Evolution of short trichomes and long hairs on Nigella petals through co-option of bHLH and non-MIXTA MYB genes

Conical cells (CCs), trichomes, and hairs are all protrusive epidermal cells of plants, yet the differences and relationships among them remain largely unclear. Here, we show that the unicellular long hairs (LHs) and short trichomes (STs) on Nigella damascena petals differ from CCs in shape, length, number, distribution pattern, relative nuclear size, ploidy level, developmental process, and molecular basis. Specifically, NidaMIXTA, an ortholog of the famous CC/trichome identity gene in many species, is involved in CC development but does not affect STs and LHs. The identities of STs and LHs, however, are specified by genes encoding components in the MYB-bHLH-WDR (MBW) complex-GL2 module. STs, which serve as tiny pillars to prop open the upper and lower petal lips and facilitate pollinators' access to nectar, require the function of NidaMYB5-1, NidaGL3, NidaTT8, and NidaGL2, whereas the formation of LHs is determined by NidaMYB5-1/-2, NidaGL3, and NidaGL2. The evolution of STs and LHs from LH-like ancestors in the genus Nigella, therefore, was likely caused by independent co-option of the TT8 and MYB5-2 genes, respectively, followed by refining of their expression patterns.