New insight on enzymatic browning mechanism by comparative transcriptome analysis of romaine lettuce cultivars with different browning potentials

Enzymatic browning on the cut edge of lettuce significantly limits its quality and shelf life. To characterize its molecular mechanisms, we performed comparative transcriptome analysis of three Romaine lettuce cultivars, Parris Island Cos (PC) with low browning potential, Tall Guzmaine (TG) with high degree of browning and Clemente (CL), a medium degree of browning cultivar derived from TG x PC. Before cutting, the phenylpropanoid and oxidative stress genes, such as PAL1, MYB1 and PPO were up-regulated in PC as compared to TG, while the expressions of genes involved in auxin, cytokinin hormone signalings and defense, such as ARF, AHP and PTI in PC were higher than TG. The transcript levels of all these genes in CL were somewhere between TG and PC. On day 3 after cutting when browning was shown up in TG, the expression of these phenylpropanoid and oxidative stress genes were remarkably increased in all three cultivars, even though their levels in TG were still higher than those in PC and CI. In comparison, expressions of the hormone and defense genes were reduced in all three cultivars, but their levels in PC were higher than CI and TG. Exogenous application of IAA and 6-BA inhibited lettuce browning possibly by alleviating the burst of those browning related genes, such as PAL1 and PPO1. These results demonstrate that lettuce tissue browning is controlled by reduced growth hormone (mainly auxin) level and increased phenolics biosynthesis and oxidation. This study provides the useful knowledge and functional markers for lettuce breeders and industry to select low tissue browning cultivar and manage lettuce quality during storage and processing. Overall design: These accessions are Parris Island Cos (PC), Tall Guzmaine (TG), and Clemente (CL). The cultivars PC and TG are major ancestors of romaine lettuce, and CL was selected from progeniesy of a cross between PC and TG. TG exhibited a relatively high degree of browning, PC possessed low browning potential and CL had medium browning potential. Immediately after harvesting, the lettuce was taken to a local distributor and stored in a 5°C cold room with forced air cooling. The cooled lettuce was transported via commercial refrigerated truck (2–4 °C) to the USDA facilities in Beltsville, MD within three days. The lettuce was then stored at 5 °C for 24 h before sample preparation. On the day of sample preparation, wrapper leaves and stems were removed manually from each head of lettuce. Lettuce leaf midribs were obtained by carefully excising the green leaves. Representative samples were taken from all locations of each head. The ribs were then cut manually into 2.5-cm pieces using an paring knife. Then samples were stored at 5 °C , and frozen on day 0, 2 and 3 in liquid nitrogen,