Mixed-Linkage Glucan Is the Main Carbohydrate Source and Starch Is an Alternative Source during Brachypodium Grain Germination

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.

Glycosyl transferase GT2 genes mediate the biosynthesis of an unusual (1,3;1,4)‐β‐glucan exopolysaccharide in the bacterium Sarcina ventriculi

“…MLGs have been implicated in resistance of barley cultivars to infection by the cereal cyst nematode (Aditya et al, 2015). MLGs in the grasses are also used for both long-term and short-term secondary sources of metabolizable glucose, where their synthesis and depolymerization are more direct, simpler and potentially faster in comparison to starch (Francin-Allami et al, 2023;Roulin et al, 2002;Trafford et al, 2013). Whether these factors contribute to any competitive advantage in the grasses remains to be demonstrated.…”

The plant cell wall—dynamic, strong, and adaptable—is a natural shapeshifter

“…MLG is also found as a reserve polysaccharide in the seeds of cereal grains ( Morrall and Briggs 1978 ). Use of MLG as a reserve polysaccharide has been studied in detail in the model grass Brachypodium distachyon , where it is present in thick endosperm cell walls and is mobilized during seedling germination ( Francin-Allami et al 2023 ). Although xylans are also found in seeds of some plants, the arabinoxylan from the seed husks of Plantago ovata has received the most attention ( Belorio and Gomez 2022 ).…”

Genetic Approaches to Increase Arabinoxylan and β-Glucan Content in Wheat

“…β-glucan content varies between genus, within species, between varieties, and in relation to developmental stage and growing environment [ 98 , 99 , 100 , 101 ]. It is suggested that β-glucans are not essential structural components of cells walls but rather represent a secondary source of metabolizable energy in the form of glucose [ 102 , 103 , 104 ]. Any attempt at adjusting the amount of β-glucan could therefore have a knock-on effect on the carbon pool in the plant.…”