纽荷尔脐橙树体中黄龙病菌的分布规律及其对叶片组织结构和木质素合成的影响

为明确纽荷尔脐橙树体不同部位叶片在不同季节的黄龙病菌Candidatus Liberibacter asiaticus感染情况，利用实时荧光定量PCR技术检测采集自不同季节及树体不同部位叶片样品中的黄龙病菌，并对秋季采集的相同叶龄且黄龙病菌含量不同的叶片进行叶脉显微结构观察、木质素含量测定以及木质素合成相关基因表达模式分析。结果显示，夏季采集的 122 份叶片样品的阳性率为16.02%，秋季采集的486份叶片样品的阳性率为33.02%，后者的阳性率显著高于前者；其中178份采集自秋季枝条顶端的叶片样品阳性率最高，为36.88%，308份采集自秋季枝条底端的叶片样品阳性率为26.84%，二者均显著高于采集自夏季枝条的60份顶端叶片的阳性率（22.98%）和62份底端叶片的阳性率（8.96%）。对秋季不同含菌量叶片样品进行比较后发现，含菌量较高的叶片其韧皮部厚度较对照显著增加；叶片内木质素含量随着黄龙病菌含量的增加而增加，同时3个木质素合成相关基因Cs4g17860、orange1.1t02755和orange1.1t04489的表达量也随着黄龙病菌含量的增加而上调表达。表明感染黄龙病的纽荷尔脐橙在秋季时其枝条顶端的叶片的阳性率相较于底端叶片以及夏季叶片更高，且黄龙病菌入侵后叶片韧皮部厚度、木质素含量及部分木质素合成相关基因表达量均随着叶片中菌含量的升高而增加。

Distribution patterns of Candidatus Liberibacter asiaticus in Newhall navel orange trees and their impact on the leaves tissue structural changes and lignin synthesis

To ascertain the infection of huanglongbing pathogen (Candidatus Liberibacter asiaticus, C. Las) in different parts of Newhall navel orange trees across various seasons, this study conducted quantitative real-time PCR (qPCR) testing for the C. Las in leaves samples collected from different parts of trees and in different seasons. Further research was conducted on leaves of the same age but with varying content of C. Las, which were collected in the autumn. This includes paraffin sections to observe the microstructure of leaf veins, determination of lignin content, and analysis of the expression pattern of genes related to lignin synthesis. The results showed that out of the 122 samples collected in the summer, the positive rate of C. Las was 16.02%; while for the 486 samples collected in the autumn, the positive rate was 33.02%, which was significantly higher than in the summer. Among these, 178 leaf samples taken from the apical leaves of branches in the autumn had the highest positive rate of 36.88%. Additionally, 308 samples from the basal leaves of autumn branches had a positive rate of 26.84%, both of which were significantly higher than the positive rate for the 60 apical leaves samples (22.98%) and 62 basal leaves samples (8.96%) from branches collected in the summer. Observations of vascular bundle structures in leaves with varying content of C. Las also revealed a significant increase in the thickness of the phloem in leaves with higher content of C. Las compared with the controls. A comparison of autumn leaves with varying content of C. Las revealed that the lignin content in the leaves increased with the content of C. Las. The expression of three lignin synthesis-related genes Cs4g17860, orange1.1t02755, and orange1.1t04489 was found to be significantly upregulated in the leaves. This study indicates that Newhall navel orange trees infected with C. Las exhibit a higher positive rate in the apical leaves compared with the basal leaves of the same autumn branches, as well as in comparison to leaves from the summer season. The phloem thickness, lignin content and expression of some synthesis-related genes of leaves in response to the invasion of C. Las increased with the leaf bacterial content.