The nitrate transporter NRT2.1 directly antagonizes PIN7-mediated auxin transport for root growth adaptation.

As a crucial nitrogen source, nitrate (NO3-) is a key nutrient for plants. Accordingly, root systems adapt to maximize NO3- availability, a developmental regulation also involving the phytohormone auxin. Nonetheless, the molecular mechanisms underlying this regulation remain poorly understood. Here, we identify low-nitrate-resistant mutant (lonr) in Arabidopsis (Arabidopsis thaliana), whose root growth fails to adapt to low-NO3- conditions. lonr2 is defective in the high-affinity NO3- transporter NRT2.1. lonr2 (nrt2.1) mutants exhibit defects in polar auxin transport, and their low-NO3--induced root phenotype depends on the PIN7 auxin exporter activity. NRT2.1 directly associates with PIN7 and antagonizes PIN7-mediated auxin efflux depending on NO3- levels. These results reveal a mechanism by which NRT2.1 in response to NO3- limitation directly regulates auxin transport activity and, thus, root growth. This adaptive mechanism contributes to the root developmental plasticity to help plants cope with changes in NO3- availability.

Root-knot nematodes produce functional mimics of tyrosine-sulfated plant peptides.

Root-knot nematodes (Meloidogyne spp.) are highly evolved obligate parasites threatening global food security. These parasites have a remarkable ability to establish elaborate feeding sites in roots, which are their only source of nutrients throughout their life cycle. A wide range of nematode effectors have been implicated in modulation of host pathways for defense suppression and/or feeding site development. Plants produce a diverse array of peptide hormones including PLANT PEPTIDE CONTAINING SULFATED TYROSINE (PSY)-family peptides, which promote root growth via cell expansion and proliferation. A sulfated PSY-like peptide RaxX (required for activation of XA21 mediated immunity X) produced by the biotrophic bacterial pathogen (Xanthomonas oryzae pv. oryzae) has been previously shown to contribute to bacterial virulence. Here, we report the identification of genes from root-knot nematodes predicted to encode PSY-like peptides (MigPSYs) with high sequence similarity to both bacterial RaxX and plant PSYs. Synthetic sulfated peptides corresponding to predicted MigPSYs stimulate root growth in Arabidopsis. MigPSY transcript levels are highest early in the infection cycle. Downregulation of MigPSY gene expression reduces root galling and egg production, suggesting that the MigPSYs serve as nematode virulence factors. Together, these results indicate that nematodes and bacteria exploit similar sulfated peptides to hijack plant developmental signaling pathways to facilitate parasitism.

Root Walker: an automated pipeline for large scale quantification of early root growth responses at high spatial and temporal resolution.

Plants are sessile organisms that constantly adapt to their changing environment. The root is exposed to numerous environmental signals ranging from nutrients and water to microbial molecular patterns. These signals can trigger distinct responses including the rapid increase or decrease of root growth. Consequently, using root growth as a readout for signal perception can help decipher which external cues are perceived by roots, and how these signals are integrated. To date, studies measuring root growth responses using large numbers of roots have been limited by a lack of high-throughput image acquisition, poor scalability of analytical methods, or low spatiotemporal resolution. Here, we developed the Root Walker pipeline, which uses automated microscopes to acquire time-series images of many roots exposed to controlled treatments with high spatiotemporal resolution, in conjunction with fast and automated image analysis software. We demonstrate the power of Root Walker by quantifying root growth rate responses at different time and throughput scales upon treatment with natural auxin and two mitogen-associated protein kinase cascade inhibitors. We find a concentration-dependent root growth response to auxin and reveal the specificity of one MAPK inhibitor. We further demonstrate the ability of Root Walker to conduct genetic screens by performing a genome-wide association study on 260 accessions in under 2 weeks, revealing known and unknown root growth regulators. Root Walker promises to be a useful toolkit for the plant science community, allowing large-scale screening of root growth dynamics for a variety of purposes, including genetic screens for root sensing and root growth response mechanisms.

