Expression profiles of 12 drought responsive genes in two European (deciduous) oak species during a two-year drought experiment with consecutive drought periods

Abstract

Based on an earlier comparative transcriptomics study (Madritsch et al., 2019, in revision) 12 representative genes were selected that were differentially expressed in response to drought stress in two major European oak species, pedunculate oak (Quercus robur) and pubescent oak (Q. pubescens). RT-qPCR technique was used to validate the quantification data from the transcriptomics study and to generate detailed gene expression profiles over the course of a two-year drought experiment with repeated long-term drought events comprising eight sampling dates. The 12 genes were selected based on quantification data and on their functional annotation and belong to different pathways and phytohormone signaling networks. PP2C27 (Protein phosphatase 2C 27), RD22 (Response to desiccation 22) and STP13 (Sugar transport protein 13) are key molecules of the abscisic acid (ABA) signaling pathway that plays a major role in the modulation of the molecular drought stress response. AOS1 (Allene oxide synthase 1) is the first enzyme in the biosynthesis of jasmonates (JAs), ERF4 (Ethylen responsive transcription factor 4) is part of the ethylene-activated pathway and CDR1 (Constitutive disease resistance 1) is part of the salicylic acid (SA) dependent pathway. Two genes related to the phenylpropanoid pathway were included, PAL (Phenylalanine ammoniase-lyase) catalyzes the first step of the pathway and UGT73C6 (UDP-glycosyltransferase 73C6) plays a role in the biosynthesis of quercetins and other flavonoids, these contribute to tolerance against oxidative stress. NRX2 (Nucleoredoxin 2) also plays a role in protection against oxidative stress. Also, two genes that are related to photosynthesis, BCA2 (Beta carbonic anhydrase 2) and GPT2 (Glucose-6-phosphate/phosphate translocator 2) and the nutrient transporter BOR2 (Boron transporter 2) were included. Most genes revealed clearly different expression profiles over the course of the experiment, indicating an adjustment process during the long-term repeated drought periods. Despite being part of the same signaling network, the ABA-related genes showed divers expression profiles, reflecting the complexity of the drought response and the modulation of the response over the course of the experiment in the two Quercus species. The expression profiles for AOS1, ERF4 and PAL indicate that they were upregulated by JA pathway in Q. pubescens independently from drought stress treatment, contradicting an involvement of JA pathway to the drought stress response. BCA2, NRX2, PP2C27 showed a contrary regulation in the two species that might be linked to their different drought tolerance while BOR2 and UGT73C6 were shown to be drought responsive for the first time.