Arabidopsis plants may posses a genetic backup to deal with faulty parental DNA.
Cress overturns textbook genetics
Helen Pearson
Surprise finding shows that plants rewrite genetic code.
In a discovery that has flabbergasted geneticists, researchers have shown that plants can overwrite the genetic code they inherit from their parents, and revert to that of their grandparents.
The finding challenges textbook rules of inheritance, which state that children simply receive combinations of the genes carried by their parents. The principle was famously established by Austrian monk Gregor Mendel in his nineteenth-century studies on pea plants.
The study, published this week in Nature1, shows that not all genes are so well behaved. It suggests that plants, and perhaps other organisms including humans, might possess a back-up mechanism that can bypass unhealthy sequences from their parents and revert to the healthier genetic code possessed by their grandparents or great-grandparents.
Robert Pruitt and his colleagues at Purdue University in West Lafayette, Indiana, hit upon the discovery when studying a particular strain of the cress plant Arabidopsis, which carries a mutation in both copies of a gene called HOTHEAD. In mutated plants, the petals and other flower parts are abnormally fused together.
Because these plants pass the mutant gene on to their offspring, conventional genetics dictates that they will also have fused flowers. Not so: Pruitt's team has known for some time that around 10% of the offspring have normal flowers.
Back to the future
Using genetic sequencing, the researchers showed that this second generation of plants had rewritten the DNA sequence of one or both of their HOTHEAD genes. They had replaced the abnormal code of their parents with the regular code possessed by earlier generations.
"It's really weird."
Steven Jacobsen
University of California, Los Angeles
And when the team studied numerous other genes, it found that the plants had often edited those back to their ancestral form too. "It was a huge surprise," Pruitt says.
The discovery has left geneticists reeling. "It's really quite stunning," says Detlef Weigel, who studies plant genetics at the Max Planck Institute for Developmental Biology in Tübingen, Germany. "It's a mechanism that no one had any idea existed."
And geneticist Steven Jacobsen at the University of California, Los Angeles, sums it up even more succinctly. "It's really weird," he says.
Hidden inheritance
Pruitt and other researchers are struggling to explain exactly how the plants could rewrite their genetic code. To do that, they need a template (a version of their grandparents' code) that can be passed from one generation to the next.
One possibility is that the plants use an extra copy of a gene perched elsewhere in their DNA. But this seems unlikely, because the team found that the plants can rewrite the code of genes that have no similar copies elsewhere in the genome.
Instead, Pruitt speculates that the plants carry a previously undiscovered store of the related molecule RNA, that acts as a backup copy of DNA. Such molecules could be passed into pollen or seeds along with DNA and used as a template to correct certain genes. "It's the most likely explanation," Weigel agrees.
Stressed out
Pruitt speculates that this type of gene correction goes on in Arabidopsis under normal conditions, just very rarely. He suggests that it is ramped up when the HOTHEAD gene is mutated, perhaps because the plant becomes stressed.
Indeed, the process could exist because it helps plants to survive whenever they find themselves in difficult condition, such as when water or nutrients become scarce. Such stress could trigger plants to revert to the genetic code of their ancestors, which is perhaps more hardy than that of their parents. To test this, Pruitt is examining whether stressful situations do indeed prompt the same phenomenon.
A similar process might even go on in humans. This is suggested by rare cases of children who inherit disease-causing mutations but show only mild symptoms, perhaps because some of their cells have reverted to a normal and healthier genetic code.
If humans do correct their genes in this way, Pruitt suggests that the procedure might be usefully hijacked by researchers or doctors. They might be able to identify the RNA molecules that carry out the repair and use them to correct harmful mutations in patients.
But for now, Pruitt and other researchers in the field are expecting the paper to prompt a lot of scepticism. "The immediate response is that they must have made a mistake," Weigel says, "but I don't think so."
References
1.Lolle S. J., Victor J. L., Young J. M. & Pruitt R. E. Nature, published online, doi:10.1038/nature03380 (2004).
