The modified flavonol glycosylation profile in the Arabidopsis rol1 mutants results in alterations in plant growth and cell shape formation

UDP-glucosyltransferase 71C4 controls the flux of phenylpropanoid metabolism to shape cotton seed development

Seeds play a crucial role in plant reproduction, making it essential to identify genes that affect seed development. In this study, we focused on UDP-glucosyltransferase 71C4 (UGT71C4) in cotton, a member of the glycosyltransferase family that shapes seed width and length, thereby influencing seed index and seed cotton yield. Overexpression of UGT71C4 results in seed enlargement owing to its glycosyltransferase activity on flavonoids, which redirects metabolic flux from lignin to flavonoid metabolism. This shift promotes cell proliferation in the ovule via accumulation of flavonoid glycosides, significantly enhancing seed cotton yield and increasing the seed index from 10.66 g to 11.91 g. By contrast, knockout of UGT71C4 leads to smaller seeds through activation of the lignin metabolism pathway and redirection of metabolic flux back to lignin synthesis. This redirection leads to increased ectopic lignin deposition in the ovule, inhibiting ovule growth and development, and alters yield components, increasing the lint percentage from 41.42% to 43.40% and reducing the seed index from 10.66 g to 8.60 g. Our research sheds new light on seed size development and reveals potential pathways for enhancing seed yield.

Ocimum kilimandscharicum 4CL11 negatively regulates adventitious root development via accumulation of flavonoid glycosides

4-Coumarate-CoA Ligase (4CL) is an important enzyme in the phenylpropanoid biosynthesis pathway. Multiple 4CLs are identified in Ocimum species; however, their in planta functions remain enigmatic. In this study, we independently overexpressed three Ok4CL isoforms from Ocimum kilimandscharicum (Ok4CL7, -11, and -15) in Nicotiana benthamiana. Interestingly, Ok4CL11 overexpression (OE) caused a rootless or reduced root growth phenotype, whereas overexpression of Ok4CL15 produced normal adventitious root (AR) growth. Ok4CL11 overexpression in N. benthamiana resulted in upregulation of genes involved in flavonoid biosynthesis and associated glycosyltransferases accompanied by accumulation of specific flavonoid-glycosides (kaempferol-3-rhamnoside, kaempferol-3,7-O-bis-alpha-l-rhamnoside [K3,7R], and quercetin-3-O-rutinoside) that possibly reduced auxin levels in plants, and such effects were not seen for Ok4CL7 and -15. Docking analysis suggested that auxin transporters (PINs/LAXs) have higher binding affinity to these specific flavonoid-glycosides, and thus could disrupt auxin transport/signaling, which cumulatively resulted in a rootless phenotype. Reduced auxin levels, increased K3,7R in the middle and basal stem sections, and grafting experiments (intra and inter-species) indicated a disruption of auxin transport by K3,7R and its negative effect on AR development. Supplementation of flavonoids and the specific glycosides accumulated by Ok4CL11-OE to the wild-type N. benthamiana explants delayed the AR emergence and also inhibited AR growth. While overexpression of all three Ok4CLs increased lignin accumulation, flavonoids, and their specific glycosides were accumulated only in Ok4CL11-OE lines. In summary, our study reveals unique indirect function of Ok4CL11 to increase specific flavonoids and their glycosides, which are negative regulators of root growth, likely involved in inhibition of auxin transport and signaling.

Mechanism of flavonols on detoxification, migration and transformation of indium in rhizosphere system

Soil contamination by toxic heavy metal induces serious environmental hazards. In recent years, the use of indium (In) in semiconductor products has increased considerably and the release of In is inevitable, which will pose great risk to the ecosystem. The interaction between metal and plants which are the fundamental components of all ecosystems are an indispensable aspect of indium assessment and remediation. The role of flavonols, which is essential to plant resistance to In stress, remains largely unknown. FLS1 related lines of A. thaliana (Col, fls1-3 and OE) were exposed to In stress in soil and flavonols as root exudates were analyzed in exogenous application test. The accumulation and release of flavonols could be induced by In stress. However, flavonols exhibited different function in vivo and in vitro of plant. The basic function of flavonols was to affect root morphology via regulating auxin, but being intervened by In stress. The synthesis and accumulation of flavonols in vivo could activate the antioxidant system and the metal detoxification system to alleviate the toxic effects of In on plant. In addition, plants could make phone calls to rhizosphere microbes for help when exposed to In. Flavonols in vitro might act as the information transmission. Combination of endogenous and exogenous flavonols could affect the migration and transformation of In in soil-plant system via metal complexation and transportation pathway.

Tyrosine-sulfated peptide hormone induces flavonol biosynthesis to control elongation and differentiation in Arabidopsis primary root

In Arabidopsis roots, growth initiation and cessation are organized into distinct zones. How regulatory mechanisms are integrated to coordinate these processes and maintain proper growth progression over time is not well understood. Here, we demonstrate that the peptide hormone PLANT PEPTIDE CONTAINING SULFATED TYROSINE 1 (PSY1) promotes root growth by controlling cell elongation. Higher levels of PSY1 lead to longer differentiated cells with a shootward displacement of characteristics common to mature cells. PSY1 activates genes involved in the biosynthesis of flavonols, a group of plant-specific secondary metabolites. Using genetic and chemical approaches, we show that flavonols are required for PSY1 function. Flavonol accumulation downstream of PSY1 occurs in the differentiation zone, where PSY1 also reduces auxin and reactive oxygen species (ROS) activity. These findings support a model where PSY1 signals the developmental-specific accumulation of secondary metabolites to regulate the extent of cell elongation and the overall progression to maturation.

Natural variation of warm temperature-induced raffinose accumulation identifies TREHALOSE-6-PHOSPHATE SYNTHASE 1 as a modulator of thermotolerance

High-temperature stress limits plant growth and reproduction. Exposure to high temperature, however, also elicits a physiological response, which protects plants from the damage evoked by heat. This response involves a partial reconfiguration of the metabolome including the accumulation of the trisaccharide raffinose. In this study, we explored the intraspecific variation of warm temperature-induced raffinose accumulation as a metabolic marker for temperature responsiveness with the aim to identify genes that contribute to thermotolerance. By combining raffinose measurements in 250 Arabidopsis thaliana accessions following a mild heat treatment with genome-wide association studies, we identified five genomic regions that were associated with the observed trait variation. Subsequent functional analyses confirmed a causal relationship between TREHALOSE-6-PHOSPHATE SYNTHASE 1 (TPS1) and warm temperature-dependent raffinose synthesis. Moreover, complementation of the tps1-1 null mutant with functionally distinct TPS1 isoforms differentially affected carbohydrate metabolism under more severe heat stress. While higher TPS1 activity was associated with reduced endogenous sucrose levels and thermotolerance, disruption of trehalose 6-phosphate signalling resulted in higher accumulation of transitory starch and sucrose and was associated with enhanced heat resistance. Taken together, our findings suggest a role of trehalose 6-phosphate in thermotolerance, most likely through its regulatory function in carbon partitioning and sucrose homoeostasis.

Kaempferol-3-rhamnoside overaccumulation in flavonoid 3'-hydroxylase tt7 mutants compromises seed coat outer integument differentiation and seed longevity

Seeds slowly accumulate damage during storage, which ultimately results in germination failure. The seed coat protects the embryo from the external environment, and its composition is critical for seed longevity. Flavonols accumulate in the outer integument. The link between flavonol composition and outer integument development has not been explored. Genetic, molecular and ultrastructural assays on loss-of-function mutants of the flavonoid biosynthesis pathway were used to study the effect of altered flavonoid composition on seed coat development and seed longevity. Controlled deterioration assays indicate that loss of function of the flavonoid 3' hydroxylase gene TT7 dramatically affects seed longevity and seed coat development. Outer integument differentiation is compromised from 9 d after pollination in tt7 developing seeds, resulting in a defective suberin layer and incomplete degradation of seed coat starch. These distinctive phenotypes are not shared by other mutants showing abnormal flavonoid composition. Genetic analysis indicates that overaccumulation of kaempferol-3-rhamnoside is mainly responsible for the observed phenotypes. Expression profiling suggests that multiple cellular processes are altered in the tt7 mutant. Overaccumulation of kaempferol-3-rhamnoside in the seed coat compromises normal seed coat development. This observation positions TRANSPARENT TESTA 7 and the UGT78D1 glycosyltransferase, catalysing flavonol 3-O-rhamnosylation, as essential players in the modulation of seed longevity.

