Transcriptome analysis of historic olives reveal stress-specific biomarkers

Hamad A. Alkhatatbeh¹Monther T. Sadder^2,3*Nizar Haddad⁴Ibrahim Al-Amad⁴Mohammad Brake⁵Nawal A Alsakarneh⁶Abdulsalam M. Alnajjar⁷

• ¹Al-Balqa Applied University, Al-Salt, Balqa, Jordan
• ²The University of Jordan, Aljubeiha, Jordan
• ³School of Agriculture, The University of Jordan, Aljubeiha, Amman, Jordan
• ⁴National Agricultural Research Center, Baq'a, Jordan
• ⁵Jerash University, Jerash, Jerash, Jordan
• ⁶Al-Huson University College, Al-Balqa Applied University, Irbid, Jordan, Irbid, Jordan
• ⁷Aqaba Customs Center, Aqaba, Jordan, Aqaba, Jordan

Water scarcity and soil salinization are increasingly becoming limiting factors for food production, including olives. Four historic olive cultivars (‘Nabali’, ‘Mehras’, ‘Frantoio’ and ‘Manzanillo’), were investigated under drought and salinity stresses. Investigating historical olives, which are the last resort for genetic resources, is essential due to their natural resilience, making them valuable for breeding sustainable production. They also preserve local biodiversity, and offer economic opportunities through unique, heritage-based olive oils. Drought and salt stress in olives are assessed through physiological (Fv/Fm, RWC), biochemical (proline content), and molecular (stress-responsive genes) analyses to evaluate stress tolerance. Under salinity and drought stress, RWC decreased in all olive cultivars, with drought having the most severe impact. Nabali exhibited the highest salinity tolerance, while all cultivars showed similar sensitivity to drought. Proline levels remained stable in Mehras but decreased under salinity stress in Frantoio, Manzanillo, and Nabali. Higher proline accumulation under drought suggested better drought tolerance than salinity in these cultivars. Photosynthetic efficiency declined under salinity and drought stress in all cultivars, with drought causing a more significant reduction. Manzanillo and Mehras exhibited the highest number of differentially expressed genes (DEGs) under both drought and salinity stress, with Manzanillo showing 2934 DEGs under drought and 664 under salinity, while Mehras had 2034 and 2866 DEGs, respectively. Nabali demonstrated the strongest salinity-specific response, with 3803 DEGs under salinity stress compared to 1346 under drought. Frantoio consistently had the lowest number of DEGs, with 345 under drought and 512 under salinity, indicating a more stable transcriptional response. Comparative analysis between drought and salinity conditions revealed significant variations, with Manzanillo showing 2599 unique DEGs under drought relative to salinity, while Nabali exhibited 2666 DEGs under salinity relative to drought. Major novel up-regulated genes under salinity stress were Xyloglucan endotransglucosylase hydrolase (7 fold in Nabali and 6.9 fold in Meharas). Drought novel genes detected in Frantoio included Phytosulfokines 3 (4.9 fold), while Allene oxide synthase (6.5 fold) and U-box domain-containing (6.4 fold) were detected in Manzanillo. The data revealed both novel unique and common stress specific biomarker, which potentially can be utilized in breeding programs of olive tree.