Changes In Root Architecture Research Articles

Plants Under Stress: Exploring Physiological and Molecular Responses to Nitrogen and Phosphorus Deficiency

Nitrogen (N) and phosphorus (P) are essential mineral macronutrients critical for plant structure and function. Both contribute to processes ranging from cellular integrity to signal transduction. Since plants require these nutrients in high concentrations, replenishing them in soil often involves chemical fertilizers. However, the main source of P, rock phosphate, is non-renewable and in decline. N, second only to carbon, oxygen, and hydrogen in plant requirements, is vital for synthesizing proteins, nucleic acids, and plant pigments. Although N is available to plants through biological fixation or fertilizer application, the frequent application of N is not a sustainable solution due to environmental concerns like groundwater contamination and eutrophication. Plants have developed sophisticated mechanisms to adapt to nutrient deficiencies, such as changes in root architecture, local signaling, and long-distance signaling through the phloem. A dual deficiency of N and P is common in the field. In addition to individual N and P deficiency responses, this review also highlights some of the most recent discoveries in the responses of plants to the combined N and P deficiencies. Understanding the molecular and physiological responses in plants to mineral deficiency will help implement strategies to produce plants with high mineral use efficiency, leading to the reduced application of fertilizers, decreased mineral runoff, and improved environment.

Research on the Mechanisms of Phytohormone Signaling in Regulating Root Development.

Phytohormones are organic compounds produced in trace amounts within plants that regulate their physiological processes. Their physiological effects are highly complex and diverse. They influence processes ranging from cell division, elongation, and differentiation to plant germination and rooting. Therefore, phytohormones play a crucial regulatory role in plant growth and development. Recently, various studies have highlighted the role of PHs, such as auxin, cytokinin (CK), and abscisic acid (ABA), and newer classes of PHs, such as brassinosteroid (BR) and peptide hormone, in the plant responses toward environmental stresses. These hormones not only have distinct roles at different stages of plant growth but also interact to promote or inhibit each other, thus effectively regulating plant development. Roots are the primary organs for water and mineral absorption in plants. During seed germination, the radicle breaks through the seed coat and grows downward to form the primary root. This occurs because the root needs to quickly penetrate the soil to absorb water and nutrients, providing essential support for the plant's subsequent growth. Root development is a highly complex and precisely regulated process influenced by various signals. Changes in root architecture can affect the plant's ability to absorb nutrients and water, which in turn impacts crop yield. Thus, studying the regulation of root development is of great significance. Numerous studies have reported on the role of phytohormones, particularly auxins, in root regulation. This paper reviews recent studies on the regulation of root development by various phytohormones, both individually and in combination, providing a reference for researchers in this field and offering perspectives on future research directions for improving crop yields.

Auxin-Mediated Lateral Root Development in Root Galls of Cucumber under Meloidogyne incognita Stress.

Root-knot nematodes induce the formation of feeding sites within the host roots and the relocation of auxin into galls results in abnormal lateral root growth. Here, we analyzed the changes in cucumber root architecture under Meloidogyne incognita stress and the distribution of auxin in these morphological and molecular root changes. The number of root tips significantly decreased, and regression analysis showed a positive relationship between the size of root galls and the numbers of nematodes in galls compared with the lateral roots on galls, emphasizing the effect of nematode parasitism on root development. Data generated via a promoter-reporter system using the transgenic hairy root system first characterized the auxin distribution during nematode parasitism in cucumber. Using DR5:GUS staining of root galls, we further detected the expression of CsPIN1 and CsAUX1, which regulate polar auxin transport. The results showed that both CsPIN1 and CsAUX1 were induced in galls, and the relative expression of the two genes significantly increased at 21 DAI. The TIBA treatment, which can disrupt polar auxin transport inhibited the numbers of cucumber root tips and total length following increasing concentration gradients. Moreover, the numbers of galls were significantly affected by TIBA treatment, which showed the vital role of auxin during nematode parasitism. Our findings suggest that the transportation of auxin plays an important role during gall formation and induces cucumber lateral root development within nematode feeding sites.

