Navigation Links
Pollen cells keep memory to control jumping genes

In any living organism, all cells have the same DNA, but each cell's identity is defined by the combination of genes that are turned on or off, any given moment in time. In animals, this cellular memory is erased between generations, so that the new egg has no memory and, as such, has the potential to become any type of cell. In flowering plants, on the contrary, cellular memory passes from generation to generation, with potentially harmful implications for the development of new plants. In the latest issue of the journal Cell*, scientists from Instituto Gulbenkian de Cincia (IGC), in Portugal, and Cold Spring Harbor Laboratory (CSHL), in the USA, describe a novel mechanism whereby potentially mutagenic sequences of mobile DNA are silenced in the pollen grain and in seeds, thus avoiding damage to new plants.

One of the main mechanisms that contributes to cellular memory is the addition of a chemical group - the methyl group - to DNA sequences (a process called methylation). DNA methylation turns a gene off. These changes in gene expression that are heritable, but not directly written in the DNA sequence, are called epigenetics. Using the plant model Arabidopsis thaliana, Jrg Becker, Jos Feij and their team, at the IGC, and Robert Martienssen and colleagues, at CSHL, analysed the genome of pollen grains and their precursor cells, the microspores, and pinpointed the sequences of DNA that were methylated. Pollen grains contain two sperm cells (the sexual cells) and an accompanying vegetative nucleus, whose DNA is not passed on to the next generation. Thanks to the technique developed by the IGC team, the researchers were able to separate the two sperm cells and the vegetative nucleus of the pollen grain and look at their methylation status separately.

Joseph Calarco (in the Martienssen lab) and Filipe Borges (in the Becker lab) observed that DNA methylation is largely maintained in the microspores and pollen grains. But there are differences between the different cell types. In the pollen grain, some DNA sequences are methylated in sperm cells but not in the vegetative nucleus, and vice versa. Amongst these non-methylated genes are mobile sequences of DNA, called transposable elements, which could become active and lead to mutagenic effects.

The research team discovered that the situation is rescued by small sequences of RNA (called siRNAs) that restore methylation of transposable elements in the embryo. Indeed, they have found siRNA in sperm cells that silence the transposable elements even before fertilisation, in at least some cases. Transposable elements are very common in all known genomes. In the human genome, for example, they make up 45% of the total genome. They are involved in the evolution of genomes, since when integrated back into the genome they can affect the function and organisation of other genes. However, transposable elements are mutagens, and, therefore, their activation needs to be under tight control, as it may be harmful to the cell and the organism. If such harmful mutations occur in sexual cells, they will be transmitted to the progeny and spread in the population.

Says Jrg Becker, 'We have unveiled a mechanism in the sexual cells that can prevent the activation of potentially harmful transposable elements, while at the same time, upon fusion of sperm cell and egg cell, allowing the formation of a cell with full capacity to become any cell type, that will give rise to a new generation. On the other hand, if female siRNAs in the egg cell do not match incoming transposable elements from the male, they might escape silencing in the developing embryo, with potentially harmful implications for the new plant that is generated. Such an uncontrolled activation of transposable elements might at least in part explain existing hybridization barriers, in which crosses between species result in seed abortion or infertility. Breaking such barriers would increase plant breeder's chances to improve crop species by making use of the phenomenon of hybrid vigor, in which the offspring shows qualities superior to its parents, well exemplified in widely used corn and rice hybrids.'

It was known that flowering plants are an exception to the rule of resetting cellular memory, since modifications may be inherited for hundreds of generations. But the extent to which this happened in the plant sexual cells and how the epigenetic reprogramming of the genome might contribute remained unclear until now. The mechanism now described may also become a strong argument to explain why sexual reproduction evolved and became so prevalent in most higher organisms.


Contact: Ana Mena
Instituto Gulbenkian de Ciencia

Related biology news :

1. Pollen can protect mahogany from extinction
2. Precision motion tracking -- thousands of cells at a time
3. Study in mice discovers injection of heat-generating cells reduces belly fat
4. UCLA researchers discover missing link between stem cells and immune system
5. Keep your distance! Why cells and organelles dont get stuck
6. A new look at proteins in living cells
7. Astrocytes control the generation of new neurons from neural stem cells
8. CYCLOPS genes may serve as an Achilles heel in tumor cells
9. Stem cells may prevent post-injury arthritis
10. Anti-aging elixir for solar cells
11. Embryonic blood vessels that make blood stem cells can also make beating heart muscles
Post Your Comments:
Related Image:
Pollen cells keep memory to control jumping genes
(Date:6/22/2016)... American College of Medical Genetics and Genomics was once again ... of the fastest-growing trade shows during the Fastest 50 Awards ... Las Vegas . Winners are ... of the following categories: net square feet of paid exhibit ... 2015 ACMG Annual Meeting was ranked 23 out of 50 ...
(Date:6/16/2016)... The global Biometric ... USD 1.83 billion by 2024, according to a ... proliferation and increasing demand in commercial buildings, consumer ... the market growth.      (Logo: ... of advanced multimodal techniques for biometric authentication and ...
(Date:6/3/2016)... , June 3, 2016 ... von Nepal hat ... Lieferung hochsicherer geprägter Kennzeichen, einschließlich Personalisierung, Registrierung ... in der Produktion und Implementierung von Identitätsmanagementlösungen. ... Ausschreibung im Januar teilgenommen, aber Decatur wurde ...
Breaking Biology News(10 mins):
(Date:6/27/2016)... ... 27, 2016 , ... Parallel 6 , the leading software as a ... Reach Virtual Patient Encounter CONSULT module which enables both audio and video telemedicine ... team. , Using the CONSULT module, patients and physicians can schedule a face to ...
(Date:6/24/2016)... June 24, 2016  Regular discussions on a range of ... between the two entities said Poloz. Speaking at ... Ottawa , he pointed to the country,s inflation target, ... government. "In certain ... institutions have common economic goals, why not sit down and ...
(Date:6/23/2016)... ... June 23, 2016 , ... UAS LifeSciences, ... launch of their brand, UP4™ Probiotics, into Target stores nationwide. The company, which ... to add Target to its list of well-respected retailers. This list includes such ...
(Date:6/23/2016)... ... June 23, 2016 , ... Charm ... Mold) microbial test has received AOAC Research Institute approval 061601. , “This is ... last year,” stated Bob Salter, Vice President of Regulatory and Industrial Affairs. “The ...
Breaking Biology Technology: