P-641 - Integrated Analysis of 'Parson Brown' Orange Reveals Multilayered Defense Mechanisms Against Citrus Greening Disease

‘Parson Brown’ orange is an early-maturing variety and considered a resilient "survivor" tree in the face of Citrus Greening disease. Its ability to maintain productivity over 30 years under Citrus greening conditions has solidified its capability as a savior for growers battling the disease. This study compared the metabolomic profiling, transcriptomic analysis, and physiological measurements of three early-maturing sweet oranges; ‘Hamlin’, ‘Roble’, and 'Parson Brown'. Healthy greenhouse-grown trees were exposed to ‘Candidatus Liberibacter asiaticus’ via psyllid infestation. We recorded a significant decrease of landed psyllids on 'Parson Brown' (20.58%) compared to 'Hamlin' (34.38%) and 'Roble' (45.04%), in addition to a lower ‘Ca. L. asiaticus’ titers in ‘Parson Brown’. RNA-seq analysis generated 18 transcriptomes from healthy and infected trees, yielding ~26.52 million reads per sample. Differential gene expression and enrichment analyses identified cultivar-specific responses to ‘Ca. L. asiaticus’-infection. ‘Parson Brown,’ showed significant upregulation of genes involved in secondary metabolism, including phenolic and flavonoid biosynthesis and terpene biosynthesis-related genes. Physiological analyses revealed reduced callose deposition and starch accumulation in ‘Parson Brown,’ contrasting with elevated levels in ‘Hamlin’ and ‘Roble’. The tolerance observed in ‘Parson Brown’ oranges to citrus greening disease is attributed to a complex cascade of defense mechanisms rather than a single genetic factor.