Changes in root architecture and endogenous hormone levels in alfalfa under phosphorus stress

Changes in root architecture and endogenous hormone levels in alfalfa under phosphorus stress

ABSCISIC ACID INSENSITIVE 3 promotes auxin signalling by regulating SHY2 expression to control primary root growth in response to dehydration stress.

Plants combat dehydration stress through different strategies including root architectural changes. Here we show that when exposed to varying levels of dehydration stress, primary root growth in Arabidopsis is modulated by regulating root meristem activity. Abscisic acid (ABA) in concert with auxin signalling adjust primary root growth according to stress levels. ABSCISIC ACID INSENSITIVE 3 (ABI3), an ABA-responsive transcription factor, stands at the intersection of ABA and auxin signalling and fine-tunes primary root growth in response to dehydration stress. Under low ABA or dehydration stress, induction of ABI3 expression promotes auxin signalling by decreasing expression of SHY2, a negative regulator of auxin response. This further enhances the expression of auxin transporter gene PIN1 and cell cycle gene CYCB1;1, resulting in an increase in primary root meristem size and root length. Higher levels of dehydration stress or ABA repress ABI3 expression and promote ABSCISIC ACID INSENSITIVE 5 (ABI5) expression. This elevates SHY2 expression, thereby impairing primary root meristem activity and retarding root growth. Notably, ABI5 can promote SHY2 expression only in the absence of ABI3. Such ABA concentration-dependent expression of ABI3 therefore functions as a regulatory sensor of dehydration stress levels and orchestrates primary root growth by coordinating its downstream regulation.

Mycorrhizal signals promote root development dependent on LysM-receptor like kinases in rice

Roots play a fundamental role in plant growth and development, serving various functions, including anchoring, water absorption, nutrient uptake, and adaptation diverse environmental conditions, such as abiotic stresses and biotic interactions. Arbuscular mycorrhizal (AM) fungi release diffusible signaling molecules known as mycorrhizal factors (Myc factors) to establish communication with plants. Extensive research has established that Myc factors play a pivotal role in orchestrating root architectural changes before fungal colonization occurs. In this study, we investigate the impact of the Myc factor CO4 on the architectural modifications of rice roots. Our findings reveal that CO4 actively promotes the development of crown roots and lateral roots in wild-type rice plants. Furthermore, we have identified that pivotal role of receptors such as OsCERK1, OsMYR1, and OsCEBiP in mediating the stimulatory effects of CO4. Knockout mutants of these receptors exhibit a significant reduction in the number of lateral roots and crown roots with lateral roots, along with decreased sensitivity to CO4. Conversely, the overexpression of OsMYR1 leads to a substantial increase in lateral roots and crown roots with lateral roots, even in the absence of CO4 treatment. We propose that CO4-induced root architecture development offers promising opportunities for enhancing lateral root growth, which, in turn, can promote nutrient uptake through direct Myc factor application.

Tobacco roots increasing diameter and secondary lateral density in response to drought stress

Exploring the responses of root morphology and its physiological mechanisms under drought stress is significant for further improving water and nutrient absorption in roots. Here, we simulated drought through hydroponics combined with PEG treatments in tobacco to characterize the changes in tobacco root architecture. Our results showed the total root length, first lateral root number, and first lateral root length were significantly reduced upon increasing drought severity, but the average root diameter and secondary lateral root density increased under certain drought conditions. The change of auxin content in roots under drought stress was correlated with the root diameter and second lateral root density responses. Exogenous addition of the auxin analog (NAA) and the auxin transport inhibitor (NPA), as well as DR5:GUS staining experiments further demonstrated that auxin participated in this physiological process. Meanwhile, brassinolide (BR) exhibited a similar trend. Exogenous addition of BR (EBR) and the BR synthesis inhibitor BRZ experiments demonstrated that BR may participate upstream of auxin under drought stress. PEG treatment significantly up-regulated NtBRI1 at 9–24 h, and promoted the up-regulation of NtBSK2 and NtBSK3 at 48 h and 24 h, respectively, these genes may contribute to the change in root morphology under drought stress. This study shows that auxin and BR are involved in the changes in root morphology in tobacco exposed to drought stress. The elucidation of the molecular mechanism at play thus represents a future target for breeding drought-tolerant tobacco varieties.

