![]() Some of the polarity genes are expressed very early, and REV and KAN1 in particular might define a prepattern ( Caggiano et al., 2017) even before the upregulation of organ founder cell markers, such as DORNROESCHEN-LIKE (DRNL) ( Chandler et al., 2011) or ARABIDOPSIS PHOSPHOTRANSFER PROTEIN 6 (AHP6) ( Besnard et al., 2014). The network also involves regulation via small RNAs ( Kuhlemeier and Timmermans, 2016). These include, for example: abaxial determinants, such as FILAMENTOUS FLOWER (FIL) ( Sawa et al., 1999 Siegfried et al., 1999) or KANADI1 (KAN1) ( Kerstetter et al., 2001) adaxial determinants, such as REVOLUTA (REV) or PHABULOSA (PHB) ( McConnell et al., 2001) and the so-called margin genes, including PRESSED FLOWER ( PRS) and WUSCHEL RELATED HOMEOBOX1 ( WOX1) ( Scanlon et al., 1996 Nakata et al., 2012), expressed in the future leaf margin and in the middle domain separating the abaxial and adaxial domains. Key are the regulators involved in establishing leaf polarity and which are activated before or during organ outgrowth. Several genes have been associated with leaf shape and size, which are determined from the earliest stages of development onwards (e.g. Although results in Solanaceae suggested that dorsoventral patterning was unstable during early development, we found that, in Arabidopsis, the local information contained within and around the primordium is able to withstand major invasive perturbations, long before polarity is fully established. Using specific markers for organ founder cells and dorsoventral identity, we were unable to perturb the polarity of leaves and sepals long before organ outgrowth. We revisited these experiments in Arabidopsis, in which a range of markers are available to target the precise site of ablation. Although these results could be reproduced in tomato, this concept has been debated. In a classical set of surgical experiments in potato meristems, Sussex provided evidence that dorsoventrality depends on a signal emanating from the meristem center. Subsequent growth usually occurs mainly in the plane of the leaf blade, which leads to the formation of a bifacial leaf with dorsoventral identities. In many species, leaves are initiated at the flanks of shoot meristems.
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