Flavonoids are indispensable for complete male fertility in rice

Recent insights into anthocyanin biosynthesis, gene involvement, distribution regulation, and domestication process in rice (Oryza sativa L.)

Anthocyanins are water-soluble natural pigments found broadly in plants. As members of the flavonoid family, they are widely distributed in various tissues and organs, including roots, leaves, and flowers, responsible for purple, red, blue, and orange colors. Beyond pigmentation, anthocyanins play a role in plant propagation, stress response, defense mechanisms, and human health benefits. Anthocyanin biosynthesis involves a series of conserved enzymes encoded by structural genes regulated by various transcription factors. In rice, anthocyanin-mediated pigmentation serves as an important morphological marker for varietal identification and purification, a critical nutrient source, and a key trait in studying rice domestication. Anthocyanin biosynthesis in rice is regulated by a ternary conserved MBW transcriptional complexes comprising MYB transcription factors (TFs), basic-helix-loop-helix (bHLH) TFs, and WD40 repeat protein, which activate the expression of structure genes. Wild rice (Oryza rufipogon) commonly has purple hull, purple stigma, purple apiculus, purple leaf, and red pericarp due to the accumulations of anthocyanin or proanthocyanin. However, most cultivated rice (Oryza sativa) varieties lose the anthocyanin phenotypes due to the function variations of some regulators including OsC1, OsRb, and Rc and the structure gene OsDFR. Over the past decades, significant progress has been made in understanding the molecular and genetic mechanisms of anthocyanin biosynthesis. This review summarizes research progress in rice anthocyanin biosynthetic pathways, genes involvements, distribution regulations, and domestication processes. Furthermore, it discusses future prospects for anthocyanin biosynthesis research in rice, aiming to provide a theoretical foundation for future investigations and applications, and to assist in breeding new rice varieties with organ-targeted anthocyanin deposition.

Flavonols improve tomato pollen thermotolerance during germination and tube elongation by maintaining reactive oxygen species homeostasis

Elevated temperatures impair pollen performance and reproductive success, resulting in lower crop yields. The tomato (Solanum lycopersicum) anthocyanin reduced (are) mutant harbors a mutation in FLAVANONE 3-HYDROXYLASE (F3H), resulting in impaired flavonol antioxidant biosynthesis. The are mutant has reduced pollen performance and seed set relative to the VF36 parental line, phenotypes that are accentuated at elevated temperatures. Transformation of are with the wild-type F3H gene, or chemical complementation with flavonols, prevented temperature-dependent reactive oxygen species (ROS) accumulation in pollen and restored the reduced viability, germination, and tube elongation of are to VF36 levels. Overexpression of F3H in VF36 prevented temperature-driven ROS increases and impaired pollen performance, revealing that flavonol biosynthesis promotes thermotolerance. Although stigmas of are had reduced flavonol and elevated ROS levels, the growth of are pollen tubes was similarly impaired in both are and VF36 pistils. RNA-seq was performed at optimal and stress temperatures in are, VF36, and the F3H overexpression line at multiple timepoints across pollen tube elongation. The number of differentially expressed genes increased over time under elevated temperatures in all genotypes, with the greatest number in are. These findings suggest potential agricultural interventions to combat the negative effects of heat-induced ROS in pollen that lead to reproductive failure.

BvCGT1-mediated differential distribution of flavonoid C-glycosides contributes to plant's response to UV-B stress

C-glycosides are a predominant class of flavonoids that demonstrate diverse medical properties and plant physiological functions. The chemical stability, structural diversity, and differential aboveground distribution of these compounds in plants make them ideal protectants. However, little is known about the transcriptional regulatory mechanisms that play these diverse roles in plant physiology. In this study, chard was selected from 69 families for its significantly different flavonoid C-glycosides distributions between the aboveground and underground parts to investigate the role and regulatory mechanism of flavonoid C-glycosides in plants. Our results indicate that flavonoid C-glycosides are affected by various stressors, especially UV-B. Through cloning and validation of key biosynthetic genes of flavonoid C-glycosides in chard (BvCGT1), we observed significant effects induced by UV-B radiation. This finding was further confirmed by resistance testing in BvCGT1 silenced chard lines and in Arabidopsis plants with BvCGT1 overexpression. Yeast one-hybrid and dual-luciferase assays were employed to determine the underlying regulatory mechanisms of BvCGT1 in withstanding UV-B stress. These results indicate a potential regulatory role of BvDof8 and BvDof13 in modulating flavonoid C-glycosides content, through their influence on BvCGT1. In conclusion, we have effectively demonstrated the regulation of BvCGT1 by BvDof8 and BvDof13, highlighting their crucial role in plant adaptation to UV-B radiation. Additionally, we have outlined a comprehensive transcriptional regulatory network involving BvDof8 and BvDof13 in response to UV-B radiation.

The transcription factor LBD10 sustains pollen tube growth and integrity by modulating reactive oxygen species homeostasis via the regulation of flavonol biosynthesis in Arabidopsis

Reproduction in angiosperms relies on the precise growth of pollen tubes, facilitating the delivery of sperm cells to the ovule for double fertilization. LATERAL ORGAN BOUNDARIES DOMAIN10 (LBD10), a plant-specific transcription factor, plays a pivotal role in Arabidopsis pollen development. Here, we uncovered LBD10's function in sustaining pollen tube growth and integrity. The lbd10 mutant exhibited elevated levels of reactive oxygen species (ROS) and hydrogen peroxide (H2O2) in both pollen grains and tubes, leading to compromised pollen tube growth. The inhibition of ROS synthesis and scavenging of excess ROS with an antioxidant treatment each alleviated these defects in lbd10. The lbd10 mutant displayed reduced flavonol accumulation in both pollen grains and tubes. All the altered phenotypes of lbd10 were complemented by expressing LBD10 under its native promoter. Exogenous application of flavonoids recused the defects in pollen tube growth and integrity in lbd10, along with reducing the excess levels of ROS and H2O2. LBD10 directly binds the promoters of key flavonol biosynthesis genes in chromatin and promotes reporter gene expression in Arabidopsis mesophyll protoplasts. Our findings indicate that LBD10 modulates ROS homeostasis by transcriptionally activating genes crucial for flavonol biosynthesis, thereby maintaining pollen tube growth and integrity.

Temperature change regulates pollen fertility of a PTGMS rice line PA64S by modulating the ROS homeostasis and PCD within the tapetum

Photoperiod and temperature-sensitive male sterility rice is an important line for two-line hybrid rice, and the changes in the cultivation temperature strictly control its pollen fertility. However, the mechanism by which temperature variation regulates pollen fertility is still unclear. This study obtained stable fertile PA64S(F) and sterile PA64S(S) rice from PA64S by controlling temperature changes. PA64S(F) shows a normal anther development and fertile pollen under low temperature (21°C), and PA64S(S) shows delayed degradation of the tapetum cells, leading to abnormal pollen wall formation and ubisch development under normal temperature (28°C). The accumulation of reactive oxygen species (ROS) positively correlates with the programmed cell death (PCD) process of tapetum cells. The delayed accumulation of ROS in the PA64S(S) tapetum at early stages leads to a delayed initiation of the PCD process. Importantly, we localized ascorbic acid (ASA) accumulation in the tapetum cells and determined that ASA is a major antioxidant for ROS homeostasis. ROS-inhibited accumulation plants (PA64S-ASA) demonstrated pollen sterility, higher ASA and lower ROS accumulation in the tapetum, and the absence of PCD processes in the tapetum cell. Abnormal changes in the tapetum of PA64S(S) rice disrupted metabolic pathways such as lipid metabolism, cutin and wax synthesis, sugar accumulation, and phenylpropane, affecting pollen wall formation and substance accumulation, suggesting that the timely accumulation of ROS is critical for male fertility. This study highlights the central role of ROS homeostasis in fertility alteration and also provides an avenue to address the effect of environmental temperature changes on pollen fertility in rice.

