Plant Growth and Hormones

Plant growth regulators also called as phytohormones are the plant hormones that help in the various processes of plant-like flowering, ripening of fruit etc. They are secreted in the root, shoot and include all part of the plant body and any tissue is able to release and any tissue is able to accept them and work accordingly. These are auxin, gibberellin, cytokinesis etc.

Chlorophyll molecules are light trapping green pigments in plants. They are made up of a cyclic tetrapyrrole head and a long phytol tail attached to the head. One atom of magnesium is directly coordinated to the four pyrrole rings in the cyclic tetrapyrrole head. Thus, magnesium is an important structural component of chlorophyll molecules. The iron is an essential micronutrient for plants. It is required for synthesis of chlorophyll and has several other functions also. Plants also require an exposure to light in order to synthesize chlorophyll.  Apparently because in the absence of light chlorophylls are not required at all; trapping of light being there chief function. In absence of light plants will be thin and pale. Such plants are called etiolated plants.

Light and temperature are important for the life of plants. This paves the way for the importance of photoperiodism and vernalization in flowering plants.

Plant organs can detect a change in their orientation with respect of gravity because atleast they have to be kept for a week. When it is kept for a week, slowly changes in orientation of gravity could be sensed. This sensation is shown by them as they posses several tropic movements. Geotropism makes the changes in the plant. Shoots are negatively geotropic and roots are positively geotropic.

Short day plants require the long duration of the dark period. If the dark period is interrupted with a brief exposure to red light (660 nm), the flowering will not take place. The inhibitory effect of red light on flowering during the critical dark period in short day plants can be overcome by a subsequent exposure to far-red rays. This is due to the fact that when exposed to far-red light, P$ _fr$ is converted to P$ _r$.

1. Extrinsic factors are those factors like light, water, temperature, availability of gases is the abiotic or environmental factors that control the growth of a plant. These are not controlled by the plant.
   
2. Intrinsic factors like growth regulators are released by the plant and are in direct control under the plant. They regulate process like cell differentiation, cell division etc.
   

• Plant growth regulators function as chemical messengers for intercellular communication. There are currently five recognized groups of plant hormones: auxins, gibberellins, cytokinins, abscisic acid ($ABA$) and ethylene.
• Indole-3-acetic acid ($IAA$, $3-IAA$) is the most common, naturally occurring plant growth regulator of the auxin class. 
  
• $2IP$ (Isopentenyl adenine) is a naturally occurring cytokinin that regulates cell division, development, and nutrient processing in plants.
• Zinc ($Zn$) is not a plant growth regulator. It is one of the eight essential micronutrients. It is needed by plants in small amounts, but yet crucial to plant development.
• So, the correct answer is '$IAA, 2IP, Zn$'.

When the tip of the seedlings was covered with agar gel and is placed on seedling, The hormones which were later found to be auxins move from base to the apex. Thus the growth of the seedling was in the direction of more concentration of agar. As they were kept in dark, there was no light for growth so the seedlings have taken agar gel as the source of growth. While in those seedlings whose tips were again placed back on seedling or drop of water was placed on top of each seedling, growth does not occur as there is no medium for growth.

• Mutation breeding is the process of exposing seeds to certain chemicals and radiation in order to generate mutants with desirable traits to bred with other cultivators. This will change the base sequence of genes of crop plants. In mung bean, resistance to yellow mosaic virus and powdery mildew were induced by mutations.
  
• Recombinant DNA technology is the process of joining DNA molecules together from 2 different species that are inserted into a host organism to produce new genetic recombinations.
• Transgenic mechanism is the mechanism of random integration of foreign DNA in transgenic animals.
• Gene therapy is a therapeutic process of delivery of nucleic acid into a patient's cell as a drug to treat a disease.

Photomorphogenesis, particularly the phytochrome-mediated response, is most sensitive to red (660 nm) and far-red (730 nm) light. Far-red light (730 nm) is highly effective at converting Pfr back to Pr, influencing germination and growth.

Morphogenesis is a process that controls the organized spatial distribution of cells during embryonic development. It can also take place in a matured organism in cell culture or in tumor cell masses. It is concerned with the shape of tissues. Plant growth regulators such as gibberellins, cytokinins, abscisic acid and ethylene control growth and development in plants. Gibberellins and cytokinins are the two instrumental hormones that play a key role in morphogenesis. Whereas auxins stimulate the plant cell to grow long.

Ethylene helps fruits ripen by making them softer, through the breakdown components of the cell walls, and sweeter, through the conversion of starches to sugars. Unlike the other plant hormones, ethylene is actually a gas and is distributed through the air, not through the plant body. One of the coolest things about ethylene is that it is released in a positive feedback loop, a little bit of ethylene causes more to be released, which causes even more to be released, and so on. A benefit of this fact is that you can take an unripe fruit (a pear, plum, or peach, for example) and put it in a paper bag with riper fruit (bananas work well for this) and ethylene will accumulate, making the unripe fruit soft and sweet. Indoleacetic acid, gibberellic acid and kinetin does not triggers the ripening process.