Spatial accumulation of lignin monomers and cellulose underlying stalk strength in maize.

Lodging largely affects yield, quality and mechanical harvesting of maize. Stalk strength is one of the major factors that affect maize lodging. Although plant cell wall components including lignin and cellulose were known to be associated with stalk strength and lodging resistance, spatial accumulation of specific lignin monomers and cellulose in different tissues and their association with stalk strength in maize was not clearly understood. In this study, we found that both G and S lignin monomers accumulate highest in root, stem rind and leaf vein. Consistently, most lignin biosynthetic genes were expressed higher in root and stem than in other tissues. However, cellulose appears to be lowest in root. There are only mild changes of G lignin and cellulose in different internodes. Instead, we noticed a dramatic decrease of S-lignin accumulation and lignin biosynthetic gene expression in 2nd to 4th internodes wherein stem breakage usually occurs, thereby revealing a few candidate lignin biosynthetic genes associated with stalk strength. Moreover, stalk strength is positively correlated with G, S lignin, and cellulose, but negatively correlated with S/G ratio based on data of maize lines with high or low stalk strength. Loss-of-function of a caffeic acid o-methyltransferase (COMT), which is involved in S lignin biosynthesis, in the maize bm3 mutant, leads to lower stalk strength. Our data collectively suggest that stalk strength is determined by tissue-specific accumulation of lignin monomers and cellulose, and manipulation of the cell wall components by genetic engineering is vital to improve maize stalk strength and lodging resistance.

Copine proteins are required for brassinosteroid signaling in maize and Arabidopsis.

Copine proteins are highly conserved and ubiquitously found in eukaryotes, and their indispensable roles in different species were proposed. However, their exact function remains unclear. The phytohormone brassinosteroids (BRs) play vital roles in plant growth, development and environmental responses. A key event in effective BR signaling is the formation of functional BRI1-SERK receptor complex and subsequent transphosphorylation upon ligand binding. Here, we demonstrate that BONZAI (BON) proteins, which are plasma membrane-associated copine proteins, are critical components of BR signaling in both the monocot maize and the dicot Arabidopsis. Biochemical and molecular analyses reveal that BON proteins directly interact with SERK kinases, thereby ensuring effective BRI1-SERK interaction and transphosphorylation. This study advances the knowledge on BR signaling and provides an important target for optimizing valuable agronomic traits, it also opens a way to study steroid hormone signaling and copine proteins of eukaryotes in a broader perspective.

A genome-wide association study identifies genes associated with cuticular wax metabolism in maize.

The plant cuticle is essential in plant defense against biotic and abiotic stresses. To systematically elucidate the genetic architecture of maize (Zea mays L.) cuticular wax metabolism, 2 cuticular wax-related traits, the chlorophyll extraction rate (CER) and water loss rate (WLR) of 389 maize inbred lines, were investigated and a genome-wide association study (GWAS) was performed using 1.25 million single nucleotide polymorphisms (SNPs). In total, 57 nonredundant quantitative trait loci (QTL) explaining 5.57% to 15.07% of the phenotypic variation for each QTL were identified. These QTLs contained 183 genes, among which 21 strong candidates were identified based on functional annotations and previous publications. Remarkably, 3 candidate genes that express differentially during cuticle development encode ß-ketoacyl-CoA synthase (KCS). While ZmKCS19 was known to be involved in cuticle wax metabolism, ZmKCS12 and ZmKCS3 functions were not reported. The association between ZmKCS12 and WLR was confirmed by resequencing 106 inbred lines, and the variation of WLR was significant between different haplotypes of ZmKCS12. In this study, the loss-of-function mutant of ZmKCS12 exhibited wrinkled leaf morphology, altered wax crystal morphology, and decreased C32 wax monomer levels, causing an increased WLR and sensitivity to drought. These results confirm that ZmKCS12 plays a vital role in maize C32 wax monomer synthesis and is critical for drought tolerance. In sum, through GWAS of 2 cuticular wax-associated traits, this study reveals comprehensively the genetic architecture in maize cuticular wax metabolism and provides a valuable reference for the genetic improvement of stress tolerance in maize.

