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

Abstract

In a grafted citrus tree, the leaf and root metabolic composition and the response to the phloem-limited pathogenic bacterium Candidatus Liberibacter asiaticus are influenced by the rootstock. Eleven different citrus rootstock varieties were investigated for their influence on the grafted tree response to Candidatus Liberibacter asiaticus (CLas), the pathogen associated with Huanglongbing (HLB). Rootstocks included mandarin and sweet orange cultivars, sour orange, and hybrids of citrus and trifoliate orange. Leaves and roots of healthy and CLas-infected greenhouse-grown 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). The genetic origin of the rootstock determined their metabolic profiles in the healthy state and influenced the root metabolic response to CLas. In the leaves, metabolic differences were determined by the tree response to CLas and to a lesser extent by the rootstock cultivar on which the plants were grafted. Among the leaf metabolites that responded most consistently to infection were citrulline, ornithine, and quinic acid, which were upregulated in infected plants. The reverse was found for cerotinic acid, cyanoalanine, hexadecane, and raffinose. In both leaves and roots, several metabolites associated with plant stress and disease tolerance such as benzoic acid and proline responded differently in plants on the mandarin × trifoliate hybrid rootstocks compared with the other rootstocks. This corresponds with the different HLB field tolerance of grafted trees on different rootstocks, which is generally higher in combination with some mandarin × trifoliate hybrids. Our results demonstrate the influence of rootstock on the grafted tree response to CLas and underscore the importance of rootstocks for disease management in an HLB-endemic environment.