The Influence of Solid and Liquid Systems In Vitro on the Growth and Biosynthetic Characteristics of Microshoot Culture of Spiraea betulifolia ssp. aemiliana

The paper focuses on the growth dynamics and biosynthetic characteristics of the microshoot culture of Spiraea betulifolia ssp. aemiliana obtained in vitro in agar-solidified and liquid media. Microshoots cultured in either type of media showed similar growth dynamics. The most active culture growth was observed from day 35 to day 60. A comparative analysis of the contents of flavonoids and phenol carboxylic acids showed a higher level of phenol carboxylic acids (5.3-6.84%) and a stronger 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging activity (half-maximal inhibitory concentration: 341 µg/mL) in S. betulifolia ssp. aemiliana microshoots grown in the liquid medium compared to the microshoots cultured in the solid medium. The flavonoid content of the cultured microshoot did not depend on the consistency of the medium. High-performance liquid chromatography (HPLC) was employed to study the profile and levels of phenolic compounds in microshoots, intact plants, and ex vitro-acclimated S. betulifolia ssp. aemiliana plants. The concentration of kaempferol glycosides was found to be higher in microshoots (1.33% in the solid medium, 1.06% in the liquid medium) compared to intact plants and ex vitro-acclimated plants. Thus, the microshoots of S. betulifolia ssp. aemiliana cultured in the liquid medium rapidly increase their biomass and are an inexpensive promising source of biologically active antioxidant substances, mainly phenol carboxylic acids and kaempferol glycosides.

Friends in Arms: Flavonoids and the Auxin/Cytokinin Balance in Terrestrialization

Land plants survive the challenges of new environments by evolving mechanisms that protect them from excess irradiation, nutrient deficiency, and temperature and water availability fluctuations. One such evolved mechanism is the regulation of the shoot/root growth ratio in response to water and nutrient availability by balancing the actions of the hormones auxin and cytokinin. Plant terrestrialization co-occurred with a dramatic expansion in secondary metabolism, particularly with the evolution and establishment of the flavonoid biosynthetic pathway. Flavonoid biosynthesis is responsive to a wide range of stresses, and the numerous synthesized flavonoid species offer two main evolutionary advantages to land plants. First, flavonoids are antioxidants and thus defend plants against those adverse conditions that lead to the overproduction of reactive oxygen species. Second, flavonoids aid in protecting plants against water and nutrient deficiency by modulating root development and establishing symbiotic relations with beneficial soil fungi and bacteria. Here, we review different aspects of the relationships between the auxin/cytokinin module and flavonoids. The current body of knowledge suggests that whereas both auxin and cytokinin regulate flavonoid biosynthesis, flavonoids act to fine-tune only auxin, which in turn regulates cytokinin action. This conclusion agrees with the established master regulatory function of auxin in controlling the shoot/root growth ratio.

Two homeologous MATE transporter genes, NtMATE21 and NtMATE22, are involved in the modulation of plant growth and flavonol transport in Nicotiana tabacum

The multidrug and toxic compound extrusion (MATE) protein family has been implicated in the transport of a diverse range of molecules, including specialized metabolites. In tobacco (Nicotiana tabacum), only a limited number of MATE transporters have been functionally characterized, and no MATE transporter has been studied in the context of flavonoid transport in this plant species so far. In the present study, we characterize two homeologous tobacco MATE genes, NtMATE21 and NtMATE22, and demonstrate their role in flavonol transport and in plant growth and development. The expression of these two genes was reported to be up-regulated in trichomes as compared with the trichome-free leaf. The transcript levels of NtMATE21 and NtMATE22 were found to be higher in flavonol overproducing tobacco transgenic lines as compared with wild type tobacco. The two transporters were demonstrated to be localized to the plasma membrane. Genetic manipulation of NtMATE21 and NtMATE22 led to altered growth phenotypes and modulated flavonol contents in N. tabacum. The β-glucuronidase and green fluorescent protein fusion transgenic lines of promoter regions suggested that NtMATE21 and NtMATE22 are exclusively expressed in the trichome heads in the leaf tissue and petals. Moreover, in a transient transactivation assay, NtMYB12, a flavonol-specific MYB transcription factor, was found to transactivate the expression of NtMATE21 and NtMATE22 genes. Together, our results strongly suggest the involvement of NtMATE21 and NtMATE22 in flavonol transport as well as in the regulation of plant growth and development.

Verification of the Protective Effects of Poplar Phenolic Compounds Against Poplar Anthracnose

Poplar anthracnose caused by Colletotrichum gloeosporioides is one of the most important diseases widely distributed in poplar-growing areas in China, causing serious economic and ecological losses. In this study, three poplar species showed different resistance to poplar anthracnose: Populus × canadensis was resistant, Populus tomentosa was susceptible, and P. × beijingensis showed intermediate resistance. However, it remains uncertain whether phenolic compounds in poplar are involved in this resistance. Therefore, we determined the concentrations of phenolic compounds and their antifungal activity. Before and after the C. gloeosporioides inoculation, 20 phenolic compounds were detected in P. × canadensis and the number increased from 12 to 14 in P. × beijingensis but decreased from seven to four in P. tomentosa. Thus, phenolic compounds may be positively correlated with the degree of disease resistance. We selected seven phenolic compounds for further analysis, which varied considerably in content after inoculation with C. gloeosporioides. These seven compounds were salicin, arbutin, benzoic acid, salicylic acid, chlorogenic acid, ferulic acid, and naringenin, which helped poplar trees to limit the growth of C. gloeosporioides and differed in their antifungal effects, with phenolic acids having the strongest inhibitory effect. In addition, the optimal concentrations of different substances varied. We demonstrate that these phenolic compounds produced by poplar do play a certain role in the process of poplar resistance to anthracnose. These findings lay a foundation for future research into the antifungal mechanism of poplar trees and may be useful for enhancing the prevention and control of poplar anthracnose.

PhRHMs play important roles in leaf and flower development and anthocyanin synthesis in petunia

Anthocyanins, vital metabolites in plants, are formed by anthocyanidins combined with various monosaccharides, including glucose, rhamnose, and arabinose. Rhamnose contributes greatly to the glycosylation of anthocyanidins. There are two kinds of rhamnose synthase (RS): rhamnose biosynthesis (RHM), and nucleotide-RS/epimerase-reductase (UER1). Nevertheless, no RS isoform was reported to be involved in anthocyanin synthesis. Here, three homologous PhRHM genes, namely PhRHM1, PhRHM2, and PhRHM3, and one PhUER1 gene from petunia were cloned and characterized. Green fluorescent protein fusion protein assays revealed that PhRHMs and PhUER1 are localized in the cytoplasm. We obtained PhRHM1 or/and PhRHM2 or PhUER1 silenced petunia plants and did not attempt to obtain PhRHM3 silenced plants since PhRHM3 mRNA was not detected in petunia organs examined. PhRHM1 and PhRHM2 (PhRHM1-2) silencing induced abnormal plant growth and decreased the contents of l-rhamnose, photosynthetic pigments and total anthocyanins, while PhUER1 silencing did not cause any visible phenotypic changes. Flavonoid metabolome analysis further revealed that PhRHM1-2 silencing reduced the contents of anthocyanins with rhamnose residue. These results revealed that PhRHMs contribute to the biosynthesis of rhamnose and that PhRHMs participate in the anthocyanin rhamnosylation in petunia, while PhUER1 does not.

A Brief Review of Phenolic Compounds Identified from Plants: Their Extraction, Analysis, and Biological Activity

Phenolic compounds are the most abundant secondary metabolites in plants, showing a wide range of distinct biological activities, have received more and more attention in recent years. This review aims to gather and systematize available information on the phenolic compounds from plants by discussing different types of phenolic compounds, extraction, and analysis methods, with an emphasis on their potential biological activities. The research direction and problems that should be paid attention to in the future are also put forward to provide some references for the further study of phenolic compounds.

Functional identification of a flavone synthase and a flavonol synthase genes affecting flower color formation in Chrysanthemum morifolium

Flavonoids confer a wide color range to plants, thus influencing the flower quality and commercial value of various ornamental plants. Flavones and flavonols are colorless pigments that are distinct from the colored anthocyanins. Flavones and flavonols are transformed from flavanones and dihydrokaempferol, which are catalyzed by flavone synthase (FNS) and flavonol synthase (FLS), respectively, and play important roles in regulating plant growth and development, and resistance to various stresses, in addition to coloration. However, few studies have been conducted on CmFNS and CmFLS genes in chrysanthemums. In this study, we isolated and identified CmFNS and CmFLS from Chrysanthemum morifolium. CmFNS and CmFLS were constitutively expressed at different levels in various C. morifolium organs, and in vitro catalytic activity of CmFNS and CmFLS was verified. CmFNS- and CmFLS-overexpressing tobacco plants exhibited phenotypes that accumulated more flavones and flavonols, respectively, but less anthocyanins. Moreover, the transcripts of CmFNS were negatively correlated with flower color, whereas CmFLS presented an opposite trend compared to CmFNS in five flower color cultivars with different anthocyanin levels. These findings suggest that CmFNS and CmFLS act as important regulators of flavone and flavonol biosynthesis, respectively, and dicate flower coloration in chrysanthemums.

Competition between anthocyanin and kaempferol glycosides biosynthesis affects pollen tube growth and seed set of Malus

Flavonoids play important roles in regulating plant growth and development. In this study, three kaempferol 3-O-glycosides were identified and mainly accumulated in flowers but not in leaves or fruits of Malus. In Malus, flower petal color is normally white, but some genotypes have red flowers containing anthocyanin. Anthocyanin biosynthesis appears to be in competition with kaempferol 3-O-glycosides production and controlled by the biosynthetic genes. The white flower Malus genotypes had better-developed seeds than the red flower genotypes. In flowers, the overexpression of MYB10 in Malus domestica enhanced the accumulation of anthocyanin, but decreased that of kaempferol 3-O-glycosides. After pollination the transgenic plants showed slower pollen tube growth and fewer developed seeds. Exogenous application of different flavonoid compounds suggested that kaempferol 3-O-glycosides, especially kaempferol 3-O-rhamnoside, regulated pollen tube growth and seed set rather than cyanidin or quercetin 3-O-glycosides. It was found that kaempferol 3-O-rhamnoside might regulate pollen tube growth through effects on auxin, the Rho of plants (ROP) GTPases, calcium and the phosphoinositides signaling pathway. With the inhibition of auxin transport, the transcription levels of Heat Shock Proteins (HSPs) and ROP GTPases were downregulated while the levels were not changed or even enhanced when blocking calcium signaling, suggesting that HSPs and ROP GTPases were downstream of auxin signaling, but upstream of calcium signaling. In summary, kaempferol glycoside concentrations in pistils correlated with auxin transport, the transcription of HSPs and ROP GTPases, and calcium signaling in pollen tubes, culminating in changes to pollen tube growth and seed set.

In through the out door: Biochemical mechanisms affecting flavonoid glycoside catabolism in plants

Plants are the sole source of flavonoids, a chemical category that includes flavonols. For the most part, flavonols occur as glycosides with numerous postulated biological roles in plants, including photoprotection, modulation of hormone translocation, and sequestration of reactive oxygen species. Flavonol glycosides are often considered as dead-end metabolites because related flavonoids (i.e., anthocyanins) occur in terminal tissues such as flowers and fruit, but recent evidence points to their turnover in planta, including developing photosynthetic tissues. Although microbial degradation pathways for flavonol glycosides of plant origin are well described, plant catabolic pathways are little studied by comparison. This review will address our current understanding of biochemical processes leading to the loss of flavonol glycosides in plants, with a specific emphasis on the evidence for flavonol-specific β-glucosidases. Complete elucidation of these catabolic pathways is dependent on the identification of regiospecific modifying steps, including enzymes associated with the hydrolysis of rhamnosylated flavonols, as well as flavonol peroxidation and their encoding genes. Herein, we highlight challenges for the identification of hypothetical plant α-rhamnosidases and peroxidases involved in flavonol glycoside degradation, and the potential biological role of this catabolism in mitigating oxidative stress in developing and postharvest plant tissues.

Hormonal Regulation in Different Varieties of Chenopodium quinoa Willd. Exposed to Short Acute UV-B Irradiation

Increased ultraviolet-B (UV-B) due to global change can affect plant development and metabolism. Quinoa tolerates extreme conditions including high UV levels. However, the physiological mechanisms behind its abiotic stress tolerance are unclear, especially those related to UV-B. We previously demonstrated that 9.12 kJ m⁻² d⁻¹ may induce UV-B-specific signaling while 18.24 kJ m⁻² d⁻¹ promotes a UV-B-independent response. Here, we explored the effects of these UV-B doses on hormonal regulation linked to plant morphology and defense among diverse varieties. Changes in fluorescence parameters of photosystem II, flavonoids and hormones (indoleacetic acid (IAA), jasmonic acid (JA), abscisic acid (ABA) and salicylic acid (SA)) were surveyed under controlled conditions. Here, we showed that the sensitivity to short acute UV-B doses in varieties from different habitats is influenced by their parental lines and breeding time. UV-B sensitivity does not necessarily correlate with quinoa’s geographical distribution. The role of flavonoids in the UV-B response seems to be different depending on varieties. Moreover, we found that the extent of changes in JA and SA correlate with UV-B tolerance, while the increase of ABA was mainly related to UV-B stress.

Defects in Cell Wall Differentiation of the Arabidopsis Mutant rol1-2 Is Dependent on Cyclin-Dependent Kinase CDK8

Plant cells are encapsulated by cell walls whose properties largely determine cell growth. We have previously identified the rol1-2 mutant, which shows defects in seedling root and shoot development. rol1-2 is affected in the Rhamnose synthase 1 (RHM1) and shows alterations in the structures of Rhamnogalacturonan I (RG I) and RG II, two rhamnose-containing pectins. The data presented here shows that root tissue of the rol1-2 mutant fails to properly differentiate the cell wall in cell corners and accumulates excessive amounts of callose, both of which likely alter the physical properties of cells. A surr (suppressor of the rol1-2 root developmental defect) mutant was identified that alleviates the cell growth defects in rol1-2. The cell wall differentiation defect is re-established in the rol1-2 surr mutant and callose accumulation is reduced compared to rol1-2. The surr mutation is an allele of the cyclin-dependent kinase 8 (CDK8), which encodes a component of the mediator complex that influences processes central to plant growth and development. Together, the identification of the surr mutant suggests that changes in cell wall composition and turnover in the rol1-2 mutant have a significant impact on cell growth and reveals a function of CDK8 in cell wall architecture and composition.

A Centrifuge-Based Method for Identifying Novel Genetic Traits That Affect Root-Substrate Adhesion in Arabidopsis thaliana

The physical presence of roots and the compounds they release affect the cohesion between roots and their environment. However, the plant traits that are important for these interactions are unknown and most methods that quantify the contributions of these traits are time-intensive and require specialist equipment and complex substrates. Our lab developed an inexpensive, high-throughput phenotyping assay that quantifies root-substrate adhesion in Arabidopsis thaliana. We now report that this method has high sensitivity and versatility for identifying different types of traits affecting root-substrate adhesion including root hair morphology, vesicle trafficking pathways, and root exudate composition. We describe a practical protocol for conducting this assay and introduce its use in a forward genetic screen to identify novel genes affecting root-substrate interactions. This assay is a powerful tool for identifying and quantifying genetic contributions to cohesion between roots and their environment.

Global transcriptome changes of elongating internode of sugarcane in response to mepiquat chloride

Background

Mepiquat chloride (DPC) is a chemical that is extensively used to control internode growth and create compact canopies in cultured plants. Previous studies have suggested that DPC could also inhibit gibberellin biosynthesis in sugarcane. Unfortunately, the molecular mechanism underlying the suppressive effects of DPC on plant growth is still largely unknown.

