Effects of light deficiency on genome methylation in Posidonia oceanica

Growth and distribution of seagrasses is closely related to undersea light quality and intensity. Currently, several factors linked to anthropogenic pressure, such as increased sediment runoff, resuspension of bottom sediments, and algae accumulations, reduce sub-surface light availability in most Mediterranean coastal areas, and are causing a general regression of Posidonia oceanica (L.) Delile meadows. We investigated whether plant capacity to adapt to light deficiency could be related to a repatterning of genome methylation in a natural population of P. oceanica subjected to low-light conditions. DNA methylation status was analyzed in shoot apical meristems and leaves of plants growing in a disturbed meadow, characterized by reduced light availability, versus plants growing in a preserved meadow. Two complementary approaches were applied, viz. the methylation-sensitive amplification polymorphism (MSAP) technique and immuno cytological detection of methylated sites on interphase nuclei. Genome hypermethylation was detected in the organs of plants growing at the disturbed site, indicating that de novo methylation occurred under light-related stress conditions. MSAP analysis revealed that changes in methylation status involved specific classes of genes with various functions, including light perception and harvesting genes such as PoPHYB and LHCB5. It is likely that epigenetic regulation of these 2 genes through methylation plays a role in the resistance and resilience of P. oceanica plants to critical light conditions. We propose that the status of genome methylation is a promising diagnostic tool for early detection of stress factors affecting P. oceanica meadows.