黄龙病重创世界柑橘产业，迄今仍无有效根除方法。前期发现，柑橘miR171b受黄龙病菌诱导呈极显著上调表达；其前体过表达植株的黄龙病耐病性显著提高，接种病菌3年后仍长势良好；同时预测其候选靶标基因均为SCL6基因。为揭示其靶向调控机理，本项目拟以转基因植株与非转基因植株为试材，通过比较该miRNA及其靶基因表达水平，明确两者在柑橘与黄龙病互作中的关系；利用5’RACE等方法鉴定miRNA对靶基因的切割位点；对该位点进行定点突变，观察突变对两者关系的影响，以明确该miRNA对靶基因切割的特异性；在甜橙中敲除以及过表达靶基因SCL6，通过检测基因敲除植株、过表达植株接种病菌后的抗性、miR171b及其靶基因表达水平的变化等，明确miR171b及其靶基因的功能，探明两者互作在柑橘黄龙病耐病性中的作用大小。本研究将为挖掘柑橘黄龙病耐病相关基因，研发抗病措施及分子育种提供理论依据和技术基础。

Huanglongbing has severely damaged the world citrus industry. Our preliminary data showed that miR171b was significantly up-regulated by infection of huanglongbing (HLB) bacteria. Over-expressing the precursor of miR171b rendered the transgenic plants high resistance/tolerance to HLB since the plants were still alive after having been infected for 3 years. Bioinformatics analysis indicated that the target genes of miR171b were homologous of SCL6. To unravel the underlying mechanisms, the proposed research will use both the miR171b overexpressing and the control plants to do the following work: comparing the expression levels of miR171b and its target genes between different plant materials to determine their roles in responses of citrus to huanglongbing; identifying miR171-directed cleavage site on SCL6 using 5’RACE and other methods; introducing point mutations at the cleavage site to investigate the effects of the mutations on miR171-directed cleavage of SCL6; knocking out SCL6 and investigating the changes in resistance of the SCL6-knocked out and overexpressing plants to HLB, as well as the associated changes in the expression of miR171b and SCL6, to elucidate the functions of the miR171b and its targets, as well as their roles in the interactions of citrus with HLB. It is expected that the project will provide a solid basis for further identification of HLB-resistance/tolerance genes and for the development of HLB-resistant cultivars and HLB-control strategy.