Within- and among-leaf variations in photo-physiological functions, gene expression and DNA methylation patterns in the large-sized seagrass Posidonia oceanica

The knowledge of how molecular functions vary in relation to developmental and environmental cues within and among seagrass leaves is scarce in comparison with terrestrial angiosperms. This strongly limits the mechanistic understanding of photosynthetic development and light acclimation processes in seagrasses, besides having fundamental methodological implications when small leaf sections are utilized as a proxy for assessing the photosynthetic performance and molecular responses to environmental changes for the whole plant. Here, the expression gradients of genes associated with key plant metabolic processes (i.e. photosynthesis, energy dissipation mechanisms, stress response and programmed cell death) were determined, for the first time, in three segments (i.e. basal, medium and high) along the longitudinal axis of three ranked leaves (i.e. leaf 1, 2 and 3) in the large-sized seagrass Posidonia oceanica. The evaluation of major shifts in gene expression paralleled the analysis of photo-physiological properties and global DNA methylation level of the different leaf sections. Photo-physiological and molecular results converged in suggesting that the within-leaf (vertical) gradient was stronger than the leaf-rank (horizontal) gradient, likely reflecting the sharp irradiance attenuation occurring inside the complex canopy formed by this species. Specific correlations between target gene expression and photo-physiological measurements were found, providing a first description of molecular rearrangements underlying the differential photosynthetic performance and light acclimation capacity of seagrass leaves. DNA methylation varied with tissue age, being higher in the youngest and oldest leaf sections, while decreasing in intermediate tissues. We interpreted such changes as a consequence of the interplay between developmental and light cues.