A co-opted steroid synthesis gene, maintained in sorghum but not maize, is associated with a divergence in leaf wax chemistry

Biosynthesis and functions of triterpenoids in cereals

BACKGROUND

Triterpenoids are versatile secondary metabolites with a diverse array of physiological activities, possessing valuable pharmacological effects and influencing the growth and development of plants. As more triterpenoids in cereals are unearthed and characterized, their biological roles in plant growth and development are gaining recognition.

AIM OF THE REVIEW

This review provides an overview of the structures, biosynthetic pathways, and diverse biological functions of triterpenoids identified in cereals. Our goal is to establish a basis for further exploration of triterpenoids with novel structures and functional activities in cereals, and to facilitate the potential application of triterpenoids in grain breeding, thus accelerating the development of superior grain varieties.

KEY SCIENTIFIC CONCEPTS OF THE REVIEW

This review consolidates information on various triterpenoid skeletons and derivatives found in cereals, and summarizes the pivotal enzyme genes involved, including oxidosqualene cyclase (OSC) and other triterpenoid modifying enzymes like cytochrome P450, glycosyltransferase, and acyltransferase. Triterpenoid-modifying enzymes exhibit specificity towards catalytic sites within triterpenoid skeletons, generating a diverse array of functional triterpenoid derivatives. Furthermore, triterpenoids have been shown to significantly impact the nutritional value, yield, disease resistance, and stress response of cereals.

Biosynthesis of triterpenoids in plants: Pathways, regulation, and biological functions

Plant triterpenoids, a vast and diverse group of natural compounds derived from six isoprene units, exhibit an extensive array of structural diversity and remarkable biological activities. In this review, we update the recent research progress in the catalytic mechanisms underlying triterpene synthesis and summarize the current insights into the biosynthetic pathways and regulatory mechanisms of triterpenoids. We emphasize the biosynthesis of pharmacologically active triterpenoids and the role of triterpenoid synthesis in plant growth, development, defense mechanisms, and plant-microbe interactions. This insight review offers a comprehensive perspective on the applications and future avenues of triterpenoid research.

Exploring genetics and genomics trends to understand the link between secondary metabolic genes and agronomic traits in cereals under stress

The plant metabolome is considered an important interface between the genome and its phenome, where it plays a significant role in regulating plant growth in response to various environmental cues. A wide array of specialized metabolites is produced by plants, which are essential for mediating environmental interactions and their adaptation. Notably, enhanced accumulation of these specialized metabolites, particularly plant secondary metabolites (PSMs), is a part of the chemical defense response that is directly linked to improved stress tolerance. Therefore, exploring the genetic diversity underlying the immense variation of the secondary metabolite pool could unravel the adaptation mechanisms in plants against different environmental stresses. The post-genomic profiling platforms have enabled the exploration of the link between metabolic diversity and important agronomic traits. The current review focuses on the major achievements and future challenges associated with plant secondary metabolite (PSM) research in graminaceous crops using advanced omics approaches. Given this, we briefly summarize different strategies adopted to explore the genetic diversity and evolution of PSMs in cereal crops. Further, we have discussed the recent technological advancements to integrate multi-omics approaches linking the metabolome diversity with the genome, transcriptome, and proteome of these crops under stress. Combining these data with phenomics (the omics of phenotypes) provides a holistic view of how plants respond to stress. Next, we outlined the genetic manipulation studies performed so far in cereals to engineer secondary metabolic pathways for enhanced stress tolerance. In summary, our review provides new insight into developing genetic and genomic trends in exploring the secondary metabolite diversity in graminaceous crops and discusses how this information can be utilized in designing strategies to generate future stress-resilient crops.

Functional diversity of oxidosqualene cyclases in genus Oryza

Triterpene skeletons, catalyzing by 2,3-oxidosqualene cyclases (OSCs), are essential for synthesis of steroids and triterpenoids. In japonica rice cultivars Zhonghua11, a total of 12 OsOSCs have been found. While the catalytic functions of OsOSC1, 3, 4, 9, and 10 remain unclear, the functions of the other OsOSCs have been well studied. In this study, we conducted a comprehensive analysis of 12 OSC genes within genus Oryza with the aid of 63 genomes from cultivated and wild rice. We found that OSC genes are relatively conserved within genus Oryza with a few exceptions. Collinearity analysis further suggested that, throughout the evolutionary history of genus Oryza, the OSC genes have not undergone significant rearrangements or losses. Further functional analysis of 5 uncharacterized OSCs revealed that OsOSC10 was a friedelin synthase, which affected the development of rice grains. Additionally, the reconstructed ancestral sequences of Oryza OSC3 and Oryza OSC9 had lupeol synthase and poaceatapetol synthase activity, respectively. The discovery of friedelin synthase in rice unlocks a new catalytic path and biological function of OsOSC10. The pan-genome analysis of OSCs within genus Oryza gives insights into the evolutionary trajectory and products diversity of Oryza OSCs.

