ABSTRACT: Trichomes contribute to plant tolerance of abiotic stresses, such as heat, and biotic stresses, including insect herbivory. Leaves of Brassica napus are typically glabrous or near-glabrous; however, a small number of hirsute accessions were identified in international germplasm collections that produced up to 300 trichomes on the earliest leaves. To investigate the genetic basis of this trait, trichome density on the abaxial and adaxial leaf surfaces, leaf margins, and petioles of the 3rd and 6th leaves was assessed in a doubled haploid (DH) population derived from a cross between a glabrous line (NAM-0) and a hirsute line (DOS-2). Lines were genotyped using a Brassica 60K SNP BeadChip array. A major quantitative trait locus (QTL) controlling trichome density was identified on chromosome C01 that overlapped with regions previously mapped in the C-genome species B. oleracea and B. villosa. In B. napus, this QTL spanned 5.65 Mb and with a 1.7 Mb deletion in the hirsute DOS-2 parent. A gene encoding a novel, R3 single-repeat, MYB protein related to the trichome repressor TRIPTYCHON was identified within the shared region and designated TRIPTYCHON-LIKE (TRL). DOS-2 harbors a single BnTRL gene, whereas NAM-0 contains four tandemly-linked copies, although BnTRL transcript abundance did not correlate directly with trichome density. The expanded TRL gene family is unique to the Brassicaceae and BnTRL proteins physically interact with the GL1-binding domain of GL3, a core component of the GL1–GL3–TTG1 trichome initiation complex. Analysis of additional genes within the shared QTL region or absent in DOS-2 revealed regulators associated with chromatin modification (e.g. ADA2b, RBBP6-like), hormone signaling (ARR2, GH3, JAM3, HAT4), cytoskeletal dynamics (SPIRAL2, MAP65-2, MAP70-5, SPIKE1), and cell growth (SVH3, LIP5). Hormone application experiments demonstrated genotype- and tissue-specific responses on trichome production, particularly to jasmonate and gibberellin. Together, these results indicate that the CO1 locus controlling trichome abundance in B. napus is complicated and may affect regulation of chromatin state, hormonal pathways, cytoskeletal organization, and cell growth, in addition to the assembly of the trichome regulatory complex.
