Trichomes at the Base of the Petal Are Regulated by the Same Transcription Factors as Cotton Seed Fibers.

Many polypetalous plants have a constriction at the base of the petal that leaves a small gap that can provide entry into the young flower bud before the reproductive organs are fully developed. In cotton (Gossypium hirsutum L.), this gap is occluded by tufts of short unicellular trichomes superficially resembling the fibers found on cotton seeds. We are just beginning to understand the developmental regulation of the seed fibers and have previously characterized several MIXTA-like MYB transcription factors (TFs) that are critical for correct seed fiber development but know little about the molecular regulation of other types of cotton trichomes. Here, using RNAi or dominant suppression transgenic cotton lines and natural fiber mutants, we investigated the development and regulation of the petal base trichomes. Petal base trichomes and seed trichomes were also examined across several different species within and outside of the Malvoideae. We found that the petal base trichomes are regulated by the same MYB TFs as cotton seed fibers and, since they are more widely distributed across different taxa than the seed fibers, could have preceded them in the evolution of these important textile fibers produced by some cotton species.

Trichomes control flower bud shape by linking together young petals.

Trichomes are widespread in plants and develop from surface cells on different tissues(1). They have many forms and functions, from defensive spines to physical barriers that trap layers of air to insulate against desiccation, but there is growing evidence that trichomes can also have developmental roles in regulating flower structure(2,3). We report here that the trichomes on petals of cotton, Gossypium hirsutum L., are essential for correct flower bud shape through a mechanical entanglement of the trichomes on adjacent petals that anchor the edges to counter the opposing force generated by asymmetric expansion of overlapping petals. Silencing a master regulator of petal trichomes, GhMYB-MIXTA-Like10 (GhMYBML10), by RNA interference (RNAi) suppressed petal trichome growth and resulted in flower buds forming into abnormal corkscrew shapes that exposed developing anthers and stigmas to desiccation damage. Artificially gluing petal edges together could partially restore correct bud shape and fertility. Such petal 'Velcro' is present in other Malvaceae and perhaps more broadly in other plant families, although it is not ubiquitous. This mechanism for physical association between separate organs to regulate flower shape and function is different from the usual organ shape control(4) exerted through cell-to-cell communication and differential cell expansion within floral tissues(5,6).