Persimmons are among the small club of plants with separate sexes --individual trees are either male or female. Now scientists at theUniversity of California, Davis, and Kyoto University in Japan havediscovered how sex is determined in a species of persimmon,potentially opening up new possibilities in plant breeding. The workis published Oct. 31 in the journal Science.
Most plants have both male and female sex organs in the sameindividual. Some, like tomato, rice, beans and other cultivatedspecies, cast pollen from male to female organs in the same flower.Others employ ingenious schemes to ensure that one individualpollinates the flower of another. Only about 5 percent of plantspecies have separate sexes, a condition called dioecy, or "twohouses."
"Think of it as nature's best trick to ensure that reproductioninvolves two individuals, thus maximizing the mixing of genes.Persimmon, pistachio, wild grapevine, kiwi, hops, spinach and evenmarijuana are dioecious," said Luca Comai, professor of plant biologyat UC Davis and senior author on the paper.
In mammals, sex is determined by X and Y chromosomes: Males have an Xand a Y; females have two X's. A single gene on the Y is responsiblefor triggering the development of male traits. Most dioecious plantsresemble the human system, with XY males and XX females. What genemay be responsible for determining plant sex has been a long-standingmystery.
Takashi Akagi, Isabelle Henry and Comai at UC Davis worked on afamily of persimmon trees (Diospyros lotus) established by RyutaroTao at Kyoto University. They combed through the genomes of some ofthese trees looking for genes that were exclusive to males and foundan unusual gene they called OGI (Japanese for male tree). Unlike mostgenes, OGI does not encode a protein, they found. Instead, it codesfor a very small piece of RNA that acts as "molecular scissors,"cutting down expression of another gene, called MeGI (Japanese forfemale tree).
Most flowering plants combine male and female in the same individual, but a few, including persimmons, have separate sexes. Male persimmon trees have XY sex chromosomes, while female trees have XX chromosomes. Scientists at UC Davis and Kyoto University discovered that a gene called OGI on the Y chromosome specifies the production of a small RNA that targets a feminizing gene called MEGI. OGI suppresses MEGI to rescue anther fertility. Without OGI, the male organs in the flower are incapacitated by excess MEGI. So, after substantial molecular squabbling, separate sexes and, presumably, 'tree love' emerge.
(Photo Credit: Luca Comai, UC Davis)
In females, MeGI builds to high level and acts like a neuteringagent, repressing pollen formation. In males, OGI preventsaccumulation of MeGI. Regulation by RNA scissors can be fickle, andthis may help explain why plants that are genetically one sex butfunctionally another can arise in dioecious species.
Discovery of the OGI-MeGI system in persimmon provides a comparisonfor parallel studies in other dioecious plant species, Comai said.
"Because separate sexes evolved independently many times in plants,we can effectively replay the evolutionary game and ask whetherplants invent different solutions to the same problem or whether thesame regulatory system is recruited over and over," he said.
The findings may also have practical applications.
"Separate sexes are the most effective way to produce plant hybrids,and hybrids are key to agricultural productivity," said Henry. "Inthe future, we may be able to breed dioecy into new species andfacilitate hybrid production through exploitation of a naturalsystem."
Source: University of California - Davis