ABA plays a central role in mediating the regulatory effects of nitrate on root branching in Arabidopsis

The formation of lateral roots (LR) is a major post-embryonic developmental event in plants. In Arabidopsis thaliana, LR development is inhibited by high concentrations of NO3-. Here we present strong evidence that ABA plays an important role in mediating the effects of NO3- on LR formation. Firstly, the inhibitory effect of NO3- is significantly reduced in three ABA,insensitive mutants, abi4-1, abi4-2 and abi5-1, but not in abi1-1, abi2-1 and abi3-1. Secondly, inhibition by NO3- is significantly reduced, but not completely abolished, in four ABA synthesis mutants, aba1-1, aba2-3, aba2-4 and aba3-2. These results indicate that there are two regulatory pathways mediating the inhibitory effects of NO3- in A. thaliana roots. One pathway is ABA-dependent and involves ABI4 and ABI5, whereas the second pathway is ABA-independent. In addition, ABA also, plays a role in mediating the stimulation of LR elongation by local NO3- applications.