Stress responses in citrus peel: Comparative analysis of host responses to Huanglongbing disease and puffing disorder

• Puffing and Huanglongbing affect starch and sucrose metabolism.
• Puffing mostly alters primary metabolism while Huanglongbing also affects secondary metabolism.
• Heat shock protein regulation was involved in the general stress responses.
• Specific sets of genes may act as biomarkers to improve diagnosis of puffing or Huanglongbing.

A comparison between transcriptomic responses to puffing disorder and Huanglongbing disease was conducted to decipher differences and similarities in gene and pathway regulation induced by abiotic (puffing) and biotic stresses (Huanglongbing) in citrus peel tissues. We functionally analyzed two previously published datasets: the first obtained for the study of puffing disorder using an Affymetrix citrus microarray and the second consisting of a deep sequencing analysis of symptomatic responses to Huanglongbing disease. Transcriptomic data were mined using bioinformatic tools to highlight genes and pathways playing a key role in modulating responses to different types of stress in citrus fruit. Puffing disorder was linked to altered expression of genes involved in abiotic stress, vesicle transport, and protein targeting while Huanglongbing disease induced biotic stress responses and transport pathways. Sucrose and starch metabolism were the most significantly regulated pathways in both the two stresses. Huanglongbing disease significantly affected secondary metabolism (phenylpropanoid, flavonoid, and terpenoid pathways) while puffing disorder was more linked to primary metabolism (fatty acid, pentose phosphate, and glycerolipid pathways). Key genes were analyzed by qRT-PCR to define possible host biomarkers specific to each stress or which could act as general indicators of stress. Ethylene-related genes in the fruit peel were more affected by Huanglongbing than puffing. Gibberellin signaling genes (GASA1 and GASA5) were repressed under both stresses. Huanglongbing upregulated key genes involved in biotroph responses such as methylsalicylate and WRKY70. A protein–protein network revealed that heat shock proteins were major, transcriptionally regulated hubs under stress conditions as shown by the repression of HSP82.