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.

The FUSED LEAVES1-ADHERENT1 regulatory module is required for maize cuticle development and organ separation.

All aerial epidermal cells in land plants are covered by the cuticle, an extracellular hydrophobic layer that provides protection against abiotic and biotic stresses and prevents organ fusion during development. Genetic and morphological analysis of the classic maize adherent1 (ad1) mutant was combined with genome-wide binding analysis of the maize MYB transcription factor FUSED LEAVES1 (FDL1), coupled with transcriptional profiling of fdl1 mutants. We show that AD1 encodes an epidermally-expressed 3-KETOACYL-CoA SYNTHASE (KCS) belonging to a functionally uncharacterized clade of KCS enzymes involved in cuticular wax biosynthesis. Wax analysis in ad1 mutants indicates that AD1 functions in the formation of very-long-chain wax components. We demonstrate that FDL1 directly binds to CCAACC core motifs present in AD1 regulatory regions to activate its expression. Over 2000 additional target genes of FDL1, including many involved in cuticle formation, drought response and cell wall organization, were also identified. Our results identify a regulatory module of cuticle biosynthesis in maize that is conserved across monocots and eudicots, and highlight previously undescribed factors in lipid metabolism, transport and signaling that coordinate organ development and cuticle formation.

Structure-function analysis of the maize bulliform cell cuticle and its potential role in dehydration and leaf rolling.

The hydrophobic cuticle of plant shoots serves as an important interaction interface with the environment. It consists of the lipid polymer cutin, embedded with and covered by waxes, and provides protection against stresses including desiccation, UV radiation, and pathogen attack. Bulliform cells form in longitudinal strips on the adaxial leaf surface, and have been implicated in the leaf rolling response observed in drought-stressed grass leaves. In this study, we show that bulliform cells of the adult maize leaf epidermis have a specialized cuticle, and we investigate its function along with that of bulliform cells themselves. Bulliform cells displayed increased shrinkage compared to other epidermal cell types during dehydration of the leaf, providing a potential mechanism to facilitate leaf rolling. Analysis of natural variation was used to relate bulliform strip patterning to leaf rolling rate, providing further evidence of a role for bulliform cells in leaf rolling. Bulliform cell cuticles showed a distinct ultrastructure with increased cuticle thickness compared to other leaf epidermal cells. Comparisons of cuticular conductance between adaxial and abaxial leaf surfaces, and between bulliform-enriched mutants versus wild-type siblings, showed a correlation between elevated water loss rates and presence or increased density of bulliform cells, suggesting that bulliform cuticles are more water-permeable. Biochemical analysis revealed altered cutin composition and increased cutin monomer content in bulliform-enriched tissues. In particular, our findings suggest that an increase in 9,10-epoxy-18-hydroxyoctadecanoic acid content, and a lower proportion of ferulate, are characteristics of bulliform cuticles. We hypothesize that elevated water permeability of the bulliform cell cuticle contributes to the differential shrinkage of these cells during leaf dehydration, thereby facilitating the function of bulliform cells in stress-induced leaf rolling observed in grasses.

A maize LIPID TRANSFER PROTEIN may bridge the gap between PHYTOCHROME-mediated light signaling and cuticle biosynthesis.

Plant epidermal cuticles are composed of hydrophobic lipids that provide a barrier to non-stomatal water loss, and arose in land plants as an adaptation to the dry terrestrial environment. The expanding maize adult leaf displays a dynamic, proximodistal gradient of cuticle development, from the leaf base to the tip. Recently, our gene co-expression network analyses together with reverse genetic analyses suggested a previously undescribed function for PHYTOCHROME-mediated light signaling during cuticular wax deposition. The present work extends these findings by identifying a role for a specific LIPID TRANSFER PROTEIN (LTP) in cuticle development, and validating it via transgenic experiments in Arabidopsis. Given that LTPs and cuticles both evolved in land plants and are absent from aquatic green algae, we propose that during plant evolution, LTPs arose as one of the innovations of land plants that enabled development of the cuticle.

Transcriptomic network analyses shed light on the regulation of cuticle development in maize leaves.

Plant cuticles are composed of wax and cutin and evolved in the land plants as a hydrophobic boundary that reduces water loss from the plant epidermis. The expanding maize adult leaf displays a dynamic, proximodistal gradient of cuticle development, from the leaf base to the tip. Laser microdissection RNA Sequencing (LM-RNAseq) was performed along this proximodistal gradient, and complementary network analyses identified potential regulators of cuticle biosynthesis and deposition. A weighted gene coexpression network (WGCN) analysis suggested a previously undescribed function for PHYTOCHROME-mediated light signaling during the regulation of cuticular wax deposition. Genetic analyses reveal that phyB1 phyB2 double mutants of maize exhibit abnormal cuticle composition, supporting the predictions of our coexpression analysis. Reverse genetic analyses also show that phy mutants of the moss Physcomitrella patens exhibit abnormal cuticle composition, suggesting an ancestral role for PHYTOCHROME-mediated, light-stimulated regulation of cuticle development during plant evolution.

