| Summary: | Huanglongbing (HLB), also known as citrus greening, is the most destructive disease of citrus worldwide. In the United States, this disease is associated with a phloem-restricted bacterium, <i>Candidatus</i> Liberibacter asiaticus. Commercial citrus cultivars are susceptible to HLB, but <i>Poncirus trifoliata</i>, a close relative of <i>Citrus</i>, is highly tolerant of HLB. Isolating <i>P. trifoliata</i> gene(s) controlling its HLB tolerance followed by expressing the gene(s) in citrus is considered a potential cisgenic approach to engineering citrus for tolerance to HLB. Previous gene expression studies indicated that the <i>constitutive disease resistance</i> (<i>CDR</i>) genes in <i>P. trifoliata</i> (<i>PtCDRs</i>) may play a vital role in its HLB tolerance. This study was designed to use <i>Arabidopsis</i> mutants as a model system to confirm the function of <i>PtCDRs</i> in plant disease resistance. <i>PtCDR2</i> and <i>PtCDR8</i> were amplified from <i>P. trifoliata</i> cDNA and transferred into the <i>Arabidopsis cdr1</i> mutant, whose resident <i>CDR1</i> gene was disrupted by T-DNA insertion. The <i>PtCDR2</i> and <i>PtCDR8</i> transgenic <i>Arabidopsis cdr1</i> mutant restored its hypersensitive response to the bacterial pathogen <i>Pseudomonas syringae</i> pv. <i>tomato</i> strain DC3000 (<i>Pst</i> DC3000) expressing <i>avrRpt2</i>. The defense marker gene <i>PATHOGENESIS RELATED 1</i> (<i>PR1</i>) expressed at much higher levels in the <i>PtCDR2</i> or <i>PtCDR8</i> transgenic <i>cdr1</i> mutant than in the non-transgenic <i>cdr1</i> mutant with or without pathogen infection. Multiplication of <i>Pst</i> DC3000 bacteria in <i>Arabidopsis</i> was inhibited by the expression of <i>PtCDR2</i> and <i>PtCDR8</i>. Our results showed that <i>PtCDR2</i> and <i>PtCDR8</i> were functional in <i>Arabidopsis</i> and played a positive role in disease resistance and demonstrated that <i>Arabidopsis</i> mutants can be a useful alternate system for screening <i>Poncirus</i> genes before making the time-consuming effort to transfer them into citrus, a perennial woody plant that is highly recalcitrant for <i>Agrobacterium</i> or biolistic-mediated transformation.
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