Size matters to a plant, but how is it controlled?

Scientists at the John Innes Centre(JIC), Norwich, UK [1] today report a discovery that explains how plants control the size and development of their cells. Published on-line by the international journal PNAS [2] the report describes how a gene (called RHL1)[3] affects a plant cell’s ability to make multiple copies of its DNA, in turn affecting cell growth and overall plant development.

“Unlike the cells of animals, plant cells typically expand to up to 1,000 times their original size as they develop” says Dr Keiko Sugimoto-Shirasu (project leader at JIC). “We don’t know a lot about how this is controlled but it is often the result of the cell making multiple copies of its DNA (so-called endoreduplication), which then stimulates this massive cell growth. Our discovery has given us an exciting insight into how plant cells actually manage all these extra copies of DNA and shown us that the process is much more sophisticated than we imagined”.

The researchers at JIC were using the common weed Thale Cress (Arabidopsis thaliana) to study the control of cell size and the endoreduplication process. They identified a mutation (called hyp7) that caused seedlings to be dwarf, because their cells did not expand normally. When they compared the damaged gene, which causes the mutation, with known genes they discovered it is a gene that is known to affect root hair production on the roots of plants – hence its name root hairless or rhl.

Hyp7 (or rhl) has similarities to other known genes from animals and bacteria. In this case the genes are known to be important in DNA replication. In fact they have a very specific role in unravelling the DNA double helix to allow it to be copied (replicated). So it seems that the hyp7/rhl gene has a key part in enabling plant cells to make multiple copies of their DNA and so is vital to cell expansion and normal development[4].

Dr Sugimoto-Shirasu concludes, “our discovery is an exciting and important piece in the much larger jigsaw of understanding how plants, which are full of complex tissues and organs, develop from the same relatively simple starting point – the cell. The more we understand about plant development the better placed we are to modify crop plants to be more productive and less dependent on chemical and other inputs”.

1. The John Innes Centre (JIC), Norwich, UK is an independent, world-leading research centre in plant and microbial sciences. The JIC has over 800 staff and students. JIC carries out high quality fundamental, strategic and applied research to understand how plants and microbes work at the molecular, cellular and genetic levels. The JIC also trains scientists and students, collaborates with many other research laboratories and communicates its science to end-users and the general public. The JIC is grant-aided by the Biotechnology and Biological Sciences Research Council.

2. PNAS – Proceedings of the National Academy of Sciences USA. Online early edition is available at http://www.pnas.org/papbyrecent.shtml.
PNAS can be contacted at 202-334-1310 or PNASNews@nas.edu

3. RHL – roothairless – is one of a family of at least 3 plant genes that are important in determining root hair formation on plant roots. RHL1 produces a protein that is targeted to the nucleus of the plant cell but its function is unknown.

4. One strategy to increase cell size is for the plant cells to amplify their chromosomal DNA content through endoreduplication cycles. Although
endoreduplication is widespread in eukaryotes, we know very little about its molecular mechanisms. Successful progression of the endoreduplication cycle in Arabidopsis requires a plant homologue of archaeal DNA topoisomerase (topo) VI.

Hyp7 (hypocotyls 7) is a dwarf Arabidopsis mutant in which various large cell types that in the wild type normally endoreduplicate multiple times complete only the first two rounds of endoreduplication and stall at 8C. It was identified from a mutant screen for dark-grown short-hypocotyl phenotypes. HYP7 encodes the RHL1 (ROOT HAIRLESS 1) protein, and sequence analysis reveals that RHL1 has similarity to the mammalian DNA topoisomerase II. RHL1 shows DNA binding activity in vitro, and we have in vivo evidence that RHL1 forms a multiprotein complex with plant topoisomerase VI. RHL1 plays an essential role in the topoisomerase VI complex to modulate its function and that plant topoisomerase II and topoisomerase VI play distinct but overlapping roles during the mitotic cell cycle and endoreduplication cycle.

It seems the RHL protein plays an essential role in successive endocycles as a component of the plant topoisomerase VI complex and enables plant cells to unravel entangled chromosomes during endocycles above 8C.