Auxin controls quiescent centre formation in the adventitious root apex of Arabidopsis, and the switching between adventitious rooting and xylogenesis

Adventitious root (ARs) are essential for the survival of numerous plant species, because they contribute to anchorage, water-use efficiency and extraction of nutrients from the soil. Moreover, the regulation of AR induction is crucial for successful micropropagation in vitro and breeding programs. In Arabidopsis, ARs are formed by the hypocotyl pericycle, in planta, and by the stem endodermis in in vitro cultured thin cell layers (TCLs) (Falasca et al., 2004). The indeterminate growth of primary root (PR) and lateral roots (LRs) is supported by the quiescent centre (CQ), which co-ordinates the activity of the surrounding stem cells, with a central role in establishment, maintenance and elaboration of patterns in the apical meristem, with an involvement of both auxin and cytokinin (Jang and Feldman, 2005). In contrast, the organization and activity of the QC in the ARs still needs investigation, the same as the possible roles of auxin and cytokinin on its establishment and maintenance. The transcription factors SHORT ROOT (SHR) and SCARECROW (SCR) are key regulators jointly controlling PR and LR stem cell definition/maintenance, and radial patterning (Helariutta et al., 2000; Sabatini et al., 2003), but their involvement in AR-formation is uncertain. AUXIN RESISTANT1 (AUX1) and LIKE AUXIN RESISTANT3 (LAX3) are auxin influx carriers contributing to PR and LR development. LAX3 expression is known to be regulated by SHR (Sozzani et al., 2010), and SCR is known to be under auxin control. Moreover, the QC marker WOX5 is controlled by the SHR/SCR complex. The interaction of SHR/SCR in AR-identity acquisition in unknown, as the role of the complex on the auxin-influx carriers and QC markers of the ARs. Xylogenesis is known to be induced by auxin, but the genes involved are largely unknown. However, the SHR-SCR complex affects PR vascular differentiation regulating the microRNA165/6. The first aim of this research was to investigate whether the QC is established in Arabidopsis ARs, in planta and in TCLs, and whether its establishment and maintenance are under auxin-control. To this aim, the activity of PR/LR QC markers (QC25::GUS, pAGL42::GFP, pWOX5::GFP), the expression patterns of DR5::GUS line, of PIN1 and LAX3 auxin carriers expression, and YUCCA6 transcription were monitored. Subsequently, the research was aimed to determine SHR/SCR activities on AR-formation, and their relationship with the auxin-influx carriers AUX1 and LAX3 in AR-formation, but also in xylogenesis, in hypocotyls and stem TCLs of Arabidopsis. Results show that the QC is established in the ARs. Independently of the founder cells, auxin accumulation and WOX5 expression characterize the early derivative cells involved in AR-formation. By the activities of auxin influx/efflux carriers, an auxin-maximum is determined at the AR-tip, where WOX5 expression is restricted, positioning the QC. An auxin biosynthesis by YUCCA6 gene is required in the apex for QC maintenance. The SHR/SCR complex is early active during AR-formation, and AUX1, mediating the auxin influx in the AR-initiating cells, seems related to the priming activity of the complex. Interestingly, AR-formation and xylogenesis are developmental programmes inversely related, with SHR, SCR and AUX1 controlling the fine-tuning between the two.