A flavonoid metabolon: cytochrome b5 enhances B-ring trihydroxylated flavan-3-ols synthesis in tea plants

Flavan-3-ols are prominent phenolic compounds found abundantly in the young leaves of tea plants. The enzymes involved in flavan-3-ol biosynthesis in tea plants have been extensively investigated. However, the localization and associations of these numerous functional enzymes within cells have been largely neglected. In this study, we aimed to investigate the synthesis of flavan-3-ols in tea plants, particularly focusing on epigallocatechin gallate. Our analysis involving the DESI-MSI method to reveal a distinct distribution pattern of B-ring trihydroxylated flavonoids, primarily concentrated in the outer layer of buds. Subcellular localization showed that CsC4H, CsF3'H, and CsF3'5'H localizes endoplasmic reticulum. Protein-protein interaction studies demonstrated direct associations between CsC4H, CsF3'H, and cytoplasmic enzymes (CHS, CHI, F3H, DFR, FLS, and ANR), highlighting their interactions within the biosynthetic pathway. Notably, CsF3'5'H, the enzyme for B-ring trihydroxylation, did not directly interact with other enzymes. We identified cytochrome b5 isoform C serving as an essential redox partner, ensuring the proper functioning of CsF3'5'H. Our findings suggest the existence of distinct modules governing the synthesis of different B-ring hydroxylation compounds. This study provides valuable insights into the mechanisms underlying flavonoid diversity and efficient synthesis and enhances our understanding of the substantial accumulation of B-ring trihydroxylated flavan-3-ols in tea plants.

Flavonols improve thermotolerance in tomato pollen during germination and tube elongation by maintaining ROS homeostasis

Elevated temperatures impair pollen performance and reproductive success, resulting in lower crop yields. The Solanum lycopersicum anthocyanin reduced ( are ) mutant has a FLAVANONE 3 HYDROXYLASE ( F3H ) gene mutation resulting in impaired synthesis of flavonol antioxidants. The are mutant has reduced pollen performance and seed set relative to the VF36 parental line, which is accentuated at elevated temperatures. Transformation of are with the wild-type F3H gene, or chemical complementation with flavonols, prevented temperature-dependent ROS accumulation in pollen and reversed are's reduced viability, germination, and tube elongation to VF36 levels. VF36 transformed with an F3H overexpression construct prevented temperature driven ROS increases and impaired pollen performance, revealing thermotolerance results from elevated flavonol synthesis. Although stigmas of are had reduced flavonols and elevated ROS, the growth of are pollen tubes were similarly impaired in both are and VF36 pistils. RNA-Seq was performed at optimal and stress temperatures in are , VF36, and the VF36 F3H overexpression line at multiple timepoints across pollen tube elongation. Differentially expressed gene numbers increased with duration of elevated temperature in all genotypes, with the largest number in are . These findings suggest potential agricultural interventions to combat the negative effects of heat-induced ROS in pollen that leads to reproductive failure.

One sentence summary

Flavonol antioxidants reduce the negative impacts of elevated temperatures on pollen performance by reducing levels of heat induced reactive oxygen species and modulation of heat-induced changes in the pollen transcriptome.

Multifunctional 5-hydroxyconiferaldehyde O-methyltransferases (CAldOMTs) in plant metabolism

Lignin, flavonoids, melatonin, and stilbenes are plant specialized metabolites with diverse physiological and biological functions, supporting plant growth and conferring stress resistance. Their biosynthesis requires O-methylations catalyzed by 5-hydroxyconiferaldehyde O-methyltransferase (CAldOMT; also called caffeic acid O-methyltransferase, COMT). CAldOMT was first known for its roles in syringyl (S) lignin biosynthesis in angiosperm cell walls and later found to be multifunctional. This enzyme also catalyzes O-methylations in flavonoid, melatonin, and stilbene biosynthetic pathways. Phylogenetic analysis indicated the convergent evolution of enzymes with OMT activities towards the monolignol biosynthetic pathway intermediates in some gymnosperm species that lack S-lignin and Selaginella moellendorffii, a lycophyte which produces S-lignin. Furthermore, neofunctionalization of CAldOMTs occurred repeatedly during evolution, generating unique O-methyltransferases (OMTs) with novel catalytic activities and/or accepting novel substrates, including lignans, 1,2,3-trihydroxybenzene, and phenylpropenes. This review summarizes multiple aspects of CAldOMTs and their related proteins in plant metabolism and discusses their evolution, molecular mechanism, and roles in biorefineries, agriculture, and synthetic biology.

Gh4CL20/20A involved in flavonoid biosynthesis is essential for male fertility in cotton (Gossypium hirsutum L.)

Flavonoids have been shown to play an essential role in plant growth and fertility. 4-Coumarate CoA ligase (4CL) is one of the indispensable enzymes involved in the biosynthesis of flavonoids. However, the role of 4CL and flavonoids in impact on cotton fertility is still unknown. In this study, on the basis of identification of an additional Gh4CL gene, Gh4CL20A, by using an updated G. hirsutum genome, we found that Gh4CL20A and its homologous Gh4CL20 were preferentially expressed in petals and stamens. The petals of the loss-of-function Gh4CL20/Gh4CL20A mutant generated by CRISPR/Cas9 gene editing remained white until wilting. Notably, the mutant showed indehiscent anthers, reduced number of pollen grains and pollen viability, leading to male sterility. Histological analysis revealed that abnormal degradation of anther tapetum at the tetrad stage and abnormal pollen grain development at the mature stage caused male sterility of the gene editing mutant. Analysis of the anther transcriptome identified a total of 10574 and 11962 genes up- and down-regulated in the mutant, respectively, compared to the wild-type. GO, KEGG, and WGCNA analyses linked the abnormality of the mutant anthers to the defective flavonoid biosynthetic pathway, leading to decreased activity of 4CL and chalcone isomerase (CHI) and reduced accumulation of flavonoids in the mutant. These results imply a role of Gh4CL20/Gh4CL20A in assuring proper development of cotton anthers by regulating flavonoid metabolism. This study elucidates a molecular mechanism underlying cotton anther development and provides candidate genes for creating cotton male sterile germplasm that has the potential to be used in production of hybrid seeds.

Overexpression of the Ginkgo biloba dihydroflavonol 4-reductase gene GbDFR6 results in the self-incompatibility-like phenotypes in transgenic tobacco

Although flavonoids play multiple roles in plant growth and development, the involvement in plant self-incompatibility (SI) have not been reported. In this research, the fertility of transgenic tobacco plants overexpressing the Ginkgo biloba dihydroflavonol 4-reductase gene, GbDFR6, were investigated. To explore the possible physiological defects leading to the failure of embryo development in transgenic tobacco plants, functions of pistils and pollen grains were examined. Transgenic pistils pollinated with pollen grains from another tobacco plants (either transgenic or wild-type), developed full of well-developed seeds. In contrast, in self-pollinated transgenic tobacco plants, pollen-tube growth was arrested in the upper part of the style, and small abnormal seeds developed without fertilization. Although the mechanism remains unclear, our research may provide a valuable method to create SI tobacco plants for breeding.