Fertiliser supplementation with a biostimulant complex of fish hydrolysate, Aloe vera extract, and kelp alters cannabis root architecture to enhance nutrient uptake

Biostimulants are non-nutritive fertiliser additives which have demonstrated improvements to growth outputs of various agricultural crops. As a result, they have gained attention as sustainable components in cultivation practices, particularly in the rapidly emerging cannabis spheres, including medicinal and industrial cannabis cultivation. One point of focus to cannabis cultivators is the plant's efficiency of nutrient utilisation, attributed to the growth vigour and activity of roots. Despite the interest in this topic, due to the concealment of plant roots within substrates, exploration of plant roots is generally challenging and as such has hampered understanding of the impacts of biostimulants to plant root activity and associated nutrient utilisation efficiency. This project aimed to characterise the impacts and modes of action of the biostimulants kelp, Aloe vera extract, and fish hydrolysate to cannabis growth through exploration of their impacts individually, and as a biostimulant complex (BC). To overcome the limitations of root analyses, substrate-free hydroponic growth systems such as aeroponics, deep water culture (DWC), and the Root-TRAPR system were utilized. These systems allowed for the measurement of changes in root architecture and nutrient utilisation induced by the biostimulants. The results demonstrated that application of BC significantly enhanced root development, as indicated by increased root branching (P = 0.038) and increased total length (P = 0.046). In addition, BC treatment resulted in significant increases in the uptake of phosphorous (P = 0.038) and potassium (P = 0.040) by the plant. Metabolite and phytohormone profiling revealed that kelp and Aloe vera extract induced general plant stress-response mechanisms, whilst fish hydrolysate induced mechanisms associated with the plant pathogen defence response. However, when applied together as BC, the induced molecular mechanisms were associated with phosphorous deficiency without induction of general stress response mechanisms, potentially driving the increased uptake of phosphorous observed. These novel characterisations of biostimulant mechanisms and efficacies provide valuable insights to establishing optimised usage of biostimulants within cannabis cultivation, leading to increased production efficiency through sustainable practises.

Nitrogen Addition Alleviates Cadmium Toxicity in Eleocarpus glabripetalus Seedlings

Cadmium (Cd) accumulation in soil is a serious form of heavy metal pollution affecting environmental safety and human health. In order to clarify the tolerance mechanisms to Cd-contaminated soils under N deposition, changes in plant growth, root architecture and physiological characteristics of Eleocarpus glabripetalus seedlings under combined nitrogen (N) and cadmium (Cd) treatments were determined in this study. The results indicated that Cd-induced negative effects inhibited the growth of E. glabripetalus seedlings through increased underground biomass allocation, and affected transpiration and respiratory processes, resulting in a decreased soluble sugars concentration in leaves and non-structural carbohydrates (NSC) in the roots. Root systems might play a major role in Cd absorption. Cd stress restricted the growth of fine roots (<0.5 mm), and affected the uptake of N and P. N addition alleviated the Cd-induced negative effect on plant growth through improving the root system, increasing starch and NSC contents in the roots and increasing total biomass. These findings have important implications for understanding the underlying tolerance mechanisms of Cd pollution under N deposition in arbor species.

