Brown-midrib genes in maize and their efficiency in dairy cow feeding. Perspectives for breeding improved silage maize targeting gene modifications in the monolignol and p-hydroxycinnamate pathways

Maize silage is widely used and comprises the largest part of roughage in dairy cows diets. However, the biological conversion of cell wall carbohydrates into fermentable sugars by rumen microorganisms is hindered by their embedding with lignins, as well as by p-ydroxycinnamic acid cross-linkages between wall components. The use of mutants inducing high cell wall digestibility, such as brown-midrib mutants and especially bm3, is a relevant alternative strategy in breeding silage maize for feeding value, somehow easier than stacking numerous favorable genomic locations with weaker effects. To date, six brown-midrib mutations have been described, but feeding value experiments with dairy cattle have mostly involved the bm3 mutation, which had appeared until now the most promising. A synthesis of 36 published articles on dairy cow feeding with regular and bm3 maize silages, allowing 43 and 42 comparisons for intake and milk production, respectively, highlighted an average extra intake by 1.16 kg per cow per day and an average extra milk production by 1.26 kg per cow per day for cows fed diets based on bm3 silages. The primary benefit of the bm3 mutation in dairy cattle feeding is thus an increased silage intake, allowing lower giving of costly concentrates in animal diets. In addition to genes inducing the brown-midrib phenotype, including the recently identified bm5 and bm6 mutations, a survey of genes involved in secondary wall assembly pointed out several putative targets to be considered in the monolignol and ferulate pathways, as well as several of their upstream regulators. Searches for mutants can be based on the transposon-tagging strategy, but also through gene deregulation, and especially gene editing with the development of the CRISPR/Cas9 technologies. The greater understanding of cell assembly and secondary wall lignification in maize together with the identification of a set of key-genes and their spatio-temporal regulations will allow moving from a limited number of brown-midrib mutants to a larger panel of genetic resources for silage maize improvement, with reduced negative side-effects.