The TOR-Auxin Connection Upstream of Root Hair Growth

Regulatory mechanisms of trichome and root hair development in Arabidopsis

In plants, cell fate determination is regulated temporally and spatially via a complex of signals consisting of a large number of genetic interactions. Trichome and root hair formation are excellent models for studying cell fate determination in plants. Nowadays, the mysteries underlying the reprograming of trichome and root hair and how nature programs the development of trichome and root hair is an interesting topic in the scientific field. In this review, we discuss the spatial and temporal regulatory networks and cross-talk between phytohormones and epigenetic modifications in the regulation of trichome and root hair initiation in Arabidopsis. The discussion in this review provides a good model for understanding the regulatory mechanism of cell differentiation processes in plants. Moreover, we summarize recent advances in the modulation of trichome and root hair initiation in plants and compare different regulatory mechanisms to help illuminate key goals for future research.

Target of rapamycin (TOR) plays a role in regulating ROS-induced chloroplast damage during cucumber (Cucumis sativus) leaf senescence

In cucumber production, delaying leaf senescence is crucial for improving cucumber yield and quality. Target of rapamycin (TOR) is a highly conserved serine/threonine protein kinase in eukaryotes, which can integrate exogenous and endogenous signals (such as cell energy state levels) to stimulate cell growth, proliferation, and differentiation. However, no studies have yet examined the regulatory role of TOR signalling in cucumber leaf senescence. In this study, the effects of TOR signalling on dark-induced cucumber leaf senescence were investigated using the TOR activator MHY1485 and inhibitor AZD8055 combined with transient transformation techniques. The results indicate that TOR responds to dark-induced leaf senescence, and alterations in TOR activity/expression influence cucumber leaf resistance to dark-induced senescence. Specifically, in plants with elevated TOR activity/expression, we observed reduced expression of senescence-related genes, less membrane lipid damage, decreased cell apoptosis, lower levels of reactive oxygen species production, and less damage to the photosynthetic system compared to the control. In contrast, in plants with reduced TOR activity/expression, we observed higher expression of senescence-related genes, increased membrane lipid damage, enhanced cell apoptosis, elevated levels of reactive oxygen species production, and more damage to the photosynthetic system. These comprehensive results underscore the critical role of TOR in regulating dark-induced cucumber leaf senescence. These findings provide a foundation for controlling premature leaf senescence in cucumber production and offer insights for further exploration of leaf senescence mechanisms and the development of more effective control methods.

Recent insights into metabolic and signalling events of directional root growth regulation and its implications for sustainable crop production systems

Roots are sensors evolved to simultaneously respond to manifold signals, which allow the plant to survive. Root growth responses, including the modulation of directional root growth, were shown to be differently regulated when the root is exposed to a combination of exogenous stimuli compared to an individual stress trigger. Several studies pointed especially to the impact of the negative phototropic response of roots, which interferes with the adaptation of directional root growth upon additional gravitropic, halotropic or mechanical triggers. This review will provide a general overview of known cellular, molecular and signalling mechanisms involved in directional root growth regulation upon exogenous stimuli. Furthermore, we summarise recent experimental approaches to dissect which root growth responses are regulated upon which individual trigger. Finally, we provide a general overview of how to implement the knowledge gained to improve plant breeding.

The proteomics and metabolomics studies of GZU001 on promoting the Merisis of maize (Zea mays L.) roots

BACKGROUND

Plant growth regulators are chemicals that regulate plant growth and development, which can regulate hormonal balance and affect plant growth, thereby increasing crop yield and improving crop quality. Our studies have revealed a new compound, GZU001, which could be used as a plant growth regulator. This compound has been observed to affect root elongation in maize significantly. However, the exact mechanism of this phenomenon is still being investigated.

RESULTS

Metabolomics and proteomics were used in unison in this study to explore the response pathway and regulation mechanism of GZU001 in promoting maize root elongation. From the appearance, we can see that both roots and plants of maize treated with GZU001 are significantly improved. Maize root metabolism revealed 101 differentially abundant proteins and 79 differentially expressed metabolites. The current study identified altered proteins and metabolites associated with physiological and biochemical processes. GZU001 treatment has been demonstrated to promote primary metabolism, essential for carbohydrates, amino acids, energy, and secondary metabolism. The result suggests that the stimulation of primary metabolism is beneficial for the growth and development of maize and plays a significant role in sustaining metabolism and growth.

