Salicylic acid

Although it acts as a painkiller for humans, salicylic acid in plants also serves as a defensive mechanism similar to jasmonic acid. It is responsible for defending against biotrophic pathogens and responding to abiotic stress, such as drought and extreme temperatures.

It is also a crucial hormone in plant innate immunity and disease resistance.

See also:

Abscisic acid

Auxin

Cytokinin