CYP86B1 is required for very long chain omega-hydroxyacid and alpha, omega -dicarboxylic acid synthesis in root and seed suberin polyester

An Endosperm-Associated Cuticle Is Required for Arabidopsis Seed Viability, Dormancy and Early Control of Germination

Cuticular layers and seeds are prominent plant adaptations to terrestrial life that appeared early and late during plant evolution, respectively. The cuticle is a waterproof film covering plant aerial organs preventing excessive water loss and protecting against biotic and abiotic stresses. Cutin, consisting of crosslinked fatty acid monomers, is the most abundant and studied cuticular component. Seeds are dry, metabolically inert structures promoting plant dispersal by keeping the plant embryo in an arrested protected state. In Arabidopsis thaliana seeds, the embryo is surrounded by a single cell endosperm layer itself surrounded by a seed coat layer, the testa. Whole genome analyses lead us to identify cutin biosynthesis genes as regulatory targets of the phytohormones gibberellins (GA) and abscisic acid (ABA) signaling pathways that control seed germination. Cutin-containing layers are present in seed coats of numerous species, including Arabidopsis, where they regulate permeability to outer compounds. However, the role of cutin in mature seed physiology and germination remains poorly understood. Here we identify in mature seeds a thick cuticular film covering the entire outer surface of the endosperm. This seed cuticle is defective in cutin-deficient bodyguard1 seeds, which is associated with alterations in endospermic permeability. Furthermore, mutants affected in cutin biosynthesis display low seed dormancy and viability levels, which correlates with higher levels of seed lipid oxidative stress. Upon seed imbibition cutin biosynthesis genes are essential to prevent endosperm cellular expansion and testa rupture in response to low GA synthesis. Taken together, our findings suggest that in the course of land plant evolution cuticular structures were co-opted to achieve key physiological seed properties.

Seeds are remarkable plant structures that appeared late during land plant evolution. Indeed, within seeds plant embryos lie in a metabolic inert and highly resistant state. Seeds allow plants to disperse and find a favorable living environment. Remarkably as well, the “near-dead” embryo is able to germinate and turn into a fragile young seedling. The fragility of this transition is betrayed by the existence of control mechanisms that block germination in response to harmful environmental conditions. Seeds therefore transform plants into time and space travellers and largely explain land plant colonization by flowering plants.

The key to this success lies in the seed’s physiological feats, a major yet unresolved question in plant biology. We show that mature seeds of the model plant Arabidopsis contain an earlier land plant evolutionary innovation: the cuticle, a waxy film covering the aerial parts of the plant preventing excessive transpiration. The seed cuticle, which contains cutin, a major lipid polymer component of the leaf cuticle, encloses all the living tissues within the seed. Seeds with cutin defects are highly oxidized and have low seed viability and dormancy. They are also unable to control their germination. Thus, land plants co-opted an ancient innovation to achieve the remarkable physiology of seeds.

Suberization - the second life of an endodermal cell

The endodermis is the innermost cortical cell layer that surrounds the central vasculature and deposits an apoplastic diffusion barrier known as the Casparian strip. Although discovered 150 years ago, the underlying mechanisms responsible for formation of the Casparian strips have only recently been investigated. However, the fate of the endodermal cell goes further than formation of Casparian strips as they undergo a second level of differentiation, defined by deposition of suberin as a secondary cell wall. The presence and function of endodermal suberin in root barriers has remained enigmatic, as its role in barrier formation is not clear, especially in respect to the already existing Casparian strips. In this review, we present recent advances in the understanding of suberin synthesis, transport to the secondary cell wall, developmental features and functions. We focus on some of the major unknown questions revolving the function of endodermal suberin, which we now have the means to investigate. We further provide thoughts on how this knowledge might expand our current models on the developmental and physiological adaptation of root in response to the environment.

Biphenyl 4-Hydroxylases Involved in Aucuparin Biosynthesis in Rowan and Apple Are Cytochrome P450 736A Proteins

Upon pathogen attack, fruit trees such as apple (Malus spp.) and pear (Pyrus spp.) accumulate biphenyl and dibenzofuran phytoalexins, with aucuparin as a major biphenyl compound. 4-Hydroxylation of the biphenyl scaffold, formed by biphenyl synthase (BIS), is catalyzed by a cytochrome P450 (CYP). The biphenyl 4-hydroxylase (B4H) coding sequence of rowan (Sorbus aucuparia) was isolated and functionally expressed in yeast (Saccharomyces cerevisiae). SaB4H was named CYP736A107. No catalytic function of CYP736 was known previously. SaB4H exhibited absolute specificity for 3-hydroxy-5-methoxybiphenyl. In rowan cell cultures treated with elicitor from the scab fungus, transient increases in the SaB4H, SaBIS, and phenylalanine ammonia lyase transcript levels preceded phytoalexin accumulation. Transient expression of a carboxyl-terminal reporter gene construct directed SaB4H to the endoplasmic reticulum. A construct lacking the amino-terminal leader and transmembrane domain caused cytoplasmic localization. Functional B4H coding sequences were also isolated from two apple (Malus × domestica) cultivars. The MdB4Hs were named CYP736A163. When stems of cv Golden Delicious were infected with the fire blight bacterium, highest MdB4H transcript levels were observed in the transition zone. In a phylogenetic tree, the three B4Hs were closest to coniferaldehyde 5-hydroxylases involved in lignin biosynthesis, suggesting a common ancestor. Coniferaldehyde and related compounds were not converted by SaB4H.

Apple russeting as seen through the RNA-seq lens: strong alterations in the exocarp cell wall

Russeting, a commercially important defect in the exocarp of apple (Malus × domestica), is mainly characterized by the accumulation of suberin on the inner part of the cell wall of the outer epidermal cell layers. However, knowledge on the underlying genetic components triggering this trait remains sketchy. Bulk transcriptomic profiling was performed on the exocarps of three russeted and three waxy apple varieties. This experimental design was chosen to lower the impact of genotype on the obtained results. Validation by qPCR was carried out on representative genes and additional varieties. Gene ontology enrichment revealed a repression of lignin and cuticle biosynthesis genes in russeted exocarps, concomitantly with an enhanced expression of suberin deposition, stress responsive, primary sensing, NAC and MYB-family transcription factors, and specific triterpene biosynthetic genes. Notably, a strong correlation (R(2) = 0.976) between the expression of a MYB93-like transcription factor and key suberin biosynthetic genes was found. Our results suggest that russeting is induced by a decreased expression of cuticle biosynthetic genes, leading to a stress response which not only affects suberin deposition, but also the entire structure of the cell wall. The large number of candidate genes identified in this study provides a solid foundation for further functional studies.

