Expression studies on AUX1-like genes in Medicago truncatula suggest that auxin is required at two steps in early nodule development

Medicago truncatula contains a family of at least five genes related to AUX 1 of Arabidopsis thaliana (termed MtLAX genes for Medicago truncatula-like AUX1 genes). The high sequence similarity between the encoded proteins and AUX1 implies that the MtLAX genes encode auxin import carriers. The MtLAX g enes are expressed in roots and other organs, suggesting that they play ple iotropic roles related to auxin uptake. In primary roots, the MtLAX genes a re expressed preferentially in the root tips, particularly in the provascul ar bundles and root caps. During lateral root and nodule development, the g enes are expressed in the primordia, particularly in cells that were probab ly derived from the pericycle. At slightly later stages, the genes are expr essed in the regions of the developing organs where the vasculature arises (central position for lateral roots and peripheral region for nodules), The se results are consistent with MtLAX being involved in local auxin transpor t and suggest that auxin is required at two common stages of lateral root a nd nodule development: development of the primordia and differentiation of the vasculature.