Seed germination of ethylene perception mutants of tomato and Arabidopsis

The involvement of ethylene in determining the time to radicle protrusion was investigated in ethylene-insensitive gain-of-function (GOF) receptor mutants in tomato and Arabidopsis, as well as in single and double loss-of-function (LOF) receptor mutants in Arabidopsis. Because ethylene evolution from seeds is coincident with radicle protrusion, and the ability to convert 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene is diagnostic for seed vigour, it was hypothesized that ethylene-insensitive mutants would require more time to complete germination compared to wild-type seeds. Mutant Never Ripe (Nr) tomato seeds from two genetic backgrounds refuted this hypothesis, while experiments with wild-type seeds, treated with the ethylene action inhibitors, 2,5-norbornadiene or silver thiosulphate, supported it. However, reciprocal crosses between wild-type and Nr demonstrated that ethylene insensitivity during seed development determined subsequent time to complete germination, rather than the ability of the embryo/endosperm to perceive ethylene in the mature seed during germination. Additionally, seed quality, determined by standard vigour tests, was reduced in Nr compared to wild-type seeds, establishing a disconnection between rapid completion of germination and seed vigour. In Arabidopsis, all ethylene-insensitive GOF, and five of six single LOF mutants, required more time to complete 50% radicle protrusion, while double LOF mutants required the same, or less, time to complete germination compared to wild-type seeds. These findings support a role for ethylene perception in determining the length of time Arabidopsis seeds remain in the lag phase prior to radicle protrusion.