Suberin: biosynthesis, regulation, and polymer assembly of a protective extracellular barrier

Suberin is a lipid-phenolic biopolyester deposited in the cell walls of certain boundary tissue layers of plants, such as root endodermis, root and tuber peridermis, and seed coats. Suberin serves as a protective barrier in these tissue layers, controlling, for example, water and ion transport. It is also a stress-induced anti-microbial barrier. The suberin polymer contains a variety of C16-C24 chain-length aliphatics, such as ω-hydroxy fatty acids, α,ω-dicarboxylic fatty acids, and primary fatty alcohols. Suberin also contains high amounts of glycerol and phenolics, especially ferulic acid. In addition, non-covalently linked waxes are likely associated with the suberin polymer. This review focusses on the suberin biosynthetic enzymes identified to date, which include β-ketoacyl-CoA synthases, fatty acyl reductases, long-chain acyl-CoA synthetases, cytochrome P450 monooxygenases, glycerol 3-phosphate acyltransferases, and phenolic acyltransferases. We also discuss recent advances in our understanding of the transport of suberin components intracellularly and to the cell wall, polymer assembly, and the regulation of suberin deposition.

Role of HXXXD-motif/BAHD acyltransferases in the biosynthesis of extracellular lipids

Terrestrial plants have evolved specific adaptations to preserve water and protect themselves from their environment. Such adaptations range from secondary metabolites and specialized structures that conduct water and nutrients, to cell wall modifications (i.e., cuticle and suberin) that prevent dehydration and provide a physical barrier to pathogens. Both the plant cuticle and suberized cell walls contain a lipid polymer framework embedded with waxes, and constitute a promising target for controlled genetic modification to improve desirable agronomic traits. Recent advances in genomic and molecular techniques coupled with the development of robust analytical methods have accelerated progress in comprehending these intractable lipid polymers. Gene products characterized in the wax, cutin and suberin pathways include a subset of HXXXD/BAHD family enzymes that catalyze acyl transfer reactions between CoA-activated hydroxycinnamic acid derivatives and hydroxylated aliphatics. This review highlights our current understanding of HXXXD/BAHD acyltransferases in extracellular lipid biosynthesis and discusses the chemical, ultrastructural and physiological ramifications of impairing the expression of BAHD acyltransferase-encoding genes related to cutin and suberin synthesis.

Long-term boron-deficiency-responsive genes revealed by cDNA-AFLP differ between Citrus sinensis roots and leaves

Seedlings of Citrus sinensis (L.) Osbeck were supplied with boron (B)-deficient (without H3BO3) or -sufficient (10 μM H3BO3) nutrient solution for 15 weeks. We identified 54 (38) and 38 (45) up (down)-regulated cDNA-AFLP bands (transcript-derived fragments, TDFs) from B-deficient leaves and roots, respectively. These TDFs were mainly involved in protein and amino acid metabolism, carbohydrate and energy metabolism, nucleic acid metabolism, cell transport, signal transduction, and stress response and defense. The majority of the differentially expressed TDFs were isolated only from B-deficient roots or leaves, only seven TDFs with the same GenBank ID were isolated from the both. In addition, ATP biosynthesis-related TDFs were induced in B-deficient roots, but unaffected in B-deficient leaves. Most of the differentially expressed TDFs associated with signal transduction and stress defense were down-regulated in roots, but up-regulated in leaves. TDFs related to protein ubiquitination and proteolysis were induced in B-deficient leaves except for one TDF, while only two down-regulated TDFs associated with ubiquitination were detected in B-deficient roots. Thus, many differences existed in long-term B-deficiency-responsive genes between roots and leaves. In conclusion, our findings provided a global picture of the differential responses occurring in B-deficient roots and leaves and revealed new insight into the different adaptive mechanisms of C. sinensis roots and leaves to B-deficiency at the transcriptional level.

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.

ABCG transporters are required for suberin and pollen wall extracellular barriers in Arabidopsis

Effective regulation of water balance in plants requires localized extracellular barriers that control water and solute movement. We describe a clade of five Arabidopsis thaliana ABCG half-transporters that are required for synthesis of an effective suberin barrier in roots and seed coats (ABCG2, ABCG6, and ABCG20) and for synthesis of an intact pollen wall (ABCG1 and ABCG16). Seed coats of abcg2 abcg6 abcg20 triple mutant plants had increased permeability to tetrazolium red and decreased suberin content. The root system of triple mutant plants was more permeable to water and salts in a zone complementary to that affected by the Casparian strip. Suberin of mutant roots and seed coats had distorted lamellar structure and reduced proportions of aliphatic components. Root wax from the mutant was deficient in alkylhydroxycinnamate esters. These mutant plants also had few lateral roots and precocious secondary growth in primary roots. abcg1 abcg16 double mutants defective in the other two members of the clade had pollen with defects in the nexine layer of the tapetum-derived exine pollen wall and in the pollen-derived intine layer. Mutant pollen collapsed at the time of anther desiccation. These mutants reveal transport requirements for barrier synthesis as well as physiological and developmental consequences of barrier deficiency.

CYP77A19 and CYP77A20 characterized from Solanum tuberosum oxidize fatty acids in vitro and partially restore the wild phenotype in an Arabidopsis thaliana cutin mutant

Cutin and suberin represent lipophilic polymers forming plant/environment interfaces in leaves and roots. Despite recent progress in Arabidopsis, there is still a lack on information concerning cutin and suberin synthesis, especially in crops. Based on sequence homology, we isolated two cDNA clones of new cytochrome P450s, CYP77A19 and CYP77A20 from potato tubers (Solanum tuberosum). Both enzymes hydroxylated lauric acid (C12:0) on position ω-1 to ω-5. They oxidized fatty acids with chain length ranging from C12 to C18 and catalysed hydroxylation of 16-hydroxypalmitic acid leading to dihydroxypalmitic (DHP) acids, the major C16 cutin and suberin monomers. CYP77A19 also produced epoxides from linoleic acid (C18:2). Exploration of expression pattern in potato by RT-qPCR revealed the presence of transcripts in all tissues tested with the highest expression in the seed compared with leaves. Water stress enhanced their expression level in roots but not in leaves. Application of methyl jasmonate specifically induced CYP77A19 expression. Expression of either gene in the Arabidopsis null mutant cyp77a6-1 defective in flower cutin restored petal cuticular impermeability. Nanoridges were also observed in CYP77A20-expressing lines. However, only very low levels of the major flower cutin monomer 10,16-dihydroxypalmitate and no C18 epoxy monomers were found in the cutin of the complemented lines.