Leucine-rich repeat receptor kinase BM41 regulates cuticular wax deposition in sorghum

Cuticular wax (CW) is the first defensive barrier of plants that forms a waterproof barrier, protects the plant from desiccation, and defends against insects, pathogens, and UV radiation. Sorghum, an important grass crop with high heat and drought tolerance, exhibits a much higher wax load than other grasses and the model plant Arabidopsis. In this study, we explored the regulation of sorghum CW biosynthesis using a bloomless mutant. The CW on leaf sheaths of the bloomless 41 (bm41) mutant showed significantly reduced very long-chain fatty acids (VLCFAs), triterpenoids, alcohols, and other wax components, with an overall 86% decrease in total wax content compared with the wild type. Notably, the 28-carbon and 30-carbon VLCFAs were decreased in the mutants. Using bulk segregant analysis, we identified the causal gene of the bloomless phenotype as a leucine-rich repeat transmembrane protein kinase. Transcriptome analysis of the wild-type and bm41 mutant leaf sheaths revealed BM41 as a positive regulator of lipid biosynthesis and steroid metabolism. BM41 may regulate CW biosynthesis by regulating the expression of the gene encoding 3-ketoacyl-CoA synthase 6. Identification of BM41 as a new regulator of CW biosynthesis provides fundamental knowledge for improving grass crops' heat and drought tolerance by increasing CW.

Herbarium specimens as tools for exploring the evolution of fatty acid-derived natural products in plants

Plants synthesize natural products via lineage-specific offshoots of their core metabolic pathways, including fatty acid synthesis. Recent studies have shed light on new fatty acid-derived natural products and their biosynthetic pathways in disparate plant species. Inspired by this progress, we set out to develop tools for exploring the evolution of fatty-acid derived products. We sampled multiple species from all major clades of euphyllophytes, including ferns, gymnosperms, and angiosperms (monocots and eudicots), and we show that the compositional profiles (though not necessarily the total amounts) of fatty-acid derived surface waxes from preserved plant specimens are consistent with those obtained from freshly collected tissue in a semi-quantitative and sometimes quantitative manner. We then sampled herbarium specimens representing 57 monocot species to assess the phylogenetic distribution and evolution, of two fatty acid-derived natural products found in that clade: beta-diketones and alkylresorcinols. These chemical data, combined with analyses of 26 monocot genomes, revealed a co-occurrence (though not necessarily a causal relationship) between whole genome duplication and the evolution of diketone synthases from an ancestral alkylresorcinol synthase-like polyketide synthase. Limitations of using herbarium specimen wax profiles as proxies for those of fresh tissue seem likely to include effects from loss of epicuticular wax crystals, effects from preservation techniques, and variation in wax chemical profiles due to genotype or environment. Nevertheless, this work reinforces the widespread utility of herbarium specimens for studying leaf surface waxes (and possibly other chemical classes) and reveals some of the evolutionary history of fatty acid-derived natural products within monocots.

A reference-grade genome of the xerophyte Ammopiptanthus mongolicus sheds light on its evolution history in legumes and drought-tolerance mechanisms

