Glucose and auxin signaling interaction in controlling Arabidopsis thaliana seedlings root growth and development

Interaction of glucose and phytohormone signaling in plants

Energy acts as a primary prerequisite for plant growth like all the other organisms. Soluble sugars function in providing enough supply of nutrients which further helps in building macromolecules and energy to carry out specific and coordinated development. Sugars functions as nutrient as well as signaling molecule to promote cell division and differentiation in plants. Intriguingly, glucose has emerged as a crucial signaling molecule where hexokinase1 acts as the conserved glucose sensor. On the molecular scale, an extensive crosstalk between glucose and phytohormone signaling has been observed where glucose signals trigger multiple hexokinase1-dependent as well as hexokinase1-independent pathways to mediate diverse developmental, physiological and molecular mechanisms. Taken together, these findings this review focused on the glucose crosstalk with several classical plant hormonal-signaling pathways and the crucial role of hexokinase1 in modulating plant physiological processes.

Modulation of auxin signalling through DIAGETROPICA and ENTIRE differentially affects tomato plant growth via changes in photosynthetic and mitochondrial metabolism

Auxin modulates a range of plant developmental processes including embryogenesis, organogenesis, and shoot and root development. Recent studies have shown that plant hormones also strongly influence metabolic networks, which results in altered growth phenotypes. Modulating auxin signalling pathways may therefore provide an opportunity to alter crop performance. Here, we performed a detailed physiological and metabolic characterization of tomato (Solanum lycopersicum) mutants with either increased (entire) or reduced (diageotropica-dgt) auxin signalling to investigate the consequences of altered auxin signalling on photosynthesis, water use, and primary metabolism. We show that reduced auxin sensitivity in dgt led to anatomical and physiological modifications, including altered stomatal distribution along the leaf blade and reduced stomatal conductance, resulting in clear reductions in both photosynthesis and water loss in detached leaves. By contrast, plants with higher auxin sensitivity (entire) increased the photosynthetic capacity, as deduced by higher V_cmax and J_max coupled with reduced stomatal limitation. Remarkably, our results demonstrate that auxin-sensitive mutants (dgt) are characterized by impairments in the usage of starch that led to lower growth, most likely associated with decreased respiration. Collectively, our findings suggest that mutations in different components of the auxin signalling pathway specifically modulate photosynthetic and respiratory processes.

Effect of glucose on the morpho-physiology, photosynthetic efficiency, antioxidant system, and carbohydrate metabolism in Brassica juncea

The present experiment was conducted to investigate the promotive effects of exogenous glucose (Glc) on the morpho-physiology in Brassica juncea. L. cv. RGN-48. The plants were treated with the different concentrations (0, 2, 4, and 8%) of glucose as foliar spray at 25 days after sowing (DAS) for 5 days consecutively. The plants were collected to analyze various growth and photosynthetic parameters at 30, 45, and 60 DAS. After 5 days exposure to Glc, the level of carbohydrate, total reducing sugars, proline, plant water status, chlorophyll content, as well as that of activities of peroxidase (EC 1.11.1.7), catalase (EC 1.11.1.6), and superoxide dismutase (EC 1.15.1.1) were increased. Glc application also enhanced the gaseous exchange parameters, i.e., stomatal conductance (g_s), internal CO₂ concentration (Ci), transpiration rate (E), and net photosynthetic rate (P_N) in intact leaf. Other enzymes, such as nitrate reductase (EC 1.7.99.4) and carbonic anhydrase (EC 4.2.1.1) were also increased. Additionally, microscopic studies further reveal a remarkable increase in the stomatal aperture on Glc exposure. Moreover, exogenous Glc decreases the levels of malondialdehyde (MDA), superoxide radical (O₂^·-) and hydrogen peroxide (H₂O₂). This indicates that exogenous Glc application has a positive effect on Brassica juncea plants.

MicroRNA 399 as a potential integrator of photo-response, phosphate homeostasis, and sucrose signaling under long day condition

BACKGROUND

Photoperiod-sensitivity is a critical endogenous regulatory mechanism for plant growth and development under specific environmental conditions, while phosphate and sucrose signaling processes play key roles in cell growth and organ initiation. MicroRNA399 is phosphate-responsive, but, whether it has roles in other metabolic processes remains unknown.

RESULTS

MicroRNA399 was determined to be sucrose-responsive through a microRNA array assay. High levels of sucrose inhibited the accumulation of microRNA399 family under phosphate starvation conditions in Arabidopsis thaliana. Similarly, exogenous sucrose supplementation also reduced microRNA399 expression in maize at developmental transition stages. RNA sequencing of a near-isogenic line(photoperiod-sensitive) line and its recurrent parent Huangzao4, a photoperiod-insensitive line, was conducted at various developmental stages. Members of microRNA399 family were down-regulated under long-day conditions in the photoperiod-sensitive near-isogenic line that accumulated more sucrose in vivo compared with the control line Huangzao4.

CONCLUSION

MicroRNA399s may play central roles in the integration of sucrose sensing and photoperiodic responses under long day conditions in maize.

CsINV5, a tea vacuolar invertase gene enhances cold tolerance in transgenic Arabidopsis

BACKGROUND

Vacuolar invertases (VINs) have been reported to regulate plant growth and development and respond to abiotic stresses such as drought and cold. With our best knowledge, the functions of VIN genes little have been reported in tea plant (Camellia sinensis L.). Therefore, it is necessary to develop research in this field.

RESULTS

Here, we identified a VIN gene, CsINV5, which was induced by cold acclimation and sugar treatments in the tea plant. Histochemical assays results showed that the 1154 bp 5'-flanking sequence of CsINV5 drove β-glucuronidase (GUS) gene expression in roots, stems, leaves, flowers and siliques of transgenic Arabidopsis during different developmental stages. Moreover, promoter deletion analysis results revealed that an LTRE-related motif (CCGAAA) and a WBOXHVISO1 motif (TGACT) within the promoter region of CsINV5 were the core cis-elements in response to low temperature and sugar signaling, respectively. In addition, overexpression of CsINV5 in Arabidopsis promoted taproot and lateral root elongation through glucose-mediated effects on auxin signaling. Based on physiological and RNA-seq analysis, we found that overexpression of CsINV5 improved cold tolerance in transgenic Arabidopsis mainly by increasing the contents of glucose and fructose, the corresponding ratio of hexose to sucrose, and the transcription of osmotic-stress-related genes (P5CS1, P5CS2, AtLEA3, COR413-PM1 and COR15B) to adjust its osmotic potential.

CONCLUSIONS

Comprehensive experimental results suggest that overexpression of CsINV5 may enhance the cold tolerance of plant through the modification of cellular sugar compounds contents and osmotic regulation related pathways.

Autophagy regulates glucose-mediated root meristem activity by modulating ROS production in Arabidopsis

Glucose produced from photosynthesis is a key nutrient signal regulating root meristem activity in plants; however, the underlying mechanisms remain poorly understood. Here, we show that, by modulating reactive oxygen species (ROS) levels, the conserved macroautophagy/autophagy degradation pathway contributes to glucose-regulated root meristem maintenance. In Arabidopsis thaliana roots, a short exposure to elevated glucose temporarily suppresses constitutive autophagosome formation. The autophagy-defective autophagy-related gene (atg) mutants have enhanced tolerance to glucose, established downstream of the glucose sensors, and accumulate less glucose-induced ROS in the root tips. Moreover, the enhanced root meristem activities in the atg mutants are associated with improved auxin gradients and auxin responses. By acting with AT4G39850/ABCD1 (ATP-binding cassette D1; Formerly PXA1/peroxisomal ABC transporter 1), autophagy plays an indispensable role in the glucose-promoted degradation of root peroxisomes, and the atg mutant phenotype is partially rescued by the overexpression of ABCD1. Together, our findings suggest that autophagy is an essential mechanism for glucose-mediated maintenance of the root meristem.

