This is my third science related blog post which is intended to give plant hobbyists a deeper appreciation of the precise nature of how their plants grow as well as a base of knowledge from which to better comprehend phenomena such as variegation and cresting. This post will focus on leaf variegation.
In my first blog post, "Secret life of plants 1: the Meristem", we learned that plants have dome clusters of embryonic stem cells (merstems) at their growing points that give rise to the different structures that make up a plant - leaves, stems, flowers, roots etc. These structures all develop from the edges of the mersitem domes as the meristem itself grows. The pattern for all these structures is programmed genetically in the meristem itself. Subsequently, in my second blog post entitled "Secret life of plants 2: photosynthesis", we learned that plants are green in color because they contain chlorophyll, a pigment essential to photosynthesis, the process by which plants use the energy of the sun to make sugars from carbon dioxide. Photosynthesis takes place in chloroplasts - complex compartments inside plant cells which contain the chlrorphyll - and that chloroplasts have their own genes - separate from the plants main genes - and that these genes are entirely inherited from the mother plant. Without chlorophyll, plants would lack the ability to make essential nutrients, and would thus starve and ultimately die.
In variegated plants, the meristem itself is variegated and gives rise to variegated structures.
The meristem itself is a mix of atypical albino (non-chlorophyll producing cells) and more typical green cells. The variegation usually appears as alternating stripes as in the plant below.
In other cases, one layer of the cells in the meristem may be variegated while the others are not. For example, the top meristem layer may be white and the layers underneath green, giving rise to a ghostly kind of variegation pattern with green underneath rather than in discreet stripes. The most common examples of these types of variants in the succulent world are the colorful Euphorbia lactea crests:
In Haworthia this phenomenon has been given the somewhat misleading name of "reverse variegation", as is the case in this plant.
How exactly does variegation arise? It's complicated and there are different mechanisms that can all lead to variegation. Here are some of the various reasons variegation can occur:
1) The meristem is a chimera (a mix of genetically different cells) of albino and green cells. With this kind of variegation, the plant itself is genetically normal but contains some mutated albino tissue resulting from the fact that at an early stage of development, a mutation occurs in a cell in the meristem that makes a cell albino. It could be a mutation in any number of genes required for chlorophyll production or chloroplast development. That cell divides to produce more daughter cells, a process eventually resulting in a meristem that is a mix of albino and normal cells. This mutation may arise spontaneously or be induced by mutagenesis (this means causing random mutations by using chemicals or X-rays). In general, this kind of mutation is not heritable (won't be transmitted through pollination and breeding) because the main genome of the plant is unaffected.
2) In some cases, transposons or "jumping genes" may cause variegation. The "jumping genes" may "jump" into or out of specific areas of DNA, but only in some and not all cells of the meristem, resulting in variegation. Read more here about leaf variegation caused by "jumping genes" in rice.
3) Mutations in the plant genome may occur that interfere with chlorophyll production or chloroplast development. In this case, the albino and non-albino cells are genetically identical. Studies in the mustard family plant Arabidopsis have identified several genes the mutation of which can lead to variegation. The following image of a variegated Arabidopsis plant is taken from a review paper you can access here. According to this paper, this plant has a mutation in a gene called IM which contains the blueprint for a "plastid member of the alternative oxidase (AOX) class of inner mitochondrial membrane proteins".
You can read more here about the molecular genetics of variegation in Arabidopsis. In some cases the albino tissue can change back to green, in other cases not.
4) Recall from the "Secret life of plants 2: photosynthesis" blog that chloroplasts have their own genome which is inherited maternally. Mutations in the chloroplast genome be a cause of variegation. An interesting study on this phenomenon in primrose can be found here. The following picture, taken from that paper, is a primrose plant with a mutation in the chloroplast gene which contains the blueprint for psaB, a protein involved in photosynthesis that resides in the chloroplast membrane.
Because the chloroplast is mainly inherited from the mother plant, this kind of variegation is only inherited when the variegated plant is used as the seed parent, not when it is used as a pollen donor. In my experience and in talking to others this seems to be the case with many haworthia variegates.
Regardless of the origin of the mutation I should note than variegation is often not strictly binary (either strictly albino or "normal" green). The mutations that cause variegation can lead to pale coloration so that a mix of dark green, light green, yellow and white variegation is sometimes seen. In general the more white a plant is, the more challenging it will be to care for because it will be making less sugars for food.
Because variegated plants demand such high prices these days, many people are trying to use chemicals to induce variegation. These may be mutagens, like EMS (ethyl methanesulfonate), which are highly toxic chemicals that cause random mutations and can produce chimeral meristems (or less often stable genetic mutations) that lead to variegation. Chemicals that inhibit chloroplast development or chlorophyll production are also used. There are several patent applications describing these kinds of treatments: Patent EP0257845A2 describes using chemicals that inhibit chlorophyll biosynthesis to make variegated plants. Patent application WO1999015001A1 describes the use of antibiotics (streptomycin and spectinomycin) to make variegated plants. Recall from the "Secret life of plants 2: photosynthesis" blog that chloroplasts are essentially bacteria that became trapped inside plant cells. Antibiotics that kill bacteria may also have the same effect on chloroplasts. Yet another patent -US20110126311A1- describes a GMO approach. The use of chemicals to induce variegation often results in variegation that cannot be transmitted to progeny and/or is unstable. Often plants from such treatments may be weak and unhealthy.
The approach we are using at Artisan Plants to make variegates is to use stable healthy plants with transmissible variegation to produce hybrid variegated seedlings which we will be offering in the coming years.
The next Secret life of plants blog post will be on crested and mostrose plants. Stay tuned.