Abstract
Huanglongbing (HLB), a bacterial disease caused by Candidatus Liberibacter asiaticus (CLas), is a major threat to the citrus industry. In a previous study conducted by our laboratory, several citrus transgenic trees expressing the Arabidopsis thaliana NPR1 (AtNPR1) gene remained HLB-free when grown in a field site under high HLB disease pressure. To determine the molecular mechanisms behind AtNPR1-mediated tolerance to HLB, a transcriptome analysis was performed using AtNPR1 overexpressing transgenic trees and non-transgenic trees as control, from which we identified 57 differentially expressed genes (DEGs). Data mining revealed the enhanced transcription of genes encoding pathogen-associated molecular patterns (PAMPs), transcription factors, leucine-rich repeat receptor kinases (LRR-RKs), and putative ankyrin repeat-containing proteins. These proteins were highly upregulated in the AtNPR1 transgenic line compared to the control plant. Furthermore, analysis of protein–protein interactions indicated that AtNPR1 interacts with CsNPR3 and CsTGA5 in the nucleus. Our results suggest that AtNPR1 positively regulates the innate defense mechanisms in citrus thereby boosting resistance and effectively protecting the plant against HLB.
Highlights
Huanglongbing (HLB), known as citrus greening, is caused by the phloem-limited pathogenic bacterium Candidatus Liberibacter asiaticus (CLas) and is the most destructive disease in citrus plants [1]
To investigate the transcriptional responses associated with Arabidopsis thaliana NONEXPRESSOR OF PATHOGENESIS RELATED GENE 1 (NPR1) (AtNPR1) overexpression in ‘Valencia’ sweet orange plants, we performed RNA sequencing (RNAseq) analyses on a selected NPR overexpressing line and a non-transgenic control plant
Our previous studies showed that the NPR1-2 transgenic line consistently expressed the highest levels of the transgene, as such, this line was the transgenic line of choice and ‘Valencia’ (Val) the non-transgenic control
Summary
Huanglongbing (HLB), known as citrus greening, is caused by the phloem-limited pathogenic bacterium Candidatus Liberibacter asiaticus (CLas) and is the most destructive disease in citrus plants [1]. To manage the disease, several plant defense inducers have been sprayed on HLB-infected field trees [9]. The premise behind these studies is that induced resistance, either in a local manner or spread systemically throughout the trees, could confer long-lasting protection against HLB by activating the salicylic acid (SA) signaling or systemic acquired resistance (SAR) pathways [10]. ‘Hamlin’ and ‘Valencia’ sweet orange transgenic lines ectopically expressing AtNPR1 under the control of the cauliflower mosaic virus (CaMV) 35S promoter or the Arabidopsis thaliana SUCROSE SYNTHASE 2 promoter (AtSUC2, a phloem specific promoter) has remained HLB-free in a site of high disease pressure [31]. An in-depth analysis, including a study of the transcriptome of the HLB resistant transgenic line, was conducted
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