Abstract The enzymatic browning of lettuce (Lactuca sativa L.) is a main cause of postharvest quality loss, and is controlled by the enzyme phenylalanine ammonia lyase (PAL). However, effective browning inhibitors that prevent lettuce butt discoloration have not been commercially developed, so the effects of such inhibitors on PAL are largely unknown. Here, we not only developed an anti-browning treatment, but also explored the mechanisms of the PAL-associated browning of Iceberg lettuce by profiling all homologs of PAL genes at transcript level. The anti-browning treatment used a combination of 0.25 M acetic acid and 200 mL L−1 ethanol and was able to repress enzymatic browning and microbial growth for two weeks. Notably, the lettuce butt discoloration in stem disks was repressed by 0.5 M acetic acid by inhibiting PAL activity, and this inhibition of PAL activity was also observed in vitro using a crude PAL enzyme extract from lettuce stems. To investigate the anti-browning mechanism at the transcriptional level, we identified and cloned six predicted LsPAL genes in the Lettuce Genome Resource, and further found that four of these (LsPAL1 to LsPAL4) were wound-inducible in the lettuce stem. Among these four wound-inducible LsPALs, LsPAL4 showed the highest wound-induced fold-change, suggesting that LsPAL4 has a key role in lettuce browning. Interestingly, wound-induction of LsPAL genes was dramatically downregulated by application of acetic acid. Taken together, acetic acid treatment of lettuce stems repressed butt discoloration by repressing PAL both enzymatically and transcriptionally, and ethanol provided complementary antimicrobial activity. A combination treatment with acetic acid and ethanol therefore has commercial potential in lettuce head processing.

中文翻译：

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通过抑制苯丙氨酸解氨酶的活性和表达，将乙酸和乙醇结合作为生菜臀部变色的抗褐变处理

摘要 生菜（Lactuca sativa L.）的酶促褐变是采后品质损失的主要原因，并受苯丙氨酸解氨酶（PAL）的控制。然而，防止生菜屁股变色的有效褐变抑制剂尚未商业开发，因此此类抑制剂对 PAL 的影响在很大程度上是未知的。在这里，我们不仅开发了一种抗褐变处理，而且还通过在转录水平上分析 PAL 基因的所有同源物来探索冰山莴苣的 PAL 相关褐变的机制。抗褐变处理使用 0.25 M 乙酸和 200 mL L-1 乙醇的组合，能够抑制酶促褐变和微生物生长两周。值得注意的是，0.5 M 乙酸通过抑制 PAL 活性来抑制茎盘中的生菜臀部变色，使用来自生菜茎的粗 PAL 酶提取物在体外也观察到这种 PAL 活性的抑制。为了研究转录水平的抗褐变机制，我们在生菜基因组资源中鉴定并克隆了六个预测的 LsPAL 基因，并进一步发现其中四个（LsPAL1 到 LsPAL4）在生菜茎中是伤口诱导的。在这四种伤口诱导型 LsPAL 中，LsPAL4 显示出最高的伤口诱导倍数变化，表明 LsPAL4 在生菜褐变中起关键作用。有趣的是，LsPAL 基因的伤口诱导通过应用乙酸显着下调。总之，用乙酸处理生菜茎通过在酶促和转录上抑制 PAL 来抑制臀部变色，乙醇提供了补充的抗菌活性。