NEW INSIGHTS ON MEDITERRANEAN TREE RESPONSE TO LOW TEMPERATURES: TRANSCRIPTOME CHANGES OF OLIVE TREE LEAVES EXPOSED TO COLD

Olea europaea is an evergreen fruit trees with thermophilic and halophilic characteristics to adapt to environmental conditions of the Mediterranean basin. Otherwise, olive tree cultivation in colder environment provides olive oil with peculiar qualitative characteristics. However, the cultivation of O. europaea in Northern regions must face challenges of severe winter, which can affect olive production as well as plant survival. Furthermore, despite the capacity of cold acclimation, winter warm spells can break the acclimation and expose olive plants to high risk of cold damages. So far, little is known about the molecular changes induced by exposition of olive trees to low temperatures. This work aims to study the short and long term transcriptional changes induced in leaves of olive trees exposed to progressive lowering of temperatures, till light freezing conditions. A RNA-seq approach based on Illumina GAIIx platform has been applied for the transcriptome analysis of the cv Leccino. Transcripts have been de novo assembled, annotated, and counted to identify differentially expressed genes (DEG). The analysis results in 5464 DEGs. We found that O. europaea activates known and conserved molecular responses to low temperatures, for instance the induction of genes encoding enzymes associated to changes in membrane composition as well as stress-related genes like cold regulated genes (cor) and dehydrins, and the downregulation of photosynthesis-related genes. However, some peculiarities characterize the cold response of olive tree. The glutathione cycle, polyamines and flavonoids are specifically exploited to scavenge reactive oxygen species (ROS), raffinose and trehalose are likely the most abundant osmolytes, the signaling pathway of abscisic acid (ABA) is strongly implicated, and the composition of cell wall changes due to synthesis of callose and lignins. The transcriptional factors regulating some of these transcriptional changes have been identified, for instance three bHLH and one MYB transcription factors regulating anthocyanin biosynthesis genes, a MYB gene known to be involved in ROS regulation and ABF transcription factors belonging to ABA signaling. Moreover, the de novo transcriptome was searched to identify CBF-like transcripts, the central switch of the cold acclimation response highly conserved among plant species from Arabidopsis to woody trees. A qRT-PCR analysis showed an early induction of the main CBF upon a cold shock. The approach was extended to several cultivars known to be contrasting for frost tolerance in field trials.