Characterization and expression analysis of chalcone synthase gene family members suggested their roles in the male sterility of a wheat temperature-sensitive genic male sterile (TGMS) line

Chalcone synthase (CHS), also known as the plants-specific type III polyketide synthases (PKSs), catalyzes the first key step in the biosynthesis of plant flavonoids. Flavonoids are one of the most important secondary metabolites which participate in flower pigmentation and pollen fertility. Recent reports have demonstrated the role of the CHS family in plant pollen exine formation. This study focused on the potential roles of CHS in the pollen exine formation of wheat. In the present study, a genome-wide investigation of the CHS family was carried out, and 87 CHS genes were identified in wheat. TaCHS3, TaCHS10, and TaCHS13 are wheat orthologs of Arabidopsis LESS ADHESIVE POLLEN (LAP5); TaCHS58, TaCHS64, and TaCHS67 are wheat orthologs of AtLAP6. TaCHS3, TaCHS10, and TaCHS67 showed anther-specific patterns. The expression of TaCHS3, TaCHS10, and TaCHS67 was positively co-expressed with sporopollenin biosynthetic genes, including TaCYP703A2, TaCYP704B1, TaDRL1, TaTKPR2, and TaMS2. Coincidently, the expression of TaCHS3, TaCHS10, and TaCHS67, together with those sporopollenin biosynthetic genes, were repressed at the tetrads and uninucleate stages in the temperature-sensitive genic male-sterile (TGMS) line BS366 under sterile conditions. Wheat anther-specific CHS genes might participate in the exine formation of BS366 through co-expressing with sporopollenin biosynthetic genes, which will undoubtedly provide knowledge of the roles of CHS in wheat pollen development.

Comparative transcriptome analysis reveals candidate genes related to the sex differentiation of Schisandra chinensis

Schisandra chinensis is a monoecious plant with unisex flowers. The fruit of S. chinensis is of high medical with economic value. The yield of S. chinensis fruit is related to the ratio of its female and male flowers. However, there is little research on its floral development and sex differentiation. To elucidate the possible mechanism for the sex differentiation of S. chinensis, we collected 18 samples of female and male flowers from three developmental stages and performed a comparative RNA-seq analysis aimed at identifying differentially expressed genes (DEGs) that may be related to sex differentiation. The results showed 936, 7179, and 6890 differentially expressed genes between female and male flowers at three developmental stages, respectively, and 466 candidate genes may play roles in sex differentiation. KEGG analysis showed genes involved in the flavonoid biosynthesis pathway and DNA replication pathway were essential for the development of female flowers. 51 MADS-box genes and 10 YABBY genes were identified in S. chinensis. The DEGs analysis indicated that MADS-box and YABBY genes were strongly related to the sex determination of S. chinensis. RT-qPCR confirmed the RNA-seq results of 20 differentially expressed genes, including three male-biased genes and 17 female-biased genes. A possible regulatory model of sex differentiation in S. chinensis was proposed according to our results. This study helps reveal the sex-differentiation mechanism of S. chinensis and lays the foundation for regulating the male-female ratio of S. chinensis in the future.

Dose effects of restorer gene modulate pollen fertility in cotton CMS-D2 restorer lines via auxin signaling and flavonoid biosynthesis

KEY MESSAGE

Dose effects of Rf1 gene regulated retrieval mechanism of pollen fertility for CMS-D2 cotton. Cytoplasmic male sterility conditioned by Gossypium harknessii cytoplasm (CMS-D2) is an economical pollination control system for producing hybrid cotton seeds compared to artificial and chemical emasculation methods. However, the unstable restoring ability of restorer lines is a main barrier in the large-scale application of "three-line" hybrid cotton in China. Our phenotypic investigation determined that the homozygous Rf1Rf1 allelic genotype had a stronger ability to generate fertile pollen than the heterozygous Rf1rf1 allelic genotype. To decipher the genetic mechanisms that control the differential levels of pollen fertility, an integrated metabolomic and transcriptomic analysis was performed at two environments using pollen grains of four cotton genotypes differing in Rf1 alleles or cytoplasm. Totally 5,391 differential metabolite features were detected, and 369 specific differential metabolites (DMs) were identified between homozygous and heterozygous Rf1 allelic genotypes with CMS-D2 cytoplasm. In addition, transcriptome analysis identified 2,490 differentially expressed genes (DEGs) and 96 unique hub DEGs with dynamic regulation in this comparative combination. Further integrated analyses revealed that several key DEGs and DMs involved in indole biosynthesis, flavonoid biosynthesis, and sugar metabolism had strong network linkage with fertility restoration. In vitro application of auxin analogue NAA and inhibitor Auxinole confirmed that over-activated auxin signaling might inhibit pollen development, whereas suppressing auxin signaling partially promoted pollen development in CMS-D2 cotton. Our results provide new insight into how the dosage effects of the Rf1 gene regulate pollen fertility of CMS-D2 cotton.

Homeostasis of flavonoids and triterpenoids most likely modulates starch metabolism for pollen tube penetration in rice

In angiosperms, the timely delivery of sperm cell nuclei by pollen tube (PT) to the ovule is vital for double fertilization. Penetration of PT into maternal stigma tissue is a critical step for sperm cell nuclei delivery, yet little is known about the process. Here, a male-specific and sporophytic mutant xt6, where PTs are able to germinate but unable to penetrate the stigma tissue, is reported in Oryza sativa. Through genetic study, the causative gene was identified as Chalcone synthase (OsCHS1), encoding the first enzyme in flavonoid biosynthesis. Indeed, flavonols were undetected in mutant pollen grains and PTs, indicating that the mutation abolished flavonoid biosynthesis. Nevertheless, the phenotype cannot be rescued by exogenous application of quercetin and kaempferol as reported in maize and petunia, suggesting a different mechanism exists in rice. Further analysis showed that loss of OsCHS1 function disrupted the homeostasis of flavonoid and triterpenoid metabolism and led to the accumulation of triterpenoid, which inhibits significantly α-amylase activity, amyloplast hydrolysis and monosaccharide content in xt6, these ultimately impaired tricarboxylic acid (TCA) cycle, reduced ATP content and lowered the turgor pressure as well. Our findings reveal a new mechanism that OsCHS1 modulates starch hydrolysis and glycometabolism through modulating the metabolic homeostasis of flavonoids and triterpenoids which affects α-amylase activity to maintain PT penetration in rice, which contributes to a better understanding of the function of CHS1 in crop fertility and breeding.

Stepwise changes in flavonoids in spores/pollen contributed to terrestrial adaptation of plants

Protecting haploid pollen and spores against UV-B light and high temperature, 2 major stresses inherent to the terrestrial environment, is critical for plant reproduction and dispersal. Here, we show flavonoids play an indispensable role in this process. First, we identified the flavanone naringenin, which serves to defend against UV-B damage, in the sporopollenin wall of all vascular plants tested. Second, we found that flavonols are present in the spore/pollen protoplasm of all euphyllophyte plants tested and that these flavonols scavenge reactive oxygen species to protect against environmental stresses, particularly heat. Genetic and biochemical analyses showed that these flavonoids are sequentially synthesized in both the tapetum and microspores during pollen ontogeny in Arabidopsis (Arabidopsis thaliana). We show that stepwise increases in the complexity of flavonoids in spores/pollen during plant evolution mirror their progressive adaptation to terrestrial environments. The close relationship between flavonoid complexity and phylogeny and its strong association with pollen survival phenotypes suggest that flavonoids played a central role in the progression of plants from aquatic environments into progressively dry land habitats.

