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Reduced vigour in clones?

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Cannabis can be propagated by taking cuttings, but eventually the vigour of the strain will diminish over successive generations; taking clones from clones over and over often results in weaker and lower yielding plants. The strain loses it’s full genetic expression over time when cloned repeatedly. What is the mechanism that causes this?

Recently “autoflowering” strains of cannabis have been introduced that have lost their photoperiodicity, and start blooming after a short period of growth, regardless of the day length. This radical change in phenotype from the normal genetic expression for cannabis is very useful, but what is at play inside the cell that we are manipulating? Scientists have discovered that the “epigenome” is where is all happens.

The expression of genetic traits in plants is much more variable than in animals; plants being sessile, have to continually adapt their growth to changing climate conditions and seasons. Whereas animals have their tissues made during the embryonic stage, plants always grow new tissue types from meristems (totipotent stem cells) that can become whatever tissue type is required, a leaf, bud, root, stem, petals or trichomes, depending on the environment.

This plasticity in plants has a genetic basis that has been studied intensely by molecular biologists, and they refer to the “epigenome” as the place where many of these growth controlling events take place. Normal growth through a cycle of vegetative development, blooming and senescence is controlled by the epigenetic biomolecules that surround and modify DNA. Responses to stress and pestilence are also under epigenetic control.

The “genome” of an organism is the DNA it inherited from its parents, the blueprint of genes that instruct cells to operate. The “epigenome” is comprised of molecules like miRNA (small RNAs), and proteins that add methyl or acetyl molecules to DNA as a way to change gene expression. These biomolecules control the expression of genes so that plant meristems can become different tissues at different times.

The molecular mechanisms controlling flowering have been shown to be epigenetic, and involve small RNA molecules called miRNA that block the function of regular RNA molecules. RNA is the “translated” readout produced from DNA, it is an epigenetic molecule and once made, RNA is then “transcribed” into proteins. The small miRNA’s block the formation of protein by attaching to the RNA molecules, silencing them; miRNA acts like a switch to turn off an “on” state or to slow down certain cellular events.

The timing of flowering is controlled by the epigenome, so if we want to alter this growth habit we must change the epigenome. An example of the research showing how we can make autoflowering plants by changing epigenetic miRNA is found here;

The epigenome can be interpreted as a molecular “memory” of plant cells and as it records what happened in the environment, it passes the epigenetic changes along with each cell division. This is what is at play when we see loss of vigour from successive cloning. Another in depth article about plant “molecular memory” is found here:

Once cannabis breeders have harnessed a fuller understanding of the epigenome, we will be able to hasten growth, increase or diminish any trait, and program plants to maintain high-vigour growth instead of losing vigour. We could develop strains that can be transformed while growing by applying a heat or cold stress, or a chemical that acts as an epigenetic enhancer, or by shining a certain wavelength of light on them. Cannabis as a species is perfectly suited for this kind of genetic engineering; it is already so highly evolved, adaptable and immediately responsive to a wide range of climates.

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