Reciprocal influences of rootstock and scion citrus cultivars challenged with Ca. Liberibacter asiaticus

Authors: ALBRECHT, UTE - UNIVERSITY OF FLORIDA; Bowman, Kim

Submitted to: Scientia Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/5/2019
Publication Date: 5/9/2019
Publication URL: https://handle.nal.usda.gov/10113/6454862
Citation: Albrecht, U., Bowman, K. 2019. Reciprocal influences of rootstock and scion citrus cultivars challenged with Ca. Liberibacter asiaticus. Scientia Horticulturae. 254:133-142. https://doi.org/10.1016/j.scienta.2019.05.010
DOI: https://doi.org/10.1016/j.scienta.2019.05.010

Interpretive Summary: Most citrus fruiting cultivars (scions) are highly susceptible to huanglongbing disease, but significant tolerance has been identified within the trifoliate orange and some of its hybrids that are commonly used as rootstocks for citrus trees. In this study, we investigated the relative influence of rootstock and scion in citrus plants composed of susceptible and tolerant varieties in both the scion and the rootstock position, on both huanglongbing disease progression and expression of stress and disease-related genes. The rate with which trees became infected with the disease, and the severity of the disease, were mainly influenced by the variety in the scion position. Although rootstock is known to play a role in tree tolerance to the disease, the study demonstrated that if a suitable tolerant scion can be found and used, this would result in a major improvement in the health of citrus trees affected by huanglongbing.

Technical Abstract: The devastating citrus disease Huanglongbing (HLB), associated with bacterial pathogens of the genus Candidatus Liberibacter, has spread across many citrus production areas worldwide causing devastating economic losses. In Florida, infection rates in most commercial citrus orchards approach 100 percent by the time trees are 3-4 years old. Most scion cultivars are highly susceptible to Ca. Liberibacter asiaticus (CLas), the pathogen prevalent in Florida, but significant tolerance has been identified within the species Poncirus trifoliata (trifoliate orange) and some of its hybrids that are commonly used as rootstocks. In this study we investigated the relative influence of rootstock and scion in citrus plants composed of susceptible and tolerant varieties in both the scion and the rootstock position on HLB disease progression and expression of stress and disease-related genes. Experiments were conducted in the greenhouse using the susceptible varieties 'Valencia' orange (Citrus sinensis) and 'Cleopatra' mandarin (Citrus reticulata), and the tolerant trifoliate hybrid varieties 'US-802' (Citrus grandis × P. trifoliata), 'US-897' (C. reticulata × P. trifoliata), and 'US-942' (C. reticulata × Poncirus trifoliata). Plants were either mock-inoculated or graft-inoculated with CLas. The rate with which trees became polymerase chain reaction PCR-positive for CLas and the severity of the disease was mainly influenced by the variety in the scion position. Typically, trees with a tolerant variety in the scion position contained a lower number of bacteria and were less damaged by infection than trees with a susceptible variety in the scion position, regardless of rootstock. Previous observations of significant influence from rootstock on field tree tolerance to CLas may be partially the result of differential rootstock ability to tolerate other stresses, or respond favorably to therapeutic treatments, under the weakened condition caused by the infection. Differential expression of defense-related and other genes found largest fold-differences between non-infected and infected plants when a susceptible variety was in the scion position. Fewer genes responded in roots than in leaves, and genes associated with starch metabolism responded strongly and in an opposite direction when comparing roots and leaves, probably indicative of carbon depletion occurring in roots of infected plants.