The Polyamine Spermine Rescues Arabidopsis from Salinity and Drought Stresses

There are accumulating reports that polyamines are involved in abiotic stress response. However, the role played by the polyamines is not fully elucidated. In the present studies, we assessed whether spermine among the polyamines plays a certain role against high salt and drought stresses using an Arabidopsis (acl5/spms) mutant plant that does not produce spermine, and found that it was hypersensitive to those stresses. In each case the hypersensitive phenotype was mitigated by application of exogenous spermine. The spermine-deficient mutant plants also showed a phenotype resembling Ca2+-deficiency. The NaCl-hypersensitivity and Ca2+-deficiency of acl5/spms double-knockout mutant resembled the phenotypes displayed by the AtGluR2- and CAX1-overexpressing transgenic plants. The two latter genes encode a glutamate receptor-type, Ca2+-ion influx channel at cytoplasmic membrane and a vacuolar Ca2+/H+ antiporter, respectively. The data suggest that regulated expression of the Ca2+-pathway members is critical to adapt to those stresses, and that spermine plays a certain role to control the stress-induced Ca2+ dynamics. Incorporating the current information from the literature, especially regarding action of polyamines on various ion channels, we present models describing a defensive role of spermine in high salt and drought stresses in Arabidopsis.