BAP vs NAA vs IBA: A Beginner’s Guide to Plant Growth Regulators
Plant growth regulators, often shortened to PGRs, are compounds that influence how plant cells grow and develop. In tissue culture, very small changes in PGR concentration can influence whether an explant produces shoots, roots, callus or very little growth at all.
Three commonly used PGRs are BAP, NAA and IBA.
Understanding their roles makes tissue culture media much easier to understand.
BAP
BAP, also known as 6-benzylaminopurine or benzyladenine, belongs to a group of plant hormones called cytokinins. Cytokinins are strongly associated with cell division and shoot development.
In tissue culture, BAP is commonly used to encourage:
- activation of dormant buds
- formation of new shoots
- multiplication of existing shoots
- reduced apical dominance
When the goal is to produce several shoots from a single explant, a cytokinin such as BAP is often included in the multiplication medium. Too much cytokinin, however, can produce undesirable effects such as very compact growth, poorly elongated shoots or hyperhydricity.
More BAP is therefore not automatically better.
NAA
NAA, or 1-naphthaleneacetic acid, is a synthetic auxin.
Auxins influence processes such as:
- cell expansion
- root initiation
- callus formation
- vascular development
- interactions between roots and shoots
In tissue culture, NAA is frequently used at low concentrations alongside a cytokinin. The balance between auxin and cytokinin can influence the developmental direction of the culture. A medium containing relatively more cytokinin may favour shoot production, while increasing the auxin influence can shift the response toward callus or root development. The exact response varies significantly between species.
IBA
IBA, or indole-3-butyric acid, is another auxin. It is especially well known for its ability to promote root initiation. For this reason, IBA is commonly associated with the rooting stage of tissue culture.
Once shoots have multiplied sufficiently, they can be transferred to a medium with a lower cytokinin level and a rooting-oriented auxin environment. This encourages the formation of a root system before acclimation outside the culture vessel.
Why the auxin-to-cytokinin ratio matters
PGRs rarely act in isolation. What often matters most is the relationship between different hormones.
A simplified way to think about it is:
Higher cytokinin influence → shoot formation
Balanced auxin and cytokinin → callus or mixed development
Higher auxin influence → root formation
Real plant responses are more complicated than this simplified model, but it is a useful starting point. Different genera, species and even individual cultivars can respond differently to the same PGR combination.
Establishment, multiplication and rooting
A typical tissue culture process may use different hormone environments during different stages.
Establishment
The goal is to obtain a clean, surviving culture. PGR concentrations are often moderate because aggressive multiplication is not yet the priority.
Multiplication
The goal is to increase the number of shoots. Cytokinins such as BAP are commonly important during this stage.
Rooting
The goal shifts from shoot production to root development. Auxins such as IBA are frequently used during this stage.
Why experimentation is still necessary
There is no universal PGR recipe that works optimally for every plant. A Monstera, Alocasia, orchid and woody plant may all respond differently. Even two cultivars of the same species can behave differently. Good tissue culture therefore combines established protocols with careful observation and controlled experimentation. Change one variable at a time whenever possible and keep records of the results. That is how a general protocol gradually becomes a reliable species-specific protocol.