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 meta...

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 meta...

Cutin and suberin are the two major lipid-based polymers of plants. Cutin is the structural polymer of the epidermal cuticle, the waterproof layer covering primary aerial organs and which is often the structure first encountered by phytopathogens. Suberin contributes to the control of diffusion of water and solutes across internal root tissues and in periderms. The enzymes responsible for assembly of the cutin polymer are largely unknown. We have identified two Arabidopsis acyltransferases essential for cutin biosynthesis, glycerol-3-phosphate acyltransferase (GPAT) 4 and GPAT8. Double knockouts gpat4/gpat8 were strongly reduced in cutin and were less resistant to desiccation and to infection by the fungus Alternaria brassicicola . They also showed striking defects in stomata structure including a lack of cuticular ledges between guard cells, highlighting the importance of cutin in stomatal biology. Overexpression of GPAT4 or GPAT8 in Arabidopsis increased the content of C16 and C18 cutin monomers in leaves and stems by 80%. In order to modify cutin composition, the acyltransferase GPAT5 and the cytochrome P450-dependent fatty acyl oxidase CYP86A1, two enzymes associated with suberin biosynthesis, were overexpressed. When both enzymes were overexpressed together the epidermal polyesters accumulated new C20 and C22 ω-hydroxyacids and α,ω-diacids typical of suberin, and the fine structure and water-barrier function of the cuticle were altered. These results identify GPATs as partners of fatty acyl oxidases in lipid polyester synthesis and indicate that their cooverexpression provides a strategy to probe the role of cutin composition and quantity in the function of plant cuticles.

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.

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 ω-hydroxy and α,ω-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 ω-hydroxyl groups. Recombinant AT5G41040p catalyzed acyl transfer from feruloyl-coenzyme A to ω-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 α,ω-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.

Summary: 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.