Abbreviation: ABA: abscisic acid; ABCD1: ATP-binding cassette D1; ABO: ABA overly sensitive; AsA: ascorbic acid; ATG: autophagy related; CFP: cyan fluorescent protein; Co-IP: co-immunoprecipitation; DAB: 3’,3’-diaininobenzidine; DCFH-DA: 2’,7’-dichlorodihydrofluorescin diacetate; DR5: a synthetic auxin response element consists of tandem direct repeats of 11 bp that included the auxin-responsive TGTCTC element; DZ: differentiation zone; EZ, elongation zone; GFP, green fluorescent protein; GSH, glutathione; GUS: β-glucuronidase; HXK1: hexokinase 1; H₂O₂: hydrogen peroxide; IAA: indole-3-acetic acid; IBA: indole-3-butyric acid; KIN10/11: SNF1 kinase homolog 10/11; MDC: monodansylcadaverine; MS: Murashige and Skoog; MZ: meristem zone; NBT: nitroblue tetrazolium; NPA: 1-N-naphtylphthalamic acid; OxIAA: 2-oxindole-3-acetic acid; PIN: PIN-FORMED; PLT: PLETHORA; QC: quiescent center; RGS1: Regulator of G-protein signaling 1; ROS: reactive oxygen species; SCR: SCARECROW; SHR, SHORT-ROOT; SKL: Ser-Lys-Leu; SnRK1: SNF1-related kinase 1; TOR: target of rapamycin; UPB1: UPBEAT1; WOX5: WUSCHEL related homeobox 5; Y2H: yeast two-hybrid; YFP: yellow fluorescent protein

The Sugar-Signaling Hub: Overview of Regulators and Interaction with the Hormonal and Metabolic Network

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.

The Sugar-Signaling Hub: Overview of Regulators and Interaction with the Hormonal and Metabolic Network

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.

Phytochrome B regulates resource allocation in Brassica rapa

Crop biomass and yield are tightly linked to how the light signaling network translates information about the environment into allocation of resources, including photosynthates. Once activated, the phytochrome (phy) class of photoreceptors signal and re-deploy carbon resources to alter growth, plant architecture, and reproductive timing. Most of the previous characterization of the light-modulated growth program has been performed in the reference plant Arabidopsis thaliana. Here, we use Brassica rapa as a crop model to test for conservation of the phytochrome-carbon network. In response to elevated levels of CO2, B. rapa seedlings showed increases in hypocotyl length, shoot and root fresh weight, and the number of lateral roots. All of these responses were dependent on nitrogen and polar auxin transport. In addition, we identified putative B. rapa orthologs of PhyB and isolated two nonsense alleles. BrphyB mutants had significantly decreased or absent CO2-stimulated growth responses. Mutant seedlings also showed misregulation of auxin-dependent genes and genes involved in chloroplast development. Adult mutant plants had reduced chlorophyll levels, photosynthetic rate, stomatal index, and seed yield. These findings support a recently proposed holistic role for phytochromes in regulating resource allocation, biomass production, and metabolic state in the developing plant.

Effects of glucose and ethylene on root hair initiation and elongation in lettuce (Lactuca sativa L.) seedlings

Root hair formation occurs in lettuce seedlings after transfer to an acidic medium (pH 4.0). This process requires cortical microtubule (CMT) randomization in root epidermal cells and the plant hormone ethylene. We investigated the interaction between ethylene and glucose, a new signaling molecule in plants, in lettuce root development, with an emphasis on root hair formation. Dark-grown seedlings were used to exclude the effect of photosynthetically produced glucose. In the dark, neither root hair formation nor the CMT randomization preceding it occurred, even after transfer to the acidic medium (pH 4.0). Adding 1-aminocyclopropane-1-carboxylic-acid (ACC) to the medium rescued the induction, while adding glucose did not. Although CMT randomization occurred when glucose was applied together with ACC, it was somewhat suppressed compared to that in ACC-treated seedlings. This was not due to a decrease in the speed of randomization, but due to lowering of the maximum degree of randomization. Despite the negative effect of glucose on ACC-induced CMT randomization, the density and length of ACC-induced root hairs increased when glucose was also added. The hair-cell length of the ACC-treated seedlings was comparable to that in the combined-treatment seedlings, indicating that the increase in hair density caused by glucose results from an increase in the root hair number. Furthermore, quantitative RT-PCR revealed that glucose suppressed ethylene signaling. These results suggest that glucose has a negative and positive effect on the earlier and later stages of root hair formation, respectively, and that the promotion of the initiation and elongation of root hairs by glucose may be mediated in an ethylene-independent manner.

Auxin production as an integrator of environmental cues for developmental growth regulation

Being sessile organisms, plants have evolved mechanisms allowing them to control their growth and development in response to environmental changes. This occurs by means of complex interacting signalling networks that integrate diverse environmental cues into co-ordinated and highly regulated responses. Auxin is an essential phytohormone that functions as a signalling molecule, driving both growth and developmental processes. It is involved in numerous biological processes ranging from control of cell expansion and cell division to tissue specification, embryogenesis, and organ development. All these processes require the formation of auxin gradients established and maintained through the combined processes of biosynthesis, metabolism, and inter- and intracellular directional transport. Environmental conditions can profoundly affect the plant developmental programme, and the co-ordinated shoot and root growth ought to be fine-tuned to environmental challenges such as temperature, light, and nutrient and water content. The key role of auxin as an integrator of environmental signals has become clear in recent years, and emerging evidence implicates auxin biosynthesis as an essential component of the overall mechanisms of plants tolerance to stress. In this review, we provide an account of auxin's role as an integrator of environmental signals and, in particular, we highlight the effect of these signals on the control of auxin production.

Control of Endogenous Auxin Levels in Plant Root Development

In this review, we summarize the different biosynthesis-related pathways that contribute to the regulation of endogenous auxin in plants. We demonstrate that all known genes involved in auxin biosynthesis also have a role in root formation, from the initiation of a root meristem during embryogenesis to the generation of a functional root system with a primary root, secondary lateral root branches and adventitious roots. Furthermore, the versatile adaptation of root development in response to environmental challenges is mediated by both local and distant control of auxin biosynthesis. In conclusion, auxin homeostasis mediated by spatial and temporal regulation of auxin biosynthesis plays a central role in determining root architecture.

Arabidopsis RSS1 Mediates Cross-Talk Between Glucose and Light Signaling During Hypocotyl Elongation Growth

Plants possess exuberant plasticity that facilitates its ability to adapt and survive under challenging environmental conditions. The developmental plasticity largely depends upon cellular elongation which is governed by a complex network of environmental and phytohormonal signals. Here, we report role of glucose (Glc) and Glc-regulated factors in controlling elongation growth and shade response in Arabidopsis. Glc controls shade induced hypocotyl elongation in a dose dependent manner. We have identified a Glc repressed factor REGULATED BY SUGAR AND SHADE1 (RSS1) encoding for an atypical basic helix-loop-helix (bHLH) protein of unknown biological function that is required for normal Glc actions. Phenotype analysis of mutant and overexpression lines suggested RSS1 to be a negative regulator of elongation growth. RSS1 affects overall auxin homeostasis. RSS1 interacts with the elongation growth-promoting proteins HOMOLOG OF BEE2 INTERACTING WITH IBH 1 (HBI1) and BR ENHANCED EXPRESSION2 (BEE2) and negatively affects the transcription of their downstream targets such as YUCs, INDOLE-3-ACETIC ACID INDUCIBLE (IAAs), LONG HYPOCOTYL IN FAR-RED1 (HFR1), HOMEOBOX PROTEIN 2 (ATHB2), XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASES (XTHs) and EXPANSINS. We propose, Glc signals might maintain optimal hypocotyl elongation under multiple signals such as light, shade and phytohormones through the central growth regulatory bHLH/HLH module.

Trehalose 6-phosphate is involved in triggering axillary bud outgrowth in garden pea (Pisum sativum L.)

