The Biologist Is In: Domesticating Garlic Mustard

Tuesday, March 4, 2014

Domesticating Garlic Mustard

Garlic Mustard (Aliara petiolate) is considered a noxious invasive weed throughout North America. Very few animals find the garlicky taste palatable (deer will preferentially browse on native plants), so it spreads unchecked and can take over in areas where it gets a foothold. It was brought to this continent by European immigrants, who planted it in their gardens for use as a potherb (imagine boiled spinach).

The plant is entirely edible, from root to flower. Analysis has shown it to be high in vitamin C, fiber, and other nutrients. Garlic Mustard does contain cyanogenic compounds and so should be cooked to break down these compounds if it is consumed routinely. This trait puts it in the good company of lima beans, cassava, and flaxseed.

As long as you don't routinely eat it raw in large quantities, you shouldn't have a problem with the cyanide levels you would be exposed to. Remember, the dose makes the poison.

The Lettuce (Lactuca scariola/serriola) and Endive (Cichorium intybus) we enjoy as delectable salad greens were once spiny, bitter weeds. These plants have been shaped by our desires and have hitched their evolutionary destiny to our own. This process is referred to as 'domestication' and can happen with or without the conscious action of the people growing the plants.

The idea of domesticating random plants amuses me. Garlic Mustard already makes a decent vegetable and I suspect that with some work, it could be made into an excellent vegetable.

A key requirement for breeding a new variety of a plant is having some genetic diversity to work with. To get this starting diversity in Garlic Mustard, I could wait a few hundred years in cryostasis while my henchmen scour the planet looking for some interesting variations of the plant… or I could induce a bunch of mutations and be done with this step in a few years.

I've decided to go with the quick option.

'Kinnow' -> 'KinnowLS'

Agricultural research labs routinely use various chemicals, particle radiation, or x-rays to induce mutations in plants. They then screen the mutated results for improvements in some characteristic to make the plant more useful/tasty/nutritious/etc. The process is known generally as 'mutation breeding' and has been widely used since the 1970s. The seedless orange in the figure at right was generated by mutating the seeded orange, then screening through the mutated progeny for the desired improvements.

I don't feel like dealing with aggressively mutagenic chemicals and I don't have ready access to a particle accelerator or an X-ray generating machine, but I do have ready access to another form of mutagenic radiation. Ultraviolet (UV) light is the component of sunlight which is responsible for giving us a sun-burn. The thymine bases in our DNA absorbs the UV light and becomes chemically altered. The alterations can result in base mismatches or strand breaks as the excess energy from the UV dissipates. Shortwave UV, or UV-C, is particularly good at damaging DNA and can be produced by commercially available bactericidal-UV fluorescent light bulbs.

Garlic Mustard seeds are small enough that sufficient UV-C light should get to the embryo and cause the mutations I'm looking for. There's no risk to me, as long as I don't get exposed to the UV-C bulbs, so it is a major plus over X-rays or mutagenic chemicals.

After acquiring some UV-C lamps and setting up a light-tight enclosure (so I don't sunburn my eyes/etc.), I will have to determine the radiation dosage needed to successfully mutate the seeds. A reasonable method is to irradiate batches of seeds for different times, then compare their germination to an un-irradiated control. The dosage corresponding to a 50% reduction in germination will give me the most bang for my buck… producing lots of mutations, but still giving me lots of viable seeds to work with.

Damaged DNA by itself is not a mutation. The damaged DNA has to be repaired with some new change to count. A method to encourage the repair of the damaged DNA is to wake up the seeds by soaking them in water a few days before mutating them, allowing them to restart their paused metabolism. This should result in a higher survival rate with more actual mutations, rather than dead seeds with shredded DNA.

Any dominant-effect mutations will be apparent the first generation after mutation (M1), but many mutations are likely to be recessive and will only become apparent in later generations (M2, etc.).

The first thing to consider relates to the current problems caused by the plant. It is highly invasive, spreading seeds everywhere and taking over wild-lands. An ideal garden vegetable would be a bit more polite and stay where we want it. How would we select for a more polite weed?

A primary difference between wild and domesticated plants is that wild plants drop their seed (referred to as 'shattering') and domesticated plants retain them until we intervene. This single trait would go a long way to convert the noxious, invasive weed into a polite garden inhabitant.

Fortunately, all I need to do to get this trait is to break the existing system the plant uses to drop its seeds. Since I'm going with a mutagenic approach, which essentially is breaking things randomly, I can expect to break this system in some plants. They key detail is to actively screen the M1 and M2 plants for modifications to the shattering trait and then to only work with their descendants. This will keep my potentially interesting garden plants from spreading and taking over wild lands like the wild Garlic Mustard plant.

At this stage of the project, I will have to develop some idea of what my breeding goals are.

Food quality related traits are likely to be easy to examine and select for. Larger roots/leaves/flowers? More delicate, sweeter leaves? Making the plant better for something that people already use it for will produce it something that someone will want to use. People already use Garlic Mustard for a wide range of recipes, so there are many potential directions.