Role of root hydrophobic barriers in salt exclusion of a mangrove plant Avicennia officinalis

Salt exclusion at the roots and salt secretion in the leaves were examined in a mangrove, Avicennia officinalis. The non-secretor mangrove Bruguiera cylindrica was used for comparative study of hydrophobic barrier formation in the roots. Bypass flow was reduced when seedlings were previously treated with high salt concentration. A biseriate exodermis was detected in the salt-treated roots, along with an enhanced deposition of hydrophobic barriers in the endodermis. These barriers reduced Na(+) loading into the xylem, accounting for a 90-95% salt exclusion in A. officinalis. Prominent barriers were found in the roots of B. cylindrica even in the absence of salt treatment. A cytochrome P450 gene that may regulate suberin biosynthesis was up-regulated within hours of salt treatment in A. officinalis roots and leaves, corresponding with increased suberin deposition. X-ray microanalysis showed preferential deposition of Na(+) and Cl(-) in the root cortex compared with the stele, suggesting that the endodermis is the primary site of salt exclusion. Enhanced salt secretion and increased suberin deposition surrounding the salt glands were seen in the leaves with salt treatment. Overall, these data show that the deposition of apoplastic barriers increases resistance to bypass flow leading to efficient salt exclusion at the roots in mangroves.

Pigmentation in sand pear (Pyrus pyrifolia) fruit: biochemical characterization, gene discovery and expression analysis with exocarp pigmentation mutant

Exocarp color of sand pear is an important trait for the fruit production and has caused our concern for a long time. Our previous study explored the different expression genes between the two genotypes contrasting for exocarp color, which indicated the different suberin, cutin, wax and lignin biosynthesis between the russet- and green-exocarp. In this study, we carried out microscopic observation and Fourier transform infrared spectroscopy analysis to detect the differences of tissue structure and biochemical composition between the russet- and green-exocarp of sand pear. The green exocarp was covered with epidermis and cuticle which was replaced by a cork layer on the surface of russet exocarp, and the chemicals of the russet exocarp were characterized by lignin, cellulose and hemicellulose. We explored differential gene expression between the russet exocarp of 'Niitaka' and its green exocarp mutant cv. 'Suisho' using Illumina RNA-sequencing. A total of 559 unigenes showed different expression between the two types of exocarp, and 123 of them were common to the previous study. The quantitative real time-PCR analysis supports the RNA-seq-derived gene with different expression between the two types of exocarp and revealed the preferential expression of these genes in exocarp than in mesocarp and fruit core. Gene ontology enrichment analysis revealed divorced expression of lipid metabolic process genes, transport genes, stress responsive genes and other biological process genes in the two types of exocarp. Expression changes in lignin metabolism-related genes were consistent with the different pigmentation of russet and green exocarp. Increased transcripts of putative genes involved the suberin, cutin and wax biosynthesis in 'Suisho' exocarp could facilitate deposition of the chemicals and take a role in the mutant trait responsible for the green exocarp. In addition, the divorced expression of ATP-binding cassette transporters involved in the trans-membrane transport of lignin, cutin, and suberin precursors suggests that the transport process could also affect the composition of exocarp and take a role in the regulation of exocarp pigmentation. Results from this study provide a base for the analysis of the molecular mechanism underlying sand pear russet/green exocarp mutation, and presents a comprehensive list of candidate genes that could be used to further investigate the trait mutation at the molecular level.

Acyl-lipid thioesterase1-4 from Arabidopsis thaliana form a novel family of fatty acyl-acyl carrier protein thioesterases with divergent expression patterns and substrate specificities

Hydrolysis of fatty acyl thioester bonds by thioesterases to produce free fatty acids is important for dictating the diversity of lipid metabolites produced in plants. We have characterized a four-member family of fatty acyl thioesterases from Arabidopsis thaliana, which we have called acyl-lipid thioesterase1 (ALT1), ALT2, ALT3, and ALT4. The ALTs belong to the Hotdog fold superfamily of thioesterases. ALT-like genes are present in diverse plant taxa, including dicots, monocots, lycophytes, and microalgae. The four Arabidopsis ALT genes were found to have distinct gene expression profiles with respect to each other. ALT1 was expressed specifically in stem epidermal cells and flower petals. ALT2 was expressed specifically in root endodermal and peridermal cells as well as in stem lateral organ boundary cells. ALT3 was ubiquitously expressed in aerial and root tissues and at much higher levels than the other ALTs. ALT4 expression was restricted to anthers. All four proteins were localized in plastids via an N-terminal targeting sequence of about 48 amino acids. When expressed in Escherichia coli, the ALT proteins used endogenous fatty acyl-acyl carrier protein substrates to generate fatty acids that varied in chain length (C6-C18), degree of saturation (saturated and monounsaturated), and oxidation state (fully reduced and β-ketofatty acids). Despite their high amino acid sequence identities, each enzyme produced a different profile of lipids in E. coli. The biological roles of these proteins are unknown, but they potentially generate volatile lipid metabolites that have previously not been reported in Arabidopsis.

Apoplastic diffusion barriers in Arabidopsis

During the development of Arabidopsis and other land plants, diffusion barriers are formed in the apoplast of specialized tissues within a variety of plant organs. While the cuticle of the epidermis is the primary diffusion barrier in the shoot, the Casparian strips and suberin lamellae of the endodermis and the periderm represent the diffusion barriers in the root. Different classes of molecules contribute to the formation of extracellular diffusion barriers in an organ- and tissue-specific manner. Cutin and wax are the major components of the cuticle, lignin forms the early Casparian strip, and suberin is deposited in the stage II endodermis and the periderm. The current status of our understanding of the relationships between the chemical structure, ultrastructure and physiological functions of plant diffusion barriers is discussed. Specific aspects of the synthesis of diffusion barrier components and protocols that can be used for the assessment of barrier function and important barrier properties are also presented.

