Is the auxin influx carrier LAX3 essential for plant growth and development in the model plants Medicago truncatula, Lotus japonicus and Arabidopsis thaliana?

The phytohormone auxin is transported by two distinct pathways in plants. Indole-3-acetic acid is mainly transported throughout the plant by an unregulated bulk flow in the mature phloem. The major auxin distribution is regulated via direct transport from cell to cell, known as polar auxin transport (PAT). PAT is maintained by the coordinated action of efflux (PIN) and auxin influx (AUX/LAX) carrier proteins. In this study, we examine, compare and localize the expression of a gene encoding an auxin influx carrier (MtLAX3) from Medicago truncatula in the model plants M. truncatula, Lotus japonicus and Arabidopsis thaliana. Transgenic plants with overexpression and down-regulation of MtLAX3, as well as with expressed promMtLAX3 transcriptional reporters, were constructed for the three model species, using Agrobacterium-mediated transformation. Histochemical and transcriptional analyses revealed the expression of MtLAX3 during various stages of somatic embryogenesis and plant development, as well as during formation of symbiotic nodules. The alteration of the MtLAX3 expression, as well as its overexpression in the analysed model species, results in various abnormal phenotypes and disturbance of leaf and root development. The reported results show that MtLAX3 plays an important role in proper plant growth and development, modelling of the root system and the number of formed nodules and seeds.