tuning of stress-responsive root auxin signaling.

Lateral root formation and nutrients: nitrogen in the spotlight

Lateral roots are important to forage for nutrients due to their ability to increase the uptake area of a root system. Hence, it comes as no surprise that lateral root formation is affected by nutrients or nutrient starvation, and as such contributes to the root system plasticity. Understanding the molecular mechanisms regulating root adaptation dynamics toward nutrient availability is useful to optimize plant nutrient use efficiency. There is at present a profound, though still evolving, knowledge on lateral root pathways. Here, we aimed to review the intersection with nutrient signaling pathways to give an update on the regulation of lateral root development by nutrients, with a particular focus on nitrogen. Remarkably, it is for most nutrients not clear how lateral root formation is controlled. Only for nitrogen, one of the most dominant nutrients in the control of lateral root formation, the crosstalk with multiple key signals determining lateral root development is clearly shown. In this update, we first present a general overview of the current knowledge of how nutrients affect lateral root formation, followed by a deeper discussion on how nitrogen signaling pathways act on different lateral root-mediating mechanisms for which multiple recent studies yield insights.

OsPHR2 modulates phosphate starvation-induced OsMYC2 signalling and resistance to Xanthomonas oryzae pv. oryzae

Phosphate (Pi) and MYC2-mediated jasmonate (JA) pathway play critical roles in plant growth and development. In particular, crosstalk between JA and Pi starvation signalling has been reported to mediate insect herbivory resistance in dicot plants. However, its roles and mechanism in monocot-bacterial defense systems remain obscure. Here, we report that Pi starvation in rice activates the OsMYC2 signalling and enhances resistance to Xanthomonas oryzae pv. oryzae (Xoo) infection. The direct regulation of OsPHR2 on the OsMYC2 promoter was confirmed by yeast one-hybrid, electrophoretic mobility shift, dual-luciferase and chromatin immunoprecipitation assays. Molecular analyses and infection studies using OsPHR2-Ov1 and phr2 mutants further demonstrated that OsPHR2 enhances antibacterial resistance via transcriptional regulation of OsMYC2 expression, indicating a positive role of OsPHR2-OsMYC2 crosstalk in modulating the OsMYC2 signalling and Xoo infection. Genetic analysis and infection assays using myc2 mutants revealed that Pi starvation-induced OsMYC2 signalling activation and consequent Xoo resistance depends on the regulation of OsMYC2. Together, these results reveal a clear interlink between Pi starvation- and OsMYC2- signalling in monocot plants, and provide new insight into how plants balance growth and defence by integrating nutrient deficiency and phytohormone signalling. We highlighted a molecular link connecting OsMYC2-mediated JA pathway and phosphate starvation signalling in monocot plant. We demonstrated that phosphate starvation promoted OsMYC2 signalling to enhance rice defence to bacterial blight via transcriptional regulation of OsPHR2 on OsMYC2.

Metabolic Profiling and Metabolite Correlation Network Analysis Reveal That Fusarium solani Induces Differential Metabolic Responses in Lotus japonicus and Lotus tenuis against Severe Phosphate Starvation

Root fungal endophytes are essential mediators of plant nutrition under mild stress conditions. However, variations in the rhizosphere environment, such as nutrient depletion, could result in a stressful situation for both partners, shifting mutualistic to nonconvenient interactions. Mycorrhizal fungi and dark septate endophytes (DSEs) have demonstrated their ability to facilitate phosphate (Pi) acquisition. However, few studies have investigated other plant-fungal interactions that take place in the root environment with regard to phosphate nutrition. In the present research work, we aimed to analyze the effect of extreme Pi starvation and the fungal endophyte Fusarium solani on the model Lotus japonicus and the crop L. tenuis. We conducted metabolomics analysis based on gas chromatography-mass spectrometry (GC-MS) on plant tissues under optimal conditions, severe Pi starvation and F.solani presence. By combining statistical and correlation network analysis strategies, we demonstrated the differential outcomes of the two plant species against the combination of treatments. The combination of nutritional stress and Fusarium presence activated significant modifications in the metabolism of L. japonicus affecting the levels of sugars, polyols and some amino acids. Our results display potential markers for further inspection of the factors related to plant nutrition and plant-fungal interactions.

