Abstract
Asymmetric cell division (ACD) and positional signals play critical roles in the tissue patterning process. In the Arabidopsis (Arabidopsis thaliana) root meristem, two major phloem cell types arise via ACDs of distinct origins: one for companion cells (CCs) and the other for proto- and metaphloem sieve elements (SEs). The molecular mechanisms underlying each of these processes have been reported; however, how these are coordinated has remained elusive. Here, we report a new phloem development process coordinated via the SHORTROOT (SHR) transcription factor in Arabidopsis. The movement of SHR into the endodermis regulates the ACD for CC formation by activating microRNA165/6, while SHR moving into the phloem regulates the ACD generating the two phloem SEs. In the phloem, SHR sequentially activates NAC-REGULATED SEED MORPHOLOGY 1 (NARS1) and SECONDARY WALL-ASSOCIATED NAC DOMAIN PROTEIN 2 (SND2), and these three together form a positive feedforward loop. Under this regulatory scheme, NARS1, generated in the CCs of the root differentiation zone, establishes a top-down signal that drives the ACD for phloem SEs in the meristem. SND2 appears to function downstream to amplify NARS1 via positive feedback. This new regulatory mechanism expands our understanding of the sophisticated vascular tissue patterning processes occurring during postembryonic root development.plantcell;32/5/1519/FX1F1fx1.
Highlights
The evolutionary success of multicellular organisms raises the key developmental question of how their complex morphogenesis is regulated
In the Arabidopsis root, phloem Sieve elements (SEs) and companion cells (CCs) are generated by the asymmetric cell division (ACD) of different precursors rather than a single precursor (Baum et al, 2002; Bonke et al, 2003)
SHR that moves into the phloem pole is primarily responsible for the ACD of the SE precursor, while SHR moving into the endodermis is responsible for the ACD of the procambium neighboring SE precursor and pericycle, which generates the CC and procambium
Summary
The evolutionary success of multicellular organisms raises the key developmental question of how their complex morphogenesis is regulated. Studies have shown that asymmetric cell division (ACD) and positional information play critical roles in the temporal and spatial regulation of tissue patterning during morphogenesis (Berger et al, 1998; Kerszberg and Wolpert, 2007; Abrash and Bergmann, 2009; De Smet and Beeckman, 2011; Kajala et al, 2014). Plant cells, caged in the rigid cell wall, largely rely on direct cell-to-cell communication through plasmodesmata to exchange positional information. Phloem is a tissue evolved to facilitate long-distance nutrient transport from source to sink. Sieve elements (SEs) and companion cells (CCs) form a functional unit for phloem (reviewed in Oparka and Turgeon, 1999).
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