Lateral root

Lateral roots extend horizontally from the primary root and serve to anchor the plant securely into the soil. This branching of roots also contributes to water uptake, and facilitates the extraction of nutrients required for the growth and development of the plant.

Many different factors are involved in the formation of lateral roots. Regulation of root formation is tightly controlled by plant hormones such as auxin, and by the precise control of aspects of the cell cycle. Such control can be particularly useful: increased auxin levels, which help to promote lateral root development, occur when young leaf primordia form and are able to synthesise the hormone. This allows coordination of root development with leaf development, enabling a balance between carbon and nitrogen metabolism to be established.

Early Morphological Changes

The following description is for early events in lateral root formation of the model organism "Arabidopsis thaliana", where lateral roots typically form when the plant is between seven and nine days old.
*Stage I: The first morphologically identifiable stage is the asymmetric division of two cells of the pericycle, termed pericycle founder cells, which are adjacent to the protoxylem poles and from which the lateral roots are derived entirely. These cells then undergo further division, causing radial expansion.
*Stage II: The small, central cells then divide periclinally (parallel to the surface of the plant body) in a series of transverse, asymmetric divisions such that the young primordium becomes visible as a projection made up of an inner layer and an outer layer.
*Stages III and IV: At the third stage, the outer layer of cells divide so that the primordium is now made of three layers. The fourth stage is then characterised by the inner layer undergoing a similar division, such that four cell layers are visible.
*Stages V to VIII: Expansion and further division of these four layers eventually results in the emergence of the young lateral root from the parent tissue (the overlying tissue of the primary root) at stage eight.

References

*Malamy, JE. And Benfey, P.N. (1997) Down and out in Arabidopsis: The formation of lateral roots. Trends in Plant Sciences 2: 390–396
*Casimiro, I., Beeckman, T., Graham, N., Bhalerao, R., Zang, H., Casero, P., Sandberg, G. and Bennet, M. (2003) Dissecting Arabidopsis Lateral Root Development. Trends in Plant Science 8, 4: 165-169.