Dynamic Development of White Lupin Rootlets Along a Cluster Root

White lupin produces cluster roots in response to phosphorus deficiency. Along the cluster root, numerous short rootlets successively appear, creating a spatial and temporal gradient of developmental stages that constitutes a powerful biological model to study the dynamics of the structural and functional evolution of these organs. The present study proposes a fine histochemical, transcriptomic and functional analysis of the rootlet development from its emergence to its final length. Between these two stages, the tissue structures of the rootlets were observed, the course of transcript expressions for the genes differentially expressed was monitored and some physiological events linked to Pi nutrition were followed. A switch between (i) a growing phase, in which a normal apical meristem is present and (ii) a specialized phase for nutrition, in which the rootlet is completely differentiated, was highlighted. In the final stage of its determinate growth, the rootlet is an organ with a very active metabolism, especially for the solubilization and absorption of several nutrients. This work discusses how the transition between a growing to a determinate state in response to nutritional stresses is found in other species and underlines the fundamental dilemma of roots between soil exploration and soil exploitation.

Greater morphological and primary metabolic adaptations in roots contribute to phosphate-deficiency tolerance in the bread wheat cultivar Kenong199

BACKGROUND

Phosphate (Pi) deficiency severely affects crop growth and productivity, including wheat, therefore it is necessary to develop cultivars with enhanced Pi-deficiency tolerance. However, the underlying mechanism of Pi-deficiency tolerance in wheat is still elusive. Two contrasting wheat cultivars, low-Pi tolerant Kenong199 (KN199) and low-Pi sensitive Chinese Spring (CS) were used to reveal adaptations in response to Pi deficiency at the morphological, physiological, metabolic, and molecular levels.

RESULTS

KN199 was more tolerant to Pi deficiency than CS with significantly increased root biomass and R/S ratio. Root traits, the total root length, total root surface area, and total root volume, were remarkably enhanced by Pi deficiency in KN199. The shoot total P and soluble Pi concentrations of KN199 were significantly higher than those of CS, but not in roots. In KN199, high Pi level in shoots is a higher priority than that in roots under Pi deficiency. It was probably due to differentially regulation in the miR399-mediated signaling network between the shoots of the two cultivars. The Pi deficiency-induced root architecture adaptation in KN199 was attributed to the regulation of the hormone-mediated signaling (ethylene, gibberellin, and jasmonates). The expression of genes associated with root development and Pi uptake was enhanced in KN199. Some primary metabolites (amino acids and organic acids) were significantly accumulated in roots of KN199 under Pi deficiency.

CONCLUSIONS

The low-Pi tolerant wheat cultivar KN199 possessed greater morphological and primary metabolic adaptations in roots than CS under Pi deficiency. The adaption and the underlying molecular mechanisms in wheat provide a better understanding of the Pi-deficiency tolerance and the strategies for improving Pi efficiency in wheat.

The role of strigolactones in P deficiency induced transcriptional changes in tomato roots

BACKGROUND

Phosphorus (P) is an essential macronutrient for plant growth and development. Upon P shortage, plant responds with massive reprogramming of transcription, the Phosphate Starvation Response (PSR). In parallel, the production of strigolactones (SLs)-a class of plant hormones that regulates plant development and rhizosphere signaling molecules-increases. It is unclear, however, what the functional link is between these two processes. In this study, using tomato as a model, RNAseq was used to evaluate the time-resolved changes in gene expression in the roots upon P starvation and, using a tomato CAROTENOID CLEAVAGE DIOXYGENASES 8 (CCD8) RNAi line, what the role of SLs is in this.

