Effects of light deficiency on genome methylation in Posidonia oceanica

Growth and distribution of seagrasses is tightly related to undersea light quality and intensity. Currently, several factors linked to anthropogenic pressure, such as increased sediment runoff, resuspension of bottom sediments, algae accumulations, reduce sub-surface light availability in most Mediterranean coastal areas, causing a general regression of Posidonia oceanica (L.) Delile meadows. In the present work we investigated whether in a natural population of P. oceanica subjected to low-light conditions, plant capacity to adapt to light deficiency could be related to a repatterning of genome methylation. DNA methylation status was analyzed in shoot apical meristem and leaves of plants growing in a disturbed meadow, characterized by reduced light availability, versus plants growing in a preserved one. Two complementary approaches were applied: (1) the Methylation-Sensitive Amplification Polymorphism (MSAP) technique, and (2) immunocytological 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 harvest such as PHYTOCHROMEB and LIGHT HARVESTING COMPLEX OF PHOTOSYSTEM II 5 genes. It is likely that epigenetic regulation of these two genes through methylation plays a role in the resistance and resilience of P. oceanica plants to critical light conditions. From a more general point of view, we propose that the status of genome methylation features as a promising "diagnostic" tool for an early detection of stress factors impending on P. oceanica meadows.