Plants that grow in extreme environments represent unique sources of stress-resistance genes and mechanisms. Ammopiptanthus mongolicus (Leguminosae) is a xerophytic evergreen broadleaf shrub native to semi-arid and desert regions; however, its drought-tolerance mechanisms remain poorly understood. Here, we report the assembly of a reference-grade genome for A. mongolicus, describe its evolutionary history within the legume family, and examine its drought-tolerance mechanisms. The assembled genome is 843.07 Mb in length, with 98.7% of the sequences successfully anchored to the nine chromosomes of A. mongolicus. The genome is predicted to contain 47 611 protein-coding genes, and 70.71% of the genome is composed of repetitive sequences; these are dominated by transposable elements, particularly long-terminal-repeat retrotransposons. Evolutionary analyses revealed two whole-genome duplication (WGD) events at 130 and 58 million years ago (mya) that are shared by the genus Ammopiptanthus and other legumes, but no species-specific WGDs were found within this genus. Ancestral genome reconstruction revealed that the A. mongolicus genome has undergone fewer rearrangements than other genomes in the legume family, confirming its status as a "relict plant". Transcriptomic analyses demonstrated that genes involved in cuticular wax biosynthesis and transport are highly expressed, both under normal conditions and in response to polyethylene glycol-induced dehydration. Significant induction of genes related to ethylene biosynthesis and signaling was also observed in leaves under dehydration stress, suggesting that enhanced ethylene response and formation of thick waxy cuticles are two major mechanisms of drought tolerance in A. mongolicus. Ectopic expression of AmERF2, an ethylene response factor unique to A. mongolicus, can markedly increase the drought tolerance of transgenic Arabidopsis thaliana plants, demonstrating the potential for application of A. mongolicus genes in crop improvement.

Classification, biosynthesis, and biological functions of triterpene esters in plants

Triterpene esters comprise a class of secondary metabolites that are synthesized by decorating triterpene skeletons with a series of oxidation, glycosylation, and acylation modifications. Many triterpene esters with important bioactivities have been isolated and identified, including those with applications in the pesticide, pharmaceutical, and cosmetic industries. They also play essential roles in plant defense against pests, diseases, physical damage (as part of the cuticle), and regulation of root microorganisms. However, there has been no recent summary of the biosynthetic pathways and biological functions of plant triterpene esters. Here, we classify triterpene esters into five categories based on their skeletons and find that C-3 oxidation may have a significant effect on triterpenoid acylation. Fatty acid and aromatic moieties are common ligands present in triterpene esters. We further analyze triterpene ester synthesis-related acyltransferases (TEsACTs) in the triterpene biosynthetic pathway. Using an evolutionary classification of BAHD acyltransferases (BAHD-ATs) and serine carboxypeptidase-like acyltransferases (SCPL-ATs) in Arabidopsis thaliana and Oryza sativa, we classify 18 TEsACTs with identified functions from 11 species. All the triterpene-skeleton-related TEsACTs belong to BAHD-AT clades IIIa and I, and the only identified TEsACT from the SCPL-AT family belongs to the CP-I subfamily. This comprehensive review of the biosynthetic pathways and bioactivities of triterpene esters provides a foundation for further study of their bioactivities and applications in industry, agricultural production, and human health.

Morphology and chemical composition of Taiwan oil millet (Eccoilopus formosanus) epicuticular wax

MAIN CONCLUSION

Taiwan oil millet has two types of epicuticular wax: platelet wax composed primarily of octacosanol and filament wax constituted essentially by the singular compound of octacosanoic acid. Taiwan oil millet (TOM-Eccoilopus formosanus) is an orphan crop cultivated by the Taiwan indigenous people. It has conspicuous white powder covering its leaf sheath indicating abundant epicuticular waxes, that may contribute to its resilience. Here, we characterized the epicuticular wax secretion in TOM leaf blade and leaf sheath using various microscopy techniques, as well as gas chromatography to determine its composition. Two kinds of waxes, platelet and filaments, were secreted in both the leaf blades and sheaths. The platelet wax is secreted ubiquitously by epidermal cells, whereas the filament wax is secreted by a specific cell called epidermal cork cells. The newly developed filament waxes were markedly re-synthesized by the epidermal cork cells through papillae protrusions on the external periclinal cell wall. Ultrastructural images of cork cell revealed the presence of cortical endoplasmic reticulum (ER) tubules along the periphery of plasma membrane (PM) and ER-PM contact sites (EPCS). The predominant wax component was a C28 primary alcohol in leaf blade, and a C28 free fatty acid in the leaf sheath, pseudopetiole and midrib. The wax morphology present in distinct plant organs corresponds to the specific chemical composition: platelet wax composed of alcohols exists mainly in the leaf blade, whereas filament wax constituted mainly by the singular compound C28 free fatty acids is present abundantly in leaf sheath. Our study clarifies the filament wax composition in relation to a previous study in sorghum. Both platelet and filament waxes comprise a protection barrier for TOM.

