Plant Hormones!

         All plants need specific hormones to grow healthy, function and most importantly reach stable homeostasis. One type of hormone that is crucial for plant growth and development is called Auxins. 

Auxins were the first of plant hormones to be discovered. These hormones were found to stimulate cell elongation, cell division in the cambium and in combination with cytokinins in tissue culture. It also stimulates differentiation of xylem from phloem. They also Stimulates root initiation on stem cuttings and lateral root development in tissue culture and mediates the tropistic response of bending in response to gravity and light. The auxin supply from the apical bud suppresses growth of lateral buds. Auxins delay leaf senescence, can inhibit or promote (via ethylene stimulation) leaf and fruit abscission and can induce fruit setting and growth in some plants. Auxin can delay fruit ripening and also stimulates growth of flower parts. All these functions would not be possible without Auxins that are released to reach homeostasis within the plant. 

Above describes the effect of auxin on strawberry development. The achenes produce auxin. When removed the strawberry does not develop. 

Another type of hormone is called Abscisic Acid. Unlike animals, plants cannot flee from potentially harmful conditions such as droughts, freezing, flooding and exposure to salinated soil. Plants must adapt or die. Abscisic Acid is the major player in mediating the adaptation of the plants to stress due to environmental conditions. ABA signaling turns on the expression of genes encoding proteins that protect cells - in seeds as well as in vegetative tissues-from damage when they become dehydrated. This hormone can also stimulate root growth in plants that need to increase their ability to extract water from the soil. ABA mediates the conversion of the apical meristem into a dormant bud. The newly developing leaves growing above the meristem become converted into stiff bud scales that wrap the meristem closely and will protect it from mechanical damage and drying out during the winter.

ABA in the bud also acts to enforce dormancy so if an unseasonably warm spell occurs before winter is over, the buds will not sprout prematurely. Only after a prolonged period of cold or the lengthening days of spring (phototropism) will bud dormancy be lifted. Seeds are not only important agents of reproduction and dispersal, but they are also essential to the survival of annual and biennial plants. These angiosperms die after flowering and seed formation is complete. ABA is essential for seed maturation and also enforces a period of seed dormancy. As we saw for buds, it is important the seeds not germinate prematurely during unseasonably mild conditions prior to the onset of winter or a dry season. ABA in the seed enforces this dormancy. Not until the seed has been exposed to a prolonged cold spell and/or sufficient water to support germination is dormancy lifted.ABA also promotes abscission of leaves and fruits (in contrast to auxin, which inhibits abscission). ABA also inhibits stem elongation. ABA helps the plant adapt to its environment and reach homeostasis.  

     Another important plant hormone is called Ethylene. Ethylene differs from other hormones in being a gas. In plants that produce fruit, ethylene is released when the fruit is approaching maturity. Ethylene promotes the ripening of the fruit. The presence of ethylene is detected by transmembrane receptors in the endoplasmic reticulum (ER) of the cells. Binding of ethylene to these receptors unleashes a signaling cascade that leads to activation of transcription factors and the turning on of gene transcription.

 Ethylene also affects many other plant functions such as absiccion of leaves, fruits, and flower petals, drooping of leaves, sprouting of potato buds, seed germination, stem elongation in rice (by promoting the breakdown of abscisic acid (ABA) and thus relieving ABA's inhibition of gibberellic acid);

flower formation in some species. Ethylene is essential in reaching plant homeostasis.