BBMB Research Seminars

October 4, 2007
Robert Thornburg
Professor
Department of Biochemistry, Biophysics, and Molecular Biology
Iowa State University
"Development and function of the ornamental tobacco nectary”
1414 Molecular Biology Buidling
4:10 p.m.

Abstract:

Many angiosperms have an usual sexual habit. They utilize animals as intermediaries in the transfer of gametes between flowers. These angiosperms attract these visiting pollinators by offering a reward of metabolite-rich nectar to these visiting pollinators. Our interests lie in understanding the biochemistry, development and function of the floral organ that is the source of this metabolite-rich nectar, the nectary gland. Nectar is a rich concoction of sugars, amino acids, vitamins, metal ions and proteins. Our analysis indicates that the proteins mediate a novel biochemical pathway we call the Nectar Redox Cycle. This pathway produces extremely high levels of hydrogen peroxide to protect the gynoecium from invading microorganisms vectored to the flower by visiting pollinators. This pathway is expressed late in nectary gland development. Analysis of the Nectarin I (nec1) promoter has permitted the isolation of the transcription factor, MYB305, that binds to the nec1 promoter. RNAi knockouts of MYB305 demonstrate that this factor regulates nec1 expression. We have also examined the nectary developmental processes that lead to nectar formation. We have found that the ornamental tobacco nectaries store sugar as starch during the first eight days of floral development. Then 24 hours before anthesis, the starch is broken down to produce high levels of sugar for nectar. In addition, the sugar from starch also flows into the formation of potent antioxidants (ascorbate and ¾-carotene). These antioxidants are thought to protect the nectary and gynoecium from the high levels of hydrogen peroxide generated by the nectar redox cycle. Analysis of the genes involved in nectary development, demonstrate that starch metabolism appears to be regulated at the gene level and we have identified a major developmental event termed the metabolic shift. The result of this shift is that the starch anabolic genes are this down-regulated and coordinately starch catabolic genes are up-regulated. The mechanism of this coordinated metabolic shift is the current focus of research in the Thornburg laboratory. Surprisingly, the MYB305 knockout plants show that this factor also participates in the metabolic shift.