《自然》雜志3月23日消息，科學(xué)家近日的發(fā)現(xiàn)使得遺傳學(xué)家啞然失色,?？茖W(xué)家們發(fā)現(xiàn)植物可以把自身遺傳的父系遺傳密碼改寫為祖系遺傳密碼,。
這項新發(fā)現(xiàn)推翻了教科書中的一項經(jīng)典遺傳定律,，即子系植物只是簡單地繼承和綜合此植物父系的遺傳密碼,。這條定律是由十九世紀(jì)的奧地利孟德爾在種植豌豆植物時總結(jié)出來的,。
此項研究結(jié)果表明，并不是所有動植物中的基因都被表現(xiàn)出來,。植物與許多其它有機體,，包括人類,，都有可能擁有一種備份機制,。這種機制可以讓動植物繞開由父系遺傳的不健康的基因，取而代之的是祖系中健康的遺傳基因,。
此項發(fā)現(xiàn)是由美國普渡大學(xué)的羅伯特與他的同事得出的,。他們在一次偶然的機會中發(fā)現(xiàn)水芹可以攜帶兩份基因。在水芹的變異實驗過程中,，當(dāng)父系水芹的花瓣及其花器凝聚時,，只有部分子系水芹出現(xiàn)正常的凝聚。
按照經(jīng)典遺傳學(xué)定律,，這些繼承父系遺傳基因的水芹,，應(yīng)當(dāng)顯現(xiàn)出與父系水芹相同的特征，即花瓣及其花器出現(xiàn)凝聚,。但是,，羅伯特的研究小組發(fā)現(xiàn)仍然有10% 的子系水芹開出了非凝聚的花朵。
此小組對這些開出非凝聚花朵的水芹進行遺傳序列分析后發(fā)現(xiàn),，這些水芹擁有兩組遺傳基因,，其中一組基因被他們命名為HOTHEAD，此組基因可以替代那些出現(xiàn)異常的父系基因序列,。當(dāng)父系基因出現(xiàn)異常時,，HOTHEAD就自動發(fā)揮作用，使得植物轉(zhuǎn)而繼承祖系的正?；?。
此研究小組對其它大量的植物進行同樣的實驗后發(fā)現(xiàn)，這些植物基本都擁有此種備份機制，以幫助它們維持正常的基因序列,。羅伯特說：“這是一項偉大的發(fā)現(xiàn),。”
Weigel是德國Tübingen市“Max Planck發(fā)展生物學(xué)研究院”的研究員。Weigel說：“此項發(fā)現(xiàn)確實讓遺傳學(xué)家感到頭疼和不解,，這種備份機制以前從未被發(fā)現(xiàn)過,。”
洛杉磯加利福尼亞大學(xué)古生物博物館的遺傳學(xué)家Jacobsen感嘆道，它真是太神秘了,！
羅伯特及其小組正在努力研究,，以解釋這些植物是怎樣改寫其自身的遺傳基因。他們的研究方法是從祖系植物中取得一份基因樣本,，然后讓這些基因傳到父系以及子系植物中去,。
有一種可能性是這些植物在自身基因序列外某個地方存有額外的基因副本。但是這似乎又不太可能,，因為此小組發(fā)現(xiàn)這些植物所改寫的基因序列中,，并沒有在其它地方找到與其相似的基因。
據(jù)羅伯特推測,，這些植物有可能存有未被發(fā)現(xiàn)的核糖核酸分子,，這些核糖核酸分子作用于出現(xiàn)異常的脫氧核糖核酸。這些分子隨著脫氧核糖核酸一起被傳到下一代中,，并起到改寫異?；虻淖饔谩eigel也同意他的觀點,，他說道這個解釋似乎比較可行,。
羅伯特推測說，這種基因改寫機制在正常情況下極少發(fā)生,。只有當(dāng)植物在特殊的環(huán)境中生長時,，這種機制才有可能被激活。
的確,這項機制可以使得植物在惡劣的情況下,，例如水或營養(yǎng)及其缺乏時,仍然能夠生存,。當(dāng)出現(xiàn)惡劣環(huán)境時，此項機制就被激發(fā),，使得植物轉(zhuǎn)而繼承其祖系的遺傳基因,。為了證實此項推測，羅伯特準(zhǔn)備將這些植物放置在惡劣的環(huán)境中,，以觀察他們是否出現(xiàn)與推測一致的現(xiàn)象,。
人類也極可能有類似的機制。此推測是源于有些孩子繼承家族中的遺傳疾病后,，僅僅顯現(xiàn)出輕微的癥狀,或許他們基因中的一部分已經(jīng)被這種機制修復(fù),。
羅伯特建議道,，如果人類真的可以正確使用此種機制，那么遺傳學(xué)家和醫(yī)生就應(yīng)更徹底的研究此種機制的作用過程,。他們也許能識別出那些執(zhí)行改寫異?；虻暮颂呛怂岱肿?，并且把它們使用在病人身上,。
但是現(xiàn)在，羅伯特及其研究小組正受到遺傳學(xué)界大量的置疑,。Weigel說道,，他并不認(rèn)為這是動植物的固有機制，這只不過是他們研究時出錯所造成的,。
