Auxins and cytokinins are two major classes of plant growth regulators frequently discussed in propagation and tissue culture. They are sometimes reduced to “root hormone” and “shoot hormone”, but their effects depend on concentration, tissue sensitivity, developmental state and interactions with other signalling pathways.
What auxin does
Auxin is involved in cell expansion, apical dominance, vascular development, tropic responses and root initiation. In propagation, synthetic or naturally occurring auxins can promote adventitious root formation in responsive tissues.
Common rooting auxins
Indole-3-butyric acid (IBA) and 1-naphthaleneacetic acid (NAA) are widely used in propagation. Their effectiveness varies by species, tissue and application method. More auxin is not automatically better: excessive exposure can inhibit subsequent root elongation or damage sensitive tissue.
What cytokinins do
Cytokinins influence cell division, shoot development, nutrient allocation and senescence. In micropropagation they are frequently used to stimulate axillary shoot proliferation or adventitious shoot formation.
Common tissue-culture cytokinins
6-benzylaminopurine (BAP or BA) and kinetin are common examples. Different genotypes can respond very differently to the same concentration, which is one reason tissue-culture protocols usually require optimisation rather than direct transfer between species.
The auxin–cytokinin relationship
Classic tissue-culture models describe relative auxin and cytokinin activity as influencing whether cultured tissue favours roots, shoots or undifferentiated growth. Modern plant biology shows that the relationship is more complex, involving transport, receptor signalling, local biosynthesis and interactions with many other regulators.
Why concentration matters
Plant growth regulators operate at biologically active concentrations and their dose-response curves are not necessarily linear. A concentration that stimulates organogenesis in one explant may suppress growth or cause abnormal morphology in another.
Propagation without added hormones
Many cuttings already contain sufficient endogenous signalling capacity to root without exogenous auxin. Added hormone is a tool, not a requirement. Cutting maturity, carbohydrate reserves, temperature, oxygen, water status and genotype can be equally important determinants of success.
Understanding plant growth regulators as components of signalling networks—rather than simple on/off switches—leads to more reliable propagation decisions.
Related guides
Continue with our micropropagation guide or our guide to rooting biology and aroid cutting rot.