Characterization and fine mapping of a maize lesion mimic mutant (Les8) with enhanced resistance to Curvularia leaf spot and southern leaf blight.

KEY MESSAGE: A novel light-dependent dominant lesion mimic mutant with enhanced multiple disease resistance was physiologically, biochemically, and genetically characterized; the causal gene was fine mapped to a 909 kb interval containing 38 genes. Identification of genes that confer multiple disease resistance (MDR) is crucial for the improvement of maize disease resistance. However, very limited genes are identified as MDR genes in maize. In this study, we characterized a dominant disease lesion mimics 8 (Les8) mutant that had chlorotic lesions on the leaves and showed enhanced resistance to both curvularia leaf spot and southern leaf blight. Major agronomic traits were not obviously altered, while decreased chlorophyll content was observed in the mutant, and the genetic effect of the Les8 mutation was stable in different genetic backgrounds. By BSR-seq analysis and map-based cloning, the LES8 gene was mapped into a 909 kb region containing 38 candidate genes on chromosome 9 wherein no lesion mimic or disease-resistance genes were previously reported. Using transcriptomics analysis, we found that genes involved in defense responses and secondary metabolite biosynthesis were enriched in the significantly up-regulated genes, while genes involved in photosynthesis and carbohydrate-related pathways were enriched in the significantly down-regulated genes in Les8. In addition, there was an overaccumulation of jasmonic acid and lignin but not salicylic acid in Les8. Taken together, this study revealed candidate genes and potential mechanism underlying Les8-conferred MDR in maize.

Tissue-preferential recruitment of electron transfer chains for cytochrome P450-catalyzed phenolic biosynthesis.

Cytochrome P450 system consists of P450 monooxygenase and redox pattern(s). While the importance of monooxygenases in plant metabolism is well documented, the metabolic roles of the related redox components have been largely overlooked. Here, we show that distinct electron transfer chains are recruited in phenylpropanoid-monolignol P450 systems to support the synthesis and distribution of different classes of phenolics in different plant tissues. While Arabidopsis cinnamate 4-hydroxylase adopts conventional NADPH-cytochrome P450 oxidoreductase (CPR) electron transfer chain for its para-hydroxylation reaction, ferulate 5-hydroxylase uses both NADPH-CPR-cytochrome b5 (CB5) and NADH-cytochrome b5 reductase-CB5 chains to support benzene ring 5-hydroxylation, in which the former route is primarily recruited in the stem for syringyl lignin synthesis, while the latter dominates in the syntheses of 5-hydroxylated phenolics in seeds and seed coat suberin. Our study unveils an additional layer of complexity and versatility of P450 system that the plants evolved for diversifying phenolic repertoires.

Quantitative disease resistance: Multifaceted players in plant defense.

In contrast to large-effect qualitative disease resistance, quantitative disease resistance (QDR) exhibits partial and generally durable resistance and has been extensively utilized in crop breeding. The molecular mechanisms underlying QDR remain largely unknown but considerable progress has been made in this area in recent years. In this review, we summarize the genes that have been associated with plant QDR and their biological functions. Many QDR genes belong to the canonical resistance gene categories with predicted functions in pathogen perception, signal transduction, phytohormone homeostasis, metabolite transport and biosynthesis, and epigenetic regulation. However, other "atypical" QDR genes are predicted to be involved in processes that are not commonly associated with disease resistance, such as vesicle trafficking, molecular chaperones, and others. This diversity of function for QDR genes contrasts with qualitative resistance, which is often based on the actions of nucleotide-binding leucine-rich repeat (NLR) resistance proteins. An understanding of the diversity of QDR mechanisms and of which mechanisms are effective against which classes of pathogens will enable the more effective deployment of QDR to produce more durably resistant, resilient crops.