RESULTS

Gene ontology (GO)-term enrichment and KEGG analysis of the genes regulated by P starvation and P replenishment revealed that metabolism is an important component of the P starvation response that is aimed at P homeostasis, with large changes occurring in glyco-and galactolipid and carbohydrate metabolism, biosynthesis of secondary metabolites, including terpenoids and polyketides, glycan biosynthesis and metabolism, and amino acid metabolism. In the CCD8 RNAi line about 96% of the PSR genes was less affected than in wild-type (WT) tomato. For example, phospholipid biosynthesis was suppressed by P starvation, while the degradation of phospholipids and biosynthesis of substitute lipids such as sulfolipids and galactolipids were induced by P starvation. Around two thirds of the corresponding transcriptional changes depend on the presence of SLs. Other biosynthesis pathways are also reprogrammed under P starvation, such as phenylpropanoid and carotenoid biosynthesis, pantothenate and CoA, lysine and alkaloids, and this also partially depends on SLs. Additionally, some plant hormone biosynthetic pathways were affected by P starvation and also here, SLs are required for many of the changes (more than two thirds for Gibberellins and around one third for Abscisic acid) in the gene expression.

CONCLUSIONS

Our analysis shows that SLs are not just the end product of the PSR in plants (the signals secreted by plants into the rhizosphere), but also play a major role in the regulation of the PSR (as plant hormone).

Root developmental responses to phosphorus nutrition

Phosphorus is an essential macronutrient for plant growth and development. Root system architecture (RSA) affects a plant's ability to obtain phosphate, the major form of phosphorus that plants uptake. In this review, I first consider the relationship between RSA and plant phosphorus-acquisition efficiency, describe how external phosphorus conditions both induce and impose changes in the RSA of major crops and of the model plant Arabidopsis, and discuss whether shoot phosphorus status affects RSA and whether there is a universal root developmental response across all plant species. I then summarize the current understanding of the molecular mechanisms governing root developmental responses to phosphorus deficiency. I also explore the possible reasons for the inconsistent results reported by different research groups and comment on the relevance of some studies performed under laboratory conditions to what occurs in natural environments.

A Network of Phosphate Starvation and Immune-Related Signaling and Metabolic Pathways Controls the Interaction between Arabidopsis thaliana and the Beneficial Fungus Colletotrichum tofieldiae

The beneficial root-colonizing fungus Colletotrichum tofieldiae mediates plant growth promotion (PGP) upon phosphate (Pi) starvation in Arabidopsis thaliana. This activity is dependent on the Trp metabolism of the host, including indole glucosinolate (IG) hydrolysis. Here, we show that C. tofieldiae resolves several Pi starvation-induced molecular processes in the host, one of which is the downregulation of auxin signaling in germ-free plants, which is restored in the presence of the fungus. Using CRISPR/Cas9 genome editing, we generated an Arabidopsis triple mutant lacking three homologous nitrilases (NIT1 to NIT3) that are thought to link IG-hydrolysis products with auxin biosynthesis. Retained C. tofieldiae-induced PGP in nit1/2/3 mutant plants demonstrated that this metabolic connection is dispensable for the beneficial activity of the fungus. This suggests that either there is an alternative metabolic link between IG-hydrolysis products and auxin biosynthesis, or C. tofieldiae restores auxin signaling independently of IG metabolism. We show that C. tofieldiae, similar to pathogenic microorganisms, triggers Arabidopsis immune pathways that rely on IG metabolism as well as salicylic acid and ethylene signaling. Analysis of IG-deficient myb mutants revealed that these metabolites are, indeed, important for control of in planta C. tofieldiae growth: however, enhanced C. tofieldiae biomass does not necessarily negatively correlate with PGP. We show that Pi deficiency enables more efficient colonization of Arabidopsis by C. tofieldiae, possibly due to the MYC2-mediated repression of ethylene signaling and changes in the constitutive IG composition in roots.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Auxin-Environment Integration in Growth Responses to Forage for Resources

Plant fitness depends on the adequate morphological adjustment to the prevailing conditions of the environment. Therefore, plants sense environmental cues through their life cycle, including the presence of full darkness, light, or shade, the range of ambient temperatures, the direction of light and gravity vectors, and the presence of water and mineral nutrients (such as nitrate and phosphate) in the soil. The environmental information impinges on different aspects of the auxin system such as auxin synthesis, degradation, transport, perception, and downstream transcriptional regulation to modulate organ growth. Although a single environmental cue can affect several of these points, the relative impacts differ significantly among the various growth processes and cues. While stability in the generation of precise auxin gradients serves to guide the basic developmental pattern, dynamic changes in the auxin system fine-tune body shape to optimize the capture of environmental resources.

