Quiescent center organization and auxin role in the root apical meristem of Arabidopsis thaliana adventitious roots in planta and in vitro

Adventitious rooting is a process essential for the survival of numerous plant species because provides anchorage and contributes to water-use efficiency and extraction of nutrients from the soil. In Arabidopsis seedlings adventitious root (AR) formation is increased by exogenous auxin, darkness and specific sucrose concentrations [Takahashi et al, J Plant Res (2003); Falasca and Altamura, Plant Biosystems (2003)]. Stem thin cell layers (TCLs) produce ARs in vitro under darkness combined with of auxin (IBA/NAA) and, mainly, with auxin (IBA) and cytokinin (Kin) [Falasca et al, Plant Cell Rep (2004)]. Indeterminate growth in the primary root (PR), and the organization of all the tissues of the primary body of PR, lateral, and adventitious roots is determined by the stem niche cells, i.e. the initial cells and the quiescent center (QC) cells in the apical meristem. However, the organization and activity of the stem cell niche in the ARs still needs investigation. To this aim, we investigated the organization of stem cell niche and the possible role of auxin in its maintenance in the ARs, in planta and in TCLs. We selected QC marker lines previously used for PR and lateral roots (LRs) (i.e., QC25, pAGL42:GFP, and pWOX5:GFP), [Sabatini et al, Cell (1999); Nawy et al, Plant Cell (2005); Ding and Friml, PNAS (2010)]. Moreover, a DR5:GUS line (harbouring the uidA gene driven by the auxin-inducible DR5 promoter) and a PIN1:GUS line were used to monitor auxin localization in the apical meristem. The seedlings were grown under darkness either without hormones (HF) or with NAA, Kin, IBA, and IBA plus Kin, and the developmental stages of ARs were characterized. The definition of the QC, and the localization of the auxin maximum, were also investigated in the ARs from TCLs cultured with IBA plus Kin. AR formation in planta was enhanced by NAA, IBA and IBA plus Kin, in comparison with the HF condition, whereas it was sporadic with Kin alone. However, NAA treatment induced anomalies in comparison with IBA+Kin treatment. DR5:GUS and PIN1:GUS activities were observed in the dividing cells involved in AR formation, independently on the hormonal treatment. During AR development, a gradient of DR5 activity was established, and it was similar to the pattern of DR5 activity previously observed during LR formation [Benková et al, Cell (2003)]. In planta the QC was defined in the AR primordia at the same stage in which its definition occurs in the LR primordia [Malamy and Benfey, Development (1997)]. Anomalous expression of the QC markers, DR5:GUS and PIN1:GUS constructs occurred in the anomalous ARs formed in the NAA treatment. In vitro, the ARs produced by the TCLs showed DR5:GUS activity at the apex, and the expression of the QC markers was the same as in ARs in planta, sustaining the important role of auxin and cytokinin in the stem cell niche formation and activity in the ARs of Arabidopsis.