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

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 (C. reticulata x P. trifoliata), and US-942 (C. reticulata x P. 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 RNA were extracted and subjected to qPCR and 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 P. trifoliata parentage, rather than with induced expression in response to bacterial infection.