Effects of Phosphate Shortage on Root Growth and Hormone Content of Barley Depend on Capacity of the Roots to Accumulate ABA

Although changes in root architecture in response to the environment can optimize mineral and water nutrient uptake, mechanisms regulating these changes are not well-understood. We investigated whether P deprivation effects on root development are mediated by abscisic acid (ABA) and its interactions with other hormones. The ABA-deficient barley mutant Az34 and its wild-type (WT) were grown in P-deprived and P-replete conditions, and hormones were measured in whole roots and root tips. Although P deprivation decreased growth in shoot mass similarly in both genotypes, only the WT increased primary root length and number of lateral roots. The effect was accompanied by ABA accumulation in root tips, a response not seen in Az34. Increased ABA in P-deprived WT was accompanied by decreased concentrations of cytokinin, an inhibitor of root extension. Furthermore, P-deficiency in the WT increased auxin concentration in whole root systems in association with increased root branching. In the ABA-deficient mutant, P-starvation failed to stimulate root elongation or promote branching, and there was no decline in cytokinin and no increase in auxin. The results demonstrate ABA’s ability to mediate in root growth responses to P starvation in barley, an effect linked to its effects on cytokinin and auxin concentrations.

Smart Root Search (SRS): A Novel Nature-Inspired Search Algorithm

In this paper, a novel heuristic search algorithm called Smart Root Search (SRS) is proposed. SRS employs intelligent foraging behavior of immature, mature and hair roots of plants to explore and exploit the problem search space simultaneously. SRS divides the search space into several subspaces. It thereupon utilizes the branching and drought operations to focus on richer areas of promising subspaces while extraneous ones are not thoroughly ignored. To achieve this, the smart reactions of the SRS model are designed to act based on analyzing the heterogeneous conditions of various sections of different search spaces. In order to evaluate the performance of the SRS, it was tested on a set of known unimodal and multimodal test functions. The results were then compared with those obtained using genetic algorithms, particle swarm optimization, differential evolution and imperialist competitive algorithms and then analyzed statistically. The results demonstrated that the SRS outperformed comparative algorithms for 92% and 82% of the investigated unimodal and multimodal test functions, respectively. Therefore, the SRS is a promising nature-inspired optimization algorithm.

Translation of Strigolactones from Plant Hormone to Agriculture: Achievements, Future Perspectives, and Challenges

Strigolactones (SLs) control plant development, enhance symbioses, and act as germination stimulants for some of the most destructive species of parasitic weeds, making SLs a potential tool to improve crop productivity and resilience. Field trials demonstrate the potential use of SLs as agrochemicals or genetic targets in breeding programs, with applications in improving drought tolerance, increasing yields, and controlling parasitic weeds. However, for effective translation of SLs into agriculture, understanding and exploiting SL diversity and the development of economically viable sources of SL analogs will be critical. Here we review how manipulation of SL signaling can be used when developing new tools and crop varieties to address some critical challenges, such as nutrient acquisition, resource allocation, stress tolerance, and plant-parasite interactions.

