The root system is responsible for water and nutrients uptake from the soil, and therefore, its extension is basic for an efficient acquisition. The maize root system is formed by different types of roots, and the lateral root branching substantially increases the surface for nutrient uptake. Therefore, the regulation of lateral root formation is fundamental in the development of root functions. Root architecture is basically controlled by auxin and cytokinins, which antagonize in the formation of lateral roots (LR) along the primary root axis, with auxin, a stimulator, and cytokinins inhibitors of LR development. This interaction has been analyzed in several zones along the primary root where LRs in different developmental stages were located. The root has been divided into several zones, such as meristem, elongation zone, and mature zone, according to the developmental processes occurring in each one. As Arabidopsis root elongated more slowly than maize root, these zones are shorter, and its delimitation is more difficult. However, these zones have previously been delimitated clearly in maize, and therefore, they analyze the effect of exogenous hormones in several LR developmental stages. The inhibitory effect of cytokinin on lateral root formation was observed in already elongated primary root zones in which initial events to form new lateral roots are taking place. Contrarily, auxin increased LR formation in the primary root segments elongated in the presence of the hormone. The inhibitory effect of cytokinin was reversed by auxin in a concentration-dependent manner when both hormones were combined. However, auxin is unable to recover LR development in primary root zones that have been previously elongated only in the presence of cytokinin. This antagonistic auxin-cytokinin effect on LR development depended on the balance between both hormones, which controls the root system architecture and determines the formation of LR during the process of initiation.

Studies concerning abscisic acid (ABA) involvement in root system architecture (RSA) and the interaction of ABA with auxin have reported contrasting results. In this study, the effects of exogenous ABA application and withdrawal as well as a combined treatment of ABA with the synthetic auxin 1-naphthaleneacetic acid (NAA) were thoroughly investigated in maize. The results showed that ABA reduced both the primary root (PR) elongation and the lateral root density (LRD), whereas NAA inhibited PR elongation but increased LRD. The combined treatment involving ABA and NAA inhibited PR elongation. Regarding ABA withdrawal, PR elongation was restored when ABA was removed from the growth media, but LRD was not restored after ABA withdrawal. However, the results of the combined treatment showed that auxin can reverse the inhibitory effect of ABA on LRD. A more in-depth analysis revealed that the inhibitory effect of ABA on lateral root (LR) formation depends on the stage of development. Exogenously added ABA blocked the development of lateral root primordia (LRPs) in the early stages, but was unable to inhibit the elongation of developed LRPs. These results suggest that ABA arrests the formation of LRPs rather than the growth and emergence of LRPs and their subsequent elongation.

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