Hormonal control of seedling growth & Gene Expression,

Hormonal control of seedling growth

In many seeds, the presence of embryo is necessary for, or for increase, in breakdown of products in the storage tissue; it shows that some stimulating factor is supplied by the embryo. Researches on barley seeds indicate that Gibberellic acid, a growth hormone, is involved in this process. 

The embryo of seeds of cereal grains is surrounded by food reserves present in the endosperm. The endosperm is surrounded by a thin layer of living cells, high in protein content, called the aleuron layer. At the time of germination, seeds absorb moisture and the aleuron cells become active. They provide the hydrolytic enzymes that digest the starch, proteins and RNA and some cell wall materials present in the endosperm cells. These smaller soluble compounds produced are then used by the embryo and it starts to develop into seedling. These enzymes are β-amylase (already present in the aleuron layer) α- amylase (synthesized when germination begins), ribonucleases and various proteases (some are synthesized upon germination).

The developing embryo provides a gibberellins GA3, which induces the increase in content of  α-amylase and proteases in the aleuron layer, & activates β -amylase. Actually, gibberllic acid act by causing formerly repressed genes to become active and new enzymes are synthesized.

Gibberlic acid GA3, increases the rate of production of certain DNA molecules, essential for growth. These DNA molecules are required for cell division and cell elongation.

Some scientists have suggested that gibberellins act by promoting auxin production. As the action of gibberllic acid upon gene activation in endosperm is similar to the auxins. Both auxins, gibberellins along with cytokinins stimulate cell division. The relative concentration of auxin and cytokinins together decide the type of tissue to be differentiated.     

The growth of the plant is a function of cell - wall formation. To achieve a certain form, the walls usually grow at various rates, but still in pace of adjoining cells. So cells divide, differentiate and form various types of meristems and there by tissues. They all together form a seedling.

Gene Expression -

Any plant cell is capable of reproducing an entire plant. Because the genetic make up of all the cells is identical, and so each cell possess the same genetic information in the form of genes. All the cellular activities are direct products of genes and of the enzymes produced under their control.

           

Then how is it possible that the cells arising from the same initials can have different shapes and perform different functions? Why do not all cells with the same genetic constitution possess identical amounts of identical enzymes at all times?

           

Let us consider the example of a single cell zygote which develops into a complete organism. The control of development might be primarily internal, through information within the zygote. Here, the DNA is activated in order to get ready for division. DNA replicates in the presence of enzyme DNA polymerase, and by the help of RNA polymerase, it forms mRNA, which then codes for proteins. So at any given time, not all the genes are functional and only a few express themselves through mRNA, which can produce specific enzymes. These specific enzymes catalyze the specific reactions. There are two basic ways in which a cell can control its production of a given substance -

1) A cell may change the effectiveness by which given enzyme can act,

2) It may alter the number of molecules of enzymes present to catalyze these reactions.

Gibberellins particularly GA3, is mainly responsible for enzyme production at the time of germination. Studies indicate that it stimulates the synthesis of enzyme α-amylase, which is necessary for starch utilization. The stimulation of enzyme synthesis would be blocked by adding inhibitors of protein synthesis. Inhibitors of RNA synthesis such as actinomycin D also prevent the GA3 stimulation. Abscissic acid is also effective. So, Gibberellins act by causing formerly repressed genes to become active, and form a new m RNA, molecules, and then new enzymes responsible for digesting and releasing products of the endosperm. Similar effect is known to be caused by auxins before they increase cell elongation. Both auxins and gibberellins along with cytokinins influence the activity of repressor proteins through allosteric transitions.