Phosphate Deprivation Can Impair Mechano-Stimulated Cytosolic Free Calcium Elevation in Arabidopsis Roots

The root tip responds to mechanical stimulation with a transient increase in cytosolic free calcium as a possible second messenger. Although the root tip will grow through a heterogeneous soil nutrient supply, little is known of the consequence of nutrient deprivation for such signalling. Here, the effect of inorganic phosphate deprivation on the root’s mechano-stimulated cytosolic free calcium increase is investigated. Arabidopsisthaliana (cytosolically expressing aequorin as a bioluminescent free calcium reporter) is grown in zero or full phosphate conditions, then roots or root tips are mechanically stimulated. Plants also are grown vertically on a solid medium so their root skewing angle (deviation from vertical) can be determined as an output of mechanical stimulation. Phosphate starvation results in significantly impaired cytosolic free calcium elevation in both root tips and whole excised roots. Phosphate-starved roots sustain a significantly lower root skewing angle than phosphate-replete roots. These results suggest that phosphate starvation causes a dampening of the root mechano-signalling system that could have consequences for growth in hardened, compacted soils.

Variable Light Condition Improves Root Distribution Shallowness and P Uptake of Soybean in Maize/Soybean Relay Strip Intercropping System

In this study, soybean root distribution in an inter-cropping system was influenced by various environmental and biotic cues. However, it is still unknown how root development and distribution in inter-cropping responds to aboveground light conditions. Herein, soybeans were inter- and monocropped with P (phosphorus) treatments of 0 and 20 kg P ha yr⁻¹ (P0 and P20, respectively) in field experiment over 4 years. In 2019, a pot experiment was conducted as the supplement to the field experiment. Shade from sowing to V5 (Five trifoliolates unroll) and light (SL) was used to imitate the light condition of soybeans in a relay trip inter-cropping system, while light then shade from V5 to maturity (LS) was used to imitate the light condition of soybeans when monocropped. Compared to monocropping, P uptake and root distribution in the upper 0–15 cm soil layer increased when inter-cropped. Inter-cropped soybeans suffered serious shade by maize during a common-growth period, which resulted in the inhibition of primary root growth and a modified auxin synthesis center and response. During the solo-existing period, plant photosynthetic capacity and sucrose accumulation increased under ameliorated light in SL (shade-light). Increased light during the reproductive stage significantly decreased leaf P concentration in SL under both P-sufficient and P-deficient conditions. Transcripts of a P starvation response gene (GmPHR25) in leaves and genes (GmEXPB2) involved in root growth were upregulated by ameliorated light during the reproductive stage. Furthermore, during the reproductive stage, more light interception increased the auxin concentration and expression of GmYUCCA14 (encoding the auxin synthesis) and GmTIR1C (auxin receptor) in roots. Across the field and pot experiments, increased lateral root growth and shallower root distribution were associated with inhibited primary root growth during the seedling stage and ameliorated light conditions in the reproductive stage. Consequently, this improved topsoil foraging and P uptake of inter-cropped soybeans. It is suggested that the various light conditions (shade-light) mediating leaf P status and sucrose transport can regulate auxin synthesis and respond to root formation and distribution.

Integrated analysis on biochemical profiling and transcriptome revealed nitrogen-driven difference in accumulation of saponins in a medicinal plant Panax notoginseng

The medicinal plant Panax notoginseng is considered a promising source of secondary metabolites due to its saponins. However, there are relatively few studies on the response of saponins to nitrogen (N) availability and the mechanisms underlying the N-driven regulation of saponins. Saponins content and saponins -related genes were analyzed in roots of P. notoginseng grown under low N (LN), moderate N (MN) and high N (HN). Saponins was obviously increased in LN individuals with a reduction in β-glucosidase activity. LN facilitated root architecture and N uptake rate. Compared with the LN individuals, 2872 and 1122 genes were incorporated into as differently expressed genes (DEGs) in the MN and HN individuals. Clustering and enrichment showed that DEGs related to "carbohydrate biosynthesis", "plant hormone signal transduction", "terpenoid backbone biosynthesis", "sesquiterpenoid and triterpenoid biosynthesis" were enriched. The up-regulation of some saponins-related genes and microelement transporters was found in LN plants. Whereas the expression of IPT3, AHK4 and GS2 in LN plants fell far short of that in HN ones. Anyways, LN-induced accumulation of C-based metabolites as saponins might derive from the interaction between N and phytohormones in processing of N acquisition, and HN-induced reduction of saponins might be result from an increase in the form of β-glucosidase activity and N-dependent cytokinins (CKs) biosynthesis.