Cell-type-specific transcriptomics reveals that root hairs and endodermal barriers play important roles in beneficial plant-rhizobacterium interactions

Growth- and health-promoting bacteria can boost crop productivity in a sustainable way. Pseudomonas simiae WCS417 is such a bacterium that efficiently colonizes roots, modifies the architecture of the root system to increase its size, and induces systemic resistance to make plants more resistant to pests and pathogens. Our previous work suggested that WCS417-induced phenotypes are controlled by root cell-type-specific mechanisms. However, it remains unclear how WCS417 affects these mechanisms. In this study, we transcriptionally profiled five Arabidopsis thaliana root cell types following WCS417 colonization. We found that the cortex and endodermis have the most differentially expressed genes, even though they are not in direct contact with this epiphytic bacterium. Many of these genes are associated with reduced cell wall biogenesis, and mutant analysis suggests that this downregulation facilitates WCS417-driven root architectural changes. Furthermore, we observed elevated expression of suberin biosynthesis genes and increased deposition of suberin in the endodermis of WCS417-colonized roots. Using an endodermal barrier mutant, we showed the importance of endodermal barrier integrity for optimal plant-beneficial bacterium association. Comparison of the transcriptome profiles in the two epidermal cell types that are in direct contact with WCS417—trichoblasts that form root hairs and atrichoblasts that do not—implies a difference in potential for defense gene activation. While both cell types respond to WCS417, trichoblasts displayed both higher basal and WCS417-dependent activation of defense-related genes compared with atrichoblasts. This suggests that root hairs may activate root immunity, a hypothesis that is supported by differential immune responses in root hair mutants. Taken together, these results highlight the strength of cell-type-specific transcriptional profiling to uncover “masked” biological mechanisms underlying beneficial plant-microbe associations.

Lysinibacillus spp.: an IAA-producing endospore forming-bacteria that promotes plant growth

Lysinibacillus is a bacterial genus that has generated recent interest for its biotechnological potential in agriculture. Strains belonging to this group are recognized for their mosquitocidal and bioremediation activity. However, in recent years some reports indicate its importance as plant growth promoting rhizobacteria (PGPR). This research sought to provide evidence of the PGP activity of Lysinibacillus spp. and the role of the indole-3-acetic acid (IAA) production associated with this activity. Twelve Lysinibacillus spp. strains were evaluated under greenhouse conditions, six of which increased the biomass and root architecture of corn plants. In most cases, growth stimulation was evident at 108 CFU/mL inoculum concentration. All strains produced IAA with high variation between them (20–70 µg/mL). The bioinformatic identification of predicted genes associated with IAA production allowed the detection of the indole pyruvic acid pathway to synthesize IAA in all strains; additionally, genes for a tryptamine pathway were detected in two strains. Extracellular filtrates from all strain’s cultures increased the corn coleoptile length in an IAA-similar concentration pattern, which demonstrates the filtrates had an auxin-like effect on plant tissue. Five of the six strains that previously showed PGPR activity in corn also promoted the growth of Arabidopsis thaliana (col 0). These strains induced changes in root architecture of Arabidopsis mutant plants (aux1-7/axr4-2), the partial reversion of mutant phenotype indicated the role of IAA on plant growth. This work provided solid evidence of the association of Lysinibacillus spp. IAA production with their PGP activity, which constitutes a new approach for this genus. These elements contribute to the biotechnological exploration of this bacterial genus for agricultural biotechnology.

A stratagem for primary root elongation under moderate salt stress in the halophyte Schrenkiella parvula.

