Comparative analysis of Wanjincheng orange leaf and root responses to ‘Candidatus liberibacter asiaticus’ infection using leaf-disc grafting

Abstract

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.