| Plants develop the bulk of their root system post-embryonically through the continued elongation of the primary root, and the growth of lateral roots that are established along the length of the primary root. The mechanisms that control primary and lateral root growth, and that modulate root system architecture in response to environmental signals, are still poorly understood. Herein I describe two different approaches that I used to explore the regulation of post-embryonic root system development in the model flowering plant Arabidopsis thaliana. First, I participated in an investigation of the genetic determinants of natural variation in root system size between the closely-related Arabidopsis accessions Columbia (Col) and Landsberg erecta (Ler). My work confirmed that a statistically-predicted quantitative trait locus (QTL) on chromosome 3 was indeed involved in the modulation of root system size compared to parental lines, and provided further insight into the physiological mechanisms underlying these phenotypes. Second, I performed a forward genetic screen in the Col background to identify mutants with altered root system architecture compared to wild-type plants. I observed that one particular mutant, lrd3 (lateral root development 3), showed an increase in lateral root development and a decrease in primary root growth compared to wild-type plants. My work determined that lrd3 is a mutant allele of a previously-uncharacterized gene, and that the LRD3 gene is expressed in phloem cells throughout the plant, including primary and lateral roots. Phloem cells form a continuous conduit which is responsible for the delivery of sucrose, plant hormones, and other molecules from source (e.g. leaf) to sink (e.g. root) tissues. I discovered that lrd3 plants showed and early deficiency and later increase in the delivery of phloem content to primary root tips compared to wild-type plants, and that this delivery was correlated with the extent of growth of the primary root. Intriguingly, both experimental approaches point to an important role for long distance signaling from the aerial tissues to the root system in altering plant root system architecture. |
