Mixed-linkage glucan is the main carbohydrate source and starch is an alternative source during Brachypodium grain germination

Authors: FRANCIN-ALLAMI, MATHILDE - Inrae; BOUDER, AXELLE - Inrae; GEAIRON, AUDREY - Inrae; ALVARADO, CAMILLE - Inrae; LE-BOT, LUCIE - Inrae; DANIEL, SYLVIANE - Inrae; SHAO, MINGQIN - Department Of Energy Joint Genome; L Chingcuanco, Debbie; VOGEL, JOHN - Department Of Energy Joint Genome; GUILLON, FABIENNE - Inrae; BONNIN, ESTELLE - Inrae; SAULNIER, LUC - Inrae; SIBOUT, RICHARD - Inrae

Submitted to: International Journal of Molecular Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2023
Publication Date: 4/6/2023
Citation: Francin-Allami, M., Bouder, A., Geairon, A., Alvarado, C., Le-Bot, L., Daniel, S., Shao, M., Chingcuanco, D.L., Vogel, J.P., Guillon, F., Bonnin, E., Saulnier, L., Sibout, R. 2023. Mixed-linkage glucan is the main carbohydrate source and starch is an alternative source during Brachypodium grain germination. International Journal of Molecular Sciences. 24(7). Article 6821. https://doi.org/10.3390/ijms24076821.
DOI: https://doi.org/10.3390/ijms24076821

Interpretive Summary: Grains of cereals such as wheat, maize or sorghum, and most wild grasses have high starch contents (30–70%) and relatively low mixed-linkage (1,3 ;1,4) ß-glucan (MLG) contents (generally <2%). Barley and oat have the highest MLG content among cereal grains (4–10%, w/w) and the lower starch content. In the same way, the grain of the wild grass Brachypodium distachyon (Brachypodium), commonly used as grass plant model, has been shown to have a particularly high MLG content (45%, w/w) and a very low starch content (6% of grain weight). Brachypodium endosperm cell walls are very much thicker compare to other grass species. These findings lead to suggest that as well as having a structural role, MLG may supplement starch as a storage carbohydrate and may be mobilised during germination. By using mutants targeting MLG biosynthesis cellulase synthase-like gene families (cslf), the disruption of the CSLF6 gene led to the total loss of MLG in grains and an increase of starch granules available for germination. Brachypodium cslf6 mutants are able to adapt their metabolism to the lack of MLG by modifying the energy source for germination and the expression of genes dedicated to their use.

Technical Abstract: Seeds of the model grass Brachypodium distachyon are unusual because they contain very little starch and high levels of mixed-linkage glucan (MLG) accumulated in thick cell walls. It was suggested that MLG might supplement starch as a storage carbohydrate and may be mobilised during germination. In this work, we observed massive degradation of MLG during germination in both endosperm and nucellar epidermis. The enzymes responsible for the MLG degradation were identified in germinated grains and characterized using heterologous expression. By using mutants targeting MLG biosynthesis genes, we showed that the expression level of genes coding for MLG and starch-degrading enzymes was modified in the germinated grains of knocked-out cslf6 mutants depleted in MLG but with higher starch content. Our results suggest a substrate-dependent regulation of the storage sugars during germination. These overall results demonstrated the function of MLG as the main carbohydrate source during germination of Brachypodium grain. More astonishingly, cslf6 Brachypodium mutants are able to adapt their metabolism to the lack of MLG by modifying the energy source for germination and the expression of genes dedicated for its use.