Arabidopsis 3-ketoacyl-coenzyme a synthase9 is involved in the synthesis of tetracosanoic acids as precursors of cuticular waxes, suberins, sphingolipids, and phospholipids

Very-long-chain fatty acids (VLCFAs) with chain lengths from 20 to 34 carbons are involved in diverse biological functions such as membrane constituents, a surface barrier, and seed storage compounds. The first step in VLCFA biosynthesis is the condensation of two carbons to an acyl-coenzyme A, which is catalyzed by 3-ketoacyl-coenzyme A synthase (KCS). In this study, amino acid sequence homology and the messenger RNA expression patterns of 21 Arabidopsis (Arabidopsis thaliana) KCSs were compared. The in planta role of the KCS9 gene, showing higher expression in stem epidermal peels than in stems, was further investigated. The KCS9 gene was ubiquitously expressed in various organs and tissues, including roots, leaves, and stems, including epidermis, silique walls, sepals, the upper portion of the styles, and seed coats, but not in developing embryos. The fluorescent signals of the KCS9::enhanced yellow fluorescent protein construct were merged with those of BrFAD2::monomeric red fluorescent protein, which is an endoplasmic reticulum marker in tobacco (Nicotiana benthamiana) epidermal cells. The kcs9 knockout mutants exhibited a significant reduction in C24 VLCFAs but an accumulation of C20 and C22 VLCFAs in the analysis of membrane and surface lipids. The mutant phenotypes were rescued by the expression of KCS9 under the control of the cauliflower mosaic virus 35S promoter. Taken together, these data demonstrate that KCS9 is involved in the elongation of C22 to C24 fatty acids, which are essential precursors for the biosynthesis of cuticular waxes, aliphatic suberins, and membrane lipids, including sphingolipids and phospholipids. Finally, possible roles of unidentified KCSs are discussed by combining genetic study results and gene expression data from multiple Arabidopsis KCSs.

Acyl-lipid metabolism

Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.

Acyl-lipid metabolism

Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.

Impacts of diversification of cytochrome P450 on plant metabolism

Cytochrome P450 monooxygenases (P450s) catalyze a wide variety of monooxygenation reactions in primary and secondary metabolism in plants. The share of P450 genes in each plant genome is estimated to be up to 1%. This implies that the diversification of P450 has made a significant contribution to the ability to acquire the emergence of new metabolic pathways during land plant evolution. The P450 families conserved universally in land plants contribute to their chemical defense mechanisms. Several P450s are involved in the biosynthesis and catabolism of plant hormones. Species-specific P450 families are essential for the biosynthetic pathways of phytochemicals such as terpenoids and alkaloids. Genome wide analysis of the gene clusters including P450 genes will provide a clue to defining the metabolic roles of orphan P450s. Metabolic engineering with plant P450s is an important technology for large-scale production of valuable phytochemicals such as medicines.

Endodermal cell-cell contact is required for the spatial control of Casparian band development in Arabidopsis thaliana

BACKGROUND AND AIMS

Apoplasmic barriers in plants fulfil important roles such as the control of apoplasmic movement of substances and the protection against invasion of pathogens. The aim of this study was to describe the development of apoplasmic barriers (Casparian bands and suberin lamellae) in endodermal cells of Arabidopsis thaliana primary root and during lateral root initiation.

METHODS

Modifications of the endodermal cell walls in roots of wild-type Landsberg erecta (Ler) and mutants with defective endodermal development - scarecrow-3 (scr-3) and shortroot (shr) - of A. thaliana plants were characterized by light, fluorescent, confocal laser scanning, transmission and cryo-scanning electron microscopy.

KEY RESULTS

In wild-type plant roots Casparian bands initiate at approx. 1600 µm from the root cap junction and suberin lamellae first appear on the inner primary cell walls at approx. 7000-8000 µm from the root apex in the region of developing lateral root primordia. When a single cell replaces a pair of endodermal and cortical cells in the scr-3 mutant, Casparian band-like material is deposited ectopically at the junction between this 'cortical' cell and adjacent pericycle cells. Shr mutant roots with an undeveloped endodermis deposit Casparian band-like material in patches in the middle lamellae of cells of the vascular cylinder. Endodermal cells in the vicinity of developing lateral root primordia develop suberin lamellae earlier, and these are thicker, compared wih the neighbouring endodermal cells. Protruding primordia are protected by an endodermal pocket covered by suberin lamellae.

CONCLUSIONS

The data suggest that endodermal cell-cell contact is required for the spatial control of Casparian band development. Additionally, the endodermal cells form a collet (collar) of short cells covered by a thick suberin layer at the base of lateral root, which may serve as a barrier constituting a 'safety zone' protecting the vascular cylinder against uncontrolled movement of water, solutes or various pathogens.

Solving the puzzles of cutin and suberin polymer biosynthesis

Cutin and suberin are insoluble lipid polymers that provide critical barrier functions to the cell wall of certain plant tissues, including the epidermis, endodermis and periderm. Genes that are specific to the biosynthesis of cutins and/or aliphatic suberins have been identified, mainly in Arabidopsis thaliana. They notably encode acyltransferases, oxidases and transporters, which may have either well-defined or more debatable biochemical functions. However, despite these advances, important aspects of cutin and suberin synthesis remain obscure. Central questions include whether fatty acyl monomers or oligomers are exported, and the extent of extracellular assembly and attachment to the cell wall. These issues are reviewed. Greater emphasis on chemistry and biochemistry will be required to solve these unknowns and link structure with function.

Chloroplast lipid synthesis and lipid trafficking through ER–plastid membrane contact sites

Plant chloroplasts contain an intricate photosynthetic membrane system, the thylakoids, and are surrounded by two envelope membranes at which thylakoid lipids are assembled. The glycoglycerolipids mono- and digalactosyldiacylglycerol, and sulfoquinovosyldiacylglycerol as well as phosphatidylglycerol, are present in thylakoid membranes, giving them a unique composition. Fatty acids are synthesized in the chloroplast and are either directly assembled into thylakoid lipids at the envelope membranes or exported to the ER (endoplasmic reticulum) for extraplastidic lipid assembly. A fraction of lipid precursors is reimported into the chloroplast for the synthesis of thylakoid lipids. Thus polar lipid assembly in plants requires tight co-ordination between the chloroplast and the ER and necessitates inter-organelle lipid trafficking. In the present paper, we discuss the current knowledge of the export of fatty acids from the chloroplast and the import of chloroplast lipid precursors assembled at the ER. Direct membrane contact sites between the ER and the chloroplast outer envelopes are discussed as possible conduits for lipid transfer.

