Functional screening of willow alleles in A rabidopsis combined with QTL mapping in willow ( S alix ) identifies S x MAX 4 as a coppicing response gene

Summary Willows ( Salix spp.) are important biomass crops due to their ability to grow rapidly with low fertilizer inputs and ease of cultivation in short‐rotation coppice cycles. They are relatively undomesticated and highly diverse, but functional testing to identify useful allelic variation is time‐consuming in trees and transformation is not yet possible in willow. A rabidopsis is heralded as a model plant from which knowledge can be transferred to advance the improvement of less tractable species. Here, knowledge and methodologies from A rabidopsis were successfully used to identify a gene influencing stem number in coppiced willows, a complex trait of key biological and industrial relevance. The strigolactone‐related More AX illary growth ( MAX ) genes were considered candidates due to their role in shoot branching. We previously demonstrated that willow and A rabidopsis show similar response to strigolactone and that transformation rescue of A rabidopsis max mutants with willow genes could be used to detect allelic differences. Here, this approach was used to screen 45 S x MAX 1, S x MAX 2 , S x MAX 3 and S x MAX 4 alleles cloned from 15 parents of 11 mapping populations varying in shoot‐branching traits. Single‐nucleotide polymorphism ( SNP ) frequencies were locus dependent, ranging from 29.2 to 74.3 polymorphic sites per kb. S x MAX alleles were 98%–99% conserved at the amino acid level, but different protein products varying in their ability to rescue A rabidopsis max mutants were identified. One poor rescuing allele, S x MAX 4 D , segregated in a willow mapping population where its presence was associated with increased shoot resprouting after coppicing and colocated with a QTL for this trait.