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Title: Comparison of gene expression changes in susceptible, tolerant, and resistant hosts in response to infection with citrus tristeza virus and huanglongbing

Author
item Bowman, Kim
item Albrecht, Ute

Submitted to: Journal of Citrus Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2015
Publication Date: 11/5/2015
Citation: Bowman, K.D., Albrecht, U. 2015. Comparison of gene expression changes in susceptible, tolerant, and resistant hosts in response to infection with citrus tristeza virus and huanglongbing. Journal of Citrus Pathology. 30450.

Interpretive Summary: Two serious citrus diseases, Huanglongbing (HLB) and citrus tristeza virus (CTV), are similar in that they both primarily affect the phloem of the tree. Another particularly interesting similarity between the two diseases is that the citrus close relative Poncirus trifoliata has exhibited resistance or tolerance to both diseases. We have hypothesized that there may be some similar factors responsible for the resistance to both diseases in Poncirus trifoliata. To investigate this, we have used three citrus rootstock genotypes, noted for their different levels of susceptibility or tolerance to the diseases, two of which are hybrids of Poncirus trifoliata. We compared the gene expression of healthy plants to those infected by HLB or CTV, using controlled inoculation and quantitative polymerase chain reaction (qPCR) analysis. There were differences in gene responses to the two diseases, but the most striking differences among the genotypes were higher expression of numerous defense related genes in the two hybrids of Poncirus trifoliata even without infection. It is hypothesized that these genes play a role in the resistance or tolerance of trifoliate orange and its hybrids to HLB and CTV.

Technical Abstract: The pathogens Candidatus Liberibacter asiaticus (Las) and Citrus tristeza virus (CTV) are both phloem limited and have significant economic impact on citrus production wherever they are found. Studies of host resistance have indicated that Poncirus trifoliata has tolerance or resistance to both pathogens, suggesting that there may be some common factors in the 2 kinds of resistance. We have conducted studies of host gene expression changes that occur in response to infection to gain further insight. Controlled inoculation by grafting infected budwood was used to infect potted greenhouse plants of Cleopatra mandarin (Citrus reticulata), US-897 (Citrus reticulata x Poncirus trifoliata), and US-942 (Citrus reticulata x Poncirus trifoliata) with CTV and with Las, the pathogen associated with the disease huanglongbing (HLB). Stem and leaf tissue was collected at 10, 20, and 30 weeks after inoculation, DNA and Ribonucleic acid (RNA) were extracted and subjected to quantitative polymerase chain reaction (qPCR) and Real time quantitative polymerase chain reaction (RT-qPCR) analysis. Few differences in gene expression were observed between mock-inoculated and CTV-inoculated plants. Differences between mock-inoculated and Las-inoculated plants were most pronounced in susceptible Cleopatra plants and at the later stages of infection. Notable was the higher expression of a gene for miraculin-like protein 2 and other defense-related genes in US-897 and US-942 plants independent of infection. It is hypothesized that tolerance or resistance of US-897 and US-942 is associated with a higher constitutive expression of defense - related or other genes associated with the Poncirus trifoliata parentage, rather than with induced expression in response to bacterial infection.