Schrenkiella parvula, an Arabidopsis-related halophyte, grows around Lake Tuz (Salt) in Turkey and can survive up to 600 mM NaCl. Here, we performed physiological studies on the roots of S. parvula and A. thaliana seedlings cultivated under a moderate salt condition (100 mM NaCl). Interestingly, S. parvula germinated and grew at 100 mM NaCl, but germination did not occur at salt concentrations above 200 mM. In addition, primary roots elongated much faster at 100 mM NaCl, while being thinner with fewer roots hair, than under NaCl-free conditions. Salt-induced root elongation was due to epidermal cell elongation, but meristem size and meristematic DNA replication were reduced. The expression of genes related to auxin response and biosynthesis was also reduced. Application of exogenous auxin abolished the changes in primary root elongation, suggesting that auxin reduction is the main trigger for root architectural changes in response to moderate salinity in S. parvula. In A. thaliana seeds, germination was maintained up to 200 mM NaCl, but post-germination root elongation was significantly inhibited. Furthermore, primary roots did not promote elongation even under fairly low salt conditions. Compared to A. thaliana, cell death and ROS content in primary roots of salt-stressed plants were significantly lower in S. parvula. These changes in the roots of S. parvula seedlings may be an adaptive strategy to reach lower salinity by advancing into deeper soils, while being impaired by moderate salt stress. This article is protected by copyright. All rights reserved.

The Role of IAA in Regulating Root Architecture of Sweetpotato (Ipomoea batatas [L.] Lam) in Response to Potassium Deficiency Stress.

Plants can adapt to the spatial heterogeneity of soil nutrients by changing the morphology and architecture of the root system. Here, we explored the role of auxin in the response of sweetpotato roots to potassium (K+) deficiency stress. Two sweetpotato cultivars, Xushu 32 (low-K-tolerant) and Ningzishu 1 (low-K-sensitive), were cultured in low K+ (0.1 mmol L-1, LK) and normal K+ (10 mmol L-1, CK) nutrient solutions. Compared with CK, LK reduced the dry mass, K+ content, and K+ accumulation in the two cultivars, but the losses of Xushu 32 were smaller than those of Ningzishu 1. LK also affected root growth, mainly impairing the length, surface area, forks number, and crossings number. However, Xushu 32 had significantly higher lateral root length, density, and surface area than Ningzishu 1, closely related to the roots' higher indole-3-acetic acid (IAA) content. According to the qPCR results, Xushu 32 synthesized more IAA (via IbYUC8 and IbTAR2) in leaves but transported and accumulated in roots through polar transport (via IbPIN1, IbPIN3, and IbAUX1). It was also associated with the upregulation of auxin signaling pathway genes (IbIAA4 and IbIAA8) in roots. These results imply that IAA participates in the formation of lateral roots and the change in root architecture during the tolerance to low K+ stress of sweetpotato, thus improving the absorption of K+ and the formation of biomass.

Long term nitrogen deficiency alters expression of miRNAs and alters nitrogen metabolism and root architecture in Indian dwarf wheat (Triticum sphaerococcum Perc.) genotypes

The important roles of plant microRNAs (miRNAs) in adaptation to nitrogen (N) deficiency in different crop species especially cereals (rice, wheat, maize) have been under discussion since last decade with little focus on potential wild relatives and landraces. Indian dwarf wheat (Triticum sphaerococcum Percival) is an important landrace native to the Indian subcontinent. Several unique features, especially high protein content and resistance to drought and yellow rust, make it a very potent landrace for breeding. Our aim in this study is to identify the contrasting Indian dwarf wheat genotypes based on nitrogen use efficiency (NUE) and nitrogen deficiency tolerance (NDT) traits and the associated miRNAs differentially expressed under N deficiency in selected genotypes. Eleven Indian dwarf wheat genotypes and a high NUE bread wheat genotype (for comparison) were evaluated for NUE under control and N deficit field conditions. Based on NUE, selected genotypes were further evaluated under hydroponics and miRNome was compared by miRNAseq under control and N deficit conditions. Among the identified, differentially expressed miRNAs in control and N starved seedlings, the target gene functions were associated with N metabolism, root development, secondary metabolism and cell-cycle associated pathways. The key findings on miRNA expression, changes in root architecture, root auxin abundance and changes in N metabolism reveal new information on the N deficiency response of Indian dwarf wheat and targets for genetic improvement of NUE.

