Recorded: 20 Feb 2001
This is an example of just one of the mutants we work on, it's a classical mutation called ramosa 1. It was first identified as a mutant colony in 1911 by Emerson and, in fact, when it was first identified (actually when Emerson first mapped it, I guess someone else actually found it first)... it was thought to be a different species. It was called Zea ramosa instead of Zea mays. And it turns out that it's a mutation in a single gene, which we've isolated molecularly now. What that gene does, it controls the distinction between the stem cells that make a branch, like this branch here, and the floral meristem that eventually gives rise to this kernel here.
You're probably familiar with what a typical ear of corn, corn on the cob, would look like, and of course such an ear would be a single branch, a single spike, that's covered in those kernels. In this mutation, some of these kernels, instead of differentiating into a flower, continue to have a stem cell-like function, produc[ing] these branches instead. These branches reiterate and actually make a tassel in the male flower of the plant. It is an even more dramatic effect than it is in the ear. So this is really a key mutation in that process. It turns out that this mutation—
the gene that is responsible for this mutation—is closely related to a gene in Arabidopsis that has been previously characterized called SUPERMAN.
We believe, in Arabidopsis, it has a rather more subtle phenotype actually, but its also involved in this stem cell determinacy property. So we're now looking at this gene; we're looking at it not only in maize but also throughout the grasses. It's very straightforward now to look at this gene, and variation in it, in plants like sorghum or even in plants like the rushes behind me in the harbor, the Phragmites. We've actually made some DNA from that, because the flower, the tassel, the male flower looks very much like ramosa. So this form of branching is a definitive evolutionary property of a group of grasses called panicoid grasses, which include things like millets, sorghum, as well as maize, and is really a key gene in evolution We think that it's a key regulator.
We're also studying a number of genes that control stem cell function in Arabidopsis. There's two in particular. One, we call asymmetric leaf, which we just published in Nature last December, which turns out to be a transcription factor that regulates the production of other transcription factors that are required for stem cell identity. And, in fact, the interaction between that gene and stem cell homeobox transcription factor genes is the subject of that paper. It's an interesting story that allows us to form a model of how stem cells and founder cells are really distinguished. We believe that there are actually interactions between that class of genes and the class of genes that I just described in maize and we're still following those up.
Rob Martienssen is a plant molecular geneticist and professor at Cold Spring Harbor Laboratory. He received his Ph.D. from Cambridge University in 1986 and did postdoctoral research at the University of California, Berkeley.
As a young scientist, he worked closely with Barbara McClintock. He currently studies plant epigenetics and development using functional genomics. He was awarded the Kumho International Science Award in Plant Biology and Biotechnology (2001).