Dynamic changes to the plant cuticle include the production of volatile cuticular wax-derived compounds

The cuticle is a hydrophobic structure that seals plant aerial surfaces from the surrounding environment. To better understand how cuticular wax composition changes over development, we conducted an untargeted screen of leaf surface lipids from black cottonwood (Populus trichocarpa). We observed major shifts to the lipid profile across development, from a phenolic and terpene-dominated profile in young leaves to an aliphatic wax-dominated profile in mature leaves. Contrary to the general pattern, levels of aliphatic cis-9-alkenes decreased in older leaves following their accumulation. A thorough examination revealed that the decrease in cis-9-alkenes was accompanied by a concomitant increase in aldehydes, one of them being the volatile compound nonanal. By applying exogenous alkenes to P. trichocarpa leaves, we show that unsaturated waxes in the cuticle undergo spontaneous oxidative cleavage to generate aldehydes and that this process occurs similarly in other alkene-accumulating systems such as balsam poplar (Populus balsamifera) leaves and corn (Zea mays) silk. Moreover, we show that the production of cuticular wax-derived compounds can be extended to other wax components. In bread wheat (Triticum aestivum), 9-hydroxy-14,16-hentriacontanedione likely decomposes to generate 2-heptadecanone and 7-octyloxepan-2-one (a caprolactone). These findings highlight an unusual route to the production of plant volatiles that are structurally encoded within cuticular wax precursors. These processes could play a role in modulating ecological interactions and open the possibility for engineering bioactive volatile compounds into plant waxes.

Recent advances in triterpenoid pathway elucidation and engineering

Triterpenoids are among the most assorted class of specialized metabolites found in all the taxa of living organisms. Triterpenoids are the leading active ingredients sourced from plant species and are utilized in pharmaceutical and cosmetic industries. The triterpenoid precursor 2,3-oxidosqualene, which is biosynthesized via the mevalonate (MVA) pathway is structurally diversified by the oxidosqualene cyclases (OSCs) and other scaffold-decorating enzymes such as cytochrome P450 monooxygenases (P450s), UDP-glycosyltransferases (UGTs) and acyltransferases (ATs). A majority of the bioactive triterpenoids are harvested from the native hosts using the traditional methods of extraction and occasionally semi-synthesized. These methods of supply are time-consuming and do not often align with sustainability goals. Recent advancements in metabolic engineering and synthetic biology have shown prospects for the green routes of triterpenoid pathway reconstruction in heterologous hosts such as Escherichia coli, Saccharomyces cerevisiae and Nicotiana benthamiana, which appear to be quite promising and might lead to the development of alternative source of triterpenoids. The present review describes the biotechnological strategies used to elucidate complex biosynthetic pathways and to understand their regulation and also discusses how the advances in triterpenoid pathway engineering might aid in the scale-up of triterpenoid production in engineered hosts.

Epicuticular wax accumulation and regulation of wax pathway gene expression during bioenergy Sorghum stem development

Bioenergy sorghum is a drought-tolerant high-biomass C4 grass targeted for production on annual cropland marginal for food crops due primarily to abiotic constraints. To better understand the overall contribution of stem wax to bioenergy sorghum's resilience, the current study characterized sorghum stem cuticular wax loads, composition, morphometrics, wax pathway gene expression and regulation using vegetative phase Wray, R07020, and TX08001 genotypes. Wax loads on sorghum stems (~103-215 µg/cm2) were much higher than Arabidopsis stem and leaf wax loads. Wax on developing sorghum stem internodes was enriched in C28/30 primary alcohols (~65%) while stem wax on fully developed stems was enriched in C28/30 aldehydes (~80%). Scanning Electron Microscopy showed minimal wax on internodes prior to the onset of elongation and that wax tubules first appear associated with cork-silica cell complexes when internode cell elongation is complete. Sorghum homologs of genes involved in wax biosynthesis/transport were differentially expressed in the stem epidermis. Expression of many wax pathway genes (i.e., SbKCS6, SbCER3-1, SbWSD1, SbABCG12, SbABCG11) is low in immature apical internodes then increases at the onset of stem wax accumulation. SbCER4 is expressed relatively early in stem development consistent with accumulation of C28/30 primary alcohols on developing apical internodes. High expression of two SbCER3 homologs in fully elongated internodes is consistent with a role in production of C28/30 aldehydes. Gene regulatory network analysis aided the identification of sorghum homologs of transcription factors that regulate wax biosynthesis (i.e., SbSHN1, SbWRI1/3, SbMYB94/96/30/60, MYS1) and other transcription factors that could regulate and specify expression of the wax pathway in epidermal cells during cuticle development.

