Genome-wide identification of Fagus sylvatica aquaporins and their comparative spring and summer expression profiles

European beech (Fagus sylvatica) has been widely studied in terms of its water relations and local adaptation. However, to date, the underlying basis conferring adaptation to differences in water availability are unknown. Therefore, we examined the expression of aquaporins in trees of four different beech provenances representing the southern and northern range margins, as well as core populations, grown in a common garden. We sampled their xylem, phloem and leaf tissue, when leaves had fully expanded, and in late summer. A total of 45 aquaporin isoforms were identified in the beech genome, of which 35 were detected across all sampled tissues. In our phylogenetic analysis, beech aquaporins clustered into the five subfamilies found in other woody species. Members of the plasma membrane intrinsic protein subfamily generally displayed the highest levels of expression, followed by tonoplast intrinsic proteins. Isoforms of the remaining subfamilies, Noduline-26-like intrinsic proteins, small basic intrinsic proteins and uncharacterised intrinsic proteins, were expressed at very low to moderate levels. The expression of most isoforms was stable or declined from spring to summer. Leaves followed a different expression profile from that of vascular tissues, whereas both phloem and xylem were found to express the same Fs MIPs. Tissue-specific aquaporin expression was very similar amongst the four beech provenances, indicating that there is no inherent difference in the capability of these provenances to regulate aquaporin activity. The general decrease in Fs MIP expression toward the end of the growing period indicates that aquaporins are involved in tree water relations and growth.