Citrus Research Institute, Southwest University/Chinese Academy of Agricultural Sciences, Chongqing 400716, China.
Int J Mol Sci. 2021 Mar 10;22(6):2803. doi: 10.3390/ijms22062803.
Citrus Huanglongbing (HLB) disease or citrus greening is caused by Liberibacter asiaticus (Las) and is the most devastating disease in the global citrus industry. Salicylic acid (SA) plays a central role in regulating plant defenses against pathogenic attack. SA methyltransferase (SAMT) modulates SA homeostasis by converting SA to methyl salicylate (MeSA). Here, we report on the functions of the citrus () gene from HLB-susceptible Wanjincheng orange ( (L.) Osbeck) in plant defenses against Las infection. The cDNA was expressed in yeast. Using in vitro enzyme assays, yeast expressing CsSAMT1 was confirmed to specifically catalyze the formation of MeSA using SA as a substrate. Transgenic Wanjincheng orange plants overexpressing had significantly increased levels of SA and MeSA compared to wild-type controls. HLB resistance was evaluated for two years and showed that transgenic plants displayed significantly alleviated symptoms including a lack of chlorosis, low bacterial counts, reduced hyperplasia of the phloem cells, and lower levels of starch and callose compared to wild-type plants. These data confirmed that overexpression confers an enhanced tolerance to Las in citrus fruits. RNA-seq analysis revealed that overexpression significantly upregulated the citrus defense response by enhancing the transcription of disease resistance genes. This study provides insight for improving host resistance to HLB by manipulation of SA signaling in citrus fruits.
黄龙病（HLB）又称柑橘绿化病，由亚洲韧皮杆菌（Las）引起，是全球柑橘产业中最具破坏性的病害。水杨酸（SA）在调节植物对病原攻击的防御中起着核心作用。SA 甲基转移酶（SAMT）通过将 SA 转化为甲基水杨酸（MeSA）来调节 SA 动态平衡。在这里，我们报告了来自 HLB 易感品种万锦橙（（L.）Osbeck）的基因在植物抵御 Las 感染中的功能。该 cDNA 在酵母中表达。通过体外酶活性测定，证实表达 CsSAMT1 的酵母能特异性地利用 SA 作为底物催化 MeSA 的形成。与野生型对照相比，过表达的转基因万锦橙植物的 SA 和 MeSA 水平显著升高。两年的 HLB 抗性评估表明，与野生型植物相比，转基因植物表现出明显减轻的症状，包括缺绿症减轻、细菌计数降低、韧皮部细胞增生减少以及淀粉和胼胝质水平降低。这些数据证实，过表达可增强柑橘果实对 Las 的耐受性。RNA-seq 分析显示，过表达通过增强抗病基因的转录，显著上调了柑橘的防御反应。本研究为通过操纵柑橘果实中的 SA 信号来提高对 HLB 的宿主抗性提供了新的见解。
