We all remember how at the end of Star Wars: Attack of the Clones, the sinister motive behind producing the clones is revealed. How would you feel to know that clones are a part of our everyday life? Some of the fruits you have been having for breakfast, the avocado and olives that you love to add to your salad and sandwiches, and even the magnificent orchid plant by your window are all produced by clonal propagation.
Clonal or vegetative propagation is a form of asexual reproduction in plants wherein a new plant arises without the formation of seeds or spores. Thus, they are genetically identical to the parent and form a clonal colony. In nature, it is a trait more commonly found in perennial plants that live for many years and have the ability to undergo flowering several times during their life. It is the Plan B, in case the plant is not able to produce enough seeds in each season due to injury or harsh environment. It facilitates the growth of a new plant from a vegetative organ such as stem, root, or leaf. Thus, the plant makes sure that during the next flowering season, it can make up for the loss with clones.
Perennial plants can propagate by producing clones through specialized structures like runners in strawberry, adventitious buds in poplar, suckers in rose, bulbs in potato and orchid. The production of adventitious roots on stem cuttings or branches is one of the ways to reproduce vegetatively. The genus Bryophyllum is known for adventitious seedlings that are produced along the margin of the leaves.
Clonal propagation requires dedifferentiation of cells into meristematic cells, which are the plant cells that can later re-differentiate into shoot and root. It has been an enigma for decades as to how plants can so easily reset all the biological processes while it is a major hurdle faced during cloning of animals.
In the plant breeding industry, vegetative reproduction has been used to since long to fix agriculturally valuable characteristics. For instance, the olive cultivars have been selected based on its taste or oil produced. Different cultivars of olive can take anywhere between 3-12 years to produce fruits. Clonal propagation helps reduce the time required to have plants which can reproduce. This is a highly sought after feature by breeders, especially in perennial plants. It also helps increase survival rate since the mortality rate of seedlings is higher in nature.
Several ways that are adopted to propagate plants commercially are based on natural propagation methods, which have been adjusted to increase plant yield. These include grafting, cuttings, layering, leaf cuttings, suckers, and tissue culture. Several plants can be propagated in more than one way. For example, grapes can be propagated by stem cuttings, grafting and air layering.
Cuttings are the most common technique used for propagation. A stem cutting is usually a piece of the stem with a leaf, which generate roots when planted on soil and might require hormonal treatment. Grapes, blackberries, fig, tea, jasmine, and plum are some of the common crops propagated by stem cuttings. Leaf cuttings are in a similar way used to multiply ornamental plants in the genus Bryophyllum and Sansevieria. Another form of vegetative propagation which uses cuttings is grafting. Although grafting is not a very commonly found propagation method in nature, horticulturists have often exploited it. In grafting, plant parts from different plants are joined together to get desirable characteristics. This could be done by joining the shoot to the root or another shoot as often seen in apple or cherry blossoms.
Strawberries are propagated by modified stems called stolons or runners – present close to the soil surface – which produce roots and buds giving rise to a new plant. In contrast, plants such as asparagus, ginger and turmeric are grown using rhizomes – modified roots that produce shoots above ground. Layering is another form of propagation in which an intact stem is induced to produce roots. In this case, the intact branch is either pinned to the ground or encased in rooting medium. Once the branch starts to root, it can be cut out from the mother plant. Bonsai plants are propagated using layering. Suckering plants like apple produce plants from the buds at the bottom of the trunk.
The selection pressure due to diverse environmental conditions followed by domestication of plants has led to large number of crops being competent to undergo clonal propagation. It is of more importance now that there are more mouths to feed and less land space to grow food. This might be one of the ways to fight world hunger but the little knowledge that we have on the fundamentals of this process is a major hurdle that we need to overcome. The exciting aspect is that these methods do not require special equipment or techniques, and thus need very low investments. There is an indestructible forest in Utah comprising of aspen trees which are around 80000 years old. Even after a forest fire, the unaffected roots sprout shoots which take over the old shoot. Considering the minimal manpower and money required, this is the cheapest and fastest way to promote reforestation. For once, perhaps we should let the world be taken over by clones and let them save us.