Role of smoke stimulatory and inhibitory biomolecules in phytochrome-regulated seed germination of Lactuca sativa.

The biologically active molecules karrikinolide (KAR1) and trimethylbutenolide (TMB) present in wildfire smoke play a key role in regulating seed germination of many plant species. To elucidate the physiological mechanism by which smoke-water (SW), KAR1 and TMB regulate seed germination in photosensitive 'Grand Rapids' lettuce (Lactuca sativa L.), we investigated levels of the dormancy-inducing hormone abscisic acid (ABA), 3 auxin catabolites and cytokinins (26 isoprenoid and 4 aromatic) in response to these compounds. Activity of the hydrolytic enzymes α-amylase and lipase along with stored food reserves (lipids, carbohydrate, starch and protein) were also assessed. The smoke compounds precisely regulated ABA and hydrolytic enzymes under all light conditions. ABA levels under red (R)-light were not significantly different in seeds treated with TMB or water. However, TMB-treated seeds showed significantly inhibited germination (33%) compared to water controls (100%). KAR1 significantly enhanced total isoprenoid cytokinins under dark conditions in comparison to other treatments; however, there was no significant effect under R-light. Enhanced levels of indole-3-aspartate (IAAsp) (an indicator of high indole-3-acetic acid [IAA] accumulation, which inhibits lettuce seed germination) and absence of trans-zeatin and trans-zeatin riboside (the most active cytokinins) in TMB-treated seeds might be responsible for reduced germination under R-light. Our results demonstrate that SW and KAR1 significantly promote lettuce seed germination by reducing levels of ABA and enhancing activity of hydrolytic enzymes, which aids in mobilizing stored reserves. However, TMB inhibits germination by enhancing ABA levels and reducing the activity of hydrolytic enzymes.