How Do Plants Synthesise Hormones Without An Endocrine System?

Phytohormones

Like human beings, plants perceive external stimuli, following which chemical messengers or hormones are released in the body known as phytohormones. The reaction of plants or sensory responses towards stimuli depends on these hormones. These chemical messengers are of various types and thus regulate different processes or pathways in the body. The types of phytohormones are auxins, cytokinins, gibberellins, ethylene, and abscisic acid. Each type has its own particular role and is thus important in the successful life cycle of any plant.

Plant Phytohormones direct the physiological processes occurring in plants. These hormones perform the primary and secondary plant functions either directly or indirectly. The growth of stem, root, and leaves, fruit ripening, apical dominance, protection from pathogens, senescence, and other vital processes are all regulated by phytohormones.

Since hormones regulate the biological reactions and the products, their deficiency could interrupt the pathways and produce a diseased state. A slight disbalance in the secretion of hormones causes major changes in the biological system.

Hormone Synthesis In Plants

Unlike humans and other animals, hormone secretion in plants is not via the endocrine system. Yet, the presence of the proper hormone concentrations regulates every process, from metabolism to plant growth and development. The synthesis of phytohormones occurs in the cells of different parts, like roots, stems, and leaves.

Plants can synthesise the hormones locally, or they may be synthesised at one site and transported to the target site. Unlike the transportation of hormones via the bloodstream in animals, the transportation of the hormone to the target site is done via the vascular tissues, namely the xylem and phloem, in plants. We will now study the biosynthesis of each hormone one by one in the next section.

Biosynthesis Of Phytohormones

Auxin

This hormone, also known as indole-3-acetic acid (IAA), is mainly associated with the physiology of plant growth by influencing the division and growth of plant tissues. The growth of the shoot is mainly regulated by IAA, and hence the biosynthesis of this hormone occurs in the shoot apical meristems, new leaves, and growing fruits and seeds. The most probable precursors for IAA synthesis are thought to be tryptophan dependent pathways.

Gibberellins

These phytohormones are associated with plant height as they regulate stem growth. So, these are used to induce height and large plants. They are synthesised via terpenoid pathways, and the common sites are leaves with non-mature chloroplasts. The enzymes involved in the production of gibberellins are found in the proplastids; therefore, leaves with mature chloroplasts will be unable to secrete them.

Abscisic Acid

This hormone mainly regulates the growth and development of the plant during stress. This means it regulates the growth of plants under stress by suppressing the growth of certain parts, like seed and bud dormancy. Due to this, it is known as the stress hormone. The sites of biosynthesis of this stress hormone are chloroplasts and other plastids.

Cytokinin

It regulates the growth of roots and shoots by promoting cell division, differentiation, and growth in these regions. Due to this, apical dominance and senescence are delayed. Generally, roots are the main synthesis sites; however, these are not the only sites. Cytokinin is isoprene, and the most common precursor for its synthesis is mevalonic acid or pyruvate.

Ethylene

This plant hormone regulates both growth and inhibition and thus can be called a multifunctional hormone. Fruit ripening, senescence, flower blooming, and closing are some of the functions controlled by ethylene. The biosynthesis of ethylene and abscisic acid is somewhat dependent on each other. Due to their multifunction, they are synthesised in almost every region, like leaves, fruits, seeds, flowers, roots, and more, depending on the requirement.

Externally Supplied Phytohormones

Agriculture plays a crucial role in the management of food production across the planet. This is done in large quantities, and hence, quick growth and development of plants are necessary. It is also required so that the plants exhibit appropriate stress responses, protection from predators, and other unique abilities for a higher yield. For this reason, phytohormones are used commercially by farmers to improve the quality of plants, fruits, and vegetables obtained from them and enhance the yield.

The use of phytohormones is also done because the plants may sometimes secrete hormones in deficient quantities, which may alter the plant’s growth and development. To prevent diseases or any physiological disorder, farmers use phytohormones in agriculture. This also increases the life span of perishable products obtained from agriculture.

Consequently, phytohormones are hormones found in plants that perform similarly to animal hormones, despite the fact that the latter is produced by glands. Plants don’t have any glands or an endocrine system. The secretion of phytohormones occurs in different regions of plants depending on the requirement for hormones. They can also be supplied by farmers to enhance the overall health of plants and crop yields.