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》杂志3月23日消息，科学家近日的发现使得遗传学家哑然失色。科学家们发现植物可以把自身遗传的父系遗传密码改写为祖系遗传密码。
这项新发现推翻了教科书中的一项经典遗传定律，即子系植物只是简单地继承和综合此植物父系的遗传密码。这条定律是由十九世纪的奥地利孟德尔在种植豌豆植物时总结出来的。
此项研究结果表明，并不是所有动植物中的基因都被表现出来。植物与许多其它有机体，包括人类，都有可能拥有一种备份机制。这种机制可以让动植物绕开由父系遗传的不健康的基因，取而代之的是祖系中健康的遗传基因。
此项发现是由美国普渡大学的罗伯特与他的同事得出的。他们在一次偶然的机会中发现水芹可以携带两份基因。在水芹的变异实验过程中，当父系水芹的花瓣及其花器凝聚时，只有部分子系水芹出现正常的凝聚。
按照经典遗传学定律，这些继承父系遗传基因的水芹，应当显现出与父系水芹相同的特征，即花瓣及其花器出现凝聚。但是，罗伯特的研究小组发现仍然有10% 的子系水芹开出了非凝聚的花朵。
此小组对这些开出非凝聚花朵的水芹进行遗传序列分析后发现，这些水芹拥有两组遗传基因，其中一组基因被他们命名为HOTHEAD，此组基因可以替代那些出现异常的父系基因序列。当父系基因出现异常时，HOTHEAD就自动发挥作用，使得植物转而继承祖系的正常基因。
此研究小组对其它大量的植物进行同样的实验后发现，这些植物基本都拥有此种备份机制，以帮助它们维持正常的基因序列。罗伯特说：“这是一项伟大的发现。”
Weigel是德国Tübingen市“Max Planck发展生物学研究院”的研究员。Weigel说：“此项发现确实让遗传学家感到头疼和不解，这种备份机制以前从未被发现过。”
洛杉矶加利福尼亚大学古生物博物馆的遗传学家Jacobsen感叹道，它真是太神秘了！
罗伯特及其小组正在努力研究，以解释这些植物是怎样改写其自身的遗传基因。他们的研究方法是从祖系植物中取得一份基因样本，然后让这些基因传到父系以及子系植物中去。
有一种可能性是这些植物在自身基因序列外某个地方存有额外的基因副本。但是这似乎又不太可能，因为此小组发现这些植物所改写的基因序列中，并没有在其它地方找到与其相似的基因。
据罗伯特推测，这些植物有可能存有未被发现的核糖核酸分子，这些核糖核酸分子作用于出现异常的脱氧核糖核酸。这些分子随着脱氧核糖核酸一起被传到下一代中，并起到改写异常基因的作用。Weigel也同意他的观点，他说道这个解释似乎比较可行。
罗伯特推测说，这种基因改写机制在正常情况下极少发生。只有当植物在特殊的环境中生长时，这种机制才有可能被激活。
的确,这项机制可以使得植物在恶劣的情况下，例如水或营养及其缺乏时,仍然能够生存。当出现恶劣环境时，此项机制就被激发，使得植物转而继承其祖系的遗传基因。为了证实此项推测，罗伯特准备将这些植物放置在恶劣的环境中，以观察他们是否出现与推测一致的现象。
人类也极可能有类似的机制。此推测是源于有些孩子继承家族中的遗传疾病后，仅仅显现出轻微的症状,或许他们基因中的一部分已经被这种机制修复。
罗伯特建议道，如果人类真的可以正确使用此种机制，那么遗传学家和医生就应更彻底的研究此种机制的作用过程。他们也许能识别出那些执行改写异常基因的核糖核酸分子，并且把它们使用在病人身上。
但是现在，罗伯特及其研究小组正受到遗传学界大量的置疑。Weigel说道，他并不认为这是动植物的固有机制，这只不过是他们研究时出错所造成的。