Results

In the present study, we first obtained high-quality long transcripts from the internodes of sugarcane using the PacBio Sequel System. A total of 72,671 isoforms, with N50 at 3073, were generated. These long isoforms were used as a reference for the subsequent RNA-seq. Afterwards, short reads generated from the Illumina HiSeq 4000 platform were used to compare the differentially expressed genes in both the DPC and the control groups. Transcriptome profiling showed that most significant gene changes occurred after six days post DPC treatment. These genes were related to plant hormone signal transduction and biosynthesis of several metabolites, indicating that DPC affected multiple pathways, in addition to suppressing gibberellin biosynthesis. The network of DPC on the key stage was illustrated by weighted gene co-expression network analysis (WGCNA). Among the 36 constructed modules, the top positive correlated module, at the stage of six days post spraying DPC, was sienna3. Notably, Stf0 sulfotransferase, cyclin-like F-box, and HOX12 were the hub genes in sienna3 that had high correlation with other genes in this module. Furthermore, the qPCR validated the high accuracy of the RNA-seq results.

Conclusion

Taken together, we have demonstrated the key role of these genes in DPC-induced growth inhibition in sugarcane.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07352-w.

Rhamnose in plants - from biosynthesis to diverse functions

In plants, the deoxy sugar l-rhamnose is widely present as rhamnose-containing polymers in cell walls and as part of the decoration of various specialized metabolites. Here, we review the current knowledge on the distribution of rhamnose, highlighting the differences between what is known in dicotyledoneuos compared to commelinid monocotyledoneous (grasses) plants. We discuss the biosynthesis and transport of UDP-rhamnose, as well as the transfer of rhamnose from UDP-rhamnose to various primary and specialized metabolites. This is carried out by rhamnosyltransferases, enzymes that can use a large variety of substrates. Some unique characteristics of rhamnose synthases, the multifunctional enzymes responsible for the conversion of UDP-glucose into UDP-rhamnose, are considered, particularly from the perspective of their ability to convert glucose present in flavonoids. Finally, we discuss how little is still known with regards to how plants rescue rhamnose from the many compounds to which it is linked, or how rhamnose is catabolized.

Flavonols modulate lateral root emergence by scavenging reactive oxygen species in Arabidopsis thaliana

Flavonoids are a class of specialized metabolites with subclasses including flavonols and anthocyanins, which have unique properties as antioxidants. Flavonoids modulate plant development, but whether and how they impact lateral root development is unclear. We examined potential roles for flavonols in this process using Arabidopsis thaliana mutants with defects in genes encoding key enzymes in flavonoid biosynthesis. We observed the tt4 and fls1 mutants, which produce no flavonols, have increased lateral root emergence. The tt4 root phenotype was reversed by genetic and chemical complementation. To more specifically define the flavonoids involved, we tested an array of flavonoid biosynthetic mutants, eliminating roles for anthocyanins and the flavonols quercetin and isorhamnetin in modulating lateral root development. Instead, two tt7 mutant alleles, with defects in a branchpoint enzyme blocking quercetin biosynthesis, formed reduced numbers of lateral roots and tt7-2 had elevated levels of kaempferol. Using a flavonol-specific dye, we observed that in the tt7-2 mutant, kaempferol accumulated within lateral root primordia at higher levels than wild-type. These data are consistent with kaempferol, or downstream derivatives, acting as a negative regulator of lateral root emergence. We examined ROS accumulation using ROS-responsive probes and found reduced fluorescence of a superoxide-selective probe within the primordia of tt7-2 compared with wild-type, but not in the tt4 mutant, consistent with opposite effects of these mutants on lateral root emergence. These results support a model in which increased level of kaempferol in the lateral root primordia of tt7-2 reduces superoxide concentration and ROS-stimulated lateral root emergence.

Functional analysis of PpRHM1 and PpRHM2 involved in UDP-l-rhamnose biosynthesis in Prunus persica

UDP-l-rhamnose (UDP-Rha) is an important sugar donor for glycosylation of various cell molecules in plant. Rhamnosides are widely present in different plant tissues and play important biological roles under different developmental or environmental conditions. However, enzymes involved in UDP-Rha biosynthesis and their encoding genes have been identified in few plants, which limits the functional analysis of plant rhamnosides. Here, two UDP-Rha biosynthesis genes, named PpRHM1 (2028 bp) and PpRHM2 (2016 bp), were isolated and characterized from Prunus persica, which is rich sources of flavonol rhamnosides. Both recombinant RHM proteins can catalyze the transformation from UDP-d-glucose (UDP-Glc) to UDP-Rha, which was confirmed by LC-MS and formation of flavonol rhamnosides. Biochemical analysis showed that both recombinant RHM proteins preferred alkaline conditions in pH range of 8.0-9.0 and had optimal reaction temperature between 25 and 30 °C. PpRHM1 showed the better UDP-Glc substrate affinity with K_m of 360.01 μM. Gene expression analysis showed different transcript levels of both RHMs in all plant tissues tested, indicating the involvement of rhamnosides in various tissues in plant. Such results provide better understanding of UDP-Rha biosynthesis in fruit tree and may be helpful for further investigation of various rhamnose derivatives and their biological functions.

Resistance effect of flavonols and toxicology analysis of hexabromocyclododecane based on soil-microbe-plant system

The toxicity characteristics of HBCD and resistance mechanism of flavonols are investigated based on physiological and metagenomic analysis. Toxicology research of HBCD on Arabidopsis thaliana (Col and fls1-3) not only shows the toxic effect of HBCD on plants, but also indicates that flavonols could improve plant resistance to HBCD, including root length, shoot biomass and chlorophyll content. Analysis of eggNOG and GO enrichment demonstrates that HBCD has toxic effect on both gene expression and protein function, which concentrates on energy production - conversion and amino acid transport - metabolism. Differential expressed genes in flavonols-treated groups indicates that flavonols regulate the metabolism of amino acids, cofactors and vitamins, which is involved in plant defense system against oxidative damage caused by HBCD stress. HBCD is believed to affect the synthesis of proteins via genes expression of ribosome biogenesis process. Flavonols could strengthen the plant resistance and alleviate toxic effect under HBCD stress.

Role of Flavonol Synthesized by Nucleus FLS1 in Arabidopsis Resistance to Pb Stress

Lead (Pb) is an important pollutant of worldwide concern with respect to extensive pollution sources and highly toxic effect. Flavonol can improve plant resistance to abiotic stress and is also responsible for the alleviating effect under Pb stress. The relationship between Pb stress and flavonol and the knowledge about the mechanisms of flavonol function are very limited. Pb affected the energy metabolism process and, thus, inhibited plant growth and development. Flavonol accumulation controlled by FLS1 (flavonol synthase) could alleviate the toxic effect. Importantly, nes (mutant of NES that allows FLS1 to enter the nucleus expression) showed better growth status and lighter oxidative damage than NES (N-terminal nucleus exclusion signal peptide prevents FLS1 from entering the nucleus expression), which indicated that nucleus flavonol synthesized by nucleus FLS1 plays a key role in plant resistance to Pb stress. Although FLS1 signals were detected in the cell membrane, cytoplasm, and nucleus, membrane flavonol, cytoplasm flavonol, and nucleus flavonol were not exercising their function in the corresponding position. The expression of nucleus FLS1 intervened in the total content and composition of flavonol. The results also revealed that nucleus flavonol could regulate the ascorbate metabolism for alleviating the damage on the chloroplast, thus maintaining the photophosphorylation pathway. Our findings provided new insights for the molecular basis of Pb tolerance and response mechanism of the plant.

Leaf Trichome Distribution Pattern in Arabidopsis Reveals Gene Expression Variation Associated with Environmental Adaptation

Gene expression varies stochastically even in both heterogenous and homogeneous cell populations. This variation is not simply useless noise; rather, it is important for many biological processes. Unicellular organisms or cultured cell lines are useful for analyzing the variation in gene expression between cells; however, owing to technical challenges, the biological relevance of this variation in multicellular organisms such as higher plants remain unclear. Here, we addressed the biological relevance of this variation between cells by examining the genetic basis of trichome distribution patterns in Arabidopsis thaliana. The distribution pattern of a trichome on a leaf is stochastic and can be mathematically represented using Turing’s reaction-diffusion (RD) model. We analyzed simulations based on the RD model and found that the variability in the trichome distribution pattern increased with the increase in stochastic variation in a particular gene expression. Moreover, differences in heat-dependent variability of the trichome distribution pattern between the accessions showed a strong correlation with environmental factors to which each accession was adapted. Taken together, we successfully visualized variations in gene expression by quantifying the variability in the Arabidopsis trichome distribution pattern. Thus, our data provide evidence for the biological importance of variations in gene expression for environmental adaptation.