Constructing functional cuticles: analysis of relationships between cuticle lipid composition, ultrastructure and water barrier function in developing adult maize leaves.

BACKGROUND AND AIMS: Prior work has examined cuticle function, composition and ultrastructure in many plant species, but much remains to be learned about how these features are related. This study aims to elucidate relationships between these features via analysis of cuticle development in adult maize (Zea mays L.) leaves, while also providing the most comprehensive investigation to date of the composition and ultrastructure of adult leaf cuticles in this important crop plant. METHODS: We examined water permeability, wax and cutin composition via gas chromatography, and ultrastructure via transmission electron microscopy, along the developmental gradient of partially expanded adult maize leaves, and analysed the relationships between these features. KEY RESULTS: The water barrier property of the adult maize leaf cuticle is acquired at the cessation of cell expansion. Wax types and chain lengths accumulate asynchronously over the course of development, while overall wax load does not vary. Cutin begins to accumulate prior to establishment of the water barrier and continues thereafter. Ultrastructurally, pavement cell cuticles consist of an epicuticular layer, and a thin cuticle proper that acquires an inner, osmiophilic layer during development. CONCLUSIONS: Cuticular waxes of the adult maize leaf are dominated by alkanes and alkyl esters. Unexpectedly, these are localized mainly in the epicuticular layer. Establishment of the water barrier during development coincides with a switch from alkanes to esters as the major wax type, and the emergence of an osmiophilic (likely cutin-rich) layer of the cuticle proper. Thus, alkyl esters and the deposition of the cutin polyester are implicated as key components of the water barrier property of adult maize leaf cuticles.

AtMYB41 activates ectopic suberin synthesis and assembly in multiple plant species and cell types.

Suberin is a lipid and phenolic cell wall heteropolymer found in the roots and other organs of all vascular plants. Suberin plays a critical role in plant water relations and in protecting plants from biotic and abiotic stresses. Here we describe a transcription factor, AtMYB41 (At4g28110), that can activate the steps necessary for aliphatic suberin synthesis and deposition of cell wall-associated suberin-like lamellae in both Arabidopsis thaliana and Nicotiana benthamiana. Overexpression of AtMYB41 increased the abundance of suberin biosynthetic gene transcripts by orders of magnitude and resulted in the accumulation of up to 22 times more suberin-type than cutin-type aliphatic monomers in leaves. Overexpression of AtMYB41 also resulted in elevated amounts of monolignols in leaves and an increase in the accumulation of phenylpropanoid and lignin biosynthetic gene transcripts. Surprisingly, ultrastructural data indicated that overexpression led to the formation of suberin-like lamellae in both epidermal and mesophyll cells of leaves. We further implicate AtMYB41 in the production of aliphatic suberin under abiotic stress conditions. These results provide insight into the molecular-genetic mechanisms of the biosynthesis and deposition of a ubiquitous cell wall-associated plant structure and will serve as a basis for discovering the transcriptional network behind one of the most abundant lipid-based polymers in nature.

The role of phloem sieve elements and laticifers in the biosynthesis and accumulation of alkaloids in opium poppy.

The benzylisoquinoline alkaloids of opium poppy, including the narcotic analgesics morphine and codeine, accumulate in the multinucleate cytoplasm of specialized laticifers that accompany vascular tissues throughout the plant. In mature opium poppy plants, immunofluorescence labeling using specific antibodies showed that four alkaloid biosynthetic enzymes, (S)-norcoclaurine 6-O-methyltransferase (6OMT), (S)-coclaurine N-methyltransferase (CNMT), (S)-3'-hydroxy-N-methylcoclaurine-4'-O-methyltransferase (4'OMT) and salutaridinol-7-O-acetyltransferase (SAT) were restricted to sieve elements of the phloem adjacent or proximal to laticifers. The identity of sieve elements was confirmed by (i) the specific immunogold labeling of the characteristic cytoplasm of this cell type, (ii) the co-localization of a sieve element-specific H(+)-ATPase with all biosynthetic enzymes and (iii) the strict association of sieve plates with immunofluorescent cells. The localization of laticifers was demonstrated antibodies specific to major latex protein (MLP), which is characteristic of this cell type. In situ hybridization using antisense RNA probes for 6OMT, CNMT, 4'OMT and SAT showed that the corresponding gene transcripts were found in the companion cell paired with each sieve element. Seven benzylisoquinoline alkaloid biosynthetic enzymes, (S)-N-methylcoclaurine 3'-hydroxylase (CYP80B1), berberine bridge enzyme, codeinone reductase, 6OMT, CNMT, 4'OMT and SAT were localized by immunofluorescence labeling to the sieve elements in the root and hypocotyl of opium poppy seedlings. The abundance of these enzymes increased rapidly between 1 and 3 days after seed germination. The localization of seven biosynthetic enzymes to the sieve elements provides strong support for the unique, cell type-specific biosynthesis of benzylisoquinoline alkaloids in the opium poppy.

