Polyamine-Induced Rapid Root Abscission in Azolla pinnata

Floating ferns of the genus Azolla detach their roots under stress conditions, a unique adaptive response termed rapid root abscission. We found that Azolla pinnata plants exhibited dose-dependent rapid root abscission in response to the polyamines spermidine and spermine after a substantial time lag (>20？min). The duration of the time lag decreased in response to high pH and high temperature whereas high light intensity increased the time lag and markedly lowered the rate of abscission. The oxidation products of polyamines, 1,3-diaminopropane, β-alanine and hydrogen peroxide all failed to initiate root abscission, and hydroxyethyl hydrazine, an inhibitor of polyamine oxidase, did not inhibit spermine-induced root abscission. Exposure of A. pinnata to the polyamines did not result in detectable release of NO and did not affect nitrite-dependent NO production. The finding of polyamine-induced rapid root abscission provides a facile assay for further study of the mode of action of polyamines in plant stress responses. 1. Introduction Polyamines (PAs) are small positively charged aliphatic molecules ubiquitous in almost all life forms. In plants, spermine (Spm), spermidine (Spd), and their precursor putrescine (Put) are the major PAs present in cells at micromolar-to-millimolar concentrations [1]. PAs have been implicated in a wide range of life processes in plants including seed germination, growth, floral initiation, floral development, pathogen defenses, and environmental stress responses [2–4]. Following the classic report of Richards and Coleman [5] on PA accumulation in potassium starved leaves, many investigations have demonstrated the physiological relevance of PAs in response to diverse environmental stresses, including heavy metal stress, SO2 pollution, osmotic stress, chilling stress, drought stress, pH stress, nutritional stress, biotic stress, and heat stress (reviewed by [4, 6–8]). Although the explicit physiological role of the increase in PA remains obscure, exogenous addition of PAs to plants under stress conditions has been reported to alleviate stress damage or to increase tolerance to adverse environments [6, 8]. A recent study suggested crosstalk with or direct involvement of the similarly multifunctional molecule nitric oxide (NO) in a PA-mediated response [9]. Regardless of the fact that underlying mechanism of its action remains unclear, it is evident that PAs are an integral part of plant stress responses. Plants, owing to their sessile nature, are compelled to endure stress due to perpetual environmental changes. They do so