CONCLUSIONS

This study recorded the changes of related proteins and metabolites in maize roots after GZU001 treatment and provided evidence for this compound's action mode and mechanism in plants.

Regulation of PIN-FORMED Protein Degradation

Auxin action largely depends on the establishment of auxin concentration gradient within plant organs, where PIN-formed (PIN) auxin transporter-mediated directional auxin movement plays an important role. Accumulating studies have revealed the need of polar plasma membrane (PM) localization of PIN proteins as well as regulation of PIN polarity in response to developmental cues and environmental stimuli, amongst which a typical example is regulation of PIN phosphorylation by AGCVIII protein kinases and type A regulatory subunits of PP2A phosphatases. Recent findings, however, highlight the importance of PIN degradation in reestablishing auxin gradient. Although the underlying mechanism is poorly understood, these findings provide a novel aspect to broaden the current knowledge on regulation of polar auxin transport. In this review, we summarize the current understanding on controlling PIN degradation by endosome-mediated vacuolar targeting, autophagy, ubiquitin modification and the related E3 ubiquitin ligases, cytoskeletons, plant hormones, environmental stimuli, and other regulators, and discuss the possible mechanisms according to recent studies.

The nitrification inhibitor 1,9-decanediol from rice roots promotes root growth in Arabidopsis through involvement of ABA and PIN2-mediated auxin signaling

1,9-decanediol (1,9-D) is a biological nitrification inhibitor secreted in roots, which effectively inhibits soil nitrifier activity and reduces nitrogen loss from agricultural fields. However, the effects of 1,9-D on plant root growth and the involvement of signaling pathways in the plant response to 1,9-D have not been investigated. Here, we report that 1,9-D, in the 100-400 μM concentration range, promotes primary root length in Arabidopsis seedlings at 3d and 5d, by 10.1%-33.3% and 6.9%-32.6%, and, in a range of 50-200 μM, leads to an increase in the number of lateral roots. 150 μM 1,9-D was found optimum for the positive regulation of root growth. qRT-PCR analysis reveals that 1,9-D can significantly increase AtABA3 gene expression and that a mutation in ABA3 results in insensitivity of root growth to 1,9-D. Moreover, through pharmacological experiments, we show that exogenous addition of ABA (abscisic acid) with 1,9-D enhances primary root length by 23.5%-63.3%, and an exogenous supply of 1,9-D with the ABA inhibitor Flu reduces primary root length by 1.0%-14.3%. Primary root length of the pin2/eir1-1 is shown to be insensitive to both exogenous addition of 1,9-D and ABA, indicating that the auxin carrier PIN2/EIR1 is involved in promotion of root growth by 1,9-D. These results suggest a novel for 1,9-D in regulating plant root growth through ABA and auxin signaling.

Photosynthesis, Phytohormone Signaling and Sugar Catabolism in the Culm Sheaths of Phyllostachys edulis

Culm sheaths play an important role in supporting and protecting bamboo shoots during the growth and development period. The physiological and molecular functions of bamboo sheaths during the growth of bamboo shoots remain unclear. In this study, we investigated the morphological anatomy of culm sheaths, photosynthesis in sheath blades, storage and distribution of sugars, and the transcriptome of the sheath. Respiration in the base of the culm sheath was higher than that in the sheath blades; chloroplasts matured with the development of the sheath blades, the fluorescence efficiency Fv/Fm value increased from 0.3 to 0.82; and sucrose and hexose accumulated in the sheath blade and the culm sheath. The sucrose, glucose, and fructose contents of the middle sheath blades were 10.66, 5.73, and 8.84 mg/g FW, respectively. Starches accumulated in parenchymal cells close to vascular bundles. Genes related to the plant hormone signaling pathway and sugar catabolism were highly expressed in the culm sheath base. These findings provide a research basis for further understanding the possible role of bamboo sheaths in the growth and development of bamboo shoots.