Trehalose 6-phosphate (Tre6P) is a signal of sucrose availability in plants, and has been implicated in the regulation of shoot branching by the abnormal branching phenotypes of Arabidopsis (Arabidopsis thaliana) and maize (Zea mays) mutants with altered Tre6P metabolism. Decapitation of garden pea (Pisum sativum) plants has been proposed to release the dormancy of axillary buds lower down the stem due to changes in sucrose supply, and we hypothesized that this response is mediated by Tre6P. Decapitation led to a rapid and sustained rise in Tre6P levels in axillary buds, coinciding with the onset of bud outgrowth. This response was suppressed by simultaneous defoliation that restricts the supply of sucrose to axillary buds in decapitated plants. Decapitation also led to a rise in amino acid levels in buds, but a fall in phosphoenolpyruvate and 2-oxoglutarate. Supplying sucrose to stem node explants in vitro triggered a concentration-dependent increase in the Tre6P content of the buds that was highly correlated with their rate of outgrowth. These data show that changes in bud Tre6P levels are correlated with initiation of bud outgrowth following decapitation, suggesting that Tre6P is involved in the release of bud dormancy by sucrose. Tre6P might also be linked to a reconfiguration of carbon and nitrogen metabolism to support the subsequent growth of the bud into a new shoot.

Identification of candidate genes involved in the sugar metabolism and accumulation during pear fruit post-harvest ripening of 'Red Clapp's Favorite' (Pyrus communis L.) by transcriptome analysis

Background

Pear (Pyrus spp.) is a popular fruit that is commercially cultivated in most temperate regions. In fruits, sugar metabolism and accumulation are important factors for fruit organoleptic quality. Post-harvest ripening is a special feature of ‘Red Clapp’s Favorite’.

Results

In this study, transcriptome sequencing based on the Illumina platform generated 23.8 - 35.8 million unigenes of nine cDNA libraries constructed using RNAs from the ‘Red Clapp’s Favorite’ pear variety with different treatments, in which 2629 new genes were discovered, and 2121 of them were annotated. A total of 2146 DEGs, 3650 DEGs, 1830 DEGs from each comparison were assembled. Moreover, the gene expression patterns of 8 unigenes related to sugar metabolism revealed by qPCR. The main constituents of soluble sugars were fructose and glucose after pear fruit post-harvest ripening, and five unigenes involved in sugar metabolism were discovered.

Conclusions

Our study not only provides a large-scale assessment of transcriptome resources of ‘Red Clapp’s Favorite’ but also lays the foundation for further research into genes correlated with sugar metabolism.

Electronic supplementary material

The online version of this article (10.1186/s41065-017-0046-0) contains supplementary material, which is available to authorized users.

Effects of Elevated Carbon Dioxide on Photosynthesis and Carbon Partitioning: A Perspective on Root Sugar Sensing and Hormonal Crosstalk

Plant responses to atmospheric carbon dioxide will be of great concern in the future, as carbon dioxide concentrations ([CO2]) are predicted to continue to rise. Elevated [CO2] causes increased photosynthesis in plants, which leads to greater production of carbohydrates and biomass. Which organ the extra carbohydrates are allocated to varies between species, but also within species. These carbohydrates are a major energy source for plant growth, but they also act as signaling molecules and have a range of uses beyond being a source of carbon and energy. Currently, there is a lack of information on how the sugar sensing and signaling pathways of plants are affected by the higher content of carbohydrates produced under elevated [CO2]. Particularly, the sugar signaling pathways of roots are not well understood, along with how they are affected by elevated [CO2]. At elevated [CO2], some plants allocate greater amounts of sugars to roots where they are likely to act on gene regulation and therefore modify nutrient uptake and transport. Glucose and sucrose also promote root growth, an effect similar to what occurs under elevated [CO2]. Sugars also crosstalk with hormones to regulate root growth, but also affect hormone biosynthesis. This review provides an update on the role of sugars as signaling molecules in plant roots and thus explores the currently known functions that may be affected by elevated [CO2].

Suppression of sucrose synthase affects auxin signaling and leaf morphology in tomato

Metabolic enzymes have been found to play roles in plant development. Sucrose synthase (SUS) is one of the two enzyme families involved in sucrose cleavage in plants. In tomato, six SUS genes have been found. We generated transgenic tomato plants with RNAi suppression of SlSUS1, SlSUS3 and SlSUS4 genes. Independent transgenic lines with RNAi suppression of more than one SUS gene exhibited morphological effects on their cotyledons and leaf structure, but there were no significant effects on their carbohydrate levels, demonstrating that SUS has a developmental function, in addition to its metabolic function. Shoot apices of the transgenic lines showed elevated expression of JAGGED (JAG) and the auxin transporter PIN1. In a PIN1-GFP fusion reporter/SUS-RNAi hybrid, PIN1-GFP patterns were altered in developing leaves (as compared to control plants), indicating that SlSUS suppression alters auxin signaling. These results suggest possible roles for SUS in the regulation of plant growth and leaf morphology, in association with the auxin-signaling pathway.

Striking the Right Chord: Signaling Enigma during Root Gravitropism

Plants being sessile can often be judged as passive acceptors of their environment. However, plants are actually even more active in responding to the factors from their surroundings. Plants do not have eyes, ears or vestibular system like animals, still they "know" which way is up and which way is down? This is facilitated by receptor molecules within plant which perceive changes in internal and external conditions such as light, touch, obstacles; and initiate signaling pathways that enable the plant to react. Plant responses that involve a definite and specific movement are called "tropic" responses. Perhaps the best known and studied tropisms are phototropism, i.e., response to light, and geotropism, i.e., response to gravity. A robust root system is vital for plant growth as it can provide physical anchorage to soil as well as absorb water, nutrients and essential minerals from soil efficiently. Gravitropic responses of both primary as well as lateral root thus become critical for plant growth and development. The molecular mechanisms of root gravitropism has been delved intensively, however, the mechanism behind how the potential energy of gravity stimulus converts into a biochemical signal in vascular plants is still unknown, due to which gravity sensing in plants still remains one of the most fascinating questions in molecular biology. Communications within plants occur through phytohormones and other chemical substances produced in plants which have a developmental or physiological effect on growth. Here, we review current knowledge of various intrinsic signaling mechanisms that modulate root gravitropism in order to point out the questions and emerging developments in plant directional growth responses. We are also discussing the roles of sugar signals and their interaction with phytohormone machinery, specifically in context of root directional responses.

Plant Hexokinases are Multifaceted Proteins

Sugars are the main carbon and energy source in cells, but they can also act as signaling molecules that affect the whole plant life cycle. Certain tissues can produce sugars and supply them to others, and this plant tissue heterogeneity makes sugar signaling a highly complex process that requires elements capable of perceiving changes in sugar concentrations among different tissues, cell compartments and developmental stages. In plants, the regulatory effects of glucose (Glc) have been the most studied to date. The first Glc sensor identified in plants was hexokinase (HXK), which is currently recognized as a dual-function protein. In addition to its catalytic activity, this enzyme can also repress the expression of some photosynthetic genes in response to high internal Glc concentrations. Additionally, the catalytic activity of HXKs has a profound impact on cell metabolism and other sugar signaling pathways that depend on phosphorylated hexoses and intermediate glycolytic products. HXKs are the only proteins that are able to phosphorylate Glc in plants, since no evidence has been provided to date concerning the existence of a glucokinase. Moreover, the intracellular localization of HXKs seems to be crucial to their activity and sensor functions. Recently, two new and surprising functions have been described for HXKs. In this review, we discuss the versatility of HXKs in regard to their catalytic and glucose sensor activities, intracellular location, protein-protein and hormone interactions, as well as how these HXK characteristics influence plant growth and development, in an effort to understand this enzyme's role in improving plant productivity.