WRKY6 transcription factor modulates root potassium acquisition through promoting expression of AKT1 in Arabidopsis.

Potassium (K+), being an essential macronutrient in plants, plays a central role in many aspects. Root growth is highly plastic and is affected by many different abiotic stresses including nutrient deficiency. The Shaker-type K+ channel Arabidopsis (Arabidopsis thaliana) K+ Transporter 1 (AKT1) is responsible for K+ uptake under both low and high external K+ conditions. However, the upstream transcription factor of AKT1 is not clear. Here, we demonstrated that the WRKY6 transcription factor modulates root growth to low potassium (LK) stress in Arabidopsis. WRKY6 showed a quick response to LK stress and also to many other abiotic stress treatments. The two wrky6 T-DNA insertion mutants were highly sensitive to LK treatment, whose primary root lengths were much shorter, less biomass and lower K+ content in roots than those of wild-type plants, while WRKY6-overexpression lines showed opposite phenotypes. A further investigation showed that WRKY6 regulated the expression of the AKT1 gene via directly binding to the W-box elements in its promoter through EMSA and ChIP-qPCR assays. A dual luciferase reporter analysis further demonstrated that WRKY6 enhanced the transcription of AKT1. Genetic analysis further revealed that the overexpression of AKT1 greatly rescued the short root phenotype of the wrky6 mutant under LK stress, suggesting AKT1 is epistatic to WRKY6 in the control of LK response. Further transcriptome profiling suggested that WRKY6 modulates LK response through a complex regulatory network. Thus, this study unveils a transcription factor that modulates root growth under potassium deficiency conditions by affecting the potassium channel gene AKT1 expression.

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.

AT-hook motif nuclear localized transcription factors function redundantly in promoting root growth through modulation of redox homeostasis.

Maintaining an optimal redox status is essential for plant growth and development, particularly when the plants are under stress. AT-hook motif nuclear localized (AHL) proteins are evolutionarily conserved transcription factors in plants. Much of our understanding about this gene family has been derived from studies on clade A members. To elucidate the functions of clade B genes, we first analyzed their spatial expression patterns using transgenic plants expressing a nuclear localized GFP under the control of their promoter sequences. AHL1, 2, 6, 7, and 10 were further functionally characterized owing to their high expression in the root apical meristem. Through mutant analyses and transgenic studies, we showed that these genes have the ability to promote root growth. Using yeast one-hybrid and dual luciferase assays, we demonstrated that AHL1, 2, 6, 7, and 10 are transcription regulators and this activity is required for their roles in root growth. Although mutants for these genes did not showed obvious defects in root growth, transgenic plants expressing their fusion proteins with the SRDX repressor motif exhibited a short-root phenotype. Through transcriptome analysis, histochemical staining and molecular genetics experiments, we found that AHL10 maintains redox homeostasis via direct regulation of glutathione transferase (GST) genes. When the transcript level of GSTF2, a top-ranked target of AHL10, was reduced by RNAi, the short-root phenotype in the AHL10-SRDX expressing plant was largely rescued. These results together suggest that AHL genes function redundantly in promoting root growth through direct regulation of redox homeostasis.

Rhamnogalacturonan lyase 1 (RGL1), as a suppressor of E3 ubiquitin ligase Arabidopsis thaliana ring zinc finger 1 (AtRZF1), is involved in dehydration response to mediate proline synthesis and pectin rhamnogalacturonan-I composition.

