We review current evidence for the presence and activity in plants of
several paradigmatic components of transmembrane signal transduction s
ystems. Components considered include the second messengers calcium, i
nositol 1,4,5-trisphosphate, and cyclic AMP; protein kinases and prote
in phosphatases; and G-proteins. At the current stage of development o
f the field of plant signal transduction, broad similarities between p
lant and the more well-studied animal systems are apparent. However, t
here also exist considerable differences in detail.