The interaction between glucose and cytokinin signaling in controlling Arabidopsis thaliana seedling root growth and development

Cytokinin (CK) and glucose (GLC) control several common responses in plants. There is an extensive overlap between CK and GLC signal transduction pathways in Arabidopsis. Physiologically, both GLC and CK could regulate root length in light. CK interacts with GLC via HXK1 dependent pathway for root length control. Wild-type (WT) roots cannot elongate in the GLC free medium while CK-receptor mutant ARABIDOPSIS HISTIDINE KINASE4 (ahk4) and type B ARR triple mutant ARABIDOPSIS RESPONSE REGULATOR1, 10,11 (arr1, 10,11) roots could elongate even in the absence of GLC as compared with the WT. The root hair initiation was also found defective in CK signaling mutants ahk4, arr1,10,11 and arr3,4,5,6,8,9 on increasing GLC concentration (up to 3%); and lesser number of root hairs were visible even at 5% GLC as compared with the WT. Out of 941 BAP regulated genes, 103 (11%) genes were involved in root growth and development. Out of these 103 genes, 60 (58%) genes were also regulated by GLC. GLC could regulate 5736 genes, which include 327 (6%) genes involved in root growth and development. Out of these 327 genes, 60 (18%) genes were also regulated by BAP. Both GLC and CK signaling cannot alter root length in light in auxin signaling mutant AUXIN RESPONSE3/INDOLE-3-ACETIC ACID17 (axr3/iaa17) suggesting that they may involve auxin signaling component as a nodal point. Therefore CK- and GLC- signaling are involved in controlling different aspects of root growth and development such as root length, with auxin signaling components working as downstream target.

Rice Homeodomain Protein WOX11 Recruits a Histone Acetyltransferase Complex to Establish Programs of Cell Proliferation of Crown Root Meristem

Shoot-borne crown roots are the major root system in cereals. Previous work has shown that the Wuschel-related homeobox gene WOX11 is necessary and sufficient to promote rice (Oryza sativa) crown root emergence and elongation. Here, we show that WOX11 recruits the ADA2-GCN5 histone acetyltransferase module to activate downstream target genes in crown root meristem. Rice ADA2 and GCN5 genes are highly expressed in root meristem and are shown to be essential for cell division and growth. WOX11 and ADA2-GCN5 commonly target and regulate a set of root-specific genes involved in energy metabolism, cell wall biosynthesis, and hormone response, some of which are known to be important for root development. The results indicate that the recruitment of ADA2-GCN5 by WOX11 establishes gene expression programs of crown root meristem cell division and suggest that permissive chromatin modification involving histone acetylation is a strategy for WOX11 to stimulate root meristem development.

Glucose and sucrose differentially modify cell proliferation in maize during germination

Glucose and sucrose play a dual role: as carbon and energy sources and as signaling molecules. In order to address the impact that sugars may have on maize seeds during germination, embryo axes were incubated with or without either of the two sugars. Expression of key cell cycle markers and protein abundance, cell patterning and de novo DNA synthesis in root meristem zones were analyzed. Embryo axes without added sugars in imbibition medium were unable to grow after 7 days; in sucrose, embryo axes developed seminal and primary roots with numerous root hairs, whereas in glucose axes showed a twisted morphology, no root hair formation but callus-like structures on adventitious and primary seminal roots. More and smaller cells were observed with glucose treatment in root apical meristems. de novo DNA synthesis was stimulated more by glucose than by sucrose. At 24 h of imbibition, expression of ZmCycD2;2a and ZmCycD4;2 was increased by sucrose and reduced by glucose. CDKA1;1 and CDKA2;1 expression was stimulated equally by both sugars. Protein abundance patterns were modified by sugars: ZmCycD2 showed peaks on glucose at 12 and 36 h of imbibition whereas sucrose promoted ZmCycD3 protein accumulation. In presence of glucose ZmCycD3, ZmCycD4 and ZmCycD6 protein abundance was reduced after 24 h. Finally, both sugars stimulated ZmCDKA protein accumulation but at different times. Overall, even though glucose appears to act as a stronger mitogen stimulator, sucrose stimulated the expression of more cell cycle markers during germination. This work provides evidence of a differential response of cell cycle markers to sucrose and glucose during maize germination that may affect the developmental program during plantlet establishment.

Apple dwarfing rootstocks exhibit an imbalance in carbohydrate allocation and reduced cell growth and metabolism

Apple dwarfing rootstocks cause earlier shoot termination and reduced root and shoot mass. To identify physiological factors responsible for rootstock-induced growth restriction, we compared vascular-enriched gene expression between two dwarfing rootstocks ('M27' and 'M9') and the vigorous rootstock 'M793' using RNA sequencing and quantitative reverse transcriptase PCR. Differentially expressed genes common to both dwarfing rootstocks belonged to five main biological processes: (1) primary metabolism, (2) cell wall synthesis and modification, (3) secondary metabolism, (4) hormone signalling and response and (5) redox homeostasis. Genes promoting the biosynthesis of amino acids, lipids and cell walls were downregulated in dwarfing rootstocks, whereas genes promoting the breakdown of these compounds were upregulated. The only exception to this trend was the upregulation of starch synthesis genes in dwarfing rootstocks. Non-structural carbohydrate analysis demonstrated that starch concentrations in 'M9' roots, stems and grafted 'Royal Gala' ('RG') scions were double that of equivalent tissues from 'RG' homo-grafted trees ('RG'/'RG'). Fructose and glucose concentrations were much lower in all three tissues of the 'RG'/'M9' trees. Together, these data indicate that dwarfing rootstocks are in a state of sugar depletion and reduced cellular activity despite having large starch reserves. Another significant finding was the over-accumulation of flavonoids and the downregulation of auxin influx transporters MdAUX1 and MdLAX2 in dwarfing rootstocks. We propose that both factors reduce polar auxin transport. The results of this study contribute novel information about the physiological state of dwarfing rootstocks.

Genome-Wide Identification and Expression, Protein-Protein Interaction and Evolutionary Analysis of the Seed Plant-Specific BIG GRAIN and BIG GRAIN LIKE Gene Family

BIG GRAIN1 (BG1) is an auxin-regulated gene which functions in auxin pathway and positively regulates biomass, grain size and yield in rice. However, the evolutionary origin and divergence of these genes are still unknown. In this study, we found that BG genes are probably originated in seed plants. We also identified that seed plants evolved a class of BIG GRAIN LIKE (BGL) genes which share conserved middle and C-terminal motifs with BG. The BG genes were present in all monocot and eudicot species analyzed; however, the BGL genes were absent in few monocot lineages. Both BG and BGL were found to be serine-rich proteins; however, differences in expansion and rates of retention after whole genome duplication events were observed. Promoters of BG and BGL genes were found to be enriched with auxin-responsive elements and the Arabidopsis thaliana BG and BGL genes were found to be auxin-regulated. The auxin-induced expression of AthBG2 was found to be dependent on the conserved ARF17/19 module. Protein-protein interaction analysis identified that AthBG2 interact with regulators of splicing, transcription and chromatin remodeling. Taken together, this study provides interesting insights about BG and BGL genes and incentivizes future work in this gene family which has the potential to be used for crop manipulation.

Effects of potato spindle tuber viroid infection on tomato metabolic profile

Viroids are the smallest plant pathogens consisting of a single stranded circular RNA molecule with a strong secondary structure, lacking a coat protein or any other proteins. The mechanism of viroid pathogenicity has remained unclear. Recent advances in instrumentation and data mining have made it possible to study the effects of various stresses on primary and secondary metabolisms. Here, we have utilized metabolic profiling approach to show how PSTVd infection alters tomato metabolic profile and the related pathways. Three terminal leaflets of third true leaf of 20-day-old tolerant tomato cultivar 'Moneymaker' were mechanically inoculated by PSTVd intermediate variant cDNAs and samples were taken from eighth leaf, 19days post-inoculation. Metabolites were extracted and analyzed by gas chromatography/mass spectrometry (GC/MS) and subjected to statistical data analysis. Affected pathways were identified by Pathway Tools program and were compared with microarray data previously reported. The study showed that 79 metabolites changed significantly and 23 pathways were identified in relation to these metabolites. Fourteen of these pathways were similar to those reported in other works. The altered pathways in PSTVd infected tomato leaves included, eight cutin and wax biosynthesis, seven pathways that produce defense related compounds, two energy generator pathways, three hormone biosynthesis pathways, two signal transduction pathways, and one nucleotide biosynthesis pathway. Our data on up/down-regulation of pathways supported the data produced on their corresponding gene(s) up/down-regulation.