A glutathione-dependent control of the indole butyric acid pathway supports Arabidopsis root system adaptation to phosphate deprivation

Root system architecture results from a highly plastic developmental process to adapt to environmental conditions. In particular, the development of lateral roots and root hair growth are constantly optimized to the rhizosphere properties, including biotic and abiotic constraints. The development of the root system is tightly controlled by auxin, the driving morphogenic hormone in plants. Glutathione, a major thiol redox regulator, is also critical for root development but its interplay with auxin is scarcely understood. Previous work showed that glutathione deficiency does not alter root responses to indole acetic acid (IAA), the main active auxin in plants. Because indole butyric acid (IBA), another endogenous auxinic compound, is an important source of IAA for the control of root development, we investigated the crosstalk between glutathione and IBA during root development. We show that glutathione deficiency alters lateral roots and root hair responses to exogenous IBA but not IAA. Detailed genetic analyses suggest that glutathione regulates IBA homeostasis or conversion to IAA in the root cap. Finally, we show that both glutathione and IBA are required to trigger the root hair response to phosphate deprivation, suggesting an important role for this glutathione-dependent regulation of the auxin pathway in plant developmental adaptation to its environment.

Selenite Uptake and Transformation in Rice Seedlings (Oryza sativa L.): Response to Phosphorus Nutrient Status

Selenite and phosphate share similar uptake mechanisms, as a phosphate transporter is involved in the selenite uptake process. However, the mechanism by which selenium (Se) transformation in plants is mediated by phosphorus (P) remains unclear. In this hydroponic study, the absorption, translocation, and biotransformation of Se in selenite-treated rice (Oryza sativa L.) seedlings were investigated under varying P nutrient status. The results showed that P-deficient cultivation increased the Se concentration in roots with Se-only treatment by 2.1 times relative to that of the P-normal condition. However, co-treating roots with additional P caused the Se concentration to decline by 20 and 73% compared to Se treatment alone under P-normal and P-deficient cultivation, respectively. A similar pattern was also observed in Se uptake by rice roots. With an Se-transfer factor elevated by 4.4 times, the shoot Se concentration was increased by 44% with additional P supply compared to the concentration under Se-only treatment of P deficiency; however, no significant differences were observed regarding P-normal cultivation. P deficiency increased the Se percentage by 28% within the cell wall, but reduced it by 60% in the soluble fraction of Se-only treated roots relative to that of the P-normal condition. Contrarily, compared with the Se-only treatment under P deficiency, additional P supply enhanced Se storage in the root soluble fraction by 1.3 times. The opposite tendency was observed for rice shoots. Moreover, P deficiency reduced the proportion of SeMet by 22%, but increased MeSeCys by 1.3 times in Se-only treated roots compared to those under the P-normal condition. Interestingly, MeSeCys was not detected when additional P was added to the two cultivation conditions. Unlike in the roots, only SeMet was generally detected in the rice shoots. The results demonstrate that the P nutrient status strongly affects the Se biofortification efficiency in rice seedlings by altering the Se subcellular distribution and speciation.

Same same, but different: growth responses of primary and lateral roots

The root system architecture describes the shape and spatial arrangement of roots within the soil. Its spatial distribution depends on growth and branching rates as well as directional organ growth. The embryonic primary root gives rise to lateral (secondary) roots, and the ratio of both root types changes over the life span of a plant. Most studies have focused on the growth of primary roots and the development of lateral root primordia. Comparably less is known about the growth regulation of secondary root organs. Here, we review similarities and differences between primary and lateral root organ growth, and emphasize particularly how external stimuli and internal signals differentially integrate root system growth.

A plant's diet, surviving in a variable nutrient environment

As primary producers, plants rely on a large aboveground surface area to collect carbon dioxide and sunlight and a large underground surface area to collect the water and mineral nutrients needed to support their growth and development. Accessibility of the essential nutrients nitrogen (N) and phosphorus (P) in the soil is affected by many factors that create a variable spatiotemporal landscape of their availability both at the local and global scale. Plants optimize uptake of the N and P available through modifications to their growth and development and engagement with microorganisms that facilitate their capture. The sensing of these nutrients, as well as the perception of overall nutrient status, shapes the plant's response to its nutrient environment, coordinating its development with microbial engagement to optimize N and P capture and regulate overall plant growth.