Proline metabolism plays a crucial role in both environmental stress responses and plant growth. However, the specific mechanism by which proline contributes to abiotic stress processes remains to be elucidated. In this study, we utilized atrzf1 (Arabidopsis thaliana ring zinc finger 1) as a parental line for T-DNA tagging mutagenesis and identified a suppressor mutant of atrzf1, designated proline content alterative 31 (pca31). The pca31 mutant suppressed the insensitivity of atrzf1 to dehydration stress during early seedling growth. Using Thermal Asymmetric Interlaced-PCR, we found that the T-DNA of pca31 was inserted into the promoter region of the At2g22620 gene, which encodes the cell wall enzyme rhamnogalacturonan lyase 1 (RGL1). Enzymatic assays indicated that RGL1 exhibited rhamnogalacturonan lyase activity, influencing cell wall pectin composition. The decrease in RGL1 gene expression suppressed the transcriptomic perturbation of the atrzf1 mutant. Silencing of the RGL1 gene in atrzf1 resulted in a sensitive phenotype similar to pca31 under osmotic stress conditions. Treatment with mannitol, salt, hydrogen peroxide, and abscisic acid induced RGL1 expression. Furthermore, we uncovered that RGL1 plays a role in modulating root growth and vascular tissue development. Molecular, physiological, and genetic experiments revealed that the positive modulation of RGL1 during abiotic stress was linked to the AtRZF1 pathway. Taken together, these findings establish that pca31 acts as a suppressor of atrzf1 in abiotic stress responses through proline and cell wall metabolisms.

SPIRO - the automated Petri plate imaging platform designed by biologists, for biologists.

Phenotyping of model organisms grown on Petri plates is often carried out manually, despite the procedures being time-consuming and laborious. The main reason for this is the limited availability of automated phenotyping facilities, whereas constructing a custom automated solution can be a daunting task for biologists. Here, we describe SPIRO, the Smart Plate Imaging Robot, an automated platform that acquires time-lapse photographs of up to four vertically oriented Petri plates in a single experiment, corresponding to 192 seedlings for a typical root growth assay and up to 2500 seeds for a germination assay. SPIRO is catered specifically to biologists' needs, requiring no engineering or programming expertise for assembly and operation. Its small footprint is optimized for standard incubators, the inbuilt green LED enables imaging under dark conditions, and remote control provides access to the data without interfering with sample growth. SPIRO's excellent image quality is suitable for automated image processing, which we demonstrate on the example of seed germination and root growth assays. Furthermore, the robot can be easily customized for specific uses, as all information about SPIRO is released under open-source licenses. Importantly, uninterrupted imaging allows considerably more precise assessment of seed germination parameters and root growth rates compared with manual assays. Moreover, SPIRO enables previously technically challenging assays such as phenotyping in the dark. We illustrate the benefits of SPIRO in proof-of-concept experiments which yielded a novel insight on the interplay between autophagy, nitrogen sensing, and photoblastic response.

Jasmonate signaling modulates root growth by suppressing iron accumulation during ammonium stress.

Plants adapt to changing environmental conditions by adjusting their growth physiology. Nitrate (NO3-) and ammonium (NH4+) are the major inorganic nitrogen forms for plant uptake. However, high NH4+ inhibits plant growth, and roots undergo striking changes, such as inhibition of cell expansion and division, leading to reduced root elongation. In this work, we show that high NH4+ modulates nitrogen metabolism and root developmental physiology by inhibiting iron (Fe)-dependent Jasmonate (JA) signaling and response in Arabidopsis (Arabidopsis thaliana). Transcriptomic data suggested that NH4+ availability regulates Fe and JA-responsive genes. High NH4+ levels led to enhanced root Fe accumulation, which impaired nitrogen balance and growth by suppressing JA biosynthesis and signaling response. Integrating pharmacological, physiological, and genetic experiments revealed the involvement of NH4+ and Fe-derived responses in regulating root growth and nitrogen metabolism through modulation of the JA pathway during NH4+ stress. The JA signaling transcription factor MYC2 directly bound the promoter of the NITRATE TRANSPORTER 1.1 (NRT1.1) and repressed it to optimize the NH4+/Fe-JA balance for plant adaptation during NH4+ stress. Our findings illustrate the intricate balance between nutrient and hormone-derived signaling pathways that appear essential for optimizing plant growth by adjusting physiological and metabolic responses during NH4+/Fe stress.

RALF1 peptide triggers biphasic root growth inhibition upstream of auxin biosynthesis.