Uncovering mechanisms governing stem growth in peanut (Arachis hypogaea L.) with varying plant heights through integrated transcriptome and metabolomics analyses

The mechanisms responsible for stem growth in peanut (Arachis hypogaea L.) cultivars with varying plant heights remain unclear, despite the significant impact of plant height on peanut yield. Therefore, this study aimed to investigate the underlying mechanisms of peanut stem growth using phenotypic, physiological, transcriptomic, and metabolomic analyses. The findings revealed that the tallest cultivar, HY33, exhibited the highest rate of stem growth and accumulated the most stem dry matter, followed by the intermediate cultivar, SH108, while the dwarf cultivar, Df216, displayed the lowest values. Furthermore, SH108 exhibited a higher harvest index, as well as superior pod and kernel yields compared to both HY33 and Df216. Transcriptome and metabolome analyses identified differentially expressed genes (DEGs) and differentially expressed metabolites (DEMs) associated with phenylpropanoid and flavonoid biosynthesis. Notably, downregulated DEGs in Df216/HY33 and Df216/SH108 included phenylalanine ammonia-lyase (PAL), caffeoyl-CoA O-methyltransferase (COMT), and ferulate-5-hydroxylase (F5H), while downregulated DEMs included p-coumaryl alcohol, chlorogenic acid, and L-epicatechin. Compared to HY33, the reduced activities of PAL, COMT, and F5H resulted in a decreased stem lignin content in Df216. Additionally, downregulated DEGs involved in gibberellin (GA) and brassinosteroid (BR) biosynthesis were identified in Df216/HY33, which contributed to the lowest levels of GA1, GA3, and BR contents in Df216. The results suggest that the dwarf phenotype arises from impaired GA and BR biosynthesis and signaling, resulting in a slower stem growth rate and reduced lignin accumulation.

Comparative Transcriptome Profiling of CMS-D2 and CMS-D8 Systems Characterizes Fertility Restoration Genes Network in Upland Cotton

Resolving the genetic basis of fertility restoration for cytoplasmic male sterility (CMS) can improve the efficiency of three-line hybrid breeding. However, the genetic determinants of male fertility restoration in cotton are still largely unknown. This study comprehensively compared the full-length transcripts of CMS-D2 and CMS-D8 systems to identify potential genes linked with fertility restorer genes Rf₁ or Rf₂. Target comparative analysis revealed a higher percentage of differential genes in each restorer line as compared to their corresponding sterile and maintainer lines. An array of genes with specific expression in the restorer line of CMS-D2 had functional annotations related to floral development and pathway enrichments in various secondary metabolites, while specifically expressed genes in the CMS-D8 restorer line showed functional annotations related to anther development and pathway enrichment in the biosynthesis of secondary metabolites. Further analysis identified potentially key genes located in the target region of fertility restorer genes Rf₁ or Rf₂. In particular, Ghir_D05G032450 can be the candidate gene related to restorer gene Rf₁, and Ghir_D05G035690 can be the candidate gene associated with restorer gene Rf₂. Further gene expression validation with qRT-PCR confirmed the accuracy of our results. Our findings provide useful insights into decoding the potential regulatory network that retrieves pollen fertility in cotton and will help to further reveal the differences in the genetic basis of fertility restoration for two CMS systems.

Interactive effects of changes in UV radiation and climate on terrestrial ecosystems, biogeochemical cycles, and feedbacks to the climate system

Terrestrial organisms and ecosystems are being exposed to new and rapidly changing combinations of solar UV radiation and other environmental factors because of ongoing changes in stratospheric ozone and climate. In this Quadrennial Assessment, we examine the interactive effects of changes in stratospheric ozone, UV radiation and climate on terrestrial ecosystems and biogeochemical cycles in the context of the Montreal Protocol. We specifically assess effects on terrestrial organisms, agriculture and food supply, biodiversity, ecosystem services and feedbacks to the climate system. Emphasis is placed on the role of extreme climate events in altering the exposure to UV radiation of organisms and ecosystems and the potential effects on biodiversity. We also address the responses of plants to increased temporal variability in solar UV radiation, the interactive effects of UV radiation and other climate change factors (e.g. drought, temperature) on crops, and the role of UV radiation in driving the breakdown of organic matter from dead plant material (i.e. litter) and biocides (pesticides and herbicides). Our assessment indicates that UV radiation and climate interact in various ways to affect the structure and function of terrestrial ecosystems, and that by protecting the ozone layer, the Montreal Protocol continues to play a vital role in maintaining healthy, diverse ecosystems on land that sustain life on Earth. Furthermore, the Montreal Protocol and its Kigali Amendment are mitigating some of the negative environmental consequences of climate change by limiting the emissions of greenhouse gases and protecting the carbon sequestration potential of vegetation and the terrestrial carbon pool.

Flavonols modulate plant development, signaling, and stress responses

Flavonols are plant-specialized metabolites with important functions in plant growth and development. Isolation and characterization of mutants with reduced flavonol levels, especially the transparent testa mutants in Arabidopsis thaliana, have contributed to our understanding of the flavonol biosynthetic pathway. These mutants have also uncovered the roles of flavonols in controlling development in above- and below-ground tissues, notably in the regulation of root architecture, guard cell signaling, and pollen development. In this review, we present recent progress made towards a mechanistic understanding of flavonol function in plant growth and development. Specifically, we highlight findings that flavonols act as reactive oxygen species (ROS) scavengers and inhibitors of auxin transport in diverse tissues and cell types to modulate plant growth and development and responses to abiotic stresses.

Genetics and breeding of phenolic content in tomato, eggplant and pepper fruits

Phenolic acids and flavonoids are large groups of secondary metabolites ubiquitous in the plant kingdom. They are currently in the spotlight due to the numerous health benefits associated with their consumption, as well as for their vital roles in plant biological processes and in plant-environment interaction. Tomato, eggplant and pepper are in the top ten most consumed vegetables in the world, and their fruit accumulation profiles have been extensively characterized, showing substantial differences. A broad array of genetic and genomic tools has helped to identify QTLs and candidate genes associated with the fruit biosynthesis of phenolic acids and flavonoids. The aim of this review was to synthesize the available information making it easily available for researchers and breeders. The phenylpropanoid pathway is tightly regulated by structural genes, which are conserved across species, along with a complex network of regulatory elements like transcription factors, especially of MYB family, and cellular transporters. Moreover, phenolic compounds accumulate in tissue-specific and developmental-dependent ways, as different paths of the metabolic pathway are activated/deactivated along with fruit development. We retrieved 104 annotated putative orthologues encoding for key enzymes of the phenylpropanoid pathway in tomato (37), eggplant (29) and pepper (38) and compiled 267 QTLs (217 for tomato, 16 for eggplant and 34 for pepper) linked to fruit phenolic acids, flavonoids and total phenolics content. Combining molecular tools and genetic variability, through both conventional and genetic engineering strategies, is a feasible approach to improve phenolics content in tomato, eggplant and pepper. Finally, although the phenylpropanoid biosynthetic pathway has been well-studied in the Solanaceae, more research is needed on the identification of the candidate genes behind many QTLs, as well as their interactions with other QTLs and genes.