Suberin goes genomics: use of a short living plant to investigate a long lasting polymer

Suberin is a highly persistent cell wall polymer, predominantly composed of long-chain hydroxylated fatty acids. Apoplastic suberin depositions occur in internal and peripheral dermal tissues where they generate lipophilic barriers preventing uncontrolled flow of water, gases, and ions. In addition, suberization provides resistance to environmental stress conditions. Despite this physiological importance the knowledge about suberin formation has increased slowly for decades. Lately, the chemical characterization of suberin in Arabidopsis enabled the proposal of genes required for suberin biosynthesis such as β-ketoacyl-CoA synthases (KCS) for fatty acid elongation and cytochrome P450 oxygenases (CYP) for fatty acid hydroxylation. Advantaged by the Arabidopsis molecular genetic resources the in silico expression pattern of candidate genes, concerted with the tissue-specific distribution of suberin in Arabidopsis, led to the identification of suberin involved genes including KCS2, CYP86A1, and CYP86B1. The isolation of mutants with a modified suberin composition facilitated physiological studies revealing that the strong reduction in suberin in cyp86a1 mutants results in increased root water and solute permeabilities. The enhanced suberin 1 mutant, characterized by twofold increased root suberin content, has increased water-use efficiency and is affected in mineral ion uptake and transport. In this review the most recent findings on the biosynthesis and physiological importance of suberin in Arabidopsis are summarized and discussed.

Cytochromes p450

There are 244 cytochrome P450 genes (and 28 pseudogenes) in the Arabidopsis genome. P450s thus form one of the largest gene families in plants. Contrary to what was initially thought, this family diversification results in very limited functional redundancy and seems to mirror the complexity of plant metabolism. P450s sometimes share less than 20% identity and catalyze extremely diverse reactions leading to the precursors of structural macromolecules such as lignin, cutin, suberin and sporopollenin, or are involved in biosynthesis or catabolism of all hormone and signaling molecules, of pigments, odorants, flavors, antioxidants, allelochemicals and defense compounds, and in the metabolism of xenobiotics. The mechanisms of gene duplication and diversification are getting better understood and together with co-expression data provide leads to functional characterization.

Laser stimulation of the chloroplast/endoplasmic reticulum nexus in tobacco transiently produces protein aggregates (boluses) within the endoplasmic reticulum and stimulates local ER remodeling

Does the ER subdomain that associates with the chloroplast in vivo, hereafter referred to as the chloroplast/ER nexus, play a role in protein flow within the ER? In studies of tobacco cells either constitutively or transiently expressing ER-retained luminal, GFP-HDEL, or trans-membrane, YFP-RHD3, fluorescent fusion proteins, brief 405-nm (3-6-mW) laser stimulation of the nexus causes a qualitative difference in the movement and behavior of proteins in the ER. Photostimulating the nexus produces fluorescent protein punctate aggregates (boluses) within the lumen and membrane of the ER. The aggregation propagates through the membrane network throughout the cell, but within minutes can revert to normal, with disaggregation propagating back toward the originally photostimulated nexus. In the meantime, the ER grows and anastomoses around the chloroplast, forming a dense cisternal and tubular network. If this network is again photostimulated, bolus formation does not recur and, if the photostimulation results in photobleaching, fluorescence recovery after photobleaching occurs as it would typically in areas away from the nexus. Bolus propagation is not mediated by the actin cytoskeleton, but can be reversed by pre-conditioning the cells for 30  min with high, 40-45°C, temperature (heat stress). Because it is not reversed with heat stress, the reorganization of the ER at the nexus following photostimulation is a separate event.

A comparison of suberin monomers from the multiseriate exodermis of Iris germanica during maturation under differing growth conditions

Iris germanica roots develop a multiseriate exodermis (MEX) in which all mature cells contain suberin lamellae. The location and lipophilic nature of the lamellae contribute to their function in restricting radial water and solute transport. The objective of the current work was to identify and quantify aliphatic suberin monomers, both soluble and insoluble, at specific stages of MEX development and under differing growth conditions, to better understand aliphatic suberin biosynthesis. Roots were grown submerged in hydroponic culture, wherein the maturation of up to three exodermal layers occurred over 21 days. In contrast, when roots were exposed to a humid air gap, MEX maturation was accelerated, occurring within 14 days. The soluble suberin fraction included fatty acids, alkanes, fatty alcohols, and ferulic acid, while the suberin poly(aliphatic) domain (SPAD) included fatty acids, α,ω-dioic acids, ω-OH fatty acids, and ferulic acid. In submerged roots, SPAD deposition increased with each layer, although the composition remained relatively constant, while the composition of soluble components shifted toward increasing alkanes in the innermost layers. Air gap exposure resulted in two significant shifts in suberin composition: nearly double the amount of SPAD monomers across all layers, and almost three times the alkane accumulation in the first layer. The localized and abundant deposition of C18:1 α,ω-dioic and ω-OH fatty acids, along with high accumulation of intercalated alkanes in the first mature exodermal layer of air gap-exposed roots indicate its importance for water retention under drought compared with underlying layers and with entire layers developing under water.

Suberin research in the genomics era--new interest for an old polymer

Suberin is an apoplastic biopolymer with tissue-specific deposition in the cell walls of the endo- and exodermis of roots, of periderms including wound periderm and other border tissues. Suberised cell walls contain both polyaliphatic and polyaromatic domains which are supposedly cross-linked. The predominant aliphatic components are ω-hydroxyacids, α,ω-diacids, fatty acids and primary alcohols, whereas hydroxycinnamic acids, especially ferulic acid, are the main components of the polyaromatic domain. Although the monomeric composition of suberin has been known for decades, its biosynthesis and deposition has mainly been a subject of speculation. Only recently, significant progress elucidating suberin biosynthesis has been achieved using molecular genetic approaches, especially in the model species Arabidopsis. In parallel, the long-standing hypothesis that suberin functions as an apoplastic barrier has been corroborated by sophisticated, quantitative physiological studies in the past decade. These studies demonstrated that suberised cell walls could act as barriers, minimising the movement of water and nutrients, restricting pathogen invasion and impeding toxic gas diffusion. In addition, suberised cell walls provide a barrier to radial oxygen loss from roots to the anaerobic root substrate in wetland plants. The recent onset of multidisciplinary approaches combining genetic, analytical and physiological studies has begun to deliver further insights into the physiological importance of suberin depositions in plants.

Specific accumulation of CYP94A1 transcripts after exposure to gaseous benzaldehyde: induction of lauric acid ω-hydroxylase activity in Vicia sativa exposed to atmospheric pollutants

The effects of air pollutants such as aldehydes, ozone, nitrogen dioxide and benzene on fatty acid ω-hydroxylase activity in Vicia sativa microsomes have been investigated. Four days old etiolated V. sativa seedlings were exposed to different concentrations of selected pollutants for varying exposure times. Growing etiolated V. sativa seedlings in air containing the gaseous benzaldehyde (150 nM) led to an 8-fold enhancement of lauric acid ω-hydroxylase activity in microsomes of treated plants compared to controls grown in pure air (96 ± 10 versus 12 ± 2 pmol/min/mg protein, respectively). The induction increased with increasing gas phase concentrations (10-1300 nM) and the maximum of activity was measured after 48 h of exposure. Northern blot analysis revealed that this induction occurred via transcriptional activation of the gene coding for CYP94A1. The absence of CYP94A2 and CYP94A3 transcription activation together with the missing effect on epoxide hydrolases activities indicate the specificity of CYP94A1 induction by benzaldehyde. Exposure to nitrogen dioxide, ozone and formaldehyde also stimulated lauric acid ω-hydroxylases activity while exposure to benzene did not show any effect.

Cytochrome P450 metabolizing fatty acids in plants: characterization and physiological roles

In plants, fatty acids (FA) are subjected to various types of oxygenation reactions. Products include hydroxyacids, as well as hydroperoxides, epoxides, aldehydes, ketones and α,ω-diacids. Many of these reactions are catalysed by cytochrome P450s (P450s), which represent one of the largest superfamilies of proteins in plants. The existence of P450-type metabolizing FA enzymes in plants was established approximately four decades ago in studies on the biosynthesis of lipid polyesters. Biochemical investigations have highlighted two major characteristics of P450s acting on FAs: (a) they can be inhibited by FA analogues carrying an acetylenic function, and (b) they can be enhanced by biotic and abiotic stress at the transcriptional level. Based on these properties, P450s capable of producing oxidized FA have been identified and characterized from various plant species. Until recently, the vast majority of characterized P450s acting on FAs belonged to the CYP86 and CYP94 families. In the past five years, rapid progress in the characterization of mutants in the model plant Arabidopsis thaliana has allowed the identification of such enzymes in many other P450 families (i.e. CYP703, CYP704, CYP709, CYP77, CYP74). The presence in a single species of distinct enzymes characterized by their own regulation and catalytic properties raised the question of their physiological meaning. Functional studies in A. thaliana have demonstrated the involvement of FA hydroxylases in the synthesis of the protective biopolymers cutin, suberin and sporopollenin. In addition, several lines of evidence discussed in this minireview are consistent with P450s metabolizing FAs in many aspects of plant biology, such as defence against pathogens and herbivores, development, catabolism or reproduction.

Transport barriers made of cutin, suberin and associated waxes

Cutinized leaf epidermal cells and suberized root cell walls form important lipophilic interfaces between the plant and its environment, significantly contributing to the regulation of water uptake and the transport of solutes in and out of the plant. A wealth of new molecular information on the genes and enzymes contributing to cutin, suberin and wax biosynthesis have become available within the past few years, which is examined in the context of the functional properties of these barriers in terms of transport and permeability. Recent progress made in measuring transport properties of cutinized and suberized barriers in plants is reviewed, and promising approaches obtained with Arabidopsis and potato that might link the molecular information with transport properties are suggested.

Suberized cell walls of cork from cork oak differ from other species

Plants have suberized cells that act as protective interfaces with the environment or between different plant tissues. A lamellar structure of alternating dark and light bands has been found upon transmission electron microscopy (TEM) observation of cork cells and considered a typical feature of the suberized secondary wall. We observed cork cells from periderms of Quercus suber, Quercus cerris, Solanum tuberosum, and Calotropis procera by TEM after uranyl acetate and lead citrate staining. A lamellated structure was observed in S. tuberosum and C. procera but not in Q. suber and Q. cerris where the suberized cell wall showed a predominantly hyaline aspect with only a dark dotted staining. Removal of suberin from Q. suber cells left a thinner secondary wall that lost the translucent aspect. We hypothesize that the species' specific chemical composition of suberin will result in different three-dimensional macromolecular development and in a different spatial location of lignin and other aromatics. A lamellated ultrastructure is therefore not a general feature of suberized cells.

Three Arabidopsis fatty acyl-coenzyme A reductases, FAR1, FAR4, and FAR5, generate primary fatty alcohols associated with suberin deposition

Suberin is a protective hydrophobic barrier consisting of phenolics, glycerol, and a variety of fatty acid derivatives, including C18:0-C22:0 primary fatty alcohols. An eight-member gene family encoding alcohol-forming fatty acyl-coenzyme A reductases (FARs) has been identified in Arabidopsis (Arabidopsis thaliana). Promoter-driven expression of the beta-glucuronidase reporter gene indicated that three of these genes, FAR1(At5g22500), FAR4(At3g44540), and FAR5(At3g44550), are expressed in root endodermal cells. The three genes were transcriptionally induced by wounding and salt stress. These patterns of gene expression coincide with known sites of suberin deposition. We then characterized a set of mutants with T-DNA insertions in FAR1, FAR4, or FAR5 and found that the suberin compositions of roots and seed coats were modified in each far mutant. Specifically, C18:0-OH was reduced in far5-1, C20:0-OH was reduced in far4-1, and C22:0-OH was reduced in far1-1. We also analyzed the composition of polymer-bound lipids of leaves before and after wounding and found that the basal levels of C18:0-C22:0 primary alcohols in wild-type leaves were increased by wounding. In contrast, C18:0-OH and C22:0-OH were not increased by wounding in far5-1 and far1-1 mutants, respectively. Heterologous expression of FAR1, FAR4, and FAR5 in yeast confirmed that they are indeed active alcohol-forming FARs with distinct, but overlapping, chain length specificities ranging from C18:0 to C24:0. Altogether, these results indicate that Arabidopsis FAR1, FAR4, and FAR5 generate the fatty alcohols found in root, seed coat, and wound-induced leaf tissue.

