P-ring: The conserved nature of phosphorus enriched cells in seedling roots of distantly related species

Plants require sunlight, carbon dioxide, water and mineral ions for their growth and development. Roots in vascular plants sequester water and ions from soil and transport them to the aboveground parts of the plant. Due to heterogeneous nature of soil, roots have evolved several regulatory barriers from molecular to organismic level that selectively allows certain ions to enter the vascular tissues for transport according to the physiological and metabolic demands of plant cell. Current literature profusely elaborates about apoplastic barriers, but the possibility of the existence of a symplastic regulation through phosphorous-enriched cells has not been mentioned. Recent investigations on native ion distribution in seedling roots of several species (Pinus pinea, Zea mays and Arachis hypogaea) identified an ionomic structure termed as “P-ring”. The P-ring is composed of a group of phosphorous-rich cells arranged in radial symmetry encircling the vascular tissues. Physiological investigations indicate that the structure is relatively inert to external temperature and ion fluctuations while anatomical studies indicates that they are less likely to be apoplastic in nature. Furthermore, their localization surrounding vascular tissues and in evolutionarily distinct plant lineages might indicate their conserved nature and involvement in ion regulation. Undoubtedly, this is an interesting and important observation that has significant merit for further investigations by the plant science community.