HFR1, a bHLH Transcriptional Regulator from Arabidopsis thaliana, Improves Grain Yield, Shade and Osmotic Stress Tolerances in Common Wheat.

Common wheat, Triticum aestivum, is the most widely grown staple crop worldwide. To catch up with the increasing global population and cope with the changing climate, it is valuable to breed wheat cultivars that are tolerant to abiotic or shade stresses for density farming. Arabidopsis LONG HYPOCOTYL IN FAR-RED 1 (AtHFR1), a photomorphogenesis-promoting factor, is involved in multiple light-related signaling pathways and inhibits seedling etiolation and shade avoidance. We report that overexpression of AtHFR1 in wheat inhibits etiolation phenotypes under various light and shade conditions, leading to shortened plant height and increased spike number relative to non-transgenic plants in the field. Ectopic expression of AtHFR1 in wheat increases the transcript levels of TaCAB and TaCHS as observed previously in Arabidopsis, indicating that the AtHFR1 transgene can activate the light signal transduction pathway in wheat. AtHFR1 transgenic seedlings significantly exhibit tolerance to osmotic stress during seed germination compared to non-transgenic wheat. The AtHFR1 transgene represses transcription of TaFT1, TaCO1, and TaCO2, delaying development of the shoot apex and heading in wheat. Furthermore, the AtHFR1 transgene in wheat inhibits transcript levels of PHYTOCHROME-INTERACTING FACTOR 3-LIKEs (TaPIL13, TaPIL15-1B, and TaPIL15-1D), downregulating the target gene STAYGREEN (TaSGR), and thus delaying dark-induced leaf senescence. In the field, grain yields of three AtHFR1 transgenic lines were 18.2-48.1% higher than those of non-transgenic wheat. In summary, genetic modification of light signaling pathways using a photomorphogenesis-promoting factor has positive effects on grain yield due to changes in plant architecture and resource allocation and enhances tolerances to osmotic stress and shade avoidance response.

Evolutionary characterization of miR396s in Poaceae exemplified by their genetic effects in wheat and maize.

MiR396s play important roles in regulating plant growth and stress response, and great potential for crop yield promotion was anticipated. For more comprehensive and precise understanding of miR396s in Poaceae, we analyzed the phylogenetic linkage, gene expression, and chromosomal distribution of miR396s in this study. Although the mature miR396s' sequences were mostly conserved, differential expression patterns and chromosomal distribution were found among Poaceae species including the major cereal crops rice, wheat, and maize. Consistently, in comparison with rice, wheat and maize plants transformed with the target mimicry construct of miR396 (MIM396) exhibited differential effects on grain size and disease resistance. While the TaMIM396 plants showed increased grain size, panicle length and sensitivity to B. graminis, the ZmMIM396 plants didn't show obvious changes in grain size and disease resistance. In Addition, several GROWTH-REGULATING FACTOR (GRF) genes in wheat and maize were repressed by miR396s, which could be reversed by MIM396, confirming the conserved regulatory roles of miR396 on GRFs. While providing new solution to enhance grain yield in wheat and revealing potential regulatory variations of miR396s in controlling grain size and disease resistance in different crops, this study gives clues to further explore miR396s' functions in other Poaceae species.

Barley GRIK1-SnRK1 kinases subvert a viral virulence protein to upregulate antiviral RNAi and inhibit infection.

Viruses often usurp host machineries for their amplification, but it remains unclear if hosts may subvert virus proteins to regulate viral proliferation. Here, we show that the 17K protein, an important virulence factor conserved in barley yellow dwarf viruses (BYDVs) and related poleroviruses, is phosphorylated by host GRIK1-SnRK1 kinases, with the phosphorylated 17K (P17K) capable of enhancing the abundance of virus-derived small interfering RNAs (vsiRNAs) and thus antiviral RNAi. Furthermore, P17K interacts with barley small RNA-degrading nuclease 1 (HvSDN1) and impedes HvSDN1-catalyzed vsiRNA degradation. Additionally, P17K weakens the HvSDN1-HvAGO1 interaction, thus hindering HvSDN1 from accessing and degrading HvAGO1-carried vsiRNAs. Importantly, transgenic expression of 17K phosphomimetics (17K5D ), or genome editing of SDN1, generates stable resistance to BYDV through elevating vsiRNA abundance. These data validate a novel mechanism that enhances antiviral RNAi through host subversion of a viral virulence protein to inhibit SDN1-catalyzed vsiRNA degradation and suggest new ways for engineering BYDV-resistant crops.