CuO Nanoparticle Interaction with Arabidopsis thaliana: Toxicity, Parent-Progeny Transfer, and Gene Expression

CuO nanoparticles (NPs) (20, 50 mg L(-1)) inhibited seedling growth of different Arabidopsis thaliana ecotypes (Col-0, Bay-0, and Ws-2), as well as the germination of their pollens and harvested seeds. For most of growth parameters (e.g., biomass, relative growth rate, root morphology change), Col-0 was the more sensitive ecotype to CuO NPs compared to Bay-0 and Ws-2. Equivalent Cu(2+) ions and CuO bulk particles had no effect on Arabidopsis growth. After CuO NPs (50 mg L(-1)) exposure, Cu was detected in the roots, leaves, flowers and harvested seeds of Arabidopsis, and its contents were significantly higher than that in CuO bulk particles (50 mg L(-1)) and Cu(2+) ions (0.15 mg L(-1)) treatments. Based on X-ray absorption near-edge spectroscopy analysis (XANES), Cu in the harvested seeds was confirmed as being mainly in the form of CuO (88.8%), which is the first observation on the presence of CuO NPs in the plant progeny. Moreover, after CuO NPs exposure, two differentially expressed genes (C-1 and C-3) that regulated root growth and reactive oxygen species generation were identified, which correlated well with the physiological root inhibition and oxidative stress data. This current study provides direct evidence for the negative effects of CuO NPs on Arabidopsis, including accumulation and parent-progeny transfer of the particles, which may have significant implications with regard to the risk of NPs to food safety and security.

Critical Roles of Vacuolar Invertase in Floral Organ Development and Male and Female Fertilities Are Revealed through Characterization of GhVIN1-RNAi Cotton Plants

Seed number and quality are key traits determining plant fitness and crop yield and rely on combined competence in male and female fertilities. Sucrose metabolism is central to reproductive success. It remains elusive, though, how individual sucrose metabolic enzymes may regulate the complex reproductive processes. Here, by silencing vacuolar invertase (VIN) genes in cotton (Gossypium hirsutum) reproductive organs, we revealed diverse roles that VIN plays in multiple reproductive processes. A set of phenotypic and genetic studies showed significant reductions of viable seeds in GhVIN1-RNAi plants, attributed to pollination failure and impaired male and female fertilities. The former was largely owing to the spatial mismatch between style and stamen and delayed pollen release from the anthers, whereas male defects came from poor pollen viability. The transgenic stamen exhibited altered expression of the genes responsible for starch metabolism and auxin and jasmonic acid signaling. Further analyses identified the reduction of GhVIN expression in the seed coat as the major cause for the reduced female fertility, which appeared to disrupt the expression of some key genes involved in trehalose and auxin metabolism and signaling, leading to programmed cell death or growth repression in the filial tissues. Together, the data provide an unprecedented example of how VIN is required to synchronize style and stamen development and the formation of male and female fertilities for seed development in a crop species, cotton.

Mycorrhiza alters the profile of root hairs in trifoliate orange

Root hairs and arbuscular mycorrhiza (AM) coexist in root systems for nutrient and water absorption, but the relation between AM and root hairs is poorly known. A pot study was performed to evaluate the effects of four different AM fungi (AMF), namely, Claroideoglomus etunicatum, Diversispora versiformis, Funneliformis mosseae, and Rhizophagus intraradices on root hair development in trifoliate orange (Poncirus trifoliata) seedlings grown in sand. Mycorrhizal seedlings showed significantly higher root hair density than non-mycorrhizal seedlings, irrespective of AMF species. AMF inoculation generally significantly decreased root hair length in the first- and second-order lateral roots but increased it in the third- and fourth-order lateral roots. AMF colonization induced diverse responses in root hair diameter of different order lateral roots. Considerably greater concentrations of phosphorus (P), nitric oxide (NO), glucose, sucrose, indole-3-acetic acid (IAA), and methyl jasmonate (MeJA) were found in roots of AM seedlings than in non-AM seedlings. Levels of P, NO, carbohydrates, IAA, and MeJA in roots were correlated with AM formation and root hair development. These results suggest that AMF could alter the profile of root hairs in trifoliate orange through modulation of physiological activities. F. mosseae, which had the greatest positive effects, could represent an efficient AM fungus for increasing fruit yields or decreasing fertilizer inputs in citrus production.

Shoot-root carbon allocation, sugar signalling and their coupling with nitrogen uptake and assimilation

Roots and shoots are distantly located but functionally interdependent. The growth and development of these two organ systems compete for energy and nutrient resource, and yet, they keep a dynamic balance with each other for growth and development. The success of such a relationship depends on efficient root-shoot communication. Aside from the well-known signalling processes mediated by hormones such as auxin and cytokinin, sugars have recently been shown to act as a rapid signal to co-ordinate root and shoot development in response to endogenous and exogenous clues, in parallel to their function as carbon and energy resources for biomass production. New findings from studies on vascular fluids have provided molecular insights into the role of sugars in long-distance communications between shoot and root. In this review, we discussed phloem- and xylem- translocation of sugars and the impacts of sugar allocation and signalling on balancing root-shoot development. Also, we have taken the shoot-root carbon-nitrogen allocation as an example to illustrate the communication between the two organs through multi-layer root-shoot-root signalling circuits, comprising sugar, nitrogen, cytokinin, auxin and vascular small peptide signals.

Haustorial Hairs Are Specialized Root Hairs That Support Parasitism in the Facultative Parasitic Plant Phtheirospermum japonicum

A haustorium is the unique organ that invades host tissues and establishes vascular connections. Haustorium formation is a key event in parasitism, but its underlying molecular basis is largely unknown. Here, we use Phtheirospermum japonicum, a facultative root parasite in the Orobanchaceae, as a model parasitic plant. We performed a forward genetic screen to identify mutants with altered haustorial morphologies. The development of the haustorium in P. japonicum is induced by host-derived compounds such as 2,6-dimethoxy-p-benzoquinone. After receiving the signal, the parasite root starts to swell to develop a haustorium, and haustorial hairs proliferate to densely cover the haustorium surface. We isolated mutants that show defects in haustorial hair formation and named them haustorial hair defective (hhd) mutants. The hhd mutants are also defective in root hair formation, indicating that haustorial hair formation is controlled by the root hair development program. The internal structures of the haustoria in the hhd mutants are similar to those of the wild type, indicating that the haustorial hairs are not essential for host invasion. However, all the hhd mutants form fewer haustoria than the wild type upon infection of the host roots. The number of haustoria is restored when the host and parasite roots are forced to grow closely together, suggesting that the haustorial hairs play a role in stabilizing the host-parasite association. Thus, our study provides genetic evidence for the regulation and function of haustorial hairs in the parasitic plant.

Transcriptome analysis reveals the effects of sugar metabolism and auxin and cytokinin signaling pathways on root growth and development of grafted apple

BACKGROUND

The root architecture of grafted apple (Malus spp.) is affected by various characteristics of the scions. To provide information on the molecular mechanisms underlying this influence, we examined root transcriptomes of M. robusta rootstock grafted with scions of wild-type (WT) apple (M. spectabilis) and a more-branching (MB) mutant at the branching stage.