A distinct type of glycerol-3-phosphate acyltransferase with sn-2 preference and phosphatase activity producing 2-monoacylglycerol

The first step in assembly of membrane and storage glycerolipids is acylation of glycerol-3-phosphate (G3P). All previously characterized membrane-bound, eukaryotic G3P acyltransferases (GPATs) acylate the sn-1 position to produce lysophosphatidic acid (1-acyl-LPA). Cutin is a glycerolipid with omega-oxidized fatty acids and glycerol as integral components. It occurs as an extracellular polyester on the aerial surface of all plants, provides a barrier to pathogens and resistance to stress, and maintains organ identity. We have determined that Arabidopsis acyltransferases GPAT4 and GPAT6 required for cutin biosynthesis esterify acyl groups predominantly to the sn-2 position of G3P. In addition, these acyltransferases possess a phosphatase domain that results in sn-2 monoacylglycerol (2-MAG) rather than LPA as the major product. Such bifunctional activity has not been previously described in any organism. The possible roles of 2-MAGs as intermediates in cutin synthesis are discussed. GPAT5, which is essential for the accumulation of suberin aliphatics, also exhibits a strong preference for sn-2 acylation. However, phosphatase activity is absent and 2-acyl-LPA is the major product. Clearly, plant GPATs can catalyze more reactions than the sn-1 acylation by which they are currently categorized. Close homologs of GPAT4-6 are present in all land plants, but not in animals, fungi or microorganisms (including algae). Thus, these distinctive acyltransferases may have been important for evolution of extracellular glycerolipid polymers and adaptation of plants to a terrestrial environment. These results provide insight into the biosynthetic assembly of cutin and suberin, the two most abundant glycerolipid polymers in nature.

Acyl-lipid metabolism

Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.

The Arabidopsis DSO/ABCG11 transporter affects cutin metabolism in reproductive organs and suberin in roots

Apart from its significance in the protection against stress conditions, the cuticular cover is essential for proper development of the diverse surface structures formed on aerial plant organs. This layer mainly consists of a cutin matrix, embedded and overlaid with cuticular waxes. Following their biosynthesis in epidermal cells, cutin and waxes were suggested to be exported across the plasma membrane by ABCG-type transporters such as DSO/ABCG11 to the cell wall and further to extracellular matrix. Here, additional aspects of DSO/ABCG11 function were investigated, predominantly in reproductive organs, which were not revealed in the previous reports. This was facilitated by the generation of a transgenic DSO/ABCG11 silenced line (dso-4) that displayed relatively subtle morphological and chemical phenotypes. These included altered petal and silique morphology, fusion of seeds, and changes in levels of cutin monomers in flowers and siliques. The dso-4 phenotypes corresponded to the strong DSO/ABCG11 gene expression in the embryo epidermis as well as in the endosperm tissues of the developing seeds. Moreover, the DSO/ABCG11 protein displayed polar localization in the embryo protoderm. Transcriptome analysis of the dso-4 mutant leaves and stems showed that reduced DSO/ABCG11 activity suppressed the expression of a large number of cuticle-associated genes, implying that export of cuticular lipids from the plasma membrane is a rate-limiting step in cuticle metabolism. Surprisingly, root suberin composition of dso-4 was altered, as well as root expression of two suberin biosynthetic genes. Taken together, this study provides new insights into cutin and suberin metabolism and their role in reproductive organs and roots development.

Lipid biochemists salute the genome

The biochemistry of plant metabolic pathways has been studied for many generations; nevertheless, numerous new enzymes and metabolic products have been discovered in the last 5-10 years. More importantly, many intriguing questions remain in all areas of metabolism. In this review, we consider these issues with respect to several pathways of lipid metabolism and the contributions made by the Arabidopsis genome sequence and the tools that it has spawned. These tools have allowed identification of enzymes and transporters required for the mobilization of seed storage lipids, as well as transporters that facilitate movement of lipids from the endoplasmic reticulum to the chloroplast in green leaf cells. Genomic tools were important in recognition of novel components of the cutin and suberin polymers that form water-impermeable barriers in plants. The waxes that also contribute to these barriers are exported from cells of the epidermis by transporters that are now being identified. Biochemical and genetic knowledge from yeast and animals has permitted successful homology-based searches of the Arabidopsis genome for genes encoding enzymes involved in the elongation of fatty acids and the synthesis of sphingolipids. Knowledge of the genome has identified novel enzymes for the biosynthesis of the seed storage lipid, triacylglycerol, and provided a refined understanding of how the pathways of fatty acid and triacylglycerol synthesis are integrated into overall carbon metabolism in developing seeds.

Arabidopsis seed secrets unravelled after a decade of genetic and omics-driven research

Seeds play a fundamental role in colonization of the environment by spermatophytes, and seeds harvested from crops are the main food source for human beings. Knowledge of seed biology is therefore important for both fundamental and applied issues. This review on seed biology illustrates the important progress made in the field of Arabidopsis seed research over the last decade. Access to 'omics' tools, including the inventory of genes deduced from sequencing of the Arabidopsis genome, has speeded up the analysis of biological functions operating in seeds. This review covers the following processes: seed and seed coat development, seed reserve accumulation, seed dormancy and seed germination. We present new insights in these various fields and describe ongoing biotechnology approaches to improve seed characteristics in crops.

The Arabidopsis translatome cell-specific mRNA atlas: Mining suberin and cutin lipid monomer biosynthesis genes as an example for data application

Plants consist of distinct cell types distinguished by position, morphological features and metabolic activities. We recently developed a method to extract cell-type specific mRNA populations by immunopurification of ribosome-associated mRNAs. Microarray profiles of 21 cell-specific mRNA populations from seedling roots and shoots comprise the Arabidopsis Translatome dataset. This gene expression atlas provides a new tool for the study of cell-specific processes. Here we provide an example of how genes involved in a pathway limited to one or few cell-types can be further characterized and new candidate genes can be predicted. Cells of the root endodermis produce suberin as an inner barrier between the cortex and stele, whereas the shoot epidermal cells form cutin as a barrier to the external environment. Both polymers consist of fatty acid derivates, and share biosynthetic origins. We use the Arabidopsis Translatome dataset to demonstrate the significant cell-specific expression patterns of genes involved in those biosynthetic processes and suggest new candidate genes in the biosynthesis of suberin and cutin.

