Table1_Understanding the defense mechanism of Allium plants through the onion isoallicin-omics study.xlsx

Onion (Allium cepa L.) is an important seasoning vegetable worldwide. It belongs to the Allium genus, which produces distinctive flavor compounds, allicin/isoallicin. It has been known that allicin/isoallicin is produced upon cell damage by vacuole-localized alliinase releasing. Pungent isoallicin and lachrymaroty factor (LF) are unique features of onions. To understand the isoallicin system of onions, we identified and characterized the biosynthesis-related genes by displaying transcriptional profiles and analyzing the isoallicin contents of onion plants. The DHW30006 onion genome encoded 64 alliinase (ALL) and 29 LF synthase (LFS) proteins, which are the key enzymes for producing of isoallicin and LF. Interestingly, when we analyzed the N-terminal signal peptide sequences (SP) necessary for transport to the vacuole, we found that 14 ALLs contained the SP (SP-ALL) and 50 ALLs did not (non-SP-ALL). We hypothesized that non-SP-ALLs stayed in the cytosol of onion cells, reacted with isoalliin, and generated isoallicin without cell damage. Our transcriptome and LC-MS/MS analyses reveal that isoallicin levels vary significantly across onion tissues and growth stages, with substantial production occurring in intact cells through cytosolic alliinases and an increase through vacuolar alliinases upon tissue disruption. This novel observation suggests that the isoallicin system in onions functions as a dual-defense mechanism: cytosolic alliinases maintain a constant level of defense against biotic stress in undamaged tissues, while vacuolar alliinases enhance isoallicin production in response to tissue damage by herbivory and insects. Together, these coordinated mechanisms demonstrate an adaptable and dynamic defense strategy against biotic stresses in Allium plants.

洋葱（Allium cepa L.）是全球范围内重要的香辛蔬菜。它隶属于葱属（Allium genus），该属植物可产生具有独特风味的物质——蒜氨酸（allicin）与异蒜氨酸（isoallicin）。已有研究表明，当细胞受到损伤时，定位于液泡的蒜氨酸酶（alliinase）释放后可催化生成蒜氨酸/异蒜氨酸。具有刺激性的异蒜氨酸与催泪因子（lachrymatory factor，LF）是洋葱的标志性特征。为解析洋葱的异蒜氨酸代谢系统，本研究通过分析洋葱植株的转录谱与异蒜氨酸含量，鉴定并表征了与异蒜氨酸生物合成相关的基因。DHW30006洋葱基因组共编码64个蒜氨酸酶（ALL）蛋白与29个催泪因子合酶（LFS）蛋白，二者均为异蒜氨酸与催泪因子合成的关键酶。有趣的是，在分析靶向液泡运输所需的N端信号肽序列（SP）时，我们发现其中14个蒜氨酸酶蛋白携带信号肽（SP-ALL），剩余50个蒜氨酸酶蛋白则不携带信号肽（non-SP-ALL）。我们据此提出假说：不携带信号肽的蒜氨酸酶会滞留于洋葱细胞的细胞质（cytosol）中，无需细胞损伤即可直接与异蒜氨酸底物反应生成异蒜氨酸。本研究的转录组与液相色谱-串联质谱（LC-MS/MS）分析结果显示，洋葱不同组织与生长阶段的异蒜氨酸含量存在显著差异；完整细胞可通过细胞质中的蒜氨酸酶持续合成异蒜氨酸，而当组织受损时，液泡中的蒜氨酸酶会进一步提升异蒜氨酸的产量。这一新发现表明，洋葱的异蒜氨酸系统可作为双重防御机制发挥作用：细胞质中的蒜氨酸酶可在未受损伤的组织中维持恒定的生物胁迫（biotic stress）防御水平，而液泡中的蒜氨酸酶则会在植食性动物与昆虫造成组织损伤时，增强异蒜氨酸的合成。上述协同调控机制共同揭示了葱属植物针对生物胁迫的适应性、动态防御策略。