Exogenous abscisic acid and sodium nitroprusside regulate flavonoid biosynthesis and photosynthesis of Nitraria tangutorum Bobr in alkali stress

Abscisic acid (ABA) and nitric oxide (NO) are involved in mediating abiotic stress-induced plant physiological responses. Nitraria tangutorum Bobr is a typical salinized desert plant growing in an arid environment. In this study, we investigated the effects of ABA and NO on N.tangutorum seedlings under alkaline stress. Alkali stress treatment caused cell membrane damage, increased electrolyte leakage, and induced higher production of reactive oxygen species (ROS), which caused growth inhibition and oxidative stress in N.tangutorum seedlings. Exogenous application of ABA (15μm) and Sodium nitroprusside (50μm) significantly increased the plant height, fresh weight, relative water content, and degree of succulency in N.tangutorum seedlings under alkali stress. Meanwhile, the contents of ABA and NO in plant leaves were significantly increased. ABA and SNP can promote stomatal closure, decrease the water loss rate, increase leaf surface temperature and the contents of osmotic regulator proline, soluble protein, and betaine under alkali stress. Meanwhile, SNP more significantly promoted the accumulation of chlorophyll a/b and carotenoids, increased quantum yield of photosystem II (φPSII) and electron transport rate (ETRII) than ABA, and decreased photochemical quenching (qP), which improved photosynthetic efficiency and accelerated the accumulation of soluble sugar, glucose, fructose, sucrose, starch, and total sugar. However, compared with exogenous application of SNP in the alkaline stress, ABA significantly promoted the transcription of NtFLS/NtF3H/NtF3H/NtANR genes and the accumulation of naringin, quercetin, isorhamnetin, kaempferol, and catechin in the synthesis pathway of flavonoid metabolites, and isorhamnetin content was the highest. These results indicate that both ABA and SNP can reduce the growth inhibition and physiological damage caused by alkali stress. Among them, SNP has a better effect on the improvement of photosynthetic efficiency and the regulation of carbohydrate accumulation than ABA, while ABA has a more significant effect on the regulation of flavonoid and anthocyanin secondary metabolite accumulation. Exogenous application of ABA and SNP also improved the antioxidant capacity and the ability to maintain Na⁺/K⁺ balance of N. tangutorum seedlings under alkali stress. These results demonstrate the beneficial effects of ABA and NO as stress hormones and signaling molecules that positively regulate the defensive response of N. tangutorum to alkaline stress.

OsCCRL1 is Essential for Phenylpropanoid Metabolism in Rice Anthers

Phenylpropanoid metabolism and timely tapetal degradation are essential for anther and pollen development, but the underlying mechanisms are unclear. In the current study, to investigate this, we identified and analyzed the male-sterile mutant, osccrl1 (cinnamoyl coA reductase-like 1), which exhibited delayed tapetal programmed cell death (PCD) and defective mature pollen. Map-based cloning, genetic complementation, and gene knockout revealed that OsCCRL1 corresponds to the gene LOC_Os09g32020.2, a member of SDR (short-chain dehydrogenase/reductase) family enzyme. OsCCRL1 was preferentially expressed in the tapetal cells and microspores, and localized to the nucleus and cytoplasm in both rice protoplasts and Nicotiana benthamiana leaves. The osccrl1 mutant exhibited reduced CCRs enzyme activity, less lignin accumulation, delayed tapetum degradation, and disrupted phenylpropanoid metabolism. Furthermore, an R2R3 MYB transcription factor OsMYB103/OsMYB80/OsMS188/BM1, involved in tapetum and pollen development, regulates the expression of OsCCRL1. Finally, the osmyb103 osccrl1 double mutants, exhibited the same phenotype as the osmyb103 single mutant, further indicating that OsMYB103/OsMYB80/OsMS188/BM1 functions upstream of OsCCRL1. These findings help to clarify the role of phenylpropanoid metabolism in male sterility and the regulatory network underlying the tapetum degradation.

Comparative transcriptome analysis in peaberry and regular bean coffee to identify bean quality associated genes

BACKGROUND

The peaberry bean in Arabica coffee has exceptional quality compared to the regular coffee bean. Understanding the molecular mechanism of bean quality is imperative to introduce superior coffee quality traits. Despite high economic importance, the regulatory aspects of bean quality are yet largely unknown in peaberry. A transcriptome analysis was performed by using peaberry and regular coffee beans in this study.

RESULTS

The result of phenotypic analysis stated a difference in the physical attributes of both coffee beans. In addition, transcriptome analysis revealed low genetic differences. Only 139 differentially expressed genes were detected in which 54 genes exhibited up-regulation and 85 showed down-regulations in peaberry beans compared to regular beans. The majority of differentially expressed genes had functional annotation with cell wall modification, lipid binding, protein binding, oxidoreductase activity, and transmembrane transportation. Many fold lower expression of Ca25840-PMEs1, Ca30827-PMEs2, Ca30828-PMEs3, Ca25839-PMEs4, Ca36469-PGs. and Ca03656-Csl genes annotated with cell wall modification might play a critical role to develop different bean shape patterns in Arabica. The ERECTA family genes Ca15802-ERL1, Ca99619-ERL2, Ca07439-ERL3, Ca97226-ERL4, Ca89747-ERL5, Ca07056-ERL6, Ca01141-ERL7, and Ca32419-ERL8 along lipid metabolic pathway genes Ca06708-ACOX1, Ca29177-ACOX2, Ca01563-ACOX3, Ca34321-CPFA1, and Ca36201-CPFA2 are predicted to regulate different shaped bean development. In addition, flavonoid biosynthesis correlated genes Ca03809-F3H, Ca95013-CYP75A1, and Ca42029-CYP75A2 probably help to generate rarely formed peaberry beans.

CONCLUSION

Our results provide molecular insights into the formation of peaberry. The data resources will be important to identify candidate genes correlated with the different bean shape patterns in Arabica.

Integration of semi-in vivo assays and multi-omics data reveals the effect of galloylated catechins on self-pollen tube inhibition in Camellia oleifera

Camellia oil extracted from the seeds of Camellia oleifera Abel. is a popular and high-quality edible oil, but its yield is limited by seed setting, which is mainly caused by self-incompatibility (SI). One of the obvious biological features of SI plants is the inhibition of self-pollen tubes; however, the underlying mechanism of this inhibition in C. oleifera is poorly understood. In this study, we constructed a semi-in vivo pollen tube growth test (SIV-PGT) system that can screen for substances that inhibit self-pollen tubes without interference from the genetic background. Combined with multi-omics analysis, the results revealed the important role of galloylated catechins in self-pollen tube inhibition, and a possible molecular regulatory network mediated by UDP-glycosyltransferase (UGT) and serine carboxypeptidase-like (SCPL) was proposed. In summary, galloylation of catechins and high levels of galloylated catechins are specifically involved in pollen tube inhibition under self-pollination rather than cross-pollination, which provides a new understanding of SI in C. oleifera. These results will contribute to sexual reproduction research on C. oleifera and provide theoretical support for improving Camellia oil yield in production.

Multifaceted roles of flavonoids mediating plant-microbe interactions

Plant-microbe interactions dynamically affect plant growth, health, and development. The mechanisms underpinning these associations are-to a large extent-mediated by specialized host-derived secondary metabolites. Flavonoids are one of the most studied classes of such metabolites, regulating both plant development and the interaction with commensal microbes. Here, we provide a comprehensive review of the multiple roles of flavonoids in mediating plant-microbe interactions. First, we briefly summarize the general aspects of flavonoid synthesis, transport, and exudation in plants. Then, we review the importance of flavonoids regulating plant-microbe interactions and dynamically influencing the overall community assembly of plant-root microbiomes. Last, we highlight potential knowledge gaps in our understanding of how flavonoids determine the interactions between plants and commensal microbes. Collectively, we advocate the importance of advancing research in this area toward innovative strategies to effectively manipulate plant-microbiome composition, in this case, via flavonoid production and exudation in plant roots. Video Abstract.

