Assemblage et dégradation des parois de maïs : de la plante entière à l'échelle cellulaire

Le maïs, l'une des plantes les plus cultivées au monde, sert de base dans l'alimentation humaine et des ruminants et pour la production d'agrocarburant. Sa valeur agronomique est principalement liée à la digestibilité des constituants de la paroi cellulaire (sucres, lignines, ...). De nombreuses études ont été menées afin d'améliorer la digestibilité de cette paroi, notamment sur la voie de biosynthèse des lignines. Au cours de ma thèse, nous avons démontré, chez le maïs, que la mutation du gène ZmCCR1 (Cinnamoyl-CoA reductase), codant pour une enzyme clé de la voie de biosynthèse des lignines, conduisait à une diminution des unités H associée à une meilleure digestibilité. La deuxième partie de mes recherches a porté sur l'identification des paramètres influençant la digestibilité à l'échelle cellulaire. En effet, il existe chez le maïs une très grande diversité de types cellulaires à paroi lignifiée non seulement entre différents organes, mais également au sein de chaque organe (sclérenchyme, parenchyme, vaisseaux de xylème...). Dans ce but, une étude combinant de multiples techniques biochimique, histologique et transcriptomique, a été réalisée sur deux lignées de digestibilité contrastée. Ainsi, nous avons mis en évidence l'importance de la proportion de tissus lignifiés, plus particulièrement du sclérenchyme périvasculaire. Au-delà, le développement de la microdissection laser (LCM) a permis de souligner la différence de composition des tissus lignifiés.Maize is one of the most widely grown crops in the world and is grown for grain for human consumption, feedstock for cattle and more recently biofuel. When used as fuel, the most important agronomic trait is digestibility which is dictated by cell wall composition and structure. Many studies have been undertaken in order to improve cell wall digestibility and have essentially focused on the lignin biosynthetic pathway. During my PhD research, we have shown that a mutation in ZmCCR1 (Cinnamoyl-CoA reductase 1), key enzyme in lignin biosynthesis, modified lignin structure which in turn resulted in an increase in digestibility. Beyond lignification per se, my studies focused on the identification of other potential parameters influencing digestibility at the cellular level. This is especially pertinent in maize since its stem biomass is made up of different several lignified tissues and cell types. By combining cell wall biochemistry on laser microdissected (LMD) lignified tissues, with histological studies, we have been able to show that in highly contrasting lines (Cm484 and F98902) the lignified cell types patterning and in particular, the amount and cell wall composition are critical factors in determining maize digestibility

Characterization of a cinnamoyl-CoA reductase 1 (CCR1) mutant in maize: effects on lignification, fibre development, and global gene expression

Cinnamoyl-CoA reductase (CCR), which catalyses the first committed step of the lignin-specific branch of monolignol biosynthesis, has been extensively characterized in dicot species, but few data are available in monocots. By screening a Mu insertional mutant collection in maize, a mutant in the CCR1 gene was isolated named Zmccr1–. In this mutant, CCR1 gene expression is reduced to 31% of the residual wild-type level. Zmccr1– exhibited enhanced digestibility without compromising plant growth and development. Lignin analysis revealed a slight decrease in lignin content and significant changes in lignin structure. p-Hydroxyphenyl units were strongly decreased and the syringyl/guaiacyl ratio was slightly increased. At the cellular level, alterations in lignin deposition were mainly observed in the walls of the sclerenchymatic fibre cells surrounding the vascular bundles. These cell walls showed little to no staining with phloroglucinol. These histochemical changes were accompanied by an increase in sclerenchyma surface area and an alteration in cell shape. In keeping with this cell type-specific phenotype, transcriptomics performed at an early stage of plant development revealed the down-regulation of genes specifically associated with fibre wall formation. To the present authors’ knowledge, this is the first functional characterization of CCR1 in a grass species

Maize cell walls synthesis and degradation (from whole plant to tissue level)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF