ABSTRACT: BACKGROUND:Similar to other urban trees, holm oaks (Quercus ilex L.) provide a physiological, ecological and social service in the urban environment, since they remove atmospheric pollution. However, the urban environment has several abiotic factors that negatively influence plant life, which are further exacerbated due to climate change, especially in the Mediterranean area. Among these abiotic factors, increased uptake of Na?+?and Cl?-?usually occurs in trees in the urban ecosystem; moreover, an excess of the tropospheric ozone concentration in Mediterranean cities further affects plant growth and survival. Here, we produced and annotated a de novo leaf transcriptome of Q. ilex as well as transcripts over- or under-expressed after a single episode of O3 (80?nl?l-1, 5?h), a salt treatment (150?mM for 15?days) or a combination of these treatments, mimicking a situation that plants commonly face, especially in urban environments. RESULTS:Salinity dramatically changed the profile of expressed transcripts, while the short O3 pulse had less effect on the transcript profile. However, the short O3 pulse had a very strong effect in inducing over- or under-expression of some genes in plants coping with soil salinity. Many differentially regulated genes were related to stress sensing and signalling, cell wall remodelling, ROS sensing and scavenging, photosynthesis and to sugar and lipid metabolism. Most differentially expressed transcripts revealed here are in accordance with a previous report on Q. ilex at the physiological and biochemical levels, even though the expression profiles were overall more striking than those found at the biochemical and physiological levels. CONCLUSIONS:We produced for the first time a reference transcriptome for Q. ilex, and performed gene expression analysis for this species when subjected to salt, ozone and a combination of the two. The comparison of gene expression between the combined salt + ozone treatment and salt or ozone alone showed that even though many differentially expressed genes overlap all treatments, combined stress triggered a unique response in terms of gene expression modification. The obtained results represent a useful tool for studies aiming to investigate the effects of environmental stresses in urban-adapted tree species.