Terpenes modulate bacterial and fungal growth and sorghum rhizobiome communities

Terpenes are among the oldest and largest class of plant-specialized bioproducts that are known to affect plant development, adaptation, and biological interactions. While their biosynthesis, evolution, and function in aboveground interactions with insects and individual microbial species are well studied, how different terpenes impact plant microbiomes belowground is much less understood. Here we designed an experiment to assess how belowground exogenous applications of monoterpenes (1,8-cineole and linalool) and a sesquiterpene (nerolidol) delivered through an artificial root system impacted its belowground bacterial and fungal microbiome. We found that the terpene applications had significant and variable impacts on bacterial and fungal communities, depending on terpene class and concentration; however, these impacts were localized to the artificial root system and the fungal rhizosphere. We complemented this experiment with pure culture bioassays on responsive bacteria and fungi isolated from the sorghum rhizobiome. Overall, higher concentrations (200 µM) of nerolidol were inhibitory to Ferrovibrium and tested Firmicutes. While fungal isolates of Penicillium and Periconia were also more inhibited by higher concentrations (200 µM) of nerolidol, Clonostachys was enhanced at this higher level and together with Humicola was inhibited by the lower concentration tested (100 µM). On the other hand, 1,8-cineole had an inhibitory effect on Orbilia at both tested concentrations but had a promotive effect at 100 µM on Penicillium and Periconia. Similarly, linalool at 100 µM had significant growth promotion in Mortierella, but an inhibitory effect for Orbilia. Together, these results highlight the variable direct effects of terpenes on single microbial isolates and demonstrate the complexity of microbe-terpene interactions in the rhizobiome. IMPORTANCE Terpenes represent one of the largest and oldest classes of plant-specialized metabolism, but their role in the belowground microbiome is poorly understood. Here, we used a "rhizobox" mesocosm experimental set-up to supply different concentrations and classes of terpenes into the soil compartment with growing sorghum for 1 month to assess how these terpenes affect sorghum bacterial and fungal rhizobiome communities. Changes in bacterial and fungal communities between treatments belowground were characterized, followed by bioassays screening on bacterial and fungal isolates from the sorghum rhizosphere against terpenes to validate direct microbial responses. We found that microbial growth stimulatory and inhibitory effects were localized, terpene specific, dose dependent, and transient in time. This work paves the way for engineering terpene metabolisms in plant microbiomes for improved sustainable agriculture and bioenergy crop production.

Sugars and cuticular waxes impact sugarcane aphid (Melanaphis sacchari) colonization on different developmental stages of sorghum

Sugarcane aphid (SCA; Melanaphis sacchari) is a devastating pest of sorghum (Sorghum bicolor) that colonizes sorghum plants at different growth stages. Leaf surface characteristics and sugars often influence aphid settling and feeding on host plants. However, how changes in cuticular waxes and sugar levels affect SCA establishment and feeding at different development stages of sorghum have not been explored. In this study, two- and six-week-old BTx623 plants, a reference line of sorghum, was used to evaluate plant-aphid interactions. Monitoring aphid feeding behavior using Electrical Penetration Graph (EPG) technique revealed that aphids spent more time in the sieve element phase of six-week-old plants compared to two-week-old plants. Significant differences were found in the time spent to reach the first sieve element and pathway phases between the two- and six-week-old plants. However, no-choice aphid bioassays displayed that SCA population numbers were higher in two-week-old plants compared to six-week-old plants. Differences in the abundance of wax and sugar contents were analyzed to determine how these plant components influenced aphid feeding and proliferation. Among the cuticular wax compounds analyzed, α-amyrin and isoarborinone increased after 10 days of aphid infestation only in six-week-old plants. Trehalose content was significantly increased by SCA feeding on two- and six-week-old plants. Furthermore, SCA feeding depressed sucrose content and increased levels of glucose and fructose in two-week-old but not in six-week-old plants. Overall, our study indicates that plant age is a determinant for SCA feeding, and subtle changes in triterpenoids and available sugars influence SCA establishment on sorghum plants.

Leaf-Inspired Patterned Organohydrogel Surface for Ultrawide Time-Range Open Biosensing

Droplet arrays show great significance in biosensing and biodetection because of low sample consumption and easy operation. However, inevitable water evaporation in open environment severely limits their applications in time-consuming reactions. Herein, inspired by the unique water retention features of leaves, it is demonstrated that an open droplet array on patterned organohydrogel surface with water evaporating replenishment (POWER) for ultrawide time-range biosensing, which integrated hydrophilic hydrogel domains and hydrophobic organogel background. The hydrogel domains on the surface can supply water to the pinned droplets through capillary channels formed in the nether organohydrogel bulk. The organogel background can inhibit water evaporation like the wax coating of leaves. Such a unique bioinspired design enables ultrawide time-range biosensing in open environment from a few minutes to more than five hours involving a variety of analytes such as ions, small molecules, and macromolecules. The POWER provides a feasible and open biosensing platform for ultrawide time-range reactions.

