Citrus Huanglongbing, associated with Candidatus Liberibacter asiaticus (Las), is the serious disease of citrus worldwide. Here, we compared differences between leaf and root responses in Wanjincheng Orange (Citrus sinensis Osbeck) to Las infection using leaf-disc grafting. Trees had no obvious symptoms in the first two months after grafting (MAG), but yellowing leaves, thickened midribs and decayed roots began to appear at 6 MAG. The Las growth rate in roots was greater than in midribs from 2 to 6 MAG; however, by 10 MAG, it was significantly lower in roots than in midribs. A microscopic analysis revealed that starch accumulation, callose deposition, cell wall thickness and the number of cell layers increased in the phloem of infected trees. After Las infection, the starch content in the leaves was significantly higher than that in the roots, while the callose deposition in the midribs was 3–19 times that in the roots. A gene expression analysis showed that transcripts of callose- and starch metabolism-related genes were obviously affected by Las infection, and the pathogenesis-related genes PR1, PR2 and PR5 were significantly upregulated and downregulated in midrib and root, respectively. Our results indicated that the PR-mediated resistance response was repressed in root but activated in midrib, which may result in the more rapid Las growth in roots than in midribs during the early infection stage.

柑橘黄龙病与Candidatus Liberibacter asiaticus (Las) 相关，是世界范围内柑橘的严重病害。在这里，我们比较了万金城橙（Citrus sinensisOsbeck) 到 Las 感染使用叶盘嫁接。树木在嫁接（MAG）后的前两个月没有明显症状，但在6MAG时开始出现叶片变黄、中脉增厚和根腐烂。根系Las生长速率大于中脉2~6 MAG；然而，到 10 MAG，它在根中明显低于中脉。显微镜分析表明，受感染树木韧皮部的淀粉积累、胼胝质沉积、细胞壁厚度和细胞层数增加。Las侵染后，叶片淀粉含量明显高于根部，而中脉胼胝质沉积量是根部的3～19倍。基因表达分析表明，胼胝质和淀粉代谢相关基因的转录明显受 Las 感染的影响，PR1、PR2和PR5在中脉和根中分别显着上调和下调。我们的结果表明，PR 介导的抗性反应在根中被抑制，但在中脉中被激活，这可能导致早期感染阶段根中 Las 的生长速度比中脉中更快。