Reactive oxygen species and NADPH oxidase-encoding genes underly the plant growth and developmental responses to Trichoderma.

The modulation of plant growth and development through reactive oxygen species (ROS) is a hallmark during the interactions with microorganisms, but how fungi and their molecules influence endogenous ROS production in the root remains unknown. In this report, we correlated the biostimulant effect of Trichoderma atroviride with Arabidopsis root development via ROS signaling. T. atroviride enhanced ROS accumulation in primary root tips, lateral root primordia, and emerged lateral roots as revealed by total ROS imaging through the fluorescent probe H2DCF-DA and NBT detection. Acidification of the substrate and emission of the volatile organic compound 6-pentyl-2H-pyran-2-one appear to be major factors by which the fungus triggers ROS accumulation. Besides, the disruption of plant NADPH oxidases, also known as respiratory burst oxidase homologs (RBOHs) including ROBHA, RBOHD, but mainly RBOHE, impaired root and shoot fresh weight and the root branching enhanced by the fungus in vitro. RbohE mutant plants displayed poor lateral root proliferation and lower superoxide levels than wild-type seedlings in both primary and lateral roots, indicating a role for this enzyme for T. atroviride-induced root branching. These data shed light on the roles of ROS as messengers for plant growth and root architectural changes during the plant-Trichoderma interaction.

Early lateral root formation in response to calcium and nickel shows variation within disjunct populations of Arabidopsis lyrata spp. lyrata

Root architecture is important in nutrient uptake and avoidance of toxic compounds within the soil. Arabidopsis lyrata spp. lyrata has widespread distribution in disjunct environments that encounter unique stressors starting at germination. Five populations of A. lyrata spp. lyrata show local adaptation to Nickel (Ni) but cross-tolerance to variations in Calcium (Ca) concentration within the soil. Differentiation among the populations begins early in development and appears to impact timing of lateral root formation; therefore the purpose of the study was to understand changes in root architecture and root exploration in response to Ca and Ni within the first three weeks of growth. Lateral root formation was first characterized under one concentration of Ca and Ni. Lateral root formation and tap root length were reduced in all five populations in response to Ni compared to Ca, with the least reduction in the three serpentine populations. When the populations were exposed to a gradient (either Ca or Ni) there were differences in population response based on the nature of the gradient. Start side was the greatest determinant of root exploration and lateral root formation under a Ca gradient, while population was the greatest determinant of root exploration and lateral root formation under a Ni gradient. All populations exhibited about the same frequency of root exploration under a Ca gradient, while the serpentine populations exhibited much higher levels of root exploration under a Ni gradient compared to the two non-serpentine populations. Differences among populations in response to Ca and Ni demonstrate the importance of stress responses early in development, particularly in species that have widespread distribution among disparate habitats.

The battle of crops against drought: Genetic dissection and improvement

With ongoing global climate change, water scarcity-induced drought stress remains a major threat to agricultural productivity. Plants undergo a series of physiological and morphological changes to cope with drought stress, including stomatal closure to reduce transpiration and changes in root architecture to optimize water uptake. Combined phenotypic and multi-omics studies have recently identified a number of drought-related genetic resources in different crop species. The functional dissection of these genes using molecular techniques has enriched our understanding of drought responses in crops and has provided genetic targets for enhancing resistance to drought. Here, we review recent advances in the cloning and functional analysis of drought resistance genes and the development of technologies to mitigate the threat of drought to crop production.