Three-dimensional structure of (1,4)-beta-D-mannan mannanohydrolase from tomato fruit.

The three-dimensional crystal structure of tomato (Lycopersicon esculentum) beta-mannanase 4a (LeMAN4a) has been determined to 1.5 A resolution. The enzyme adopts the (beta/alpha)(8) fold common to the members of glycohydrolase family GH5. The structure is comparable with those of the homologous Trichoderma reesei and Thermomonospora fusca beta-mannanases: There is a conserved three-stranded beta-sheet located near the N terminus that stacks against the central beta-barrel at the end opposite the active site. Three noncanonical beta-helices surround the active site. Similar helices are found in T. reesei but not T. fusca beta-mannanase. By analogy with other beta-mannanases, the catalytic acid/base residue is E204 and the nucleophile residue is E318. The active site cleft of L. esculentum beta-mannanase most closely resembles that of the T. reesei isozyme. A model of substrate binding in LeMAN4a is proposed in which the mannosyl residue occupying the -1 subsite of the enzyme adopts the (1)S(5) skew-boat conformation.

Variation in its C-terminal amino acids determines whether endo-beta-mannanase is active or inactive in ripening tomato fruits of different cultivars.

Endo-beta-mannanase cDNAs were cloned and characterized from ripening tomato (Lycopersicon esculentum Mill. cv Trust) fruit, which produces an active enzyme, and from the tomato cv Walter, which produces an inactive enzyme. There is a two-nucleotide deletion in the gene from tomato cv Walter, which results in a frame shift and the deletion of four amino acids at the C terminus of the full-length protein. Other cultivars that produce either active or inactive enzyme show the same absence or presence of the two-nucleotide deletion. The endo-beta-mannanase enzyme protein was purified and characterized from ripe fruit to ensure that cDNA codes for the enzyme from fruit. Immunoblot analysis demonstrated that non-ripening mutants, which also fail to exhibit endo-beta-mannanase activity, do so because they fail to express the protein. In a two-way genetic cross between tomato cvs Walter and Trust, all F(1) progeny from both crosses produced fruit with active enzyme, suggesting that this form is dominant and homozygous in tomato cv Trust. Self-pollination of a plant from the heterozygous F(1) generation yielded F(2) plants that bear fruit with and without active enzyme at a ratio appropriate to Mendelian genetic segregation of alleles. Heterologous expression of the two endo-beta-mannanase genes in Escherichia coli resulted in active enzyme being produced from cultures containing the tomato cv Trust gene and inactive enzyme being produced from those containing the tomato cv Walter gene. Site-directed mutagenesis was used to establish key elements in the C terminus of the endo-beta-mannanase protein that are essential for full enzyme activity.

Gel diffusion assays for endo-beta-mannanase and pectin methylesterase can underestimate enzyme activity due to proteolytic degradation: a remedy.

The accuracy of the sensitive gel-diffusion assay for endo-beta-mannanase activity was improved when protein was added to fruit extracts or into the substrate-gel matrix in which the enzyme assays were conducted. Mixing of commercially available protease inhibitors with fruit enzyme extracts also resulted in increased assayable activity. These treatments were less effective when applied to extracts from tomato seeds, which contained over three times more endogenous protein than fruit extracts. Thus the presence of added or higher amounts of endogenous proteins served as the protectant for endo-beta-mannanase during the course of the gel-diffusion assay, which required an incubation at 32 degrees C for at least 18 h. There was no difference in assayable endo-beta-mannanase activity in the presence and absence of added protein when measured rapidly by viscometry. An effective modification was made to the galactomannan substrate gel assay for endo-beta-mannanase, which is the most efficient method for assaying large numbers of extracts, to improve its accuracy when the enzyme is obtained from tissues containing a low endogenous protein content. This involved incorporating an optimal concentration of gelatin into the galactomannan assay matrix gel. Much higher enzyme activities were recorded, with up to a 10-fold increase for tomato fruit extracts, compared to the same samples assayed on gels with no gelatin added. This increased activity was also obtained using extracts from the fruit of cantaloupe, peach, and nectarine. When incorporated into esterified pectin substrate gels, gelatin also increased the assayable activity of pectin methylesterase. Thus the incorporation of protein (gelatin) into substrate gels during the assay also should be widely more useful for other cell-wall-mobilizing enzymes and hydrolases.