Underlying mechanism of accelerated cell death and multiple disease resistance in a maize lethal leaf spot 1 allele.

Multiple disease resistance (MDR) in maize has attracted increasing attention. However, the interplay between cell death and metabolite changes and their contributions to MDR remains elusive in maize. In this study, we identified a mutant named as lesion mimic 30 (les30) that showed 'suicidal' lesion formation in the absence of disease and had enhanced resistance to the fungal pathogen Curvularia lunata. Using map-based cloning, we identified the causal gene encoding pheophorbide a oxidase (PAO), which is known to be involved in chlorophyll degradation and MDR, and is encoded by LETHAL LEAF SPOT1 (LLS1). LLS1 was found to be induced by both biotic and abiotic stresses. Transcriptomics analysis showed that genes involved in defense responses and secondary metabolite biosynthesis were mildly activated in leaves of the les30 mutant without lesions, whilst they were strongly activated in leaves with lesions. In addition, in les30 leaves with lesions, there was overaccumulation of defense-associated phytohormones including jasmonic acid and salicylic acid, and of phytoalexins including phenylpropanoids, lignin, and flavonoids, suggesting that their biosynthesis was activated in a lesion-dependent manner. Taken together, our study implies the existence of an interactive amplification loop of interrupted chlorophyll degradation, cell death, expression of defense-related genes, and metabolite changes that results in suicidal lesion formation and MDR, and this has the potential to be exploited by genetic manipulation to improve maize disease resistance.

Commonly and Specifically Activated Defense Responses in Maize Disease Lesion Mimic Mutants Revealed by Integrated Transcriptomics and Metabolomics Analysis.

Disease lesion mimic (Les/les) mutants display disease-like spontaneous lesions in the absence of pathogen infection, implying the constitutive activation of defense responses. However, the genetic and biochemical bases underlying the activated defense responses in those mutants remain largely unknown. Here, we performed integrated transcriptomics and metabolomics analysis on three typical maize Les mutants Les4, Les10, and Les17 with large, medium, and small lesion size, respectively, thereby dissecting the activated defense responses at the transcriptional and metabolomic level. A total of 1,714, 4,887, and 1,625 differentially expressed genes (DEGs) were identified in Les4, Les10, and Les17, respectively. Among them, 570, 3,299, and 447 specific differentially expressed genes (SGs) were identified, implying a specific function of each LES gene. In addition, 480 common differentially expressed genes (CGs) and 42 common differentially accumulated metabolites (CMs) were identified in all Les mutants, suggesting the robust activation of shared signaling pathways. Intriguingly, substantial analysis of the CGs indicated that genes involved in the programmed cell death, defense responses, and phenylpropanoid and terpenoid biosynthesis were most commonly activated. Genes involved in photosynthetic biosynthesis, however, were generally repressed. Consistently, the dominant CMs identified were phenylpropanoids and flavonoids. In particular, lignin, the phenylpropanoid-based polymer, was significantly increased in all three mutants. These data collectively imply that transcriptional activation of defense-related gene expression; increase of phenylpropanoid, lignin, flavonoid, and terpenoid biosynthesis; and inhibition of photosynthesis are generalnatures associated with the lesion formation and constitutively activated defense responses in those mutants. Further studies on the identified SGs and CGs will shed new light on the function of each LES gene as well as the regulatory network of defense responses in maize.

A high-quality genome assembly highlights rye genomic characteristics and agronomically important genes.