Reactive oxygen species function as signaling molecules in controlling plant development and hormonal responses

Reactive oxygen species (ROS) serve as second messengers in plant signaling pathways to remodel plant growth and development. New insights into how enzymatic ROS-producing machinery is regulated by hormones or localized during development have provided a framework for understanding the mechanisms that control ROS accumulation patterns. Signaling-mediated increases in ROS can then modulate the activity of proteins through reversible oxidative modification of specific cysteine residues. Plants also control the synthesis of antioxidants, including plant-specialized metabolites, to further define when, where, and how much ROS accumulate. The availability of sophisticated imaging capabilities, combined with a growing tool kit of ROS detection technologies, particularly genetically encoded biosensors, sets the stage for improved understanding of ROS as signaling molecules.

Integrated analysis of metabolome and transcriptome reveals the cytoplasmic effects of CMS-D2 on pollen fertility resulting from disrupted lipid metabolism

Using cytoplasmic male sterility of Gossypium harknesii (CMS-D2) is an economical and effective method to produce cotton hybrids. However, the detrimental cytoplasmic effects of CMS-D2 on pollen fertility and fiber yields greatly limit the further development of three-line hybrid cotton in China. In this study, an integrated non-targeted metabolomics and transcriptome analysis was performed on mature pollens of maintainer line NB, isonuclear alloplasmic near-isogenic restorer lines NH and SH under two environments. A total of 820 metabolites were obtained, of which lipids and lipid-like molecules were the most, followed by organic acids derivatives, phenylpropanoids, and polyketides. Transcriptome analysis revealed significantly more differentially expressed genes (DEGs) in SH versus NH both in Anyang and Jiujiang, and most of the DEGs were significantly upregulated. Further KEGG analysis showed that most DEGs were enriched in the biosynthesis of unsaturated fatty acids, phenylalanine metabolism, and phagosome. Based on the weighted gene co-expression network analysis, totally 74 hub genes were also identified, of which three transcription factors, i.e., WRKY22, WRKY53, and ARF18 were significantly upregulated in SH and may play a negative regulatory role in pollen development by directly or indirectly regulating the jasmonic acid synthesis and signal transduction. Moreover, we found that the negative effects of CMS-D2 cytoplasm on pollen fertility were mainly due to disturbed lipid metabolism, especially the metabolic balance of unsaturated fatty acids, ultimately resulting in the decline of pollen fertility. Meanwhile, in the presence of CMS-D2 sterile cytoplasm, the cytoplasmic-nucleus interaction effects generated a substantial quantity of flavonoids involved in the fertility restoration process. This study preliminarily clarified some of the reasons for the negative effects of CMS-D2 cytoplasm on pollen fertility, and our results will provide an important theoretical reference for further breeding and improvement of three-line hybrid cotton in the future.

High-energy-level metabolism and transport occur at the transition from closed to open flowers

During the maturation phase of flower development, the onset of anthesis visibly marks the transition from buds to open flowers, during which petals stretch out, nectar secretion commences, and pollination occurs. Analysis of the metabolic changes occurring during this developmental transition has primarily focused on specific classes of metabolites, such as pigments and scent emission, and far less on the whole network of primary and secondary metabolites. To investigate the metabolic changes occurring at anthesis, we performed multi-platform metabolomics alongside RNA sequencing in individual florets harvested from the main inflorescence of Arabidopsis (Arabidopsis thaliana) ecotype Col-0. To trace metabolic fluxes at the level of the whole inflorescence and individual florets, we further integrated these studies with radiolabeled experiments. These extensive analyses revealed high-energy-level metabolism and transport of carbohydrates and amino acids, supporting intense metabolic rearrangements occurring at the time of this floral transition. These comprehensive data are discussed in the context of our current understanding of the metabolic shifts underlying flower opening. We envision that this analysis will facilitate the introgression of floral metabolic traits promoting pollination in crop species for which a comprehensive knowledge of flower metabolism is still limited.

OsSNDP3 Functions for the Polar Tip Growth in Rice Pollen Together with OsSNDP2, a Paralog of OsSNDP3

Understanding pollen tube growth is critical for crop yield maintenance. The pollen tube provides a path for sperm cells for fertilization with egg cells. Cells must be subdivided into functionally and structurally distinct compartments for polar tip growth, and phosphoinositides are thought to be one of the facilitators for polarization during pollen tube growth. OsSNDP3 encodes Sec14-nodulin domain-containing protein and localizes in the nucleus and the microdomains of the plasma membrane in tobacco leaf epidermis cells. OsSNDP3 is thought to bind with phosphatidylinositol 4,5-bisphosphate based on the data including the information of basic amino acids in the C-terminal and colocalization with 2X Pleckstrin homology domain of Phospholipase C delta-1. OsSNDP3 interacts with a protein that contains a class I nodulin domain. We discovered that OsSNDP3 plays a significant role in pollen tube germination using CRISPR/Cas9 systems, whereas another pollen-preferential Sec14-nodulin domain-containing protein, OsSNDP2, additively functions with OsSNDP3 during pollen tube germination. Gene Ontology analysis using downregulated genes in ossndp3 indicated that the expression of genes involved in the phosphatidylinositol metabolic process and tip growth was significantly altered in ossndp3. OsSNDP3 aids pollen polar tip growth by binding with phosphatidylinositol 4,5-bisphosphate. We can better understand the roles of phosphoinositides during pollen tube growth by studying the functions of OsSNDP3 and OsSNDP2. And downregulated genes in ossndp3 might be useful targets for future research on polar tip growth.

Alternative Splicing and Its Roles in Plant Metabolism

Plant metabolism, including primary metabolism such as tricarboxylic acid cycle, glycolysis, shikimate and amino acid pathways as well as specialized metabolism such as biosynthesis of phenolics, alkaloids and saponins, contributes to plant survival, growth, development and interactions with the environment. To this end, these metabolic processes are tightly and finely regulated transcriptionally, post-transcriptionally, translationally and post-translationally in response to different growth and developmental stages as well as the constantly changing environment. In this review, we summarize and describe the current knowledge of the regulation of plant metabolism by alternative splicing, a post-transcriptional regulatory mechanism that generates multiple protein isoforms from a single gene by using alternative splice sites during splicing. Numerous genes in plant metabolism have been shown to be alternatively spliced under different developmental stages and stress conditions. In particular, alternative splicing serves as a regulatory mechanism to fine-tune plant metabolism by altering biochemical activities, interaction and subcellular localization of proteins encoded by splice isoforms of various genes.

Integrative Analyses of Transcriptomics and Metabolomics in Sex Differentiation of Mulberry Flowers

Sex determination and sex differentiation of plants are important physiological processes of plant development. Mulberry (Morus indica L.) is an important economic tree being cultivated in sericulture countries, and mulberry leaf is commonly used for sericulture. The transcriptomic and metabolomic differences between the staminate flowers (SFs) and pistillate flowers (PFs) of mulberry were investigated by RNA sequencing and ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Overall, we uncovered 4,230 genes and 209 metabolites are significantly differentially expressed between the SFs and PFs of mulberry. The combined transcriptomic and metabolomic analysis revealed these differentially expressed genes (DEGs) and differentially expressed metabolites (DEMs) are involved in flavonoid biosynthesis, galactose metabolism, plant–pathogen interaction, and starch and sucrose metabolism, and these detected DEGs and DEMs may be associated with sex differentiation of mulberry through the regulation of the enrichment pathways, such as the MAPK pathway, flavonoid biosynthesis, galactose metabolism, plant–pathogen interaction, and starch and sucrose metabolism. This study will provide a rich source for the analysis of the molecular mechanism of mulberry sex differentiation processes.

