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Caractères biochimiques et histologiques impliqués dans les variations de digestibilité de paroi : de l’étude de leur variabilité à celle de leur déterminisme génétique. : Mise à jour des cibles majeures d’amélioration de la digestibilité dans un contexte de fort rendement chez les hybrides modernes de maïs fourrage

Abstract : This CIFRE thesis is part of the DECLIC project (New sources of cell wall degradability improvement and better understanding of degradability using cytologic approaches on lignocellulosic biomass and quantifications of ferulic and coumaric acids in maize) financed by seven maize breeding companies that are members of Promaïs (Maïsadour Semences, Caussade Semences, Euralis Semences, Bayer, KWS, RAGT Semences and Limagrain). The central objective of this thesis is to better understand how the specificities of the maize cell wall, notably the high quantities of p-hydroxycinnamic acids, the structure of the lignin and the tissue distribution of the lignin impact the cell wall digestibility. This thesis will thus provide results and tools to improve cell wall digestibility and silage quality in maize breeding programs while maintaining good yields.In the first part of this work we present the development of two high throughput tools. Since 1998, the "Unité Fourragère Laitière, UFL" value has been used as a criterion for the registration of silage maize varieties in the French catalog. The estimation of this energy value is mainly based on the dry matter digestibility estimated by the Aufrère protocol. In this work, we shed light on the key role of the gelatinization step in this protocol and propose an improved version that is faithful, reliable, high throughput and easy to implement in all analytical laboratories. The second analytical development concerns the development of an automatic method for the analysis of fasga-stained images of internode sections based on the H, S and V properties of each image pixel. The proposed method automatically segments 44 distinct tissues: 29 tissues in the rind, 10 tissues in the pith and 5 tissues in the bundles.Using these two high-throughput methods, we characterized 80 modern early forage maize hybrids provided by the private partners of the project for their digestibilities, biochemical composition and tissue distribution of their lignification. We showed that three biochemical parameters: lignin content in the cell wall, lignin structure and p-coumaroylation and three histological parameters: TM2b, TM5b and TB5 had an important role in the explanation of cell wall digestibility variations. We thus described the maize ideotype of good digestibility as a maize limiting its lignin content in the cell wall in a range already restricted by selection, favoring condensed and low p-coumaroylated lignins and maximizing the proportion of highly digestible nonlignified tissues in its pith.In a population of recombinant lines resulting from the cross between F4 (flint line of good digestibility and with a blue pith) and F7 (flint line of good digestibility and with a red pith), we studied the genetic determinism of traits related to digestibility, cell wall composition and tissue distribution of lignification. We have identified numerous QTLs as well as important clusters in selection for digestibility. Most importantly, we deciphered the mechanism of blue or red coloration of the pith which relies on 3 QTLs located on the bottom of chromosome 5, the top of chromosome 5 and chromosome 7.In a final section, we discussed all the results obtained in this thesis work and put them in relation to the issues of digestibility and yield improvement in forage maize breeding programs. In particular, we emphasized the importance and the control of blue coloration of the pith to ensure both good digestibility and good agronomic properties of future forage maize.
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Paul-Louis Lopez--Marnet. Caractères biochimiques et histologiques impliqués dans les variations de digestibilité de paroi : de l’étude de leur variabilité à celle de leur déterminisme génétique. : Mise à jour des cibles majeures d’amélioration de la digestibilité dans un contexte de fort rendement chez les hybrides modernes de maïs fourrage. Amélioration des plantes. Université Paris-Saclay, 2021. Français. ⟨NNT : 2021UPASB069⟩. ⟨tel-03544723⟩

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