Rye is a valuable food and forage crop, an important genetic resource for wheat and triticale improvement and an indispensable material for efficient comparative genomic studies in grasses. Here, we sequenced the genome of Weining rye, an elite Chinese rye variety. The assembled contigs (7.74 Gb) accounted for 98.47% of the estimated genome size (7.86 Gb), with 93.67% of the contigs (7.25 Gb) assigned to seven chromosomes. Repetitive elements constituted 90.31% of the assembled genome. Compared to previously sequenced Triticeae genomes, Daniela, Sumaya and Sumana retrotransposons showed strong expansion in rye. Further analyses of the Weining assembly shed new light on genome-wide gene duplications and their impact on starch biosynthesis genes, physical organization of complex prolamin loci, gene expression features underlying early heading trait and putative domestication-associated chromosomal regions and loci in rye. This genome sequence promises to accelerate genomic and breeding studies in rye and related cereal crops.

Cytochrome b 5 Is an Obligate Electron Shuttle Protein for Syringyl Lignin Biosynthesis in Arabidopsis.

Angiosperms have evolved the metabolic capacity to synthesize p-hydroxyphenyl, guaiacyl (G), and syringyl (S) lignin subunits in their cell walls to better adapt to the harsh terrestrial environment. The structural characteristics of lignin subunits are essentially determined by three cytochrome P450-catalzyed reactions. NADPH-dependent cytochrome P450 oxidoreductase (CPR) is commonly regarded as the electron carrier for P450-catalyzed reactions during monolignol biosynthesis. Here, we show that cytochrome b 5 isoform D (CB5D) is an indispensable electron shuttle protein specific for S-lignin biosynthesis. Arabidopsis (Arabidopsis thaliana) CB5D localizes to the endoplasmic reticulum membrane and physically associates with monolignol P450 enzymes. Disrupting CB5D in Arabidopsis resulted in a >60% reduction in S-lignin subunit levels but no impairment in G-lignin formation compared with the wild type, which sharply contrasts with the impaired G- and S-lignin synthesis observed after disrupting ATR2, encoding Arabidopsis CPR. The defective S-lignin synthesis in cb5d mutants was rescued by the expression of the gene encoding CB5D but not with mutant CB5D devoid of its electron shuttle properties. Disrupting ATR2 suppressed the catalytic activity of both cinnamic acid 4-hydroxylase and ferulate 5-hydroxylase (F5H), but eliminating CB5D specifically depleted the latter's activity. Therefore, CB5D functions as an obligate electron shuttle intermediate that specifically augments F5H-catalyzed reactions, thereby controlling S-lignin biosynthesis.

The scaffold proteins of lignin biosynthetic cytochrome P450 enzymes.

Lignin is a complex and irregular biopolymer of crosslinked phenylpropanoid units in plant secondary cell walls. Its biosynthesis requires three endoplasmic reticulum (ER)-resident cytochrome P450 monooxygenases, C4H, C3'H and F5H, to establish the structural characteristics of its monomeric precursors. These P450 enzymes were reported to associate with each other or potentially with other soluble monolignol biosynthetic enzymes to form an enzyme complex or a metabolon. However, the molecular basis governing such enzyme or pathway organization remains elusive. Here, we show that Arabidopsis membrane steroid-binding proteins (MSBPs) serve as a scaffold to physically organize monolignol P450 monooxygenases, thereby regulating the lignin biosynthetic process. We find that although C4H, C3'H and F5H are in spatial proximity to each other on the ER membrane in vivo, they do not appear to directly interact with each other. Instead, two MSBP proteins physically interact with all three P450 enzymes and, moreover, MSBPs themselves associate as homomers and heteromers on the ER membrane, thereby organizing P450 clusters. Downregulation of MSBP genes does not affect the transcription levels of monolignol biosynthetic P450 genes but substantially impairs the stability and activity of the MSBP-interacting P450 enzymes and, consequently, lignin deposition, and the accumulation of soluble phenolics in the monolignol branch but not in the flavonoid pathway. Our study suggests that MSBP proteins are essential structural components in the ER membrane that physically organize and stabilize the monolignol biosynthetic P450 enzyme complex, thereby specifically controlling phenylpropanoid-monolignol branch biosynthesis.

Calcium Pumps and Interacting BON1 Protein Modulate Calcium Signature, Stomatal Closure, and Plant Immunity.

Calcium signaling is essential for environmental responses including immune responses. Here, we provide evidence that the evolutionarily conserved protein BONZAI1 (BON1) functions together with autoinhibited calcium ATPase10 (ACA10) and ACA8 to regulate calcium signals in Arabidopsis. BON1 is a plasma membrane localized protein that negatively regulates the expression of immune receptor genes and positively regulates stomatal closure. We found that BON1 interacts with the autoinhibitory domains of ACA10 and ACA8, and the aca10 loss-of-function (LOF) mutants have an autoimmune phenotype similar to that of the bon1 LOF mutants. Genetic evidences indicate that BON1 positively regulates the activities of ACA10 and ACA8. Consistent with this idea, the steady level of calcium concentration is increased in both aca10 and bon1 mutants. Most strikingly, cytosolic calcium oscillation imposed by external calcium treatment was altered in aca10, aca8, and bon1 mutants in guard cells. In addition, calcium- and pathogen-induced stomatal closure was compromised in the aca10 and bon1 mutants. Taken together, this study indicates that ACA10/8 and BON1 physically interact on plasma membrane and function in the generation of cytosol calcium signatures that are critical for stomatal movement and impact plant immunity.

Sumoylation E3 Ligase SIZ1 Modulates Plant Immunity Partly through the Immune Receptor Gene SNC1 in Arabidopsis.

The small ubiqutin-like modifier E3 ligase SIZ1 regulates multiple processes in Arabidopsis, including salicylic-acid-dependent immune responses. However, the targets of SIZ1 in plant immunity are not known. Here, we provide evidence that the plant immune receptor nucleotide-binding leucine-rich repeat gene SNC1 partially mediates the regulation of plant immunity by SIZ1. The siz1 loss-of-function mutant has an autoimmune phenotype that is dependent on SNC1 and temperature. Overexpression of SIZ1 partially rescues autoimmune mutant phenotypes induced by activation or overaccumulation of SNC1, and the SNC1 protein amount is attenuated by SIZ1 overexpression. In addition, overexpression of the F-box protein CPR1 that degrades the SNC1 protein inhibits the growth defects and disease resistance of the siz1 mutant. Furthermore, we found that the SNC1 protein is sumoylated in planta. Although it remains to be determined whether SIZ1 primarily modulates the SNC1 protein via sumoylation or affects SNC1 transcript level, our data indicate that SNC1 is a major mediator of defense response modulated by SIZ1 and that SNC1 is a crucial target for fine-tuning plant defense responses.

The MYB107 Transcription Factor Positively Regulates Suberin Biosynthesis.

Suberin, a lipophilic polymer deposited in the outer integument of the Arabidopsis (Arabidopsis thaliana) seed coat, represents an essential sealing component controlling water and solute movement and protecting seed from pathogenic infection. Although many genes responsible for suberin synthesis are identified, the regulatory components controlling its biosynthesis have not been definitively determined. Here, we show that the Arabidopsis MYB107 transcription factor acts as a positive regulator controlling suberin biosynthetic gene expression in the seed coat. MYB107 coexpresses with suberin biosynthetic genes in a temporal manner during seed development. Disrupting MYB107 particularly suppresses the expression of genes involved in suberin but not cutin biosynthesis, lowers seed coat suberin accumulation, alters suberin lamellar structure, and consequently renders higher seed coat permeability and susceptibility to abiotic stresses. Furthermore, MYB107 directly binds to the promoters of suberin biosynthetic genes, verifying its primary role in regulating their expression. Identifying MYB107 as a positive regulator for seed coat suberin synthesis offers a basis for discovering the potential transcriptional network behind one of the most abundant lipid-based polymers in nature.

