Lateral root formation is governed by a complex network of hormonal regulations. The plant hormone auxin dominates this process. Auxin has been shown to positively regulate lateral root initiation and development.For example, exogenous application or endogenous overproduction of auxin enhance lateral root initiation and on the other hand, mutants impaired in auxin signalling completely failed to initiate lateral roots.
Also during post-initial stages auxin transport dependent asymmetric distribution is crucial for development of primordia. This positive effect of auxin is effectively counteracted by cytokinin. Exogenously applied cytokinins inhibit lateral root formation and accordingly overexpression of CK oxidase stimulates lateral root initiation. To identify components of the molecular interaction between auxin and cytokinin involved in lateral root formation genetic approach was employed. We screened for mutants that produce lateral roots after application of auxin simultaneously with inhibiting cytokinin concentrations. So far, 17 novel primordia on auxin and cytokinin (pac) mutant alleles have been recovered from the EMS mutagenized population of 1700 M1 families. The mutants can be classified based on their lateral root formation and response to auxin and cytokinin Some mutants show enhanced primordia initiation representing probably basic regulators of lateral root formation and others show resistance to inhibitory cytokinin effect, representing known as well as novel components of cytokinin signalling. The best candidates for cross-talk components are mutants, in which the basal process of lateral root initiation is not affected and the cytokinin resistance phenotype is manifested only in the presence of auxin.