Flavonols regulate root hair development by modulating accumulation of reactive oxygen species in the root epidermis

Reactive oxygen species (ROS) are signaling molecules produced by tissue-specific respiratory burst oxidase homolog (RBOH) enzymes to drive development. In Arabidopsis thaliana, ROS produced by RBOHC was previously reported to drive root hair elongation. We identified a specific role for one ROS, H₂O₂, in driving root hair initiation and demonstrated that localized synthesis of flavonol antioxidants control the level of H₂O₂ and root hair formation. Root hairs form from trichoblast cells that express RBOHC and have elevated H₂O₂ compared with adjacent atrichoblast cells that do not form root hairs. The flavonol-deficient tt4 mutant has elevated ROS in trichoblasts and elevated frequency of root hair formation compared with the wild type. The increases in ROS and root hairs in tt4 are reversed by genetic or chemical complementation. Auxin-induced root hair initiation and ROS accumulation were reduced in an rbohc mutant and increased in tt4, consistent with flavonols modulating ROS and auxin transport. These results support a model in which localized synthesis of RBOHC and flavonol antioxidants establish patterns of ROS accumulation that drive root hair formation.

Increased Flavonol Levels in Tobacco Expressing AcFLS Affect Flower Color and Root Growth

The onion (Allium cepa L.) flavonol synthase (AcFLS-HRB) gene, encoding an enzyme responsible for flavonol biosynthesis in yellow onion, was recently identified and enzymatically characterized. Here, we performed an in vivo feeding assay involving bacterial expression of AcFLS-HRB and observed that it exhibited both flavanone 3-hydroxylase (F3H) and FLS activity. Transgenic tobacco (Nicotiana tabacum) expressing AcFLS-HRB produced lighter-pink flowers compared to wild-type plants. In transgenic petals, AcFLS-HRB was highly expressed at the mRNA and protein levels, and most AcFLS-HRB protein accumulated in the insoluble microsomal fractions. High-performance liquid chromatography (HPLC) analysis showed that flavonol levels increased but anthocyanin levels decreased in transgenic petals, indicating that AcFLS-HRB is a functional gene in planta. Gene expression analysis showed the reduced transcript levels of general phenylpropanoid biosynthetic genes and flavonoid biosynthetic genes in AcFLS-HRB overexpressed tobacco petals. Additionally, transgenic tobacco plants at the seedling stages showed increased primary root and root hair length and enhanced quercetin signals in roots. Exogenous supplementation with quercetin 3-O-rutinoside (rutin) led to the same phenotypic changes in root growth, suggesting that rutin is the causal compound that promotes root growth in tobacco. Therefore, augmenting flavonol levels affects both flower color and root growth in tobacco.

A Crucial Role of GA-Regulated Flavonol Biosynthesis in Root Growth of Arabidopsis

Flavonols have been demonstrated to play many important roles in plant growth, development, and communication with other organisms. Flavonol biosynthesis is spatiotemporally regulated by the subgroup 7 R2R3-MYB (SG7 MYB) transcription factors including MYB11/MYB12/MYB111. However, whether SG7-MYB activity is subject to post-translational regulation remains unclear. Here, we show that gibberellic acid (GA) inhibits flavonol biosynthesis via DELLA proteins in Arabidopsis. Protein-protein interaction analyses revealed that DELLAs (RGA and GAI) interacted with SG7 MYBs (MYB12 and MYB111) both in vitro and in vivo, leading to enhanced affinity of MYB binding to the promoter regions of key genes for flavonol biosynthesis and thus increasing their transcriptional levels. We observed that the level of auxin in the root tip was negatively correlated with root flavonol content. Furthermore, genetic assays showed that loss-of-function mutations in MYB12, which is predominantly expressed in roots, partially rescued the short-root phenotype of the GA-deficient mutant ga1-3 by increasing root meristem size and mature cell size. Consistent with these observations, exogenous application of the flavonol quercetin restored the root meristem size of myb12 ga1-3 to that of ga1-3. Taken together, our data elucidate a molecular mechanism by which GA promotes root growth by directly reducing flavonol biosynthesis.

RBOH-Dependent ROS Synthesis and ROS Scavenging by Plant Specialized Metabolites To Modulate Plant Development and Stress Responses

Reactive oxygen species (ROS) regulate plant growth and development. ROS are kept at low levels in cells to prevent oxidative damage, allowing them to be effective signaling molecules upon increased synthesis. In plants and animals, NADPH oxidase/respiratory burst oxidase homolog (RBOH) proteins provide localized ROS bursts to regulate growth, developmental processes, and stress responses. This review details ROS production via RBOH enzymes in the context of plant development and stress responses and defines the locations and tissues in which members of this family function in the model plant Arabidopsis thaliana. To ensure that these ROS signals do not reach damaging levels, plants use an array of antioxidant strategies. In addition to antioxidant machineries similar to those found in animals, plants also have a variety of specialized metabolites that scavenge ROS. These plant specialized metabolites exhibit immense structural diversity and have highly localized accumulation. This makes them important players in plant developmental processes and stress responses that use ROS-dependent signaling mechanisms. This review summarizes the unique properties of plant specialized metabolites, including carotenoids, ascorbate, tocochromanols (vitamin E), and flavonoids, in modulating ROS homeostasis. Flavonols, a subclass of flavonoids with potent antioxidant activity, are induced during stress and development, suggesting that they have a role in maintaining ROS homeostasis. Recent results using genetic approaches have shown how flavonols regulate development and stress responses through their action as antioxidants.

Flavonols control pollen tube growth and integrity by regulating ROS homeostasis during high-temperature stress

Plant reproduction requires long-distance growth of a pollen tube to fertilize the female gametophyte. Prior reports suggested that mutations altering synthesis of flavonoids, plant specialized metabolites that include flavonols and anthocyanins, impair pollen development in several species, but the mechanism by which flavonols enhanced fertility was not defined. Here, we used genetic approaches to demonstrate that flavonols enhanced pollen development by reducing the abundance of reactive oxygen species (ROS). We further showed that flavonols reduced high-temperature stress-induced ROS accumulation and inhibition of pollen tube growth. The anthocyanin reduced (are) tomato mutant had reduced flavonol accumulation in pollen grains and tubes. This mutant produced fewer pollen grains and had impaired pollen viability, germination, tube growth, and tube integrity, resulting in reduced seed set. Consistent with flavonols acting as ROS scavengers, are had elevated levels of ROS. The pollen viability, tube growth and integrity defects, and ROS accumulation in are were reversed by genetic complementation. Inhibition of ROS synthesis or scavenging of excess ROS with an exogenous antioxidant treatment also reversed the are phenotypes, indicating that flavonols function by reducing ROS levels. Heat stress resulted in increased ROS in pollen tubes and inhibited tube growth, with more pronounced effects in the are mutant that could be rescued by antioxidant treatment. These results are consistent with increased ROS inhibiting pollen tube growth and with flavonols preventing ROS from reaching damaging levels. These results reveal that flavonol metabolites regulate plant sexual reproduction at both normal and elevated temperatures by maintaining ROS homeostasis.

Increased chalcone synthase (CHS) expression is associated with dicamba resistance in Kochia scoparia

BACKGROUND

Resistance to the synthetic auxin herbicide dicamba is increasingly problematic in Kochia scoparia. The resistance mechanism in an inbred dicamba-resistant K. scoparia line (9425R) was investigated using physiological and transcriptomics (RNA-Seq) approaches.

RESULTS

No differences were found in dicamba absorption or metabolism between 9425R and a dicamba-susceptible line, but 9425R was found to have significantly reduced dicamba translocation. Known auxin-responsive genes ACC synthase (ACS) and indole-3-acetic acid amino synthetase (GH3) were transcriptionally induced following dicamba treatment in dicamba-susceptible K. scoparia but not in 9425R. Chalcone synthase (CHS), the gene regulating synthesis of the flavonols quertecin and kaemperfol, was found to have twofold higher transcription in 9425R both without and 12 h after dicamba treatment. Increased CHS transcription co-segregated with dicamba resistance in a forward genetics screen using an F₂ population.

