PhD Candidate, Research Assistant University of Wisconsin-Madison Madison, Wisconsin
Body of Abstract: Root growth is a key process for water and nutrient acquisition that drives plant. Roots grow when cells produced by the meristem transiently elongate, reaching peak growth rates of 60% their length per hour before abruptly slowing at the end of the elongation zone. We developed a high-throughput method for measuring this dynamic growth in Arabidopsis roots. Our goal was to identify the genetic regions responsible for the variation in the growth rates and determine their relationship to root gravitropism traits. We used a kinematic analysis framework that enables us to measure root growth traits, such as the overall growth rate, the length of the growth zone, the maximum relative elemental growth rate (REGR), and the axial position of the maximum REGR. We measured 1,575 roots representing 162 recombinant inbred lines (RILs) derived from a Cvi x Ler cross and mapped 10 significant quantitative trait loci (QTL) for these 4 kinematic traits. In three instances, the same locus was responsible for two or more growth traits. We then explored the relationship between these growth traits and root gravitropic phenotypes, as gravitropism is driven by asymmetrical cell expansion within the growth zone and forces the root to bend. When comparing the QTLs of kinematic traits to those of tip angle QTLs from the same RIL population, there were no significant overlaps. Additionally, the principal component of the tip angle data does not correlate with any one of the kinematic traits. We then extracted the max swing rate from tip angle curves to calculate the point of greatest differential between the top and bottom sides of the root during gravitropism. This also did not correlate with any of the kinematic traits. Despite gravitropism resulting from asymmetrical cell elongation within the growth zone, the kinematic traits do not determine a root’s gravitropic response.