Cytokinins, Ethylene and ABA — Definition

Definition

Plant growth regulators (PGRs) are organic substances produced naturally by plants that, in low concentrations, control physiological processes. They are often referred to as plant hormones or phytohormones. This topic focuses on three key PGRs: Cytokinins, Ethylene, and Abscisic Acid (ABA), each with distinct roles in a plant's life cycle.

Cytokinins are a class of plant hormones that primarily promote cell division (cytokinesis) in plant roots and shoots. Their discovery was linked to the search for factors that stimulate cell division in plant tissue cultures.

Chemically, most natural cytokinins are derivatives of adenine, with zeatin being a well-known example found in corn kernels and coconut milk. Beyond cell division, cytokinins are crucial for breaking apical dominance, promoting the growth of lateral buds, delaying senescence (aging) in leaves, and promoting chloroplast development.

They also play a significant role in morphogenesis in tissue culture, influencing the differentiation of shoots and roots in conjunction with auxins. The balance between auxins and cytokinins is critical for determining the developmental pathway of plant cells.

Ethylene is unique among plant hormones because it is a gaseous hormone. It is produced by most plant tissues, especially in senescing tissues and ripening fruits. Ethylene's primary role is in fruit ripening, where it triggers a cascade of biochemical changes leading to softening, color change, and aroma development.

This property is exploited commercially to ripen fruits like bananas and mangoes. Besides ripening, ethylene promotes senescence and abscission (shedding) of leaves, flowers, and fruits. It also influences root growth and root hair formation, increasing the surface area for absorption.

In germinating seedlings, ethylene is responsible for the 'triple response' – inhibition of hypocotyl elongation, swelling of the hypocotyl, and exaggeration of the apical hook – which helps the seedling push through soil.

Abscisic Acid (ABA) is often called the 'stress hormone' because its production increases significantly under adverse environmental conditions such as drought, salinity, and cold stress. ABA plays a crucial role in helping plants cope with these stresses.

One of its most well-known functions is inducing the closure of stomata, thereby reducing water loss through transpiration during drought. It also promotes seed dormancy, preventing premature germination under unfavorable conditions.

This dormancy can be overcome by factors like gibberellins or specific environmental cues. ABA also promotes abscission of leaves and fruits, and bud dormancy, acting antagonistically to growth-promoting hormones like auxins and gibberellins.

Its multifaceted roles highlight its importance in plant survival and adaptation.