Phospholipase D-derived phosphatidic acid promotes root hair development under phosphorus deficiency by suppressing vacuolar degradation of PIN-FORMED2

Root hair development is crucial for phosphate absorption, but how phosphorus deficiency affects root hair initiation and elongation remains unclear. We demonstrated the roles of auxin efflux carrier PIN-FORMED2 (PIN2) and phospholipase D (PLD)-derived phosphatidic acid (PA), a key signaling molecule, in promoting root hair development in Arabidopsis thaliana under a low phosphate (LP) condition. Root hair elongation under LP conditions was greatly suppressed in pin2 mutant or under treatment with a PLDζ2-specific inhibitor, revealing that PIN2 and polar auxin transport and PLDζ2-PA are crucial in LP responses. PIN2 was accumulated and degraded in the vacuole under a normal phosphate (NP) condition, whereas its vacuolar accumulation was suppressed under the LP or NP plus PA conditions. Vacuolar accumulation of PIN2 was increased in pldζ2 mutants under LP conditions. Increased or decreased PIN2 vacuolar accumulation is not observed in sorting nexin1 (snx1) mutant, indicating that vacuolar accumulation of PIN2 is mediated by SNX1 and the relevant trafficking process. PA binds to SNX1 and promotes its accumulation at the plasma membrane, especially under LP conditions, and hence promotes root hair development by suppressing the vacuolar degradation of PIN2. We uncovered a link between PLD-derived PA and SNX1-dependent vacuolar degradation of PIN2 in regulating root hair development under phosphorus deficiency.

The auxin-inducible phosphate transporter AsPT5 mediates phosphate transport and is indispensable for arbuscule formation in Chinese milk vetch at moderately high phosphate supply

Phosphorus is a macronutrient that is essential for plant survival. Most land plants have evolved the ability to form a mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi, which enhances phosphate (Pi) acquisition. Modulation of Pi transporter systems is the master strategy used by mycorrhizal plants to adapt to ambient Pi concentrations. However, the specific functions of PHOSPHATE TRANSPORTER 1 (PHT1) genes, which are Pi transporters that are responsive to high Pi availability, are largely unknown. Here, we report that AsPT5, an Astragalus sinicus (Chinese milk vetch) member of the PHT1 gene family, is conserved across dicotyledons and is constitutively expressed in a broad range of tissues independently of Pi supply, but is remarkably induced by indole-3-acetic acid (auxin) treatment under moderately high Pi conditions. Subcellular localization experiments indicated that AsPT5 localizes to the plasma membrane of plant cells. Using reverse genetics, we showed that AsPT5 not only mediates Pi transport and remodels root system architecture but is also essential for arbuscule formation in A. sinicus under moderately high Pi concentrations. Overall, our study provides insight into the function of AsPT5 in Pi transport, AM development and the cross-talk between Pi nutrition and auxin signalling in mycorrhizal plants.

Heterogeneous phosphate supply influences maize lateral root proliferation by regulating auxin redistribution

BACKGROUND AND AIMS

Roots take up phosphorus (P) as inorganic phosphate (Pi). Enhanced root proliferation in Pi-rich patches enables plants to capture the unevenly distributed Pi, but the underlying control of root proliferation remains largely unknown. Here, the role of auxin in this response was investigated in maize (Zea mays).

METHODS

A split-root, hydroponics system was employed to investigate root responses to Pi supply, with one (heterogeneous) or both (homogeneous) sides receiving 0 or 500 μm Pi.

