Polyamines in plant stress  Polyamines (PAs) are small, positively charged, organic molecules that are ubiquitous in all living organisms. These are considered as one of the oldest group of substances known in biochemistry. There are three common types of polyamines, putrescine, spermidine, hermospermine according to structure, universal distribution in all cellular compartments, and presumed involvement in physiological activities. Polyamine is found in all cellular compartments and physiological activities due to their simple structures. The function of polyamine is very diverse in that it performs a key macromolecule to cellular membrane. Because of their essential roles in plant, mutation of polyamines can cause critical damage on plants. Furthermore, some polyamines like putrescine inhibit biosynthetic activities in plants. The activity of polyamines can be categorized to some parts due to its signalling and growing activity. Polyamines, which are the key compounds in plant physiology, play essential role in cell proliferation, growth and differentiation. They also carry out significant effects on embryo/fetus proliferation, implantation, embryonic diapause, placentation, angiogenesis and fetal development. Even though polyamines carry out important roles like soothing and ameliorating in rice physiology,  its catabolic products can cause stress damage. Putrescine, a kind of polyamine, decreaseS phytochelatin synthesis at enzymatic and gene expression levels by increasing cadmium toxicity in rice. Polyamines provide protection against heavy metals such as inhibiting Cadmium uptake or its entry into the cells because exogenous polyamines are mainly allocated to the apoplast. Spermidine and spermine, kinds of polyamine, are found in reducing cadmium toxicity in rice. Nevertheless, putrescine leads to increasing hydrogen peroxide accumulation and inhibits root elongation in rice. In this case, huge amounts of polyamine-cadmium complexes form, so higher density of cadmium and putrescine decreases phytochelatin synthesis as well as its gene expression. Therefore, plants with decreasing phytochelatin activities can struggle with detoxification of metal ions like Zn, Cu, and Ni.  polyamines are compounds that are involved in a complex signaling system and have a key role in the regulation of stress tolerance. In the past, however, the main role of polyamines was originally assumed to be as direct protective compounds important under stress conditions because of their cationic nature (-) at physiological pH.