Diversification of P450 genes during land plant evolution

Plant cytochromes P450 (P450s) catalyze a wide variety of monooxygenation/hydroxylation reactions in primary and secondary metabolism. The number of P450 genes in plant genomes is estimated to be up to 1% of total gene annotations of each plant species. This implies that diversification within P450 gene superfamilies has led to the emergence of new metabolic pathways throughout land plant evolution. The conserved P450 families contribute to chemical defense mechanisms under terrestrial conditions and several are involved in hormone biosynthesis and catabolism. Species-specific P450 families are essential for the biosynthetic pathways of species-specialized metabolites. Future genome-wide analyses of P450 gene clusters and coexpression networks should help both in identifying the functions of many orphan P450s and in understanding the evolution of this versatile group of enzymes.

Nanoridges that characterize the surface morphology of flowers require the synthesis of cutin polyester

Distinctive nanoridges on the surface of flowers have puzzled plant biologists ever since their discovery over 75 years ago. Although postulated to help attract insect pollinators, the function, chemical nature, and ontogeny of these surface nanostructures remain uncertain. Studies have been hampered by the fact that no ridgeless mutants have been identified. Here, we describe two mutants lacking nanoridges and define the biosynthetic pathway for 10,16-dihydroxypalmitate, a major cutin monomer in nature. Using gene expression profiling, two candidates for the formation of floral cutin were identified in the model plant Arabidopsis thaliana: the glycerol-3-phosphate acyltransferase 6 (GPAT6) and a member of a cytochrome P450 family with unknown biological function (CYP77A6). Plants carrying null mutations in either gene produced petals with no nanoridges and no cuticle could be observed by either scanning or transmission electron microscopy. A strong reduction in cutin content was found in flowers of both mutants. In planta overexpression suggested GPAT6 preferentially uses palmitate derivatives in cutin synthesis. Comparison of cutin monomer profiles in knockouts for CYP77A6 and the fatty acid omega-hydroxylase CYP86A4 provided genetic evidence that CYP77A6 is an in-chain hydroxylase acting subsequently to CYP86A4 in the synthesis of 10,16-dihydroxypalmitate. Biochemical activity of CYP77A6 was demonstrated by production of dihydroxypalmitates from 16-hydroxypalmitate, using CYP77A6-expressing yeast microsomes. These results define the biosynthetic pathway for an abundant and widespread monomer of the cutin polyester, show that the morphology of floral surfaces depends on the synthesis of cutin, and identify target genes to investigate the function of nanoridges in flower biology.

Identification of an Arabidopsis feruloyl-coenzyme A transferase required for suberin synthesis

All plants produce suberin, a lipophilic barrier of the cell wall that controls water and solute fluxes and restricts pathogen infection. It is often described as a heteropolymer comprised of polyaliphatic and polyaromatic domains. Major monomers include omega-hydroxy and alpha,omega-dicarboxylic fatty acids, glycerol, and ferulate. No genes have yet been identified for the aromatic suberin pathway. Here we demonstrate that Arabidopsis (Arabidopsis thaliana) gene AT5G41040, a member of the BAHD family of acyltransferases, is essential for incorporation of ferulate into suberin. In Arabidopsis plants transformed with the AT5G41040 promoter:YFP fusion, reporter expression is localized to cell layers undergoing suberization. Knockout mutants of AT5G41040 show almost complete elimination of suberin-associated ester-linked ferulate. However, the classic lamellar structure of suberin in root periderm of at5g41040 is not disrupted. The reduction in ferulate in at5g41040-knockout seeds is associated with an approximate stoichiometric decrease in aliphatic monomers containing omega-hydroxyl groups. Recombinant AT5G41040p catalyzed acyl transfer from feruloyl-coenzyme A to omega-hydroxyfatty acids and fatty alcohols, demonstrating that the gene encodes a feruloyl transferase. CYP86B1, a cytochrome P450 monooxygenase gene whose transcript levels correlate with AT5G41040 expression, was also investigated. Knockouts and overexpression confirmed CYP86B1 as an oxidase required for the biosynthesis of very-long-chain saturated alpha,omega-bifunctional aliphatic monomers in suberin. The seed suberin composition of cyp86b1 knockout was surprisingly dominated by unsubstituted fatty acids that are incapable of polymeric linkages. Together, these results challenge our current view of suberin structure by questioning both the function of ester-linked ferulate as an essential component and the existence of an extended aliphatic polyester.

CYP704B1 is a long-chain fatty acid omega-hydroxylase essential for sporopollenin synthesis in pollen of Arabidopsis

Sporopollenin is the major component of the outer pollen wall (exine). Fatty acid derivatives and phenolics are thought to be its monomeric building blocks, but the precise structure, biosynthetic route, and genetics of sporopollenin are poorly understood. Based on a phenotypic mutant screen in Arabidopsis (Arabidopsis thaliana), we identified a cytochrome P450, designated CYP704B1, as being essential for exine development. CYP704B1 is expressed in the developing anthers. Mutations in CYP704B1 result in impaired pollen walls that lack a normal exine layer and exhibit a characteristic striped surface, termed zebra phenotype. Heterologous expression of CYP704B1 in yeast cells demonstrated that it catalyzes omega-hydroxylation of long-chain fatty acids, implicating these molecules in sporopollenin synthesis. Recently, an anther-specific cytochrome P450, denoted CYP703A2, that catalyzes in-chain hydroxylation of lauric acid was also shown to be involved in sporopollenin synthesis. This shows that different classes of hydroxylated fatty acids serve as essential compounds for sporopollenin formation. The genetic relationships between CYP704B1, CYP703A2, and another exine gene, MALE STERILITY2, which encodes a fatty acyl reductase, were explored. Mutations in all three genes resulted in pollen with remarkably similar zebra phenotypes, distinct from those of other known exine mutants. The double and triple mutant combinations did not result in the appearance of novel phenotypes or enhancement of single mutant phenotypes. This implies that each of the three genes is required to provide an indispensable subset of fatty acid-derived components within the sporopollenin biosynthesis framework.