RESULTS

The growth rate of rootstock grafted MB was repressed significantly, especially the primary root length and diameter, and root weight. Biological function categories of differentially expressed genes were significantly enriched in processes associated with hormone signal transduction and intracellular activity, with processes related to the cell cycle especially down-regulated. Roots of rootstock grafted with MB scions displayed elevated auxin and cytokinin contents and reduced expression of MrPIN1, MrARF, MrAHP, most MrCRE1 genes, and cell growth-related genes MrGH3, MrSAUR and MrTCH4. Although auxin accumulation and transcription of MrPIN3, MrALF1 and MrALF4 tended to induce lateral root formation in MB-grafted rootstock, the number of lateral roots was not significantly changed. Sucrose, fructose and glucose contents were not decreased in MB-grafted roots compared with those bearing WT scions, but glycolysis and tricarboxylic acid cycle metabolic activities were repressed. Root resistance and nitrogen metabolism were reduced in MB-grafted roots as well.

CONCLUSIONS

Our findings suggest that root growth and development of rootstock are mainly influenced by sugar metabolism and auxin and cytokinin signaling pathways. This study provides a basis that the characteristics of scions are related to root growth and development, resistance and activity of rootstocks.

The WUSCHEL Related Homeobox Protein WOX7 Regulates the Sugar Response of Lateral Root Development in Arabidopsis thaliana

Sugars promote lateral root formation at low levels but become inhibitory at high C/N or C/P ratios. How sugars suppress lateral root formation is unclear, however. Here we report that WOX7, a member of the WUSCHEL related homeobox (WOX) family transcription factors, inhibits lateral root development in a sugar-dependent manner. The number of lateral root primordia increased in wox7 mutants but decreased in plants over-expressing WOX7. Plants expressing the WOX7-VP16 fusion protein produced even more lateral roots than wox7, suggesting that WOX7 acts as a transcriptional repressor in lateral root development. WOX7 is expressed at all stages of lateral root development, but it is primarily involved in lateral root initiation. Consistent with this, the wox7 mutant had a higher mitotic activity only at early stages of lateral root development. Further studies suggest that WOX7 regulates lateral root development through direct repression of cell cycle genes, particularly CYCD6;1. WOX7 expression was enhanced by sugar, reduced by auxin, but did not respond to salt and mannitol. In the wox7 mutant, the effect of sugar on lateral root formation was mitigated. These results together suggest that WOX7 plays an important role in coupling the lateral root development program and sugar status in plants.

Brassinosteroids Regulate Root Growth, Development, and Symbiosis

Brassinosteroids (BRs) are natural plant hormones critical for growth and development. BR deficient or signaling mutants show significantly shortened root phenotypes. However, for a long time, it was thought that these phenotypes were solely caused by reduced cell elongation in the mutant roots. Functions of BRs in regulating root development have been largely neglected. Nonetheless, recent detailed analyses, revealed that BRs are not only involved in root cell elongation but are also involved in many aspects of root development, such as maintenance of meristem size, root hair formation, lateral root initiation, gravitropic response, mycorrhiza formation, and nodulation in legume species. In this review, current findings on the functions of BRs in mediating root growth, development, and symbiosis are discussed.

Photosynthate Regulation of the Root System Architecture Mediated by the Heterotrimeric G Protein Complex in Arabidopsis

Assimilate partitioning to the root system is a desirable developmental trait to control but little is known of the signaling pathway underlying partitioning. A null mutation in the gene encoding the Gβ subunit of the heterotrimeric G protein complex, a nexus for a variety of signaling pathways, confers altered sugar partitioning in roots. While fixed carbon rapidly reached the roots of wild type and agb1-2 mutant seedlings, agb1 roots had more of this fixed carbon in the form of glucose, fructose, and sucrose which manifested as a higher lateral root density. Upon glucose treatment, the agb1-2 mutant had abnormal gene expression in the root tip validated by transcriptome analysis. In addition, PIN2 membrane localization was altered in the agb1-2 mutant. The heterotrimeric G protein complex integrates photosynthesis-derived sugar signaling incorporating both membrane-and transcriptional-based mechanisms. The time constants for these signaling mechanisms are in the same range as photosynthate delivery to the root, raising the possibility that root cells are able to use changes in carbon fixation in real time to adjust growth behavior.

Inhibition of root meristem growth by cadmium involves nitric oxide-mediated repression of auxin accumulation and signalling in Arabidopsis

The root is the first plant organ to get in contact with the toxin cadmium (Cd), which is a widespread soil contaminant. Cd inhibits the growth of the primary root, but the mechanisms underlying this inhibition remain elusive. In this study, we used physiological, pharmacological and genetic approaches to investigate the roles of nitric oxide (NO) and auxin in Cd-mediated inhibition of Arabidopsis thaliana root meristem growth. Our study demonstrated that in the first 12 h of exposure, Cd inhibits primary root elongation through a decrease in the sizes of both the elongation and meristematic zones. Following Cd exposure, a decrease in auxin levels is associated with reduced PIN1/3/7 protein accumulation, but not with reduced PIN1/3/7 transcript levels. Additionally, Cd stabilized AXR3/IAA17 protein to repress auxin signalling in this Cd-mediated process. Furthermore, decreasing Cd-induced NO accumulation with either NO-specific scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) or NO synthase inhibitor N(ω) -nitro-l-Arg-methylester (l-NAME) compromised the Cd-mediated inhibition of root meristem development, reduction in auxin and PIN1/3/7 accumulation, as well as stabilization of AXR3/IAA17, indicating that NO participates in Cd-mediated inhibition of root meristem growth. Taken together, our data suggest that Cd inhibits root meristem growth by NO-mediated repression of auxin accumulation and signalling in Arabidopsis.

SIZ1-Dependent Post-Translational Modification by SUMO Modulates Sugar Signaling and Metabolism in Arabidopsis thaliana

Post-translational modification mechanisms function as switches that mediate the balance between optimum growth and the response to environmental stimuli, by regulating the activity of key proteins. SUMO (small ubiquitin-like modifier) attachment, or sumoylation, is a post-translational modification that is essential for the plant stress response, also modulating hormonal circuits to co-ordinate developmental processes. The Arabidopsis SUMO E3 ligase SAP and Miz 1 (SIZ1) is the major SUMO conjugation enhancer in response to stress, and is implicated in several aspects of plant development. Here we report that known SUMO targets are over-represented in multiple carbohydrate-related proteins, suggesting a functional link between sumoylation and sugar metabolism and signaling in plants. We subsequently observed that SUMO-conjugated proteins accumulate in response to high doses of sugar in a SIZ1-dependent manner, and that the null siz1 mutant displays increased expression of sucrose and starch catabolic genes and shows reduced starch levels. We demonstrated that SIZ1 controls germination time and post-germination growth via osmotic and sugar-dependent signaling, respectively. Glucose was specifically linked to SUMO-sugar interplay, with high levels inducing root growth inhibition and aberrant root hair morphology in siz1. The use of sugar analogs and sugar marker gene expression analysis allowed us to implicate SIZ1 in a signaling pathway dependent on glucose metabolism, probably involving modulation of SNF1-related kinase 1 (SnRK1) activity.

Comprehensive Evolutionary and Expression Analysis of FCS-Like Zinc finger Gene Family Yields Insights into Their Origin, Expansion and Divergence

Plant evolution is characterized by frequent genome duplication events. Expansion of habitat resulted in the origin of many novel genes and genome duplication events which in turn resulted in the expansion of many regulatory gene families. The plant-specific FCS-Like Zinc finger (FLZ) gene family is characterized by the presence of a FCS-Like Zinc finger (FLZ) domain which mediates the protein-protein interaction. In this study, we identified that the expansion of FLZ gene family size in different species is correlated with ancestral and lineage-specific whole genome duplication events. The subsequent gene loss found to have a greater role in determining the size of this gene family in many species. However, genomic block duplications played the significant role in the expansion of FLZ gene family in some species. Comparison of Arabidopsis thaliana and Oryza sativa FLZ gene family revealed monocot and dicot specific evolutionary trends. The FLZ genes were found to be under high purifying selection. The spatiotemporal expression analyses of Arabidopsis thaliana FLZ gene family revealed that majority of the members are highly expressed in reproductive organs. FLZ genes were also found to be highly expressed during vegetative-to-reproductive phase transition which is correlated with the proposed role of this gene family in sugar signaling. The comparison of sequence, structural and expression features of duplicated genes identified lineage-specific redundancy and divergence. This extensive evolutionary analysis and expression analysis of Arabidopsis thaliana FLZ genes will pave the way for further functional analysis of FLZ genes.

