पौधो की प्रजनन तरीकों मी बदलाव से पैँदावर बढेगी

Tampering with the sex life of plants for high yields

Desirable traits: Qualities such as high yield, pest resistance, more beautiful flowers are sought while crossing plants. Hybrid plants that produce high yields of grains, fruits and cash crops are of great value. Breeding them on a sustained basis has been a challenge since the hybrid ‘vigour’ dulls over time.

In this connection, Dr. Imran Siddiqi and associates of the Centre for Cellular & Molecular Biology, Hyderabad have published a landmark paper in the 28 February 2008 issue of Nature.

Sustained production

Here they describe a key step towards the technology of sustained production of high yielding plants. This involves a genetic switching step that alters the sex life of the plant.

How many ways are there to produce a child? The answer depends on whom you ask. Many single cell organisms like bacteria have a ready, self-sufficient way of doing so.

They simply make a copy of their entire genetic content, the genome, and divide into two cells. In turn, each daughter divides into two, and the doubling goes on and on. Soon, from a single parent cell, millions and billons of cells are produced. This is a time-tested process called mitosis and has been going on for billions of years.

Since this mode of reproduction is from a single product, and as exact copies of it (well, almost exact), are made, the offspring are clones of the parent and the process is termed clonal expansion.

In multicellular organisms, cells not only divide as exact replicas, but also differentiate to produce other, different cell types. These differentiated cells, in turn, also clonally expand producing tissues and organs.

Clonal reproduction

Clonal reproduction is efficient but occasionally error-prone. When such copying error occurs, the information content in the DNA is compromised. Over time, organisms have evolved to contain proof-reading, editing and correcting mechanisms in their cells.

DNA-repair genes are contained endogenously within the cells. Yet, despite this, if the error stays and is copied into daughter cells, we have mutations.

When a mutation, per chance, offers a trait to the organism to withstand any environmental stress, it becomes beneficial and allows the mutant cells the advantage to survive better than the normal ones.

This is how drug-resistant bacteria evolve. On the other hand, mutations might occur which weaken a cell in some manner. If the genetic defect were to be carried over through generations, we then have a not so healthy family tree of this organism. It would thus be good to have other modes of gaining new and useful genes, and of reproduction.

If the same question is posed to mammals like us, the answer is: sex, where two parents, a female and a male, are needed to make children. Besides the 200-odd cell types that a female has, she prepares a special type called the egg cell or the ovum.

Genetic material

This contains not all, but half of her genetic material. On his part, the male does likewise, packaging half his genetic material in a set of cells called the sperm.

The sperm cell meets the egg cell and fertilizes it to produce the embryo. The embryo develops in time and becomes the baby. The genes of the father and of the mother are thus passed on to the offspring.

Sexual reproduction of this kind has a great advantage over the clonal one. Brand new genes, and thus variety, are introduced. New traits and new possibilities arise.

And the female (and the male) has (in principle) the opportunity of choosing the partner to mate with, in order to produce healthy babies.

Gene mixing is the biological advantage of sex. But the cell biology involved is more complex than simple mitosis.

The splitting of the parent’s genetic content into making the germ cells (egg and sperm) first and then joining through fertilization involves two steps — meiosis and recombination.

Many plants too have taken this path. In the pistil of flowering plants are the egg cells (called ovules). The stamen is the male reproductive organ, which has anthers. Anther contains pollen, which are the male germ cells.

When pollen gets into the pistil and enters the ovule, fertilization happens. The result is the embryo, contained within seed. The surrounding cells of the ovule also divide to form the seed coat that nourishes the embryo within.

Desired qualities

Just as we would (if we could) choose our mates with desired qualities and traits, we choose, while farming, to cross plants with desired qualities in their offspring.

These qualities are high yield, pest resistance, better fruits, more beautiful flowers and so on. In agriculture and major food crops, such well-chosen crosses lead to high yielding hybrids. The Green Revolution is the result of such an exercise.

But farmers face a problem. Once they have a good hybrid, they wish to keep its genes and propagate them by self-fertilizing the hybrid, so that the cultivars have superior yields than the parental inbred lines

Reshuffling

Sadly though, this hybrid vigour decreases with each generation of hybrid self crossing. The copies of the different genes in the hybrid separate from each other during germ cell production, and get reshuffled in each succeeding generation. As a result we need to cross the parental lines anew each time — not a satisfactory situation.

But there is way out. This comes from a peculiar property of many plants (and some animals such as insects and fish). Some plants such as dandelion, some berries and grasses side-step the meiosis process altogether and their seeds reproduce clonally, asexually.

The entire gene pool of the mother is passed on straight to the daughter seed. This curious, but exciting, property is referred to as apomixis, the first step of which is called apomeiosis (the apo-referring to the missing of a component).

While berries and grasses do this, major crop plants do not; they reproduce sexually. If only they could be made apomictic, we could retain hybrid vigour forever, since no reshuffling of genes as in sexual reproduction kind would occur.

If we understand the genetic and cell biological basis behind apomeiosis, we could perhaps send wheat, rice and corn along the apomixis path and produce high yielding hybrid seeds.

Important issue

What are the genes controlling apomixis? It is this important issue that has been elegantly addressed and identified by Dr. Imran Siddiqi and co-workers, using the sexually reproducing plant Arabidopsis.

They show that alteration in the gene called DYAD leads to apomeiosis, sending the plant into the asexual mode. The DYAD gene normally regulates the organisation of chromosomes during meosis. Plants with mutation in DYAD, however, give rise to seeds that contain the full set of genes from the parent.

A single gene, whose function is known, can lead to a plant becoming apomictic, when its function is tampered with! This is truly a path-finding discovery and we should applaud the group for this breakthrough.

Imran, the soft-spoken and low-key aesthete, stoically adds: “(our) results provide impetus to search for additional genes which can be used in combination with DYAD to bring about apomixis in food crops, a major goal for agriculture biotechnology worldwide.” Tathastu!