Plant cell growth responds rapidly to various stimuli, adapting architecture to environmental changes. Two major endogenous signals regulating growth are the phytohormone auxin and the secreted peptides rapid alkalinization factors (RALFs). Both trigger very rapid cellular responses and also exert long-term effects [Du et al., Annu. Rev. Plant Biol. 71, 379-402 (2020); Blackburn et al., Plant Physiol. 182, 1657-1666 (2020)]. However, the way, in which these distinct signaling pathways converge to regulate growth, remains unknown. Here, using vertical confocal microscopy combined with a microfluidic chip, we addressed the mechanism of RALF action on growth. We observed correlation between RALF1-induced rapid Arabidopsis thaliana root growth inhibition and apoplast alkalinization during the initial phase of the response, and revealed that RALF1 reversibly inhibits primary root growth through apoplast alkalinization faster than within 1 min. This rapid apoplast alkalinization was the result of RALF1-induced net H+ influx and was mediated by the receptor FERONIA (FER). Furthermore, we investigated the cross-talk between RALF1 and the auxin signaling pathways during root growth regulation. The results showed that RALF-FER signaling triggered auxin signaling with a delay of approximately 1 h by up-regulating auxin biosynthesis, thus contributing to sustained RALF1-induced growth inhibition. This biphasic RALF1 action on growth allows plants to respond rapidly to environmental stimuli and also reprogram growth and development in the long term.

Arabidopsis TBP-ASSOCIATED FACTOR 12 ortholog NOBIRO6 controls root elongation with unfolded protein response cofactor activity.

Plant root growth is indeterminate but continuously responds to environmental changes. We previously reported on the severe root growth defect of a double mutant in bZIP17 and bZIP28 (bz1728) modulating the unfolded protein response (UPR). To elucidate the mechanism by which bz1728 seedlings develop a short root, we obtained a series of bz1728 suppressor mutants, called nobiro, for rescued root growth. We focused here on nobiro6, which is defective in the general transcription factor component TBP-ASSOCIATED FACTOR 12b (TAF12b). The expression of hundreds of genes, including the bZIP60-UPR regulon, was induced in the bz1728 mutant, but these inductions were markedly attenuated in the bz1728nobiro6 mutant. In view of this, we assigned transcriptional cofactor activity via physical interaction with bZIP60 to NOBIRO6/TAF12b. The single nobiro6/taf12b mutant also showed an altered sensitivity to endoplasmic reticulum stress for both UPR and root growth responses, demonstrating that NOBIRO6/TAF12b contributes to environment-responsive root growth control through UPR.

PIN2/3/4 auxin carriers mediate root growth inhibition under conditions of boron deprivation in Arabidopsis.

The mechanistic basis by which boron (B) deprivation inhibits root growth via the mediation of root apical auxin transport and distribution remains elusive. This study showed that B deprivation repressed root growth of wild-type Arabidopsis seedlings, which was related to higher auxin accumulation (observed with DII-VENUS and DR5-GFP lines) in B-deprived roots. Boron deprivation elevated the auxin content in the root apex, coinciding with upregulation of the expression levels of auxin biosynthesis-related genes (TAA1, YUC3, YUC9, and NIT1) in shoots, but not in root apices. Phenotyping experiments using auxin transport-related mutants revealed that the PIN2/3/4 carriers are involved in root growth inhibition caused by B deprivation. B deprivation not only upregulated the transcriptional levels of PIN2/3/4, but also restrained the endocytosis of PIN2/3/4 carriers (observed with PIN-Dendra2 lines), resulting in elevated protein levels of PIN2/3/4 in the plasma membrane. Overall, these results suggest that B deprivation not only enhances auxin biosynthesis in shoots by elevating the expression levels of auxin biosynthesis-related genes but also promotes the polar auxin transport from shoots to roots by upregulating the gene expression levels of PIN2/3/4, as well as restraining the endocytosis of PIN2/3/4 carriers, ultimately resulting in auxin accumulation in root apices and root growth inhibition.

