Understanding the evolution of dimorphic flowers in maize through combined changes in transcriptional dynamics, hormone levels and genetic networks

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

The biological rules that govern floral development are paramount, given that reproduction is critical to evolutionary success. Although most flowering plants produce only perfect flowers (those with both male and female structures), roughly 30% of species produce imperfect flowers, which have with either male (staminate) or female (carpellate) flowers. Imperfect flowers are associated with wind-pollination and increased outcrossing. Although other agriculturally domesticated plants in the grasses (ex. rice, wheat, millet, sorghum) produce perfect flowers, maize forms separate, staminate and carpellate inflorescences on the same plant. Separate male and female inflorescences have allowed plant breeding programs to generate elite, adapted maize varieties with highly productive female inflorescences and F1 hybrids. This research grant will employ multiple levels of organization to investigate floral characteristics in maize by 1) comparing the conservation of transcriptional dynamics during the development of staminate and carpellate inflorescences in maize with the perfect inflorescences of the closely related species, sorghum, 2) characterizing new floral mutants through analysis of hormone levels and molecular biology, 3) identifying candidate genes involved in the development of perfect and imperfect flowers by integrating developmental transcriptomics with hormone-responsive genetic pathways. Knowledge gained from the work will provide important insight into how reproductive structures are generated in cereal species through the coordinated activity of genes, hormones, and developmental evolutionary processes and will allow one to predict which genes can be used for manipulating floral dimorphism in other crops. Through analysis of maize mutants with altered male and female floral characteristics, it appears that at least three hormones, JA, GA, and BR, are necessary for specification of staminate and carpellate flowers in maize, often regulating pistil abortion in male flowers. In Aim 1, a transcriptional ontogeny of developing maize ears will be compared to the ontogeny of developing tassels and sorghum panicles. Unique male and female programs will be identified. In Aim 2, two new mutants, fun and Te2, will be analyzed. They are different than previously described inflorescence mutants and may provide new clues to the evolution of separate male and female inflorescences. The tassels of both have a more female architecture than the JA, BR or microRNA tasselseed mutants. They do not encode enzymes in hormone pathways or directly involve microRNAs. FUN is a nuclear localized protein of unknown function. TE2 is an ARF transcription factor. By identifying the interacting partners of FUN and the downstream targets of TE2, the network of inflorescence determination factors will be expanded. The goal of Aim 3 is to combine the approaches of Aims 1 and 2 alongside other public datasets to formulate rules of life that coordinate the evolution of imperfect flowers. Although research expects multiple evolutionary origins of imperfect flowers, known developmental mechanisms regulating organ abortion are limited and thus common specification genetic networks may be possible. With that in mind, the rules of rule that coordinate floral dimorphism in maize may apply to other grasses and angiosperms in general. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.