CkREV Enhances the Drought Resistance of Caragana korshinskii through Regulating the Expression of Auxin Synthetase Gene CkYUC5

As a common abiotic stress, drought severely impairs the growth, development, and even survival of plants. Here we report a transcription factor, Caragana korshinskii REVOLUTA(CkREV), which can bidirectionally regulate the expression of the critical enzyme gene CkYUC5 in auxin synthesis according to external environment changes, so as to control the biosynthesis of auxin and further enhance the drought resistance of plants. Quantitative analysis reveals that the expression level of both CkYUC5 and AtYUC5 is down-regulated after C. korshinskii and Arabidopsis thaliana are exposed to drought. Functional verification of CkREV reveals that CkREV up-regulates the expression of AtYUC5 in transgenic A. thaliana under common conditions, while down-regulating it under drought conditions. Meanwhile, the expression of CkYUC5 is also down-regulated in C. korshinskii leaves instantaneously overexpressing CkREV. We apply a dual-luciferase reporter system to discover that CkREV can bind to the promoter of CkYUC5 to regulate its expression, which is further proved by EMSA and Y1H esxperiments. Functional verification of CkREV in C. korshinskii and transgenic A. thaliana shows that CkREV can regulate the expression of CkYUC5 and AtYUC5 in a contrary way, maintaining the equilibrium of plants between growth and drought resisting. CkREV can positively regulate the expression of CkYUC5 to promote auxin synthesis in favor of growth under normal development. However, CkREV can also respond to external signals and negatively regulate the expression of CkYUC5, which inhibits auxin synthesis in order to reduce growth rate, lower water demands, and eventually improve the drought resistance of plants.

An Arabidopsis mutant deficient in phosphatidylinositol-4-phosphate kinases ß1 and ß2 displays altered auxin-related responses in roots

Phosphatidylinositol 4-kinases (PI4Ks) are the first enzymes that commit phosphatidylinositol into the phosphoinositide pathway. Here, we show that Arabidopsis thaliana seedlings deficient in PI4Kβ1 and β2 have several developmental defects including shorter roots and unfinished cytokinesis. The pi4kβ1β2 double mutant was insensitive to exogenous auxin concerning inhibition of root length and cell elongation; it also responded more slowly to gravistimulation. The pi4kß1ß2 root transcriptome displayed some similarities to a wild type plant response to auxin. Yet, not all the genes displayed such a constitutive auxin-like response. Besides, most assessed genes did not respond to exogenous auxin. This is consistent with data with the transcriptional reporter DR5-GUS. The content of bioactive auxin in the pi4kß1ß2 roots was similar to that in wild-type ones. Yet, an enhanced auxin-conjugating activity was detected and the auxin level reporter DII-VENUS did not respond to exogenous auxin in pi4kß1ß2 mutant. The mutant exhibited altered subcellular trafficking behavior including the trapping of PIN-FORMED 2 protein in rapidly moving vesicles. Bigger and less fragmented vacuoles were observed in pi4kß1ß2 roots when compared to the wild type. Furthermore, the actin filament web of the pi4kß1ß2 double mutant was less dense than in wild-type seedling roots, and less prone to rebuilding after treatment with latrunculin B. A mechanistic model is proposed in which an altered PI4K activity leads to actin filament disorganization, changes in vesicle trafficking, and altered auxin homeostasis and response resulting in a pleiotropic root phenotypes.

The UPR regulator IRE1 promotes balanced organ development by restricting TOR-dependent control of cellular differentiation in Arabidopsis

Proteostasis of the endoplasmic reticulum (ER) is controlled by sophisticated signaling pathways that are collectively called the unfolded protein response (UPR) and are initiated by specialized ER membrane-associated sensors. The evidence that complete loss-of-function mutations of the most conserved of the UPR sensors, inositol-requiring enzyme 1 (IRE1), dysregulates tissue growth and development in metazoans and plants raises the fundamental question as to how IRE1 is connected to organismal growth. To address this question, we interrogated the Arabidopsis primary root, an established model for organ development, using the tractable Arabidopsis IRE1 mutant ire1a ire1b, which has marked root development defects in the absence of exogenous stress. We demonstrate that IRE1 is required to reach maximum rates of cell elongation and root growth. We also established that in the actively growing ire1a ire1b mutant root tips the Target of Rapamycin (TOR) kinase, a widely conserved pro-growth regulator, is hyperactive, and that, unlike cell proliferation, the rate of cell differentiation is enhanced in ire1a ire1b in a TOR-dependent manner. By functionally connecting two essential growth regulators, these results underpin a novel and critical role of IRE1 in organ development and indicate that, as cells exit an undifferentiated state, IRE1 is required to monitor TOR activity to balance cell expansion and maturation during organ biogenesis.