A role for fermentation in aerobic conditions as revealed by computational analysis of maize root metabolism during growth by cell elongation.

The root is a well-studied example of cell specialisation, yet little is known about the metabolism that supports the transport functions and growth of different root cell types. To address this, we used computational modelling to study metabolism in the elongation zone of a maize lateral root. A functional-structural model captured the cell-anatomical features of the root and modelled how they changed as the root elongated. From these data, we derived constraints for a flux balance analysis model that predicted metabolic fluxes of the 11 concentric rings of cells in the root. We discovered a distinct metabolic flux pattern in the cortical cell rings, endodermis and pericycle (but absent in the epidermis) that involved a high rate of glycolysis and production of the fermentation end-products lactate and ethanol. This aerobic fermentation was confirmed experimentally by metabolite analysis. The use of fermentation in the model was not obligatory but was the most efficient way to meet the specific demands for energy, reducing power and carbon skeletons of expanding cells. Cytosolic acidification was avoided in the fermentative mode due to the substantial consumption of protons by lipid synthesis. These results expand our understanding of fermentative metabolism beyond that of hypoxic niches and suggest that fermentation could play an important role in the metabolism of aerobic tissues.

Arabidopsis plasma membrane H+-ATPase interacts with auxin to regulate Danger-Associated Peptide Pep1-induced root growth inhibition.

Plant elicitor peptide 1 (Pep1) is one of plant-derived damage-associated molecular patterns (DAMPs) involved in the regulation of multiple biological processes, including immune response and root growth. The exogenous application of Pep1 was shown to inhibit root growth by affecting the auxin content and extracellular pH level in the transition zone (TZ). However, the signaling relationship between extracellular pH and auxin in Pep1-regulated root growth inhibition has not been explored. Our study here suggested that both pH signaling and auxin signaling were responsible for Pep1-regulated root growth inhibition, and the Pep1-induced auxin accumulation in TZ depended on apoplastic acidification. To increase the apoplastic pH in TZ, we mutated the AHA2 and found that the mutants of aha2-4 and pin2aha2-4 both reduced Pep1-induced auxin content in TZ, thereby alleviating root growth inhibition. Thus, our results reveal a new auxin-pH signaling crosstalk mechanism in regulating root growth, and provide new insights into the function of Pep1 in regulating root growth in Arabidopsis.

Low nitrogen priming improves nitrogen uptake and assimilation adaptation to nitrogen deficit stress in wheat seedling.

MAIN CONCLUSION: Early-stage low nitrogen priming promotes root growth and delays leaf senescence through gene expression, enhancing nitrogen absorption and assimilation in wheat seedlings, thereby alleviating growth inhibition under nitrogen deficit stress and supporting normal seedling development. Verifying the strategies to reduce the amount of nitrogen (N) fertilizer while maintaining high crop yields is important for improving crop N use efficiency (NUE) and protecting the environment. To determine whether low N (LN) priming (LNP) can alleviate the impact of N-deficit stress on the growth of wheat seedlings and improve their tolerance to N-deficit stress, we conducted hydroponic experiments using two wheat cultivars, Yangmai 158 (YM158, LN tolerant) and Zaoyangmai (ZYM, LN sensitive) to study the effects of LNP on wheat seedlings under N-deficit stress. N-deficit stress decreased the plant dry weight, leaf area, and leaf N content (LNC), while LNP could significantly reduce this reduction. Distinct sensitivities to N-deficit stress were observed between the wheat cultivars, with ZYM showing an early decrease in leaf N content compared to YM158, which exhibited a late-stage reduction. LNP promoted root growth, expanded N uptake area, and upregulated the expression of TaNRT1.1, TaNRT2.1, and TaNRT2.2 in wheat seedlings, suggesting that LNP can enhance root N uptake capacity to increase N accumulation in plants. In addition, LNP improved the activity of glutamine synthase (GS) to enhance the capacity of N assimilation of plants. The relative expression of TaGS1 in the lower leaves of priming and stress (PS) was lower than that of no priming and stress (NS) after LNP, indicating that the rate of N transfer from the lower leaves to the upper leaves became slower after LNP, which alleviated the senescence of the lower leaves. The relative expression of TaGS2 was significantly increased, which might be related to the enhanced photorespiratory ammonia assimilation capacity after LNP, which reduced the N loss and maintained higher LNC. Therefore, LNP in the early stage can improve the N absorption and assimilation ability and maintain the normal N supply to alleviate the inhibition of N-deficit stress in wheat seedlings.