OsSPLs Regulate Male Fertility in Response to Different Temperatures by Flavonoid Biosynthesis and Tapetum PCD in PTGMS Rice

Photoperiod and thermo-sensitive genic male sterile (PTGMS) rice is an important resource for two line hybrid rice production. The SQUAMOSA–promoter binding, such as the (SPL) gene family, encode the plant specific transcription factors that regulate development and defense responses in plants. However, the reports about SPLs participating in male fertility regulation are limited. Here, we identified 19 OsSPL family members and investigated their involvement in the fertility regulation of the PTGMS rice lines, PA2364S and PA2864S, with different fertility transition temperatures. The results demonstrated that OsSPL2, OsSPL4, OsSPL16 and OsSPL17 affect male fertility in response to temperature changes through the MiR156-SPL module. WGCNA (weighted gene co-expression network analysis) revealed that CHI and APX1 were co-expressed with OsSPL17. Targeted metabolite and flavonoid biosynthetic gene expression analysis revealed that OsSPL17 regulates the expression of flavonoid biosynthesis genes CHI, and the up regulation of flavanones (eriodictvol and naringenin) and flavones (apigenin and luteolin) content contributed to plant fertility. Meanwhile, OsSPL17 negatively regulates APX1 to affect APX (ascorbate peroxidase) activity, thereby regulating ROS (reactive oxygen species) content in the tapetum, controlling the PCD (programmed cell death) process and regulating male fertility in rice. Overall, this report highlights the potential role of OsSPL for the regulation of male fertility in rice and provides a new insight for the further understanding of fertility molecular mechanisms in PTGMS rice.

Deficiency in flavonoid biosynthesis genes CHS, CHI, and CHIL alters rice flavonoid and lignin profiles

Lignin is a complex phenylpropanoid polymer deposited in the secondary cell walls of vascular plants. Unlike most gymnosperm and eudicot lignins that are generated via the polymerization of monolignols, grass lignins additionally incorporate the flavonoid tricin as a natural lignin monomer. The biosynthesis and functions of tricin-integrated lignin (tricin-lignin) in grass cell walls and its effects on the utility of grass biomass remain largely unknown. We herein report a comparative analysis of rice (Oryza sativa) mutants deficient in the early flavonoid biosynthetic genes encoding CHALCONE SYNTHASE (CHS), CHALCONE ISOMERASE (CHI), and CHI-LIKE (CHIL), with an emphasis on the analyses of disrupted tricin-lignin formation and the concurrent changes in lignin profiles and cell wall digestibility. All examined CHS-, CHI-, and CHIL-deficient rice mutants were largely depleted of extractable flavones, including tricin, and nearly devoid of tricin-lignin in the cell walls, supporting the crucial roles of CHS and CHI as committed enzymes and CHIL as a noncatalytic enhancer in the conserved biosynthetic pathway leading to flavone and tricin-lignin formation. In-depth cell wall structural analyses further indicated that lignin content and composition, including the monolignol-derived units, were differentially altered in the mutants. However, regardless of the extent of the lignin alterations, cell wall saccharification efficiencies of all tested rice mutants were similar to that of the wild-type controls. Together with earlier studies on other tricin-depleted grass mutant and transgenic plants, our results reflect the complexity in the metabolic consequences of tricin pathway perturbations and the relationships between lignin profiles and cell wall properties.

Combined analysis of transcriptome and metabolome reveals that sugar, lipid, and phenylpropane metabolism are essential for male fertility in temperature-induced male sterile rice

Photoperiod- and thermosensitive genic male sterility (PTGMS) rice is a vital germplasm resource consisting of two-line hybrid rice in which light and temperature strictly control their fertility changes. Variable environmental conditions present huge risks to the two-lines hybrid seed production. Explaining the regulatory mechanism of male fertility in rice PTGMS lines is an essential prerequisite to ensuring food security production. A group of near-isogenic lines (NILs) of a rice PTGMS line unique to this research group was used for this study. These lines have the same genetic background and regulate male fertility by responding to different temperature changes. Transcriptomic analysis revealed that 315 upregulated genes and 391 regulated genes regulated male fertility in response to temperature changes, and differentially expressed genes (DEGs) were mainly characterized in enrichment analysis as having roles in the metabolic pathways of sugar, lipid and phenylpropanoid. Electron microscopy analysis revealed that a lack of starch accumulation in sterile pollen grains induced by high temperature, with an abnormal exine development and a lack of inner pollen grains. Defective processes for sporopollenin synthesis, sporopollenin transport and pollen wall formation in sterile anthers were verified using qPCR. Targeted metabolomics analysis revealed that most lipids (phospholipids, sphingolipids and fatty acids) and flavonoids (flavones and flavanones) were upregulated in fertile anthers and involved in pollen wall development and male fertility formation, while lignin G units and C-type lignin were the major contributors to pollen wall development. The coding genes for trehalose 6-phosphate phosphatase, beta-1,3-glucanase, phospholipase D and 4-coumarate-CoA ligase are considered essential regulators in the process of male fertility formation. In conclusion, our results indicated that the expression of critical genes and accumulation of metabolites in the metabolism of sugar, lipid, and phenylpropanoid are essential for male fertility formation. The results provide new insights for addressing the negative effects of environmental variation on two-line hybrid rice production.

Two Is Company, but Four Is a Party-Challenges of Tetraploidization for Cell Wall Dynamics and Efficient Tip-Growth in Pollen

Some cells grow by an intricately coordinated process called tip-growth, which allows the formation of long tubular structures by a remarkable increase in cell surface-to-volume ratio and cell expansion across vast distances. On a broad evolutionary scale, tip-growth has been extraordinarily successful, as indicated by its recurrent 're-discovery' throughout evolutionary time in all major land plant taxa which allowed for the functional diversification of tip-growing cell types across gametophytic and sporophytic life-phases. All major land plant lineages have experienced (recurrent) polyploidization events and subsequent re-diploidization that may have positively contributed to plant adaptive evolutionary processes. How individual cells respond to genome-doubling on a shorter evolutionary scale has not been addressed as elaborately. Nevertheless, it is clear that when polyploids first form, they face numerous important challenges that must be overcome for lineages to persist. Evidence in the literature suggests that tip-growth is one of those processes. Here, I discuss the literature to present hypotheses about how polyploidization events may challenge efficient tip-growth and strategies which may overcome them: I first review the complex and multi-layered processes by which tip-growing cells maintain their cell wall integrity and steady growth. I will then discuss how they may be affected by the cellular changes that accompany genome-doubling. Finally, I will depict possible mechanisms polyploid plants may evolve to compensate for the effects caused by genome-doubling to regain diploid-like growth, particularly focusing on cell wall dynamics and the subcellular machinery they are controlled by.

SWATH-MS based quantitive proteomics reveal regulatory metabolism and networks of androdioecy breeding system in Osmanthus fragrans

BACKGROUND

The fragrant flower plant Osmanthus fragrans has an extremely rare androdioecious breeding system displaying the occurrence of males and hermaphrodites in a single population, which occupies a crucial intermediate stage in the evolutionary transition between hermaphroditism and dioecy. However, the molecular mechanism of androdioecy plant is very limited and still largely unknown.

