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

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 inmidribs from 2 to 6 MAG;however,by 10 MAG,it was significantly lower in roots than in midribs.Amicroscopic analysis revealed that starch accumulation,callose deposition,cellwall 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.

Cloning and Expression Analysis of Citrus Genes Cs GH3.1 and Cs GH3.6Responding to Xanthomonas axonopodis pv. citri Infection

To study the functions of the early auxin-responsive genes Cs GH3.1 and Cs GH3.6 in citrus resistance against canker disease, we cloned Cs GH3.1and Cs GH3.6 in ‘Newhall' Navel Orange(Citrus sinensis Osbeck). They are 1 797 bp and 1 887 bp and encode 598 and 629 amino acids, respectively. In vitro mature leaves from susceptible ‘Newhall' and resistant Calamondin(C. madurensis) were inoculated by a Xanthomonas axonopodis pv. citri(Xac) bacterial suspension, and expression of Cs GH3.1 and Cs GH3.6 in the two varieties were analyzed using quantitative real-time PCR(q RT-PCR). ‘Newhall' leaves were treated with different hormones for 3 days, inoculated by Xac bacterial suspension, and then the symptoms in these leaves were investigated. We used q RT-PCR to analyze the effect of different hormones on Cs GH3.1 and Cs GH3.6 expression in ‘Newhall'leaves. The expression levels of both Cs GH3.1 and Cs GH3.6 were significantly induced by Xac in ‘Newhall' leaves, compared with levels in Calamondin leaves. 1-naphthy acetic acid(NAA) increased the hypertrophy of infection sites in ‘Newhall' leaves, while naphthyl-phthalamic acid(NPA)had no visible effect on lesion development. NAA hormone greatly improved expression of Cs GH3.1 in ‘Newhall', but not Cs GH3.6. These results indicate that the auxin primary-response gene Cs GH3.1 plays an important role in citrus susceptibility to Xac.