Transcription factor OsSHR2 regulates rice architecture and yield per plant in response to nitrogen.

MAIN CONCLUSION: Mutation of OsSHR2 adversely impacted root and shoot growth and impaired plant response to N conditions, further reducing the yield per plant. Nitrogen (N) is a crucial factor that regulates the plant architecture. There is still a lack of research on it. In our study, it was observed that the knockout of the SHORTROOT 2 (OsSHR2) which was induced by N deficiency, can significantly affect the regulation of plant architecture response to N in rice. Under N deficiency, the mutation of OsSHR2 significantly reduced root growth, and impaired the sensitivity of the root meristem length to N deficiency. The mutants were found to have approximately a 15% reduction in plant height compared to wild type. But mutants showed a significant increase in tillering at post-heading stage, approximately 26% more than the wild type, particularly in high N conditions. In addition, due to reduced seed setting rate and 1000-grain weight, mutant yield was significantly decreased by approximately 33% under low N fertilizer supply. The mutation also changed the distribution of N between the vegetative and reproductive organs. Our findings suggest that the transcription factor OsSHR2 plays a regulatory role in the response of plant architecture and yield per plant to N in rice.

The tetraploid wheat (Triticum dicoccum (Schrank) Schuebl.) improves nitrogen uptake and assimilation adaptation to nitrogen-deficit stress.

MAIN CONCLUSION: The genetic diversity in tetraploid wheat provides a genetic pool for improving wheat productivity and environmental resilience. The tetraploid wheat had strong N uptake, translocation, and assimilation capacity under N deficit stress, thus alleviating growth inhibition and plant N loss to maintain healthy development and adapt to environments with low N inputs. Tetraploid wheat with a rich genetic variability provides an indispensable genetic pool for improving wheat yield. Mining the physiological mechanisms of tetraploid wheat in response to nitrogen (N) deficit stress is important for low-N-tolerant wheat breeding. In this study, we selected emmer wheat (Kronos, tetraploid), Yangmai 25 (YM25, hexaploid), and Chinese spring (CS, hexaploid) as materials. We investigated the differences in the response of root morphology, leaf and root N accumulation, N uptake, translocation, and assimilation-related enzymes and gene expression in wheat seedlings of different ploidy under N deficit stress through hydroponic experiments. The tetraploid wheat (Kronos) had stronger adaptability to N deficit stress than the hexaploid wheats (YM25, CS). Kronos had better root growth under low N stress, expanding the N uptake area and enhancing N uptake to maintain higher NO3- and soluble protein contents. Kronos exhibited high TaNRT1.1, TaNRT2.1, and TaNRT2.2 expression in roots, which promoted NO3- uptake, and high TaNRT1.5 and TaNRT1.8 expression in roots and leaves enhanced NO3- translocation to the aboveground. NR and GS activity in roots and leaves of Kronos was higher by increasing the expression of TANIA2, TAGS1, and TAGS2, which enhanced the reduction and assimilation of NO3- as well as the re-assimilation of photorespiratory-released NH4+. Overall, Kronos had strong N uptake, translocation, and assimilation capacity under N deficit stress, alleviating growth inhibition and plant N loss and thus maintaining a healthy development. This study reveals the physiological mechanisms of tetraploid wheat that improve nitrogen uptake and assimilation adaptation under low N stress, which will provide indispensable germplasm resources for elite low-N-tolerant wheat improvement and breeding.