Growth of Zea mays in response to artificial inoculation with endophytic Beauveria bassiana compared to Trichoderma sp. “atroviride B”

ABSTRACT Demand for sustainable crop production is putting pressure on growers to improve yield while using environmentally friendly solutions. Recently, entomopathogenic fungi have been reported to promote plant growth under certain conditions; however, colonisation of plants as endophytes may be inconsistent or persistence within plant tissues brief. We studied the effects of inoculating three Beauveria bassiana isolates in comparison with a known plant-growth-promoting isolate of Trichoderma sp. “atroviride B” as a positive control, on the growth of maize (Zea mays L.). We tested the plant response by measuring multiple parameters including biomass, length, chlorophyll content and root architecture following inoculation, to identify even subtle plant growth effects. Fungal isolates were inoculated by inserting hyphae through a small wound made in the stem of emerging seedlings, a technique that has not been previously used for B. bassiana. Results showed neutral effects on biomass, but plants exhibited root architecture changes because of inoculation with B. bassiana isolate (FRh2), which increased fine root length and because of T. sp. “atroviride B” which increased larger diameter root length. A higher chlorophyll content was observed for plants inoculated with B. bassiana isolate (J18), although this did not correlate with biomass differences. The inoculation technique enabled the comparison of systemic effects to the plants and may have affected subtle but important root-trait modifications. Changes to root architecture can impact resource acquisition in plants, therefore this study highlights the importance of measuring various physiological changes in the plant host when assessing for potential growth effects.

Root architecture plasticity in response to endoparasitic cyst nematodes is mediated by damage signaling.

Plant root architecture plasticity in response to biotic stresses has not been thoroughly investigated. Infection by endoparasitic cyst nematodes induces root architectural changes that involve the formation of secondary roots at infection sites. However, the molecular mechanisms regulating secondary root formation in response to cyst nematode infection remain largely unknown. We first assessed whether secondary roots form in a nematode density-dependent manner by challenging wild-type Arabidopsis plants with increasing numbers of cyst nematodes (Heterodera schachtii). Next, using jasmonate-related reporter lines and knockout mutants, we tested whether tissue damage by nematodes triggers jasmonate-dependent secondary root formation. Finally, we verified whether damage-induced secondary root formation depends on local auxin biosynthesis at nematode infection sites. Intracellular host invasion by H. schachtii triggers a transient local increase in jasmonates, which activates the expression of ERF109 in a COI1-dependent manner. Knockout mutations in COI1 and ERF109 disrupt the nematode density-dependent increase in secondary roots observed in wild-type plants. Furthermore, ERF109 regulates secondary root formation upon H. schachtii infection via local auxin biosynthesis. Host invasion by H. schachtii triggers secondary root formation via the damage-induced jasmonate-dependent ERF109 pathway. This points at a novel mechanism underlying plant root plasticity in response to biotic stress.

The locoweed endophyte Alternaria oxytropis affects root development in Arabidopsis in vitro through auxin signaling and polar transport.

Locoweeds are leguminous forbs known for their toxicity to livestock caused by the endophytic fungi Alternaria sect. Undifilum. Unlike the defensive mutualisms reported in many toxin-producing endophytes and their plant hosts, the benefits that A. sect. Undifilum can confer to it host plants remains unclear. Here, we conducted physiological and genetic analyses to show that A. (sect. Undifilum) oxytropis influences growth, especially root development, in its locoweed host Oxytropis ochrocephala and Arabidopsis. The presence of A. oxytropis significantly decreased primary root length while increasing the numbers of lateral roots and root hairs, and increasing plant leaf area and fresh weight. The fungus also increased the concentrations of plant endogenous auxin, and the expression of key genes for auxin biosynthesis, signaling, and transport. These effects on root development were abolished in mutants deficient in auxin signaling and polar transport. Alternaria oxytropis down-regulated expression of PIN1 but increased expression of PIN2, PIN7, and AUX1, which might reflect alterations in the spatial accumulation of auxin responsible for the changes in root architecture. Plant growth was insensitive to A. oxytropis when naphthylphthalamic acid was applied. Our findings indicate a function of A. oxytropis in promoting the growth and development of Arabidopsis via the regulation of auxin, which in turn suggests a possible role in benefiting its locoweed hosts via a process independent of its toxin production.