Differential Rootstock-mediated Scion Gene Expression in HLB-susceptible Versus Putatively HLB-tolerant Scion/Rootstock Combinations

Rootstocks are an invaluable part of citrus production. Citrus trees are grown commercially by grafting or budding scions onto rootstocks. Many scion traits such as disease resistance, plant vigor, fruit size and precocity are influenced by rootstocks. Thus, rootstock genotype is important in determining scion quality. Florida citrus production is consistently declining because of the presence of Huanglongbing (HLB) disease. There is a need for HLB tolerant or resistant rootstocks in Florida to achieve sustainable fruit production and to maintain plant health. The University of Florida Citrus Breeding Program has developed rootstocks that have shown the potential to reduce HLB disease severity and infection frequency based on field performance and visual symptom observations. In the present study, high throughput RNA sequencing technology was used to quantify leaf gene expression levels in ‘Valencia’ sweet orange [Citrus sinensis (L) Osbeck.] scions, and to identify the rootstock-mediated gene expression patterns comparing the industry standard ‘Swingle’ citrumelo [Citrus paradisi MacFaden X Poncirus trifoliate (L)]rootstock with selected newly developed,putatively HLB-tolerant rootstocks that visually reduce HLB severity. The results showed that significant differentially expressed genes belonged to the following gene function categories: defense induction, protein kinases, transportation, cell homeostasis, transcription factors and hormonal regulations. Based on phenotypic evaluation, we estimate that photosynthesis is upregulated in scions grafted on newly developed, putatively HLB-tolerant rootstocks. The juice analysis of fruit harvested from ‘Valencia’ grafted on new rootstocks had a higher brix-acid ratio (maximum 15) compared to the fruit harvested from ‘Valencia’ grafted on ‘Swingle’. Our findings support previous research that suggests rootstocks can differentially reprogram the scion transcriptome under biotic and abiotic pressure.