ONLINE IMAGE COLLECTION

About the Image

Light micrograph of a cucumber root tip releasing border cells after immersion in water. Recent concern about global warming and the underlying anthropogenic increase in atmospheric CO₂ has led to intense interest in carbon cycles. One component of the carbon cycle is soil sequestration of carbon, which is ecologically important. Much of the underground cycling of carbon occurs in rhizospheres, small layers of soil surrounding and influenced by the roots of higher plants. Throughout its lifetime, a plant root releases organic carbon to its rhizosphere. Carbon-containing exudates have been found to be particularly abundant at root tips and at points of initiation of branch roots. The root tips, including root caps, meristems, and elongation zones, are particularly active in secreting sugars, organic acids, and specialized compounds such as phytosiderophores. Mucilage, containing high molecular weight polysaccharides (particularly polyuronic acids), is produced copiously by root caps and cortical cells. These carbon sources sustain the rich microflora of the rhizosphere. Another source of carbon is provided by border cells, a subset of peripheral root cap cells. These cells are formed as part of the root cap and released from the exterior of the cap to live freely in the soil for a time. Border cells are known to exert antibiotic effects, protecting plants from certain pathogenic microbial infections, and stimulating the growth of other microbes. While examining the importance of border cell release to carbon cycling in soils, we made the unexpected discovery that the number of cells released is strongly dependent on the developmental stage of the root. In four species that were examined, young, short seedling roots released thousands, while roots longer than 4 cm released only tens to hundreds of border cells. This developmental program is hypothesized to protect the plant during the critical period of radicle penetration of the soil following germination and during seedling establishment.

For further details, see Odell et al.: Stage-dependent border cell and carbon flow from roots to rhizosphere, American Journal of Botany, Volume 95, Issue 4, pages 441-446, http://www.amjbot.org/cgi/content/short/95/4/441.