Multiple Interactions between Glucose and Brassinosteroid Signal Transduction Pathways in Arabidopsis Are Uncovered by Whole-Genome Transcriptional Profiling

Brassinosteroid (BR) and glucose (Glc) regulate many common responses in plants. Here, we demonstrate that under etiolated growth conditions, extensive interdependence/overlap occurs between BR- and Glc-regulated gene expression as well as physiological responses. Glc could regulate the transcript level of 72% of BR-regulated genes at the whole-genome level, of which 58% of genes were affected synergistically and 42% of genes were regulated antagonistically. Presence of Glc along with BR in medium could affect BR induction/repression of 85% of BR-regulated genes. Glc could also regulate several genes involved in BR metabolism and signaling. Both BR and Glc coregulate a large number of genes involved in abiotic/biotic stress responses and growth and development. Physiologically, Glc and BR interact to regulate hypocotyl elongation growth of etiolated Arabidopsis (Arabidopsis thaliana) seedlings in a dose-dependent manner. Glc may interact with BR via a hexokinase1 (HXK1)-mediated pathway to regulate etiolated hypocotyl elongation. Brassinosteroid insensitive1 (BRI1) is epistatic to HXK1, as the Glc insensitive2bri1-6 double mutant displayed severe defects in hypocotyl elongation growth similar to its bri1-6 parent. Analysis of Glc and BR sensitivity in mutants defective in auxin response/signaling further suggested that Glc and BR signals may converge at S-phase kinase-associated protein1-Cullin-F-box-transport inhibitor response1/auxin-related f-box-auxin/indole-3-acetic acid-mediated auxin-signaling machinery to regulate etiolated hypocotyl elongation growth in Arabidopsis.

Ready, steady, go! A sugar hit starts the race to shoot branching

In the classical theory of apical dominance, auxin depletion from the stem releases bud dormancy. Recent studies have revealed a poor correlation between the initial bud release and auxin depletion from the stem after decapitation. Sucrose mobility in plants and its accumulation in buds correlates well with the onset of bud release and is able to trigger bud outgrowth. The diversion of sugars away from axillary buds decreases bud release even where hormones are at levels generally considered conducive to bud release. This impact of sugars on bud outgrowth may be mediated by specific sugar and hormonal signalling pathways.

Sucrose is an early modulator of the key hormonal mechanisms controlling bud outgrowth in Rosa hybrida

Sugar has only recently been identified as a key player in triggering bud outgrowth, while hormonal control of bud outgrowth is already well established. To get a better understanding of sugar control, the present study investigated how sugar availability modulates the hormonal network during bud outgrowth in Rosa hybrida. Other plant models, for which mutants are available, were used when necessary. Buds were grown in vitro to manipulate available sugars. The temporal patterns of the hormonal regulatory network were assessed in parallel with bud outgrowth dynamics. Sucrose determined bud entrance into sustained growth in a concentration-dependent manner. Sustained growth was accompanied by sustained auxin production in buds, and sustained auxin export in a DR5::GUS-expressing pea line. Several events occurred ahead of sucrose-stimulated bud outgrowth. Sucrose upregulated early auxin synthesis genes (RhTAR1, RhYUC1) and the auxin efflux carrier gene RhPIN1, and promoted PIN1 abundance at the plasma membrane in a pPIN1::PIN1-GFP-expressing tomato line. Sucrose downregulated both RwMAX2, involved in the strigolactone-transduction pathway, and RhBRC1, a repressor of branching, at an early stage. The presence of sucrose also increased stem cytokinin content, but sucrose-promoted bud outgrowth was not related to that pathway. In these processes, several non-metabolizable sucrose analogues induced sustained bud outgrowth in R. hybrida, Pisum sativum, and Arabidopsis thaliana, suggesting that sucrose was involved in a signalling pathway. In conclusion, we identified potential hormonal candidates for bud outgrowth control by sugar. They are central to future investigations aimed at disentangling the processes that underlie regulation of bud outgrowth by sugar.

Interaction between glucose and brassinosteroid during the regulation of lateral root development in Arabidopsis

Glucose (Glc) plays a fundamental role in regulating lateral root (LR) development as well as LR emergence. In this study, we show that brassinosteroid (BR) signaling works downstream of Glc in controlling LR production/emergence in Arabidopsis (Arabidopsis thaliana) seedlings. Glc and BR can promote LR emergence at lower concentrations, while at higher concentrations, both have an inhibitory effect. The BR biosynthesis and perception mutants showed highly reduced numbers of emerged LRs at all the Glc concentrations tested. BR signaling works downstream of Glc signaling in regulating LR production, as in the glucose insensitive2-1brassinosteroid insensitive1 double mutant, Glc-induced LR production/emergence was severely reduced. Differential auxin distribution via the influx carriers AUXIN RESISTANT1/LIKE AUXIN RESISTANT1-3 and the efflux carrier PIN-FORMED2 plays a central role in controlling LR production in response to Glc and BR. Auxin signaling components AUXIN RESISTANT2,3 and SOLITARY ROOT act downstream of Glc and BR. AUXIN RESPONSE FACTOR7/19 work farther downstream and control LR production by regulating the expression of LATERAL ORGAN BOUNDARIES-DOMAIN29 and EXPANSIN17 genes. Increasing light flux could also mimic the Glc effect on LR production/emergence. However, increased light flux could not affect LR production in those BR and auxin signaling mutants that were defective for Glc-induced LR production. Altogether, our study suggests that, under natural environmental conditions, modulation of endogenous sugar levels can manipulate root architecture for optimized development by altering its nutrient/water uptake as well as its anchorage capacity.

Ethylene acts as a negative regulator of glucose induced lateral root emergence in Arabidopsis

Plants, being sessile organisms, are more exposed to the hazards of constantly changing environmental conditions globally. During the lifetime of a plant, the root system encounters various challenges such as obstacles, pathogens, high salinity, water logging, nutrient scarcity etc. The developmental plasticity of the root system provides brilliant adaptability to plants to counter the changes exerted by both external as well as internal cues and achieve an optimized growth status. Phytohormones are one of the major intrinsic factors regulating all aspects of plant growth and development both independently as well as through complex signal integrations at multiple levels. We have previously shown that glucose (Glc) and brassinosteroid (BR) signalings interact extensively to regulate lateral root (LR) development in Arabidopsis. (1) Auxin efflux as well as influx and downstream signaling components are also involved in Glc-BR regulation of LR emergence. Here, we provide evidence for involvement of ethylene signaling machinery downstream to Glc and BR in regulation of LR emergence.

Genome-wide survey and comprehensive expression profiling of Aux/IAA gene family in chickpea and soybean

Auxin plays a central role in many aspects of plant growth and development. Auxin/Indole-3-Acetic Acid (Aux/IAA) genes cooperate with several other components in the perception and signaling of plant hormone auxin. An investigation of chickpea and soybean genomes revealed 22 and 63 putative Aux/IAA genes, respectively. These genes were classified into six subfamilies on the basis of phylogenetic analysis. Among 63 soybean Aux/IAA genes, 57 (90.5%) were found to be duplicated via whole genome duplication (WGD)/segmental events. Transposed duplication played a significant role in tandem arrangements between the members of different subfamilies. Analysis of Ka/Ks ratio of duplicated Aux/IAA genes revealed purifying selection pressure with restricted functional divergence. Promoter sequence analysis revealed several cis-regulatory elements related to auxin, abscisic acid, desiccation, salt, seed, and endosperm, indicating their role in development and stress responses. Expression analysis of chickpea and soybean Aux/IAA genes in various tissues and stages of development demonstrated tissue/stage specific differential expression. In soybean, at least 16 paralog pairs, duplicated via WGD/segmental events, showed almost indistinguishable expression pattern, but eight pairs exhibited significantly diverse expression patterns. Under abiotic stress conditions, such as desiccation, salinity and/or cold, many Aux/IAA genes of chickpea and soybean revealed differential expression. qRT-PCR analysis confirmed the differential expression patterns of selected Aux/IAA genes in chickpea. The analyses presented here provide insights on putative roles of chickpea and soybean Aux/IAA genes and will facilitate elucidation of their precise functions during development and abiotic stress responses.

