黑料吃瓜资源

Argonaute 9 inhibits asexual reproduction, apparently by silencing transposons

Cold Spring Harbor, NY — One seemingly insurmountable obstacle to the dream of virtually limitless yields of staple crops like corn, wheat, and rice is the dependence of those plants on sexual reproduction.聽When male and female gametes—sperm and egg—combine randomly to generate a genetically unique seed, valuable parental traits painstakingly selected by breeders are erased.聽But what if plants like these could be engineered to reproduce asexually—an ability of a subset of plants, for example, the common dandelion?

In a paper to appear online in Nature February 7, plant geneticists at 黑料吃瓜资源 (CSHL) and the National Polytechnic Institute in Mexico report moving a step closer to the goal of turning plants that normally reproduce sexually into asexual reproducers, an outcome that would have profound implications for agriculture globally.

In sexually reproducing plants, the sex cells, or gametes, each bear one half of the organism鈥檚 genetic legacy, which is combined in their progeny, a single seed.聽These unique offspring stands in contrast to the offspring of asexual reproducers like the dandelion, which are clones of the parent—genetically identical. For years, Jean-Philippe Vielle-Calzada, Ph.D., who led the Mexican team, has sought to generate viable seeds in the absence of a fusion of sperm and egg.聽 His team experimented with a type of asexual reproduction called apomixis, using the small mustard plant Arabidopsis thaliana—normally a sexual reproducer—as a testbed.

The key technique was shutting down the activity of a聽protein called Argonaute 9.聽By doing this, the scientists tricked an Arabidopsis ovule into manufacturing multiple gametes, rather than one, as it usually does.聽Instead of carrying half the plant鈥檚 genetic legacy, these extra gametes carried a full load of genetic material.聽The plant, in some respects, had cloned itself, raising the possibility that some or perhaps all plants have the potential to reproduce asexually.聽If so, the secret of unlocking that potential, it would appear, is the ability to repress the activity of Argonaute 9.

Argonaute 9 binds small interfering RNA (siRNA), which are studied by the team at CSHL led by Rob Martienssen, Ph.D. The new results build on the discovery by Martienssen鈥檚 team last year of a similar mechanism in the male gametes of the plant, found in its pollen grains.聽They showed that siRNAs were passed from the cells immediately surrounding the gametes, called companion cells, into the adjacent sperm cells.聽The siRNAs inactivated, or silenced, specific DNA sequences in the sperm.聽Importantly, those sequences marked the position of transposons, the 鈥渏umping genes鈥� discovered by CSHL Nobel laureate Barbara McClintock 60 years ago.聽If activated, they have the ability to jump around the genome, wreaking havoc.

In fact, says Martienssen, 鈥渋t has long been speculated that transposons and sex co-evolved.聽But it has remained a mystery as to how transposons promote sexual reproduction.聽In the work we鈥檙e reporting today, it appears that Argonaute 9 inhibits asexual reproduction—apparently by silencing transposons.鈥澛燤artienssen says his lab will be pursuing the relationship between transposons and sex in collaboration with the Mexican team.

Written by: Communications Department | [email protected] | 516-367-8455

Citation

鈥淐ontrol of female gamete formation by a non-cell-autonomous small RNA pathway in Arabidopsis鈥� will appear online ahead of print in Nature on Sunday, March 7, 2010.聽The authors are: Vianey Olmedo-Monfil, No茅 Dur谩n-Figueroa, Mario Arteaga-V谩zquez, Edgar Demesa-Ar茅valo, Daphn茅 Autran, Daniel Grimanelli, Keith Slotkin, Robert A. Martienssen and Jean-Philippe Vielle-Calzada. The paper can be viewed online at: doi: 10.1038/nature08828.