Development of the quiescent center and definition of the auxin maximum in Arabidopsis adventitious roots in planta and in thin cell layers

In Arabidopsis adventitious roots (ARs) are induced by darkness [Takahashi et al, J Plant Res (2003)]. Stem thin cell layers (TCLs) produce ARs in vitro under darkness and with auxin (IBA or NAA) and, mainly, auxin (IBA) plus cytokinin (Kin) [Falasca et al, Plant Cell Rep (2004)]. We investigated the definition of the quiescent center (QC) and the auxin maximum in the ARs in planta and in TCLs. We used QC marker lines previously used in lateral roots (LR) (i.e., QC25 and pAGL42:GFP) and a DR5:GUS line (harbouring uidA gene driven by the auxin-inducible DR5 promoter). The seedlings were grown under darkness either without hormones (HF) or with NAA, Kin, and IBA plus Kin. The definition of the QC and the auxin maximum were also investigated in the ARs from TCLs cultured with IBA plus Kin. AR formation in planta was enhanced by NAA and IBA plus Kin in comparison with the HF medium, and was sporadic with Kin alone. At day 11 after sowing ARs at various stages were similarly present in NAA- and IBA+Kin-grown seedlings, but AR anomalies increased with NAA. DR5:GUS activity was observed in the dividing hypocotyl 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 during lateral rooting [Benkova et al, Cell (2003)].The QC was defined in the AR primordia in planta at the same stage in which it occurs in LR primordia [Malamy and Benfey Development (1997)], and the expression of the QC markers and DR5:GUS construct was concomitant. Anomalous expression of the QC markers and DR5:GUS construct occurred in the anomalous ARs of the NAA treatment. The ARs produced by the TCLs showed DR5:GUS activity at the tip, and QC markers expression as in ARs in planta. In conclusion, QC definition and auxin maximum are related events in the ARs in planta and in vitro.