CONCLUSION

Prior work has shown that the flavonols quertecin and kaemperfol compete with auxin for intercellular movement and vascular loading via ATP-binding cassette subfamily B (ABCB) membrane transporters. The results of this study support a model in which constitutively increased CHS expression in the meristem produces more flavonols that would compete with dicamba for intercellular transport by ABCB transporters, resulting in reduced dicamba translocation. © 2017 Society of Chemical Industry.

Recent advances in matrix-assisted laser desorption/ionisation mass spectrometry imaging (MALDI-MSI) for in situ analysis of endogenous molecules in plants

INTRODUCTION

Mass spectrometry imaging (MSI) as a label-free and powerful imaging technique enables in situ evaluation of a tissue metabolome and/or proteome, becoming increasingly popular in the detection of plant endogenous molecules.

OBJECTIVE

The characterisation of structure and spatial information of endogenous molecules in plants are both very important aspects to better understand the physiological mechanism of plant organism.

METHODS

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is a commonly-used tissue imaging technique, which requires matrix to assist in situ detection of a variety of molecules on the surface of a tissue section. In previous studies, MALDI-MSI was mostly used for the detection of molecules from animal tissue sections, compared to plant samples due to cell structural limitations, such as plant cuticles, epicuticular waxes, and cell walls. Despite the enormous progress that has been made in tissue imaging, there is still a challenge for MALDI-MSI suitable for the imaging of endogenous compounds in plants.

RESULTS

This review summarises the recent advances in MALDI-MSI, focusing on the application of in situ detection of endogenous molecules in different plant organs, i.e. root, stem, leaf, flower, fruit, and seed.

CONCLUSION

Further improvements on instrumentation sensitivity, matrix selection, image processing and sample preparation will expand the application of MALDI-MSI in plant research.

Sugar-responsive transcription factor bZIP3 affects leaf shape in Arabidopsis plants

Sugars are essential for plant metabolism, growth and development. Plants must therefore manage their growth and developmental processes in response to sugar availability. Sugar signaling pathways constitute a complicated molecular network and are associated with global transcriptional regulation. However, the molecular mechanisms underlying sugar signaling remain largely unclear. This study reports that the protein basic-region leucine zipper 3 (bZIP3) is a novel sugar-responsive transcription factor in Arabidopsis plants. The expression of bZIP3 was rapidly repressed by sugar. Genetic analysis indicated that bZIP3 expression was modulated by the SNF1-RELATED KINASE 1 (SnRK1) pathway. Moreover, transgenic plants overexpressing bZIP3 and dominant repressor form bZIP3-SRDX showed aberrant shaped cotyledons with hyponastic bending. These findings suggest that bZIP3 plays a role in plant responses to sugars and is also associated with leaf development.

Flavonol rhamnosylation indirectly modifies the cell wall defects of RHAMNOSE BIOSYNTHESIS1 mutants by altering rhamnose flux

Rhamnose is required in Arabidopsis thaliana for synthesizing pectic polysaccharides and glycosylating flavonols. RHAMNOSE BIOSYNTHESIS1 (RHM1) encodes a UDP-l-rhamnose synthase, and rhm1 mutants exhibit many developmental defects, including short root hairs, hyponastic cotyledons, and left-handed helically twisted petals and roots. It has been proposed that the hyponastic cotyledons observed in rhm1 mutants are a consequence of abnormal flavonol glycosylation, while the root hair defect is flavonol-independent. We have recently shown that the helical twisting of rhm1 petals results from decreased levels of rhamnose-containing cell wall polymers. In this study, we found that flavonols indirectly modify the rhm1 helical petal phenotype by altering rhamnose flux to the cell wall. Given this finding, we further investigated the relationship between flavonols and the cell wall in rhm1 cotyledons. We show that decreased flavonol rhamnosylation is not responsible for the cotyledon phenotype of rhm1 mutants. Instead, blocking flavonol synthesis or rhamnosylation can suppress rhm1 defects by diverting UDP-l-rhamnose to the synthesis of cell wall polysaccharides. Therefore, rhamnose is required in the cell wall for normal expansion of cotyledon epidermal cells. Our findings suggest a broad role for rhamnose-containing cell wall polysaccharides in the morphogenesis of epidermal cells.

Integrated omics analysis of root-preferred genes across diverse rice varieties including Japonica and indica cultivars

Plant root systems play essential roles in developmental processes, such as the absorption of water and inorganic nutrients, and structural support. Gene expression is affected by growth conditions and the genetic background of plants. To identify highly conserved root-preferred genes in rice across diverse growth conditions and varieties, we used two independent meta-anatomical expression profiles based on a large collection of Affymetrix and Agilent 44K microarray data sets available for public use. We then identified 684 loci with root-preferred expression, which were validated with in silico analysis using both meta-expression profiles. The expression patterns of four candidate genes were confirmed in vivo by monitoring expression of β-glucuronidase under control of the candidate-gene promoters, providing new tools to manipulate agronomic traits associated with roots. We also utilized real-time PCR to examine the root-preferential expression of 14 genes across four rice varieties, including japonica and indica cultivars. Using a database of rice genes with known functions, we identified the reported functions of 39 out of the 684 candidate genes. Sixteen genes are directly involved in root development, while the remaining are involved in processes indirectly related to root development (i.e., soil-stress tolerance or growth retardation). This indicates the importance of our candidate genes for studies on root development and function. Gene ontology enrichment analysis in the 'biological processes' category revealed that root-preferred genes in rice are closely associated with nutrient transport-related genes, indicating that the primary role of roots is the uptake of nutrients from soil. In addition, predicted protein-protein interaction analysis suggested a molecular network for root development composed of 215 interactions associated with 44 root-preferred or root development-related genes. Taken together, our data provide an important foundation for future research on root development in rice.

Identification of a Flavonoid Glucosyltransferase Involved in 7-OH Site Glycosylation in Tea plants (Camellia sinensis)

Flavonol glycosides, which are often converted from aglycones in a process catalyzed by UDP-glycosyltransferases (UGTs), play an important role for the health of plants and animals. In the present study, a gene encoding a flavonoid 7-O-glycosyltransferase (CsUGT75L12) was identified in tea plants. Recombinant CsUGT75L12 protein displayed glycosyltransferase activity on the 7-OH position of multiple phenolic compounds. In relative comparison to wild-type seeds, the levels of flavonol-glucosides increased in Arabidopsis seeds overexpressing CsUGT75L12. In order to determine the key amino acid residues responsible for the catalytic activity of the protein, a series of site-directed mutagenesis and enzymatic assays were performed based on the 3D structural modeling and docking analyses. These results suggested that residue Q54 is a double binding site that functions as both a sugar receptor and donor. Residues H56 and T151, corresponding to the basic active residues H20 and D119 of VvGT1, were not irreplaceable for CsUGT75L12. In addition, residues Y182, S223, P238, T239, and F240 were demonstrated to be responsible for a ‘reversed’ sugar receptor binding model. The results of single and triple substitutions confirmed that the function of residues P238, T239, and F240 may substitute or compensate with each other for the flavonoid 7-O-glycosyltransferase activity.

An Apoplastic β-Glucosidase is Essential for the Degradation of Flavonol 3-O-β-Glucoside-7-O-α-Rhamnosides in Arabidopsis

