Giberelin, Ethylene, Cytokinins, Abscisic Acid, Traunmatin and Brassinosteroid

Gibberellin 

Gibberellin is a synergetic hormone together with auxin. Gibberellin affects the development and germination of embryo. It stimulates the formation of the enzyme amylase. Amylase plays a role in breaking down starch (amylum) stored in endosperm into  glucose. Glucose is energy source for growth. A dwarf plant that is given gibberellins will grow normally. 

Gibberellins also functions in formation of seeds, to stimulate pollen and flower formation, enlarge fruit size, and end seed dormancy. In low concentrations, gibberellin does not affect root formation. To induce root formation, high concentrations are necessary. 

This hormone was first isolated from the fungus Gibberella fujikuroi. Look at Figure 

• (a) The fungus Gibberella fujikuroi that produces giberrellin is found in normal rice" plants, and 
• (b) the fungus infects rice plants and causes them to grow taller. 

There are several types of gibberellin hormones: gibberellin A, gibberellin A2, and gibberellin A3, each with a specific molecular structure and function. 

Ethylene 

Ethylene functions in fruit ripening and leaf shedding. When the proportion of ethylene concentration is very high compared to auxin and gibberellin, the formation of stems, roots, and flowers are inhibited. However, equal proportions to auxin causes ethylene to stimulate flower formation. The compound exists in plants as a gas. 

Ethylene is often used by fruit distributors or importers. Fruits are harvested and packed before they ripen (ripe fruits are not distributed since they are easily spoilt during transporting). In fruit stalls, fruits are brooded over by incubating them with ethylene gas. This will speed up ripening, and the fruits can then be sold. 

Cytokinins 

Cytokinin is a hormone with a role in cell division (cytokinesis). Its functions are: 

1. to stimulate the formation of roots and stems and formatién of root branches and stems by inhibiting apical dominance; 
2. to control the growth of leaves and shoots; 
3. to enlarge the size of young leaves; 
4. to control the formation of flowers and fruits; 
5. to slow down the aging process by stimulating the process and transportation of mineral salts and amino acids to the leaves

Cytokinin was first discovered to be contained in tobacco plant and was called kinetin. The substance is formed in the root struture and transported to all tobacco plant cells. Cytokinin found in corn plants is called zeatin. 

Abscisic acid (ABA) 

Abscisic acid is an inhibitory compound that works antagonistically to auxin and gibberelin. It plays a role in the aging process and leaf falling. The hormone functions to preserve plants from harsh environmental conditions, such as drought, by dormancy. Lack of water will increase the concentration of abscisic acid in the guard cells of stomata. As a result, stomata close and reduce transpiration in order to maintain the water balance. 

Traumatin (traumatic acid) 

Traumatin is the precursor of traumatic acid, a plant hormone that plays a role in cell regeneration as a result of tissue .damage or wounding. Damaged tissue forms a callus (undifferentiated tissue) exactly on the wounded spot. 

Besides hormones, vitamins also play a role in growth and development. Examples of vitamins are riboflavin (vitamin Bu), ascorbic acid (vitamin C), thiamine (vitamin B), pyridoxine (vitamin 86), and nicotinic acid. Vitamins function in the production of hormones and being part of coenzymes (nonprotein component of active enzymes). 

Brassinosteroid 

Brassinosteroid (BR) is a class of steroid plant hormone. It was isolated from Brassica napus (Brassicaceae). Brassinosteroid is not widely known. However, many studies conducted to identity BR and to attain informations about its biochemical pathway. Some researches by Li et a1. (1996) and Szekers (1996) brought out a wider acceptance that BR is one of important plant hormone. 

1. Brassinosteroid is involved in many plant  proccesses: 
2. together with auxin, promote cell expansion and elongation; 
3. promote vascular (xylem) differentiation; 
4. inhibit the root growth and leaf abscission; 
5. play a role in plant resistancy to stress condition. 

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