Lessons Learned from the Studies of Roots Shaded from Direct Root Illumination

The root is the below-ground organ of a plant, and it has evolved multiple signaling pathways that allow adaptation of architecture, growth rate, and direction to an ever-changing environment. Roots grow along the gravitropic vector towards beneficial areas in the soil to provide the plant with proper nutrients to ensure its survival and productivity. In addition, roots have developed escape mechanisms to avoid adverse environments, which include direct illumination. Standard laboratory growth conditions for basic research of plant development and stress adaptation include growing seedlings in Petri dishes on medium with roots exposed to light. Several studies have shown that direct illumination of roots alters their morphology, cellular and biochemical responses, which results in reduced nutrient uptake and adaptability upon additive stress stimuli. In this review, we summarize recent methods that allow the study of shaded roots under controlled laboratory conditions and discuss the observed changes in the results depending on the root illumination status.

Exogenous carbon source supplementation counteracts root and hypocotyl growth limitations under increased cotyledon shading, with glucose and sucrose differentially modulating growth curves

Plant growth is continuously modulated by endogenous and exogenous stimuli. By no means the only, but well described, signaling molecules produced in plants and distributed through the plant body to orchestrate efficient growth are photosynthates. Light is a potent exogenous stimulus that determines, first, the rate of photosynthesis, but also the rate of plant growth. Root meristem activity is reduced with direct illumination but enhanced with increased sugar levels. With reduced cotyledon illumination, the seedling increases hypocotyl elongation until adequate light exposure is again provided. If endogenous carbon sources are limited, this leads to a temporary inhibition of root growth. Experimental growth conditions include exogenous supplementation of sucrose or glucose in addition to culturing seedlings under light exposure in Petri dishes. We compared total root length and hypocotyl elongation of Arabidopsis thaliana wild type Col-0 in response to illumination status and carbon source in the growth medium. Overall, sucrose supplementation promoted hypocotyl and root length to a greater extent than glucose supplementation. Glucose promoted root length compared to non-supplemented seedlings especially when cotyledon illumination was greatly reduced.

Auxin and Target of Rapamycin Spatiotemporally Regulate Root Organogenesis

The programs associated with embryonic roots (ERs), primary roots (PRs), lateral roots (LRs), and adventitious roots (ARs) play crucial roles in the growth and development of roots in plants. The root functions are involved in diverse processes such as water and nutrient absorption and their utilization, the storage of photosynthetic products, and stress tolerance. Hormones and signaling pathways play regulatory roles during root development. Among these, auxin is the most important hormone regulating root development. The target of rapamycin (TOR) signaling pathway has also been shown to play a key role in root developmental programs. In this article, the milestones and influential progress of studying crosstalk between auxin and TOR during the development of ERs, PRs, LRs and ARs, as well as their functional implications in root morphogenesis, development, and architecture, are systematically summarized and discussed.

Understanding the Intricate Web of Phytohormone Signalling in Modulating Root System Architecture

Root system architecture (RSA) is an important developmental and agronomic trait that is regulated by various physical factors such as nutrients, water, microbes, gravity, and soil compaction as well as hormone-mediated pathways. Phytohormones act as internal mediators between soil and RSA to influence various events of root development, starting from organogenesis to the formation of higher order lateral roots (LRs) through diverse mechanisms. Apart from interaction with the external cues, root development also relies on the complex web of interaction among phytohormones to exhibit synergistic or antagonistic effects to improve crop performance. However, there are considerable gaps in understanding the interaction of these hormonal networks during various aspects of root development. In this review, we elucidate the role of different hormones to modulate a common phenotypic output, such as RSA in Arabidopsis and crop plants, and discuss future perspectives to channel vast information on root development to modulate RSA components.