Flavonol bisglycosides accumulate in plant vegetative tissues in response to abiotic stress, including simultaneous environmental perturbations (i.e. nitrogen deficiency and low temperature, NDLT), but disappear with recovery from NDLT. Previously, we determined that a recombinant Arabidopsis β-glucosidase (BGLU), BGLU15, hydrolyzes flavonol 3-O-β-glucoside-7-O-α-rhamnosides and flavonol 3-O-β-glucosides, forming flavonol 7-O-α-rhamnosides and flavonol aglycones, respectively. In this study, the transient expression of a BGLU15-Cherry fusion protein in onion epidermal cells demonstrated that BGLU15 was localized to the apoplast. Analysis of BGLU15 T-DNA insertional inactivation lines (bglu15-1 and bglu15-2) revealed negligible levels of BGLU15 transcripts, whereas its paralogs BGLU12 and BGLU16 were expressed in wild-type and bglu15 plants. The recombinant BGLU16 did not hydrolyze quercetin 3-O-β-glucoside-7-O-α-rhamnoside or rhamnosylated flavonols, but was active with the synthetic substrate, p-nitrophenyl-β-d-glucoside. In addition, shoots of both bglu15 mutants contained negligible flavonol 3-O-β-glucoside-7-O-α-rhamnoside hydrolase activity, whereas this activity increased by 223% within 2 d of NDLT recovery in wild-type plants. The levels of flavonol 3-O-β-glucoside-7-O-α-rhamnosides and quercetin 3-O-β-glucoside were high and relatively unchanged in shoots of bglu15 mutants during recovery from NDLT, whereas rapid losses were apparent in wild-type shoots. Moreover, losses of two flavonol 3-O-β-neohesperidoside-7-O-α-rhamnosides and kaempferol 3-O-α-rhamnoside-7-O-α-rhamnoside were evident during recovery from NDLT, regardless of whether BGLU15 was present. A spike in a kaempferol 7-O-α-rhamnoside occurred with stress recovery, regardless of germplasm, suggesting a contribution from hydrolysis of kaempferol 3-O-β-neohesperidoside-7-O-α-rhamnosides and/or kaempferol 3-O-α-rhamnoside-7-O-α-rhamnoside by hitherto unknown mechanisms. Thus, BGLU15 is essential for catabolism of flavonol 3-O-β-glucoside-7-O-α-rhamnosides and flavonol 3-O-β-glucosides in Arabidopsis.

Flavonol-induced changes in PIN2 polarity and auxin transport in the Arabidopsis thaliana rol1-2 mutant require phosphatase activity

The phytohormone auxin is a major determinant and regulatory component important for plant development. Auxin transport between cells is mediated by a complex system of transporters such as AUX1/LAX, PIN, and ABCB proteins, and their localization and activity is thought to be influenced by phosphatases and kinases. Flavonols have been shown to alter auxin transport activity and changes in flavonol accumulation in the Arabidopsis thaliana rol1-2 mutant cause defects in auxin transport and seedling development. A new mutation in ROOTS CURL IN NPA 1 (RCN1), encoding a regulatory subunit of the phosphatase PP2A, was found to suppress the growth defects of rol1-2 without changing the flavonol content. rol1-2 rcn1-3 double mutants show wild type-like auxin transport activity while levels of free auxin are not affected by rcn1-3. In the rol1-2 mutant, PIN2 shows a flavonol-induced basal-to-apical shift in polar localization which is reversed in the rol1-2 rcn1-3 to basal localization. In vivo analysis of PINOID action, a kinase known to influence PIN protein localization in a PP2A-antagonistic manner, revealed a negative impact of flavonols on PINOID activity. Together, these data suggest that flavonols affect auxin transport by modifying the antagonistic kinase/phosphatase equilibrium.

Involvement of three putative glucosyltransferases from the UGT72 family in flavonol glucoside/rhamnoside biosynthesis in Lotus japonicus seeds

Flavonols are one of the largest groups of flavonoids that confer benefits for the health of plants and animals. Flavonol glycosides are the predominant flavonoids present in the model legume Lotus japonicus. The molecular mechanisms underlying the biosynthesis of flavonol glycosides as yet remain unknown in L. japonicus. In the present study, we identified a total of 188 UDP-glycosyltransferases (UGTs) in L. japonicus by genome-wide searching. Notably, 12 UGTs from the UGT72 family were distributed widely among L. japonicus chromosomes, expressed in all tissues, and showed different docking scores in an in silico bioinformatics docking analysis. Further enzymatic assays showed that five recombinant UGTs (UGT72AD1, UGT72AF1, UGT72AH1, UGT72V3, and UGT72Z2) exhibit activity toward flavonol, flavone, and isoflavone aglycones. In particular, UGT72AD1, UGT72AH1, and UGT72Z2 are flavonol-specific UGTs with different kinetic properties. In addition, the overexpression of UGT72AD1 and UGT72Z2 led to increased accumulation of flavonol rhamnosides in L. japonicus and Arabidopsis thaliana. Moreover, the increase of kaempferol 3-O-rhamnoside-7-O-rhamnoside in transgenic A. thaliana inhibited root growth as compared with the wild-type control. These results highlight the significance of the UGT72 family in flavonol glycosylation and the role of flavonol rhamnosides in plant growth.

The Mechanism Forming the Cell Surface of Tip-Growing Rooting Cells Is Conserved among Land Plants

To discover mechanisms that controlled the growth of the rooting system in the earliest land plants, we identified genes that control the development of rhizoids in the liverwort Marchantia polymorpha. 336,000 T-DNA transformed lines were screened for mutants with defects in rhizoid growth, and a de novo genome assembly was generated to identify the mutant genes. We report the identification of 33 genes required for rhizoid growth, of which 6 had not previously been functionally characterized in green plants. We demonstrate that members of the same orthogroup are active in cell wall synthesis, cell wall integrity sensing, and vesicle trafficking during M. polymorpha rhizoid and Arabidopsis thaliana root hair growth. This indicates that the mechanism for constructing the cell surface of tip-growing rooting cells is conserved among land plants and was active in the earliest land plants that existed sometime more than 470 million years ago [1, 2].

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336,000 T-DNA lines and a genome assembly were generated in Marchantia polymorpha

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33 genes required for rhizoid growth were identified

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Six of the 33 genes were functionally characterized in plants for the first time

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Genes belonging to these orthogroups were active in the first land plant roots

Proteomic analysis of the mature Brassica stigma reveals proteins with diverse roles in vegetative and reproductive development

The stigma, the specialized apex of the Brassicaceae gynoecium, plays a role in pollen capture, discrimination, hydration, germination, and guidance. Despite this crucial role in reproduction, the global proteome underlying Brassicaceae stigma development and function remains largely unknown. As a contribution towards the characterization of the Brassicaceae dry stigma global proteome, more than 2500 Brassica napus mature stigma proteins were identified using three different gel-based proteomics approaches. Most stigma proteins participated in Metabolic Processes, Responses to Stimulus or Stress, Cellular or Developmental Processes, and Transport. The stigma was found to express a wide variety of proteins with demonstrated roles in cellular and organ development including proteins known to be involved in cellular expansion and morphogenesis, embryo development, as well as gynoecium and stigma development. Comparisons to a corresponding proteome from a very morphologically different Poaceae dry stigma showed a very similar distribution of proteins among different functional categories, but also revealed evident distinctions in protein composition especially in glucosinolate and carotenoid metabolism, photosynthesis, and self-incompatibility. To our knowledge, this study reports the largest Brassicaceae stigma protein dataset described to date.

Regulation of polar auxin transport by protein and lipid kinases

The directional transport of auxin, known as polar auxin transport (PAT), allows asymmetric distribution of this hormone in different cells and tissues. This system creates local auxin maxima, minima, and gradients that are instrumental in both organ initiation and shape determination. As such, PAT is crucial for all aspects of plant development but also for environmental interaction, notably in shaping plant architecture to its environment. Cell to cell auxin transport is mediated by a network of auxin carriers that are regulated at the transcriptional and post-translational levels. Here we review our current knowledge on some aspects of the 'non-genomic' regulation of auxin transport, placing an emphasis on how phosphorylation by protein and lipid kinases controls the polarity, intracellular trafficking, stability, and activity of auxin carriers. We describe the role of several AGC kinases, including PINOID, D6PK, and the blue light photoreceptor phot1, in phosphorylating auxin carriers from the PIN and ABCB families. We also highlight the function of some receptor-like kinases (RLKs) and two-component histidine kinase receptors in PAT, noting that there are probably RLKs involved in co-ordinating auxin distribution yet to be discovered. In addition, we describe the emerging role of phospholipid phosphorylation in polarity establishment and intracellular trafficking of PIN proteins. We outline these various phosphorylation mechanisms in the context of primary and lateral root development, leaf cell shape acquisition, as well as root gravitropism and shoot phototropism.