Melatonin-mediated CcARP1 alters F-actin dynamics by phosphorylation of CcADF9 to balance root growth and salt tolerance in pigeon pea.

As a multifunctional hormone-like molecule, melatonin exhibits a pleiotropic role in plant salt stress tolerance. While actin cytoskeleton is essential to plant tolerance to salt stress, it is unclear if and how actin cytoskeleton participates in the melatonin-mediated alleviation of plant salt stress. Here, we report that melatonin alleviates salt stress damage in pigeon pea by activating a kinase-like protein, which interacts with an actin-depolymerizing factor. Cajanus cajan Actin-Depolymerizing Factor 9 (CcADF9) has the function of severing actin filaments and is highly expressed under salt stress. The CcADF9 overexpression lines (CcADF9-OE) showed a reduction of transgenic root length and an increased sensitivity to salt stress. By using CcADF9 as a bait to screen an Y2H library, we identified actin depolymerizing factor-related phosphokinase 1 (ARP1), a novel protein kinase that interacts with CcADF9. CcARP1, induced by melatonin, promotes salt resistance of pigeon pea through phosphorylating CcADF9, inhibiting its severing activity. The CcARP1 overexpression lines (CcARP1-OE) displayed an increased transgenic root length and resistance to salt stress, whereas CcARP1 RNA interference lines (CcARP1-RNAi) presented the opposite phenotype. Altogether, our findings reveal that melatonin-induced CcARP1 maintains F-actin dynamics balance by phosphorylating CcADF9, thereby promoting root growth and enhancing salt tolerance.

The RALF2-FERONIA-MYB63 module orchestrates growth and defense in tomato roots.

Plant secreted peptides RAPID ALKALINISATION FACTORs (RALFs), which act through the receptor FERONIA (FER), play important roles in plant growth. However, it remains unclear whether and how RALF-FER contributes to the trade-off of plant growth-defense. Here, we used a variety of techniques such as CRISPR/Cas9, protein-protein interaction and transcriptional regulation methods to investigate the role of RALF2 and its receptor FER in regulating lignin deposition, root growth, and defense against Fusarium oxysporum f. sp. lycopersici (Fol) in tomato (Solanum lycopersicum). The ralf2 and fer mutants show reduced primary root length, elevated lignin accumulation, and enhanced resistance against Fol than the wild-type. FER interacts with and phosphorylates MYB63 to promote its degradation. MYB63 serves as an activator of lignin deposition by regulating the transcription of dirigent protein gene DIR19. Mutation of DIR19 suppresses lignin accumulation, and reverses the short root phenotype and Fol resistance in ralf2 or fer mutant. Collectively, our results demonstrate that the RALF2-FER-MYB63 module fine-tunes root growth and resistance against Fol through regulating the deposition of lignin in tomato roots. The study sheds new light on how plants maintain the growth-defense balance via RALF-FER.

In situ seasonal patterns of root auxin concentrations and meristem length in an arctic sedge.

Seasonal dynamics of root growth play an important role in large-scale ecosystem processes; they are largely governed by growth regulatory compounds and influenced by environmental conditions. Yet, our knowledge about physiological drivers of root growth is mostly limited to laboratory-based studies on model plant species. We sampled root tips of Eriophorum vaginatum and analyzed their auxin concentrations and meristem lengths biweekly over a growing season in situ in a subarctic peatland, both in surface soil and at the permafrost thawfront. Auxin concentrations were almost five times higher in surface than in thawfront soils and increased over the season, especially at the thawfront. Surprisingly, meristem length showed an opposite pattern and was almost double in thawfront compared with surface soils. Meristem length increased from peak to late season in the surface soils but decreased at the thawfront. Our study of in situ seasonal dynamics in root physiological parameters illustrates the potential for physiological methods to be applied in ecological studies and emphasizes the importance of in situ measurements. The strong effect of root location and the unexpected opposite patterns of meristem length and auxin concentrations likely show that auxin actively governs root growth to ensure a high potential for nutrient uptake at the thawfront.