RESULTS

Here, we used SWATH-MS-based quantitative approach to study the proteome changes between male and hermaphroditic O. fragrans pistils. A total of 428 proteins of diverse functions were determined to show significant abundance changes including 210 up-regulated and 218 down-regulated proteins in male compared to hermaphroditic pistils. Functional categorization revealed that the differentially expressed proteins (DEPs) primarily distributed in the carbohydrate metabolism, secondary metabolism as well as signaling cascades. Further experimental analysis showed the substantial carbohydrates accumulation associated with promoted net photosynthetic rate and water use efficiency were observed in purplish red pedicel of hermaphroditic flower compared with green pedicel of male flower, implicating glucose metabolism serves as nutritional modulator for the differentiation of male and hermaphroditic flower. Meanwhile, the entire upregulation of secondary metabolism including flavonoids, isoprenoids and lignins seem to protect and maintain the male function in male flowers, well explaining important feature of androdioecy that aborted pistil of a male flower still has a male function. Furthermore, nine selected DEPs were validated via gene expression analysis, suggesting an extra layer of post-transcriptional regulation occurs during O. fragrans floral development.

CONCLUSION

Taken together, our findings represent the first SWATH-MS-based proteomic report in androdioecy plant O. fragrans, which reveal carbohydrate metabolism, secondary metabolism and post-transcriptional regulation contributing to the androdioecy breeding system and ultimately extend our understanding on genetic basis as well as the industrialization development of O. fragrans.

Tricin Biosynthesis and Bioengineering

Tricin (3',5'-dimethoxyflavone) is a specialized metabolite which not only confers stress tolerance and involves in defense responses in plants but also represents a promising nutraceutical. Tricin-type metabolites are widely present as soluble tricin O-glycosides and tricin-oligolignols in all grass species examined, but only show patchy occurrences in unrelated lineages in dicots. More strikingly, tricin is a lignin monomer in grasses and several other angiosperm species, representing one of the "non-monolignol" lignin monomers identified in nature. The unique biological functions of tricin especially as a lignin monomer have driven the identification and characterization of tricin biosynthetic enzymes in the past decade. This review summarizes the current understanding of tricin biosynthetic pathway in grasses and tricin-accumulating dicots. The characterized and potential enzymes involved in tricin biosynthesis are highlighted along with discussion on the debatable and uncharacterized steps. Finally, current developments of bioengineering on manipulating tricin biosynthesis toward the generation of functional food as well as modifications of lignin for improving biorefinery applications are summarized.

Involvement of the R2R3-MYB transcription factor MYB21 and its homologs in regulating flavonol accumulation in Arabidopsis stamen

Commonly found flavonols in plants are synthesized from dihydroflavonols by flavonol synthase (FLS). The genome of Arabidopsis thaliana contains six FLS genes, among which FLS1 encodes a functional enzyme. Previous work has demonstrated that the R2R3-MYB subgroup 7 transcription factors MYB11, MYB12, and MYB111 redundantly regulate flavonol biosynthesis. However, flavonol accumulation in pollen grains was unaffected in the myb11myb12myb111 triple mutant. Here we show that MYB21 and its homologs MYB24 and MYB57, which belong to subgroup 19, promote flavonol biosynthesis through regulation of FLS1 gene expression. We used a combination of genetic and metabolite analysis to identify the role of MYB21 in regulating flavonol biosynthesis through direct binding to the GARE cis-element in the FLS1 promoter. Treatment with kaempferol or overexpression of FLS1 rescued stamen defects in the myb21 mutant. We also observed that excess reactive oxygen species (ROS) accumulated in the myb21 stamen, and that treatment with the ROS inhibitor diphenyleneiodonium chloride partly rescued the reduced fertility of the myb21 mutant. Furthermore, drought increased ROS abundance and impaired fertility in myb21, myb21myb24myb57, and chs, but not in the wild type or myb11myb12myb111, suggesting that pollen-specific flavonol accumulation contributes to drought-induced male fertility by ROS scavenging in Arabidopsis.

Implications of ionizing radiation on pollen performance in comparison with diverse models of polar cell growth

Research concerning the effects of ionizing radiation (IR) on plant systems is essential for numerous aspects of human society, as for instance, in terms of agriculture and plant breeding, but additionally for elucidating consequences of radioactive contamination of the ecosphere. This comprehensive survey analyses effects of x- and γ-irradiation on male gametophytes comprising primarily in vitro but also in vivo data of diverse plant species. The IR-dose range for pollen performance was compiled and 50% inhibition doses (ID₅₀ ) for germination and tube growth were comparatively related to physiological characteristics of the microgametophyte. Factors influencing IR-susceptibility of mature pollen and polarized tube growth were evaluated, such as dose-rate, environmental conditions, or species-related variations. In addition, all available reports suggesting bio-positive IR-effects particularly on pollen performance were examined. Most importantly, for the first time influences of IR specifically on diverse phylogenetic models of polar cell growth were comparatively analysed, and thus demonstrated that the gametophytic system of pollen is extremely resistant to IR, more than plant sporophytes and especially much more than comparable animal cells. Beyond that, this study develops hypotheses regarding a molecular basis for the extreme IR-resistance of the plant microgametophyte and highlights its unique rank among organismal systems.

Contribution of phenylpropanoid metabolism to plant development and plant-environment interactions

Phenylpropanoid metabolism is one of the most important metabolisms in plants, yielding more than 8,000 metabolites contributing to plant development and plant-environment interplay. Phenylpropanoid metabolism materialized during the evolution of early freshwater algae that were initiating terrestrialization and land plants have evolved multiple branches of this pathway, which give rise to metabolites including lignin, flavonoids, lignans, phenylpropanoid esters, hydroxycinnamic acid amides, and sporopollenin. Recent studies have revealed that many factors participate in the regulation of phenylpropanoid metabolism, and modulate phenylpropanoid homeostasis when plants undergo successive developmental processes and are subjected to stressful environments. In this review, we summarize recent progress on elucidating the contribution of phenylpropanoid metabolism to the coordination of plant development and plant-environment interaction, and metabolic flux redirection among diverse metabolic routes. In addition, our review focuses on the regulation of phenylpropanoid metabolism at the transcriptional, post-transcriptional, post-translational, and epigenetic levels, and in response to phytohormones and biotic and abiotic stresses.

Managing enzyme promiscuity in plant specialized metabolism: A lesson from flavonoid biosynthesis: Mission of a "body double" protein clarified

Specificities of enzymes involved in plant specialized metabolism, including flavonoid biosynthesis, are generally promiscuous. This enzyme promiscuity has served as an evolutionary basis for new enzyme functions and metabolic pathways in land plants adapting to environmental challenges. This phenomenon may lead, however, to inefficiency in specialized metabolism and adversely affect metabolite-mediated plant survival. How plants manage enzyme promiscuity for efficient specialized metabolism is, thus, an open question. Recent studies of flavonoid biosynthesis addressing this issue have revealed a conserved strategy, namely, a homolog of chalcone isomerase with no catalytic activity binds to chalcone synthase, a key flavonoid pathway enzyme, to narrow (or rectify) the enzyme's highly promiscuous product specificity. Reducing promiscuity via specific protein-protein interactions among metabolic enzymes and proteins may be a solution adopted by land plants to achieve efficient operation of specialized metabolism, while the intrinsic promiscuity of enzymes has likely been retained incidentally.