2019 ASHS Annual Conference

Huanglongbing (HLB) is a phloem limited bacterial disease caused by Candidatus Liberibacter asiaticus (CLas). This disease has devastated the Florida citrus industry and is spreading to other citrus growing parts of the USA. The citrus relative, Microcitrus australasica (Australian finger lime) has been observed to be tolerant to the disease under endemic HLB conditions while commercial sweet oranges such as 'Valencia' are not. To better understand the mechanism of tolerance to HLB, a greenhouse infection study was conducted with the finger lime DPI-50-36 and 'Valencia' SPB-1-14-19 sweet orange trees. Both scions were budded onto carrizo rootstock and 6-month old trees were side grafted with 6-inch-long HLB infected sweet orange bud sticks. HLB disease progression was monitored on a quarterly interval using qPCR. Both finger lime and 'Valencia' trees tested positive for CLas within 6 months of grafting. Finger lime trees however had a lower population of the bacterium in the phloem and remained visually symptom free even after 1 year following infection. On the other hand, 'Valencia' trees developed HLB symptoms within 9 months following infection. RNA samples were extracted from leaves of one-year old infected trees and sequenced using the Illumina HiSeq platform with a 2x150bp configuration. Analyses between the finger lime infected and control resulted in the identification of 2919 differently expressed genes, of which 1520 were down regulated in the infected group, while 1399 were up regulated. Similar analyses between the infected and control 'Valencia' resulted in 335 differently expressed genes, of which 96 were down regulated in the infected group, while 239 were up regulated. Gene ontology analyses revealed that cell wall components and cell wall supports (microtubules) where down regulated in both infected groups, while heme binding and oxidoreductase activity were overexpressed. In addition to a number of defense related genes (PR, TIR-NBS-LRR, NAC families), several primary and secondary metabolism genes as well as genes responsible for nutrient transport were highly upregulated in the infected finger lime trees. Defense and nutrition related genes were down regulated in infected 'Valencia' trees. Our results provide a novel insight into the potential mechanism of HLB tolerance in finger limes.