Lateral root development of two Arabidopsis auxin transport mutants, aux1-7 and eir1-1, in response to nitrate supplies

Lateral roots (LRs) formation is regulated by both intrinsic developmental programs and environmental conditions. Uniform supply of high NO3− has a systematic inhibitory effect on LR development. Yet, the mechanism underlying this response is still poorly understood. The role of polar auxin transport on LR formation is well established. In the present study, the possible role of polar auxin transport in high NO3−-mediated LR development was investigated. LR initiation, emergence and activation to NO3− supply (0.01–50 mmol L−1) were determined in auxin influx (aux1-7) and efflux carrier (eir1-1/atpin2) mutants of Arabidopsis in comparison with the wild type (Col.). Also, the expression of DR5::GUS (auxin response reporter gene) in LR primordia (LRPs) of wild type and two mutants was characterized. Exogenous NAA and 2,4-D supplies were used to further confirm the results. LRP emergence and subsequent activation were not affected when nitrate supply changed from 0.1 to 10 mmol L−1. At high nitrate supply (50 mmol L−1), LRP emergence and activation were inhibited. Auxin level as indicated by DR5::GUS expression was reduced with increasing nitrate level. However, the response of LRP development to high nitrate was largely similar in aux1-7 and eir1-1 mutants as in the wild type. Mutant aux1-7 had greater LRP emergence and activation rate even though the auxin levels (DR5::GUS expression) in its LRP were very low. Exogenous supply of NAA and/or 2,4-D increased LRP initiation, emergence, and activation. Again no difference was found between the low and high nitrate treatments and among the WT and the two auxin mutants. Taken together, we conclude that, although auxin transport and activity are essential for LRP development, the inhibitory effect of high nitrate on LRP emergence and activation might not depend directly on the regulation of auxin levels in LRPs.