Comparative genomics reveals the diversification of triterpenoid biosynthesis and origin of ocotillol-type triterpenes in Panax

Gene duplication is assumed to be the major force driving the evolution of metabolite biosynthesis in plants. Freed from functional burdens, duplicated genes can mutate toward novelties until fixed due to selective fitness. However, the extent to which this mechanism has driven the diversification of metabolite biosynthesis remains to be tested. Here we performed comparative genomics analysis and functional characterization to evaluate the impact of gene duplication on the evolution of triterpenoid biosynthesis using Panax species as models. We found that whole-genome duplications (WGDs) occurred independently in Araliaceae and Apiaceae lineages. Comparative genomics revealed the evolutionary trajectories of triterpenoid biosynthesis in plants, which was mainly promoted by WGDs and tandem duplication. Lanosterol synthase (LAS) was likely derived from a tandem duplicate of cycloartenol synthase that predated the emergence of Nymphaeales. Under episodic diversifying selection, the LAS gene duplicates produced by γ whole-genome triplication have given rise to triterpene biosynthesis in core eudicots through neofunctionalization. Moreover, functional characterization revealed that oxidosqualene cyclases (OSCs) responsible for synthesizing dammarane-type triterpenes in Panax species were also capable of producing ocotillol-type triterpenes. Genomic and biochemical evidence suggested that Panax genes encoding the above OSCs originated from the specialization of one OSC gene duplicate produced from a recent WGD shared by Araliaceae (Pg-β). Our results reveal the crucial role of gene duplication in diversification of triterpenoid biosynthesis in plants and provide insight into the origin of ocotillol-type triterpenes in Panax species.

Cross-species predictive modeling reveals conserved drought responses between maize and sorghum

Drought tolerance is a highly complex trait controlled by numerous interconnected pathways with substantial variation within and across plant species. This complexity makes it difficult to distill individual genetic loci underlying tolerance, and to identify core or conserved drought-responsive pathways. Here, we collected drought physiology and gene expression datasets across diverse genotypes of the C4 cereals sorghum and maize and searched for signatures defining water-deficit responses. Differential gene expression identified few overlapping drought-associated genes across sorghum genotypes, but using a predictive modeling approach, we found a shared core drought response across development, genotype, and stress severity. Our model had similar robustness when applied to datasets in maize, reflecting a conserved drought response between sorghum and maize. The top predictors are enriched in functions associated with various abiotic stress-responsive pathways as well as core cellular functions. These conserved drought response genes were less likely to contain deleterious mutations than other gene sets, suggesting that core drought-responsive genes are under evolutionary and functional constraints. Our findings support a broad evolutionary conservation of drought responses in C4 grasses regardless of innate stress tolerance, which could have important implications for developing climate resilient cereals.

Sorghum cuticular waxes influence host plant selection by aphids

Quantification of cuticular waxes coupled with insect bioassays and feeding behavior analysis demonstrate that long-chain C₃₂ fatty alcohol impacts host plant selection by aphids. Cuticular waxes constitute the first point of contact between plants and their environment, and it also protect plants from external stresses. However, the role of waxes in Sorghum bicolor (sorghum) against sugarcane aphid (Melanaphis sacchari), a relatively new and devastating pest of sorghum in the U.S., is not fully understood. In this study, we monitored sugarcane aphid behavior on two genotypes of young sorghum plants with different wax chemistry: a wild-type plant (bloom) with lower C₃₂ alcohol cuticular wax, and a mutant plant (bloomless) with 1.6 times the amount of wax compared to wild-type plants. No-choice aphid bioassays revealed that sugarcane aphid reproduction did not vary between wild-type and the bloomless plants. Electrical Penetration Graph (EPG) monitoring indicated that the sugarcane aphids spent comparable amount of time feeding from the sieve elements of the wild-type and bloomless plants. However, aphids spent more time feeding on the xylem sap of the bloomless plants compared to the wild-type plants. Furthermore, aphid choice assays revealed that the sugarcane aphids preferred to settle on bloomless compared to wild-type plants. Overall, our results suggest that cuticular waxes on young sorghum leaves play a critical role in influencing host plant selection by sugarcane aphids.

