RESUMEN
Lignin is a phenylpropanoid-derived heteropolymer important for the strength and rigidity of the plant secondary cell wall. Genetic disruption of lignin biosynthesis has been proposed as a means to improve forage and bioenergy crops, but frequently results in stunted growth and developmental abnormalities, the mechanisms of which are poorly understood. Here we show that the phenotype of a lignin-deficient Arabidopsis mutant is dependent on the transcriptional co-regulatory complex, Mediator. Disruption of the Mediator complex subunits MED5a (also known as REF4) and MED5b (also known as RFR1) rescues the stunted growth, lignin deficiency and widespread changes in gene expression seen in the phenylpropanoid pathway mutant ref8, without restoring the synthesis of guaiacyl and syringyl lignin subunits. Cell walls of rescued med5a/5b ref8 plants instead contain a novel lignin consisting almost exclusively of p-hydroxyphenyl lignin subunits, and moreover exhibit substantially facilitated polysaccharide saccharification. These results demonstrate that guaiacyl and syringyl lignin subunits are largely dispensable for normal growth and development, implicate Mediator in an active transcriptional process responsible for dwarfing and inhibition of lignin biosynthesis, and suggest that the transcription machinery and signalling pathways responding to cell wall defects may be important targets to include in efforts to reduce biomass recalcitrance.
Asunto(s)
Proteínas de Arabidopsis/genética , Arabidopsis/crecimiento & desarrollo , Arabidopsis/genética , Lignina/metabolismo , Complejo Mediador/genética , Mutación/genética , Arabidopsis/metabolismo , Proteínas de Arabidopsis/metabolismo , Biocombustibles , Biomasa , Pared Celular/química , Pared Celular/metabolismo , Celulosa/metabolismo , Regulación de la Expresión Génica de las Plantas/genética , Lignina/biosíntesis , Lignina/química , Complejo Mediador/química , Complejo Mediador/deficiencia , Complejo Mediador/metabolismo , Fenotipo , Plantas Modificadas Genéticamente , Subunidades de Proteína/genética , Subunidades de Proteína/metabolismo , Transcripción Genética/genéticaRESUMEN
Plants produce an array of metabolites (including lignin monomers and soluble UV-protective metabolites) from phenylalanine through the phenylpropanoid biosynthetic pathway. A subset of plants, including many related to Arabidopsis thaliana, synthesizes glucosinolates, nitrogen- and sulfur-containing secondary metabolites that serve as components of a plant defense system that deters herbivores and pathogens. Here, we report that the Arabidopsis thaliana reduced epidermal fluorescence5 (ref5-1) mutant, identified in a screen for plants with defects in soluble phenylpropanoid accumulation, has a missense mutation in CYP83B1 and displays defects in glucosinolate biosynthesis and in phenylpropanoid accumulation. CYP79B2 and CYP79B3 are responsible for the production of the CYP83B1 substrate indole-3-acetaldoxime (IAOx), and we found that the phenylpropanoid content of cyp79b2 cyp79b3 and ref5-1 cyp79b2 cyp79b3 plants is increased compared with the wild type. These data suggest that levels of IAOx or a subsequent metabolite negatively influence phenylpropanoid accumulation in ref5 and more importantly that this crosstalk is relevant in the wild type. Additional biochemical and genetic evidence indicates that this inhibition impacts the early steps of the phenylpropanoid biosynthetic pathway and restoration of phenylpropanoid accumulation in a ref5-1 med5a/b triple mutant suggests that the function of the Mediator complex is required for the crosstalk.
Asunto(s)
Proteínas de Arabidopsis/metabolismo , Arabidopsis/metabolismo , Regulación de la Expresión Génica de las Plantas , Glucosinolatos/metabolismo , Propanoles/metabolismo , Aldehído Deshidrogenasa/genética , Aldehído Deshidrogenasa/metabolismo , Arabidopsis/genética , Proteínas de Arabidopsis/genética , Vías Biosintéticas , Sistema Enzimático del Citocromo P-450/genética , Sistema Enzimático del Citocromo P-450/metabolismo , Ácidos Indolacéticos/metabolismo , Indoles/metabolismo , Lignina/metabolismo , Proteínas de la Membrana/genética , Proteínas de la Membrana/metabolismo , Mutación Missense , Oximas/metabolismo , Reguladores del Crecimiento de las Plantas/metabolismo , Plantas Modificadas Genéticamente , Plantones/genética , Plantones/metabolismoRESUMEN
Modifying lignin composition and structure is a key strategy to increase plant cell wall digestibility for biofuel production. Disruption of the genes encoding both cinnamyl alcohol dehydrogenases (CADs), including CADC and CADD, in Arabidopsis thaliana results in the atypical incorporation of hydroxycinnamaldehydes into lignin. Another strategy to change lignin composition is downregulation or overexpression of ferulate 5-hydroxylase (F5H), which results in lignins enriched in guaiacyl or syringyl units, respectively. Here, we combined these approaches to generate plants enriched in coniferaldehyde-derived lignin units or lignins derived primarily from sinapaldehyde. The cadc cadd and ferulic acid hydroxylase1 (fah1) cadc cadd plants are similar in growth to wild-type plants even though their lignin compositions are drastically altered. In contrast, disruption of CAD in the F5H-overexpressing background results in dwarfism. The dwarfed phenotype observed in these plants does not appear to be related to collapsed xylem, a hallmark of many other lignin-deficient dwarf mutants. cadc cadd, fah1 cadc cadd, and cadd F5H-overexpressing plants have increased enzyme-catalyzed cell wall digestibility. Given that these CAD-deficient plants have similar total lignin contents and only differ in the amounts of hydroxycinnamaldehyde monomer incorporation, these results suggest that hydroxycinnamaldehyde content is a more important determinant of digestibility than lignin content.