Feature

Biotechnology and the art of growing elite marula trees

Dr Elisabeth Goyvaerts

The marula tree is a treasure trove, providing products from marula beer to amarula, drums to furniture, jams, pickles, cosmetics and even coffee. Despite this tree's amazing array of benefits and products, to-date no commercial plantations nor community gardens exist. All fruits are harvested from the wild by communities in the Mpumalanga and Limpopo Provinces. But this is set to change and improve with the help of a little technology.

As any farmer or breeder will tell you, the productivity of the tree, its fruit bearing capacity, is critical when developing a commercially viable business. The factors which impact significantly on a tree's performance are its genetic make-up and the environment in which the tree is grown. These are very important concerns when making a decision to cultivate, especially in the case of trees such as the marula as it takes several years of investment before the trees literally begin to bear fruit.

Genetically determining the quality of the tree

But it is not enough for the tree to 
just bear fruit; the fruit needs to be abundant and of good quality. It is here where good genes rather than good farming methods make the difference. If for example a tree has a specific character that means it bears fruit which tastes bad or bears little fruit and it is found that this poor performance is genetically determined, then manipulating the tree's environment or altering the horticultural practices will only marginally change the expressed character. In other words, an orchard with such poor performing trees would not be commercially viable.

There are several ways of examining the genetic make-up of a tree. You could for example simply compare the performance of a tree with others in the same environment. But that means you will spend a lot of time waiting for the tree to mature and begin bearing fruit before comparisons can be made. A faster way may be by directly examining the genetic material of the tree itself.

For a quick overview of DNA, enter here.

Looking for the telltale DNA markers

Examination of the DNA now enables scientists to visualise the differences between sites or loci in the DNA. Once the presence or absence of a specific DNA sequence at a specific locus (DNA marker) is linked to the presence or absence of a character, this linkage can be used as a tool. This means that seedling populations can be tested since that specific DNA sequence is present in all cells of an organism, even before the character is expressed. It also means not having to wait for the whole tree to grow. The following examples illustrate the advantages of speed offered by using molecular tools in classical breeding.

Using molecular tools, marula breeders can now combine two different desirable genetic characteristics in a new elite tree more effectively and much faster. They do this by crossing two trees which each possess only one of the characters. Seeds are harvested and then sown. When the seedlings appear, just a small piece of a leaf is required to examine the presence or absence of both the DNA markers that are linked to the desired characters. The breeder is thus able to select and plant just the seedlings with both characters long before the characters are expressed! It means that no resources are wasted evaluating trees that do not have both characters. Because capacity is freed to gather more field performance data on the trees of interest, a better-performing tree will be selected faster.

Another good way of explaining this is using the sex of a tree. Let's assume that sex of a marula tree (i.e. whether a tree is female or male) is genetically determined. A DNA marker linked to sex will enable direct examination of the seedlings. Breeders can thus determine which seedling is female and which is male. Only female trees bear fruit. In the absence of DNA molecular markers, the first visual sign that a tree is male or female will be when the tree is around 8 to 10 years old and produces its first flowers which may be male or female. Field testing of a male tree in a breeding program wastes resources. Growing an orchard of male trees will be a commercial disaster!

Elite Tree to Elite Orchard!

What happens once a breeder has produced a tree which has all of the characteristics? The ideal is to move from one tree to an orchard of trees with the same desirable traits.

Trees can propagate sexually or vegetatively. Sexual propagation enables the combination and exchange of DNA and its genetic characters whereas vegetative propagation enables multiplication of the same genotype. Having trees with uniform characters, for example, the same height, the same flowering time, makes orchard management easier and cheaper. Growing elite trees increases profit.

Vegetative propagation has traditionally been used to establish orchards of elite trees. Normally branches are taken from the elite tree and dipped in a plant growth regulator solution to promote rooting, or the branches are grafted upon a rootstock. Grafting must ensure that the wood vessels of the rootstock and the grafted branch grow together to enable upward and downward transport of water and nutrients. However, there simply may not be enough branches to propagate fast enough. The speed of propagation is limited by the number of branches available on the elite tree and it may take several years to bulk sufficient material to enable orchard planting. Propagation in tissue culture offers a faster propagation alternative.

Using a technique known as tissue culture, small parts of the tree are taken, sterilized using bleach and cultured in an artificial medium that is empirically designed for each plant type. For marula as little as a cutting of 1.5 to 3cm from a young branch can give rise to one small tree. This translates in a large increase in the number of trees of the same genotype that can be generated from one elite tree per season. It dramatically speeds up the planting of community gardens and orchards with higher-yielding trees bearing better-quality fruits.

The marula tree is soon to benefit from this technology. Watch this space!

More information:

See an earlier article on the marula tree here.

Public understanding of Biotechnology website: www.pub.ac.za 

Glossary:

DNA overview
Deoxyribonucleic acids, DNA for short, are the genetic material of a cell. Four types of DNA molecules exist as determined by the bases attached to the backbone, namely A or adenine, G or guanine, C or cytosine and T or thymine. The genetic information of a cell is contained in the particular sequence of A, G, C and T which act like a four-symbol code. The DNA sequence determines which proteins are made and when. Proteins can be structural or can possess cellular activities, e.g. to carry out reactions in biochemical pathways such as movement or biosynthesis of fats. Together these proteins build the cell and are responsible for all the cell's functions. Hence, the DNA contains all the information necessary to make a cell and an organism through the careful orchestration of the synthesis of different proteins at different times and in different cells during development.

In the cell, DNA is double-stranded and A pairs with T and G with C. Thus each DNA double strand consists of two complementary copies. When a cell divides the genetic material is duplicated by using these complementary single DNA strands as templates to make copies. Then one full set of double-stranded DNA is passed on to each of the two daughter cells. So no genetic information is lost.

The DNA is organized in a specific manner with each character being encoded at a specific location or locus. Errors and minor changes may occur in the DNA during the DNA duplication and its passage to offspring. Some of these changes impact the proteins that are being synthesized and thus the character of the cell or organism. Based on this principle, individual marula trees are a little bit different, due to subtle changes in different characters as encoded in the DNA and due to environmental effects during the realization of these characters. The genotype of a tree is the sum of all the genetic characters of that tree. 

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