The Plant Fatty Acyl Reductases

Fatty acyl reductase (FAR) is a crucial enzyme that catalyzes the NADPH-dependent reduction of fatty acyl-CoA or acyl-ACP substrates to primary fatty alcohols, which in turn acts as intermediate metabolites or metabolic end products to participate in the formation of plant extracellular lipid protective barriers (e.g., cuticular wax, sporopollenin, suberin, and taproot wax). FARs are widely present across plant evolution processes and play conserved roles during lipid synthesis. In this review, we provide a comprehensive view of FAR family enzymes, including phylogenetic analysis, conserved structural domains, substrate specificity, subcellular localization, tissue-specific expression patterns, their varied functions in lipid biosynthesis, and the regulation mechanism of FAR activity. Finally, we pose several questions to be addressed, such as the roles of FARs in tryphine, the interactions between transcription factors (TFs) and FARs in various environments, and the identification of post-transcriptional, translational, and post-translational regulators.

Exploring and exploiting cuticle biosynthesis for abiotic and biotic stress tolerance in wheat and barley

Wheat and barley are widely distributed cereal crops whose yields are adversely affected by environmental stresses such as drought, salinity, extreme temperatures, and attacks of pathogens and pests. As the interphase between aerial plant organs and their environments, hydrophobic cuticle largely consists of a cutin matrix impregnated and sealed with cuticular waxes. Increasing evidence supports that the cuticle plays a key role in plant adaptation to abiotic and biotic stresses, which could be harnessed for wheat and barley improvement. In this review, we highlighted recent advances in cuticle biosynthesis and its multifaceted roles in abiotic and biotic stress tolerance of wheat and barley. Current strategies, challenges, and future perspectives on manipulating cuticle biosynthesis for abiotic and biotic stress tolerance in wheat and barley are discussed.

Integrating GWAS and TWAS to elucidate the genetic architecture of maize leaf cuticular conductance

The cuticle, a hydrophobic layer of cutin and waxes synthesized by plant epidermal cells, is the major barrier to water loss when stomata are closed. Dissecting the genetic architecture of natural variation for maize (Zea mays L.) leaf cuticular conductance (gc) is important for identifying genes relevant to improving crop productivity in drought-prone environments. To this end, we performed an integrated genome- and transcriptome-wide association studies (GWAS and TWAS) to identify candidate genes putatively regulating variation in leaf gc. Of the 22 plausible candidate genes identified, 4 were predicted to be involved in cuticle precursor biosynthesis and export, 2 in cell wall modification, 9 in intracellular membrane trafficking, and 7 in the regulation of cuticle development. A gene encoding an INCREASED SALT TOLERANCE1-LIKE1 (ISTL1) protein putatively involved in intracellular protein and membrane trafficking was identified in GWAS and TWAS as the strongest candidate causal gene. A set of maize nested near-isogenic lines that harbor the ISTL1 genomic region from eight donor parents were evaluated for gc, confirming the association between gc and ISTL1 in a haplotype-based association analysis. The findings of this study provide insights into the role of regulatory variation in the development of the maize leaf cuticle and will ultimately assist breeders to develop drought-tolerant maize for target environments.

Toward a smart skin: Harnessing cuticle biosynthesis for crop adaptation to drought, salinity, temperature, and ultraviolet stress

Drought, salinity, extreme temperatures, and ultraviolet (UV) radiation are major environmental factors that adversely affect plant growth and crop production. As a protective shield covering the outer epidermal cell wall of plant aerial organs, the cuticle is mainly composed of cutin matrix impregnated and sealed with cuticular waxes, and greatly contributes to the plant adaption to environmental stresses. Past decades have seen considerable progress in uncovering the molecular mechanism of plant cutin and cuticular wax biosynthesis, as well as their important roles in plant stress adaptation, which provides a new direction to drive strategies for stress-resilient crop breeding. In this review, we highlighted the recent advances in cuticle biosynthesis in plant adaptation to drought, salinity, extreme temperatures, and UV radiation stress, and discussed the current status and future directions in harnessing cuticle biosynthesis for crop improvement.

