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Review
, 39 (7), 524-9

Control of Asymmetric Cell Divisions During Root Ground Tissue Maturation

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Review

Control of Asymmetric Cell Divisions During Root Ground Tissue Maturation

Ji Won Choi et al. Mol Cells.

Abstract

Controlling the production of diverse cell/tissue types is essential for the development of multicellular organisms such as animals and plants. The Arabidopsis thaliana root, which contains distinct cells/tissues along longitudinal and radial axes, has served as an elegant model to investigate how genetic programs and environmental signals interact to produce different cell/tissue types. In the root, a series of asymmetric cell divisions (ACDs) give rise to three ground tissue layers at maturity (endodermis, middle cortex, and cortex). Because the middle cortex is formed by a periclinal (parallel to the axis) ACD of the endodermis around 7 to 14 days post-germination, middle cortex formation is used as a parameter to assess maturation of the root ground tissue. Molecular, genetic, and physiological studies have revealed that the control of the timing and extent of middle cortex formation during root maturation relies on the interaction of plant hormones and transcription factors. In particular, abscisic acid and gibberellin act synergistically to regulate the timing and extent of middle cortex formation, unlike their typical antagonism. The SHORT-ROOT, SCARECROW, SCARECROW-LIKE 3, and DELLA transcription factors, all of which belong to the plant-specific GRAS family, play key roles in the regulation of middle cortex formation. Recently, two additional transcription factors, SEUSS and GA- AND ABA-RESPONSIVE ZINC FINGER, have also been characterized during ground tissue maturation. In this review, we provide a detailed account of the regulatory networks that control the timing and extent of middle cortex formation during post-embryonic root development.

Keywords: asymmetric cell division; ground tissue; plant hormone; root development; transcription factor.

Figures

Fig. 1.
Fig. 1.
Arabidopsis root development. (A) Arabidopsis root development under standard conditions. As the root ages, length of the primary root increases and lateral roots emerge in the later stages. Scale bar, 1 cm. (B) Schematic presentation of the Arabidopsis root during GT maturation. The left panel illustrates a longitudinal axis of the root at an early stage in post-embryonic root development. The quiescent center (QC) and adjacent stem cells form the stem cell niche, which gives rise to cells in diverse lineages. As the root ages, the endodermis undergoes additional periclinal ACDs to generate the endodermis (EN; blue) and the middle cortex (MC; red), which is located between the endodermis and the cortex (CO; green). The right panel shows that the root has three layers in the GT: endodermis (EN), middle cortex (MC), and cortex (CO) at maturity.
Fig. 2.
Fig. 2.
Schematic model of the regulatory networks involved in MC formation. In the GA signaling pathway, bioactive GAs negatively regulate DELLA transcription factors by facilitating proteolytic degradation of DELLAs. SCL3, acting downstream of both DELLA and SHR/SCR transcription factors, serves as an endodermis-specific integrator. In addition, SEU is involved in the control of MC formation, by positively regulating the expression of SHR, SCR, and SCL3. In parallel, the ABA pathway also controls the abundance of GAZ mRNA, which plays a role in the transcriptional control of GA and ABA metabolism. GAZ is positioned downstream of SCL3, which regulates the level of GAZ expression via an unknown transcription factor (TF X). Thus, GAZ acts as a point of convergence for the ABA and GA pathways. Together, multiple inputs from plant hormone pathways (ABA and GA) and developmental pathways (SHR/SCR and SEU) should be coordinately integrated to control maturation of the Arabidopsis root GT. Arrows represent positive regulation