7-Rhamnosylated Flavonols Modulate Homeostasis of the Plant Hormone Auxin and Affect Plant Development

Flavonols are a group of secondary metabolites that affect diverse cellular processes. They are considered putative negative regulators of the transport of the phytohormone auxin, by which they influence auxin distribution and concomitantly take part in the control of plant organ development. Flavonols are accumulating in a large number of glycosidic forms. Whether these have distinct functions and diverse cellular targets is not well understood. The rol1-2 mutant of Arabidopsis thaliana is characterized by a modified flavonol glycosylation profile that is inducing changes in auxin transport and growth defects in shoot tissues. To determine whether specific flavonol glycosides are responsible for these phenotypes, a suppressor screen was performed on the rol1-2 mutant, resulting in the identification of an allelic series of UGT89C1, a gene encoding a flavonol 7-O-rhamnosyltransferase. A detailed analysis revealed that interfering with flavonol rhamnosylation increases the concentration of auxin precursors and auxin metabolites, whereas auxin transport is not affected. This finding provides an additional level of complexity to the possible ways by which flavonols influence auxin distribution and suggests that flavonol glycosides play an important role in regulating plant development.

Rapid modulation of ultraviolet shielding in plants is influenced by solar ultraviolet radiation and linked to alterations in flavonoids

The accumulation of ultraviolet (UV)-absorbing compounds (flavonoids and related phenylpropanoids) and the resultant decrease in epidermal UV transmittance (TUV ) are primary protective mechanisms employed by plants against potentially damaging solar UV radiation and are critical components of the overall acclimation response of plants to changing solar UV environments. Whether plants can adjust this UV sunscreen protection in response to rapid changes in UV, as occurs on a diurnal basis, is largely unexplored. Here, we use a combination of approaches to demonstrate that plants can modulate their UV-screening properties within minutes to hours, and these changes are driven, in part, by UV radiation. For the cultivated species Abelmoschus esculentus, large (30-50%) and reversible changes in TUV occurred on a diurnal basis, and these adjustments were associated with changes in the concentrations of whole-leaf UV-absorbing compounds and several quercetin glycosides. Similar results were found for two other species (Vicia faba and Solanum lycopersicum), but no such changes were detected in Zea mays. These findings reveal a much more dynamic UV-protection mechanism than previously recognized, raise important questions concerning the costs and benefits of UV-protection strategies in plants and have practical implications for employing UV to enhance crop vigor and quality in controlled environments.

Re-interpreting plant morphological responses to UV-B radiation

There is a need to reappraise the effects of UV-B radiation on plant morphology in light of improved mechanistic understanding of UV-B effects, particularly elucidation of the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor. We review responses at cell and organismal levels, and explore their underlying regulatory mechanisms, function in UV protection and consequences for plant fitness. UV-induced morphological changes include thicker leaves, shorter petioles, shorter stems, increased axillary branching and altered root:shoot ratios. At the cellular level, UV-B morphogenesis comprises changes in cell division, elongation and/or differentiation. However, notwithstanding substantial new knowledge of molecular, cellular and organismal UV-B responses, there remains a clear gap in our understanding of the interactions between these organizational levels, and how they control plant architecture. Furthermore, despite a broad consensus that UV-B induces relatively compact architecture, we note substantial diversity in reported phenotypes. This may relate to UV-induced morphological changes being underpinned by different mechanisms at high and low UV-B doses. It remains unproven whether UV-induced morphological changes have a protective function involving shading and decreased leaf penetration of UV-B, counterbalancing trade-offs such as decreased photosynthetic light capture and plant-competitive abilities. Future research will need to disentangle seemingly contradictory interactions occurring at the threshold UV dose where regulation and stress-induced morphogenesis overlap.

The fungal leaf endophyte Paraconiothyrium variabile specifically metabolizes the host-plant metabolome for its own benefit

Fungal endophytes live inside plant tissues and some have been found to provide benefits to their host. Nevertheless, their ecological impact is not adequately understood. Considering the fact that endophytes are continuously interacting with their hosts, it is conceivable that both partners have substantial influence on each other's metabolic processes. In this context, we have investigated the action of the endophytic fungus Paraconiothyrium variabile, isolated from the leaves of Cephalotaxus harringtonia, on the secondary metabolome of the host-plant. The alteration of the leaf compounds by the fungus was monitored through metabolomic approaches followed by structural characterization of the altered products. Out of more than a thousand molecules present in the crude extract of the plant leaf, we have observed a specific biotransformation of glycosylated flavonoids by the endophyte. In all cases it led to the production of the corresponding aglycone via deglycosylation. The deglycosylated flavonoids turned out to display significant beneficial effects on the hyphal growth of germinated spores. Our finding, along with the known allelopathic role of flavonoids, illustrates the chemical cooperation underlying the mutualistic relationship between the plant and the endophyte.

Transient silencing of CHALCONE SYNTHASE during fruit ripening modifies tomato epidermal cells and cuticle properties

Tomato (Solanum lycopersicum) fruit ripening is accompanied by an increase in CHALCONE SYNTHASE (CHS) activity and flavonoid biosynthesis. Flavonoids accumulate in the cuticle, giving its characteristic orange color that contributes to the eventual red color of the ripe fruit. Using virus-induced gene silencing in fruits, we have down-regulated the expression of SlCHS during ripening and compared the cuticles derived from silenced and nonsilenced regions. Silenced regions showed a pink color due to the lack of flavonoids incorporated to the cuticle. This change in color was accompanied by several other changes in the cuticle and epidermis. The epidermal cells displayed a decreased tangential cell width; a decrease in the amount of cuticle and its main components, cutin and polysaccharides, was also observed. Flavonoids dramatically altered the cuticle biomechanical properties by stiffening the elastic and viscoelastic phase and by reducing the ability of the cuticle to deform. There seemed to be a negative relation between SlCHS expression and wax accumulation during ripening that could be related to the decreased cuticle permeability to water observed in the regions silencing SlCHS. A reduction in the overall number of ester linkages present in the cutin matrix was also dependent on the presence of flavonoids.

The flavonoid biosynthetic enzyme chalcone isomerase modulates terpenoid production in glandular trichomes of tomato

Flavonoids and terpenoids are derived from distinct metabolic pathways but nevertheless serve complementary roles in mediating plant interactions with the environment. Here, we show that glandular trichomes of the anthocyanin free (af) mutant of cultivated tomato (Solanum lycopersicum) fail to accumulate both flavonoids and terpenoids. This pleiotropic metabolic deficiency was associated with loss of resistance to native populations of coleopteran herbivores under field conditions. We demonstrate that Af encodes an isoform (SlCHI1) of the flavonoid biosynthetic enzyme chalcone isomerase (CHI), which catalyzes the conversion of naringenin chalcone to naringenin and is strictly required for flavonoid production in multiple tissues of tomato. Expression of the wild-type SlCHI1 gene from its native promoter complemented the anthocyanin deficiency in af. Unexpectedly, the SlCHI1 transgene also complemented the defect in terpenoid production in glandular trichomes. Our results establish a key role for SlCHI1 in flavonoid production in tomato and reveal a link between CHI1 and terpenoid production. Metabolic coordination of the flavonoid and terpenoid pathways may serve to optimize the function of trichome glands in dynamic environments.

Kaempferol 3-O-rhamnoside-7-O-rhamnoside is an endogenous flavonol inhibitor of polar auxin transport in Arabidopsis shoots

Polar auxin transport (PAT) plays key roles in the regulation of plant growth and development. Flavonoids have been implicated in the inhibition of PAT. However, the active flavonoid derivative(s) involved in this process in vivo has not yet been identified. Here, we provide evidence that a specific flavonol bis-glycoside is correlated with shorter plant stature and reduced PAT. Specific flavonoid-biosynthetic or flavonoid-glycosylating steps were genetically blocked in Arabidopsis thaliana. The differential flavonol patterns established were analyzed by high-performance liquid chromatography (HPLC) and related to altered plant stature. PAT was monitored in stem segments using a radioactive [(3)H]-indole-3-acetic acid tracer. The flavonoid 3-O-glucosyltransferase mutant ugt78d2 exhibited a dwarf stature in addition to its altered flavonol glycoside pattern. This was accompanied by reduced PAT in ugt78d2 shoots. The ugt78d2-dependent growth defects were flavonoid dependent, as they were rescued by genetic blocking of flavonoid biosynthesis. Phenotypic and metabolic analyses of a series of mutants defective at various steps of flavonoid formation narrowed down the potentially active moiety to kaempferol 3-O-rhamnoside-7-O-rhamnoside. Moreover, the level of this compound was negatively correlated with basipetal auxin transport. These results indicate that kaempferol 3-O-rhamnoside-7-O-rhamnoside acts as an endogenous PAT inhibitor in Arabidopsis shoots.