Rootstock influences the metabolic response to Candidatus Liberibacter asiaticus in grafted sweet orange trees

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

Submitted to: Trees
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/2019
Publication Date: 11/5/2019
Citation: Albrecht, U., Tripathi, I., Bowman, K.D. 2019. Rootstock influences the metabolic response to Candidatus Liberibacter asiaticus in grafted sweet orange trees. Trees. 34:405-431. https://doi.org/10.1007/s00468-019-01925-3.
DOI: https://doi.org/10.1007/s00468-019-01925-3

Interpretive Summary: Most citrus fruiting cultivars 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 11 rootstocks on the metabolic chemicals that are found in tree leaf and root tissues using gas chromatography, with special focus on metabolic components that appear associated with huanglongbing disease and tree tolerance. The relative huanglongbing (HLB) field tolerance of grafted trees on different rootstocks was clearly associated with constitutive metabolic components and and metabolic changes that occurred following infection. Our results demonstrate the influence of rootstock on the grafted tree response to HLB and underscores the importance of rootstocks for disease management in an HLB-endemic environment.

Technical Abstract: Eleven different rootstock varieties were investigated for their influence on the grafted tree response to Candidatus Liberibacter asiaticus (CLas) in a controlled greenhouse environment. Rootstocks were Cleopatra mandarin, Ridge pineapple sweet orange, sour orange, and 8 different hybrids of citrus and trifoliate orange. Leaves and fibrous roots of healthy and CLas-infected plants were collected 6 months after graft inoculation and metabolite profiles were assessed by untargeted gas chromatography time-of-flight mass spectrometry (GC TOF-MS). Nearly 600 unique metabolites were identified in leaves and roots of all plants. The genetic origin of the rootstock determined the root metabolic profiles and influenced the root metabolic response to CLas. Root metabolites that responded most strongly to CLas included benzoic acid, salicylic acid, raffinose, and galactinol which were upregulated in infected plants. Root metabolites that were downregulated in infected plants included cysteine, cyanoalanine, fructose, and hexadecane. In the leaves, metabolic differences were mostly determined by the tree response to CLas. However, leaf metabolic profiles and the leaf response to CLas were also influenced by the rootstock cultivar on which the plants were grafted, demonstrating the biochemical influence of rootstock on the grafted scion. In nearly all graft combinations, more leaf metabolites were downregulated than upregulated in infected plants. Among the leaf metabolites that responded most consistently to infection were citrulline, ornithine, and quinic acid which were more highly concentrated in leaves from infected than non-infected plants. The reverse was found for cyanoalanine, hexadecane, cerotinic acid, and raffinose. Several metabolites showed an opposite response in leaves and roots such as quinic acid and raffinose. In both leaves and roots, many metabolites responded differently in plants on the mandarin x trifoliate hybrid rootstocks compared with the other rootstocks. This corresponds with the different huanglongbing (HLB) field tolerance of grafted trees on different rootstocks, which is generally higher in combination with some mandarin x trifoliate hybrids. Although it is unclear how specific metabolites affect the disease outcome, our results demonstrate the influence of rootstock on the grafted tree response to HLB and underscores the importance of rootstocks for disease management in an HLB-endemic environment.