Interplay between nitric oxide, ethylene, and gibberellic acid regulating the release of Amaranthus retroflexus seed dormancy

A very small percentage, below 10%, of Amaranthus retroflexus seeds were germinated at 25 °C in the light and thus the seeds were considered to be primary dormant. Nitric oxide (NO) applied for only 5 h stimulated germination of these non-deep physiologically dormant seeds. Likewise, ethephon, ethylene, 1-aminocyclopropane-1-carboxylic acid (ACC) and gibberellic acid (GA₃) induced dormancy release. The stimulatory effect of NO, ACC, and GA₃ on germination was associated with increased ethylene production prior to radicle protrusion. The nitric oxide scavenger, cPTIO, antagonized the stimulatory effect of GA₃, ethephon, and ACC on seed germination, indicating that the presence of endogenous NO is required for dormancy release by these compounds. This scavenger inhibited both germination and ethylene production stimulated by NO and GA₃. An inhibitor of ethylene binding to its receptor, 2,5-norbornadiene (NBD), antagonized the beneficial effect of ethylene and NO, indicating that ethylene action is necessary for the appearance of seed response to these gases. The ACC synthase inhibitor, aminoethoxyvinylglycine (AVG), and the ACC oxidase inhibitor, α-amino-isobutyric acid (AIB), strengthened the effect of NBD on the germination of NO-pretreated seeds. Induction of germination of dormant seeds by NO, ethephon or GA₃ was associated with initiation of the cell cycle prior to radicle protrusion. The data presented indicate NO crosstalk with ethylene and GA₃ in regulating dormancy release in A. retroflexus seeds.