A new insight into root responses to external cues: Paradigm shift in nutrient sensing

Higher plants are sessile and their growth relies on nutrients present in the soil. The acquisition of nutrients is challenging for plants. Phosphate and nitrate sensing and signaling cascades play significant role during adverse conditions of nutrient unavailability. Therefore, it is important to dissect the mechanism by which plant roots acquire nutrients from the soil. Root system architecture (RSA) exhibits extensive developmental flexibility and changes during nutrient stress conditions. Growth of root system in response to external concentration of nutrients is a joint operation of sensor or receptor proteins along with several other cytoplasmic accessory proteins. After nutrient sensing, sensor proteins start the cellular relay involving transcription factors, kinases, ubiquitin ligases and miRNA. The complexity of nutrient sensing is still nebulous and many new players need to be better studied. This review presents a survey of recent paradigm shift in the advancements in nutrient sensing in relation to plant roots.

Expression of Arabidopsis FCS-Like Zinc finger genes is differentially regulated by sugars, cellular energy level, and abiotic stress

Cellular energy status is an important regulator of plant growth, development, and stress mitigation. Environmental stresses ultimately lead to energy deficit in the cell which activates the SNF1-RELATED KINASE 1 (SnRK1) signaling cascade which eventually triggering a massive reprogramming of transcription to enable the plant to survive under low-energy conditions. The role of Arabidopsis thaliana FCS-Like Zinc finger (FLZ) gene family in energy and stress signaling is recently come to highlight after their interaction with kinase subunits of SnRK1 were identified. In a detailed expression analysis in different sugars, energy starvation, and replenishment series, we identified that the expression of most of the FLZ genes is differentially modulated by cellular energy level. It was found that FLZ gene family contains genes which are both positively and negatively regulated by energy deficit as well as energy-rich conditions. Genetic and pharmacological studies identified the role of HEXOKINASE 1- dependent and energy signaling pathways in the sugar-induced expression of FLZ genes. Further, these genes were also found to be highly responsive to different stresses as well as abscisic acid. In over-expression of kinase subunit of SnRK1, FLZ genes were found to be differentially regulated in accordance with their response toward energy fluctuation suggesting that these genes may work downstream to the established SnRK1 signaling under low-energy stress. Taken together, the present study provides a conceptual framework for further studies related to SnRK1-FLZ interaction in relation to sugar and energy signaling and stress response.

Interplay of sugar, light and gibberellins in expression of Rosa hybrida vacuolar invertase 1 regulation

Our previous findings showed that the expression of the Rosa hybrida vacuolar invertase 1 gene (RhVI1) was tightly correlated with the ability of buds to grow out and was under sugar, gibberellin and light control. Here, we aimed to provide an insight into the mechanistic basis of this regulation. In situ hybridization showed that RhVI1 expression was localized in epidermal cells of young leaves of bursting buds. We then isolated a 895 bp fragment of the promoter of RhVI1. In silico analysis identified putative cis-elements involved in the response to sugars, light and gibberellins on its proximal part (595 bp). To carry out functional analysis of the RhVI1 promoter in a homologous system, we developed a direct method for stable transformation of rose cells. 5' deletions of the proximal promoter fused to the uidA reporter gene were inserted into the rose cell genome to study the cell's response to exogenous and endogenous stimuli. Deletion analysis revealed that the 468 bp promoter fragment is sufficient to trigger reporter gene activity in response to light, sugars and gibberellins. This region confers sucrose- and fructose-, but not glucose-, responsive activation in the dark. Inversely, the -595 to -468 bp region that carries the sugar-repressive element (SRE) is required to down-regulate the RhVI1 promoter in response to sucrose and fructose in the dark. We also demonstrate that sugar/light and gibberellin/light act synergistically to up-regulate β-glucuronidase (GUS) activity sharply under the control of the 595 bp pRhVI1 region. These results reveal that the 127 bp promoter fragment located between -595 and -468 bp is critical for light and sugar and light and gibberellins to act synergistically.

Phosphate depletion modulates auxin transport in Triticum aestivum leading to altered root branching

Understanding the mechanisms by which nutritional signals impact upon root system architecture is a key facet in the drive for greater nutrient application efficiency in agricultural systems. Cereal plants reduce their rate of lateral root emergence under inorganic phosphate (Pi) shortage; this study uses molecular and pharmacological techniques to dissect this Pi response in Triticum aestivum. Plants were grown in coarse sand washed in high- or low-Pi nutrient solution before being assessed for their root branching density and expression of AUX/IAA and PIN genes. Seedlings were also grown on media containing [(14)C]indole acetic acid to measure basipetal auxin transport. Seedlings grown in low-Pi environments displayed less capacity to transport auxin basipetally from the seminal root apex, a reduction in root expression of PIN auxin transporter genes, and perturbed expression of a range of AUX/IAA auxin response genes. Given the known importance of basipetally transported auxin in stimulating lateral root initiation, it is proposed here that, in T. aestivum, Pi availability directly influences lateral root production through modulation of PIN expression. Understanding such processes is important in the drive for greater efficiency in crop use of Pi fertilizers in agricultural settings.

Glucose control of root growth direction in Arabidopsis thaliana

Directional growth of roots is a complex process that is modulated by various environmental signals. This work shows that presence of glucose (Glc) in the medium also extensively modulated seedling root growth direction. Glc modulation of root growth direction was dramatically enhanced by simultaneous brassinosteroid (BR) application. Glc enhanced BR receptor BRASSINOSTEROID INSENSITIVE1 (BRI1) endocytosis from plasma membrane to early endosomes. Glc-induced root deviation was highly enhanced in a PP2A-defective mutant, roots curl in naphthyl phthalamic acid 1-1 (rcn1-1) suggesting that there is a role of phosphatase in Glc-induced root-growth deviation. RCN1, therefore, acted as a link between Glc and the BR-signalling pathway. Polar auxin transport worked further downstream to BR in controlling Glc-induced root deviation response. Glc also affected other root directional responses such as root waving and coiling leading to altered root architecture. High light intensity mimicked the Glc-induced changes in root architecture that were highly reduced in Glc-signalling mutants. Thus, under natural environmental conditions, changing light flux in the environment may lead to enhanced Glc production/response and is a way to manipulate root architecture for optimized development via integrating several extrinsic and intrinsic signalling cues.

Glucose inhibits root meristem growth via ABA INSENSITIVE 5, which represses PIN1 accumulation and auxin activity in Arabidopsis

Glucose functions as a hormone-like signalling molecule that modulates plant growth and development in Arabidopsis thaliana. However, the role of glucose in root elongation remains elusive. Our study demonstrates that high concentrations of glucose reduce the size of the root meristem zone by repressing PIN1 accumulation and thereby reducing auxin levels. In addition, we verified the involvement of ABA INSENSITIVE 5 (ABI5) in this process by showing that abi5-1 is less sensitive to glucose than the wild type, whereas glucose induces ABI5 expression and the inducible overexpression of ABI5 reduces the size of the root meristem zone. Furthermore, the inducible overexpression of ABI5 in PIN1::PIN1-GFP plants reduces the level of PIN1-GFP, but glucose reduces the level of PIN1-GFP to a lesser extent in abi5-1 PIN1::PIN1-GFP plants than in the PIN1::PIN1-GFP control, suggesting that ABI5 is involved in glucose-regulated PIN1 accumulation. Taken together, our data suggest that ABI5 functions in the glucose-mediated inhibition of the root meristem zone by repressing PIN1 accumulation, thus leading to reduced auxin levels in roots.