KEY RESULTS

Maize roots proliferated in Pi-rich media, particularly with heterogeneous Pi supply. The second-order lateral root number was 3-fold greater in roots of plants receiving a heterogeneous Pi supply than in roots of plants with a homogeneous Pi supply. Root proliferation in a heterogeneous Pi supply was inhibited by the auxin transporter inhibitor 1-N-naphthylphthalamic acid (NPA). The proliferation of lateral roots was accompanied by an enhanced auxin response in the apical meristem and vascular tissues at the root tip, as demonstrated in a DR5::RFP marker line.

CONCLUSIONS

It is concluded that the response of maize root morphology to a heterogeneous Pi supply is modulated by local signals of Pi availability and systemic signals of plant P nutritional status, and is mediated by auxin redistribution.

Lateral Roots: Random Diversity in Adversity

Lateral roots are essential for soil foraging and uptake of minerals and water. They feature a large morphological diversity that results from divergent primordia or root growth and development patterns. Besides a structured diversity, resulting from the hierarchical and developmental organization of root systems, there exists a random diversity, occurring between roots of similar age, of the same hierarchical order, and exposed to uniform conditions. The physiological bases and functional consequences of this random diversity are largely ignored. Here we review the evidence for such random diversity throughout the plant kingdom, present innovative approaches based on statistical modeling to account for such diversity, and set the list of its potential benefits in front of a variable and unpredictable soil environment.

Lateral Root Formation in Arabidopsis: A Well-Ordered LRexit

Lateral roots (LRs) are crucial for increasing the surface area of root systems to explore heterogeneous soil environments. Major advances have recently been made in the model plant arabidopsis (Arabidopsis thaliana) to elucidate the cellular basis of LR development and the underlying gene regulatory networks (GRNs) that control the morphogenesis of the new root organ. This has provided a foundation for understanding the sophisticated adaptive mechanisms that regulate how plants pattern their root branching to match the spatial availability of resources such as water and nutrients in their external environment. We review new insights into the molecular, cellular, and environmental regulation of LR development in arabidopsis.

Nitric oxide and plant mineral nutrition: current knowledge

Plants under conditions of essential mineral deficiency trigger signaling mechanisms that involve common components. Among these components, nitric oxide (NO) has been identified as a key participant in responses to changes in nutrient availability. Usually, nutrient imbalances affect the levels of NO in specific plant tissues, via modification of its rate of synthesis or degradation. Changes in the level of NO affect plant morphology and/or trigger responses associated with nutrient homeostasis, mediated by its interaction with reactive oxygen species, phytohormones, and through post-translational modification of proteins. NO-related events constitute an exciting field of research to understand how plants adapt and respond to conditions of nutrient shortage. This review summarizes the current knowledge on NO as a component of the multiple processes related to plant performance under conditions of deficiency in mineral nutrients, focusing on macronutrients such as nitrogen, phosphate, potassium, and magnesium, as well as micronutrients such as iron and zinc.

Tackling Plant Phosphate Starvation by the Roots

Plant responses to phosphate deprivation encompass a wide range of strategies, varying from altering root system architecture, entering symbiotic interactions to excreting root exudates for phosphorous release, and recycling of internal phosphate. These processes are tightly controlled by a complex network of proteins that are specifically upregulated upon phosphate starvation. Although the different effects of phosphate starvation have been intensely studied, the full extent of its contribution to altered root system architecture remains unclear. In this review, we focus on the effect of phosphate starvation on the developmental processes that shape the plant root system and their underlying molecular pathways.

Regulation of Ubiquitination Is Central to the Phosphate Starvation Response

As sessile organisms, plants have developed numerous strategies to overcome the limiting availability of the essential nutrient phosphate in nature. Recent studies reveal that post-translational modification (PTM) by ubiquitination is an important and central regulation mechanism in the plant phosphate starvation response (PSR). Ubiquitination precisely modulates the stability and trafficking of proteins in response to the heterogeneous phosphate supplement. Induction of autophagy provides novel insights into the molecular mechanisms under phosphate starvation. In this review, we present and discuss novel findings on the regulation of diverse PSRs through ubiquitination. Resolving these regulation mechanisms will pave the way to improve phosphate acquisition and utilization efficiency in crops.