Polyamines: small molecules triggering pathways in plant growth and development.

Plant biologists fall into two categories: those who believe polyamines play an important role in plant growth and development, and those who are skeptical. The latter group by far exceeds the former. Why is this? Over the years polyamines have been implicated in being involved in a wide array of processes in plants, ranging from triggering organogenesis to protecting against stress. However, the problem has been that a particular response or a developmental event has generally been correlated only with changes in polyamine levels and spectra. The question of how direct or indirect the effect of polyamines is has remained open to debate. Although parallels are often drawn with animal systems in which polyamines have been linked with cell proliferation, their exact role has yet to be firmly established. It is no surprise that a formative review with the provocative title "Do Polyamines Have Roles in Plant Development?" concluded that although this was indeed likely, there were no definitive supporting conclusions (Evans and Malmberg, 1989). Since 1989, however, severa1 important advances have been made in plant polyamine research. Most of the genes encoding polyamine biosynthetic enzymes have been isolated, and antibodies to some are available. Moreover, transgenic plants, or mutants changed in polyamine metabolism, have been created. Thus, it would seem an opportune time to reconsider the issue of what role polyamines play in plant development.