Opposing Effects on Two Phases of Defense Responses from Concerted Actions of HEAT SHOCK COGNATE70 and BONZAI1 in Arabidopsis.

The plant immune system consists of multiple layers of responses targeting various phases of pathogen infection. Here, we provide evidence showing that two responses, one controlling stomatal closure and the other mediated by intracellular receptor proteins, can be regulated by the same proteins but in an antagonistic manner. The HEAT SHOCK COGNATE70 (HSC70), while previously known as a negative regulator of stomatal closure, is a positive regulator of immune responses mediated by the immune receptor protein SUPPRESSOR OF NPR1-1, CONSTITUTIVE1 (SNC1) as well as basal defense responses. In contrast to HSC70, a calcium-binding protein, BONZAI1 (BON1), promotes abscisic acid- and pathogen-triggered stomatal closure in addition to and independent of its previously known negative role in SNC1 regulation. BON1 likely regulates stomatal closure through activating SUPPESSOR OF THE G2 ALLELE OF SKP1 VARIANT B and inhibiting HSC70. New functions of BON1 and HSC70 identified in this study thus reveal opposite effects of each of them on immunity. The opposing roles of these regulators at different phases of plant immune responses exemplify the complexity in immunity regulation and suggest that immune receptors may guard positive regulators functioning at stomatal closure control.

Down-regulation of Kelch domain-containing F-box protein in Arabidopsis enhances the production of (poly)phenols and tolerance to ultraviolet radiation.

Phenylpropanoid biosynthesis in plants engenders myriad phenolics with diverse biological functions. Phenylalanine ammonia-lyase (PAL) is the first committed enzyme in the pathway, directing primary metabolic flux into a phenylpropanoid branch. Previously, we demonstrated that the Arabidopsis (Arabidopsis thaliana) Kelch domain-containing F-box proteins, AtKFB01, AtKFB20, and AtKFB50, function as the negative regulators controlling phenylpropanoid biosynthesis via mediating PAL's ubiquitination and subsequent degradation. Here, we reveal that Arabidopsis KFB39, a close homolog of AtKFB50, also interacts physically with PAL isozymes and modulates PAL stability and activity. Disturbing the expression of KFB39 reciprocally affects the accumulation/deposition of a set of phenylpropanoid end products, suggesting that KFB39 is an additional posttranslational regulator responsible for the turnover of PAL and negatively controlling phenylpropanoid biosynthesis. Furthermore, we discover that exposure of Arabidopsis to ultraviolet (UV)-B radiation suppresses the expression of all four KFB genes while inducing the transcription of PAL isogenes; these data suggest that Arabidopsis consolidates both transcriptional and posttranslational regulation mechanisms to maximize its responses to UV light stress. Simultaneous down-regulation of all four identified KFBs significantly enhances the production of (poly)phenols and the plant's tolerance to UV irradiation. This study offers a biotechnological approach for engineering the production of useful phenolic chemicals and for increasing a plant's resistance to environmental stress.

Tailoring lignin biosynthesis for efficient and sustainable biofuel production.

Increased global interest in a bio-based economy has reinvigorated the research on the cell wall structure and composition in plants. In particular, the study of plant lignification has become a central focus, with respect to its intractability and negative impact on the utilization of the cell wall biomass for producing biofuels and bio-based chemicals. Striking progress has been achieved in the last few years both on our fundamental understanding of lignin biosynthesis, deposition and assembly, and on the interplay of lignin synthesis with the plant growth and development. With the knowledge gleaned from basic studies, researchers are now able to invent and develop elegant biotechnological strategies to sophisticatedly manipulate the quantity and structure of lignin and thus to create economically viable bioenergy feedstocks. These concerted efforts open an avenue for the commercial production of cost-competitive biofuel to meet our energy needs.