Using interdisciplinary, phylogeny-guided approaches to understand the evolution of plant metabolism

To cope with relentless environmental pressures, plants produce an arsenal of structurally diverse chemicals, often called specialized metabolites. These lineage-specific compounds are derived from the simple building blocks made by ubiquitous core metabolic pathways. Although the structures of many specialized metabolites are known, the underlying metabolic pathways and the evolutionary events that have shaped the plant chemical diversity landscape are only beginning to be understood. However, with the advent of multi-omics data sets and the relative ease of studying pathways in previously intractable non-model species, plant specialized metabolic pathways are now being systematically identified. These large datasets also provide a foundation for comparative, phylogeny-guided studies of plant metabolism. Comparisons of metabolic traits and features like chemical abundances, enzyme activities, or gene sequences from phylogenetically diverse plants provide insights into how metabolic pathways evolved. This review highlights the power of studying evolution through the lens of comparative biochemistry, particularly how placing metabolism into a phylogenetic context can help a researcher identify the metabolic innovations enabling the evolution of structurally diverse plant metabolites.

A conserved mechanism affecting hydride shifting and deprotonation in the synthesis of hopane triterpenes as compositions of wax in oat

Hopanoids are a group of biologically important triterpene scaffolds found in nature, but the discovery of hopane-type triterpene synthases in plants has not been reported. We discovered two types of triterpene synthases synthesizing hopanoid skeletons from monocot and dicot plants and elucidated a mechanism involving the deprotonation at different sites by site-directed mutagenesis experiments and the quantum mechanics and molecular mechanics calculation. Our results provide a genetic element for synthesizing biologically active hopane-type triterpenoids and serve as a foundation for studying the molecular mechanisms of methyl and hydride transfer in the triterpene cyclization mechanism.

Triterpenoids are biologically active metabolites synthesized from a common linear precursor catalyzed by 2,3-oxidosqualene cyclases (OSCs) to form diverse triterpenoid skeletons. OSCs corresponding to many discovered triterpene alcohols in nature have not been functionally and mechanistically characterized due to the diversity of chemical structures and complexity of the cyclization mechanism. We carried out a genome-wide investigation of OSCs from Avena strigosa and discovered two triterpene synthases, namely, AsHS1 and AsHS2, using a Nicotiana benthamiana expression system. These synthases produce hopenol B and hop-17(21)-en-3β-ol, which are components of surface wax in oat panicles and sheathes, respectively. We demonstrated that substitutions of two to three amino acid residues in AsHS1 with corresponding residues from AsHS2 allowed it to be completely converted into a hop-17(21)-en-3β-ol synthase. AsHS2 mutants with a substitution at site 410 could synthesize hopenol B alone or mixed with a side product isomotiol. The combined quantum mechanics and molecular mechanics calculation demonstrated that the side chain size of the residue at site 410 regulated the relative orientations between the hopyl C22 cation and Phe257, leading to a difference in deprotonation positions through providing or not providing cation–π interaction between the aromatic ring of F257 and the carbocation intermediate. A similar mechanism could be applied to a hopenol B synthase from a dicotyledonous plant Aquilegia. This study provided mechanistic insight into triterpenoid synthesis and discovered key amino acid residues acting on hydride transfer and a deprotonation site to differentiate between hopane-type scaffolds in diverse plant species.

The Tea Plant Leaf Cuticle: From Plant Protection to Tea Quality

Camellia sinensis (tea tree) is a perennial evergreen woody crop that has been planted in more than 50 countries worldwide; its leaves are harvested to make tea, which is one of the most popular nonalcoholic beverages. The cuticle is the major transpiration barrier to restrict nonstomatal water loss and it affects the drought tolerance of tea plants. The cuticle may also provide molecular cues for the interaction with herbivores and pathogens. The tea-making process almost always includes a postharvest withering treatment to reduce leaf water content, and many studies have demonstrated that withering treatment-induced metabolite transformation is essential to shape the quality of the tea made. Tea leaf cuticle is expected to affect its withering properties and the dynamics of postharvest metabolome remodeling. In addition, it has long been speculated that the cuticle may contribute to the aroma quality of tea. However, concrete experimental evidence is lacking to prove or refute this hypothesis. Even though its relevance to the abiotic and biotic stress tolerance and postharvest processing properties of tea tree, tea cuticle has long been neglected. Recently, there are several studies on the tea cuticle regarding its structure, wax composition, transpiration barrier organization, environmental stresses-induced wax modification, and structure-function relations. This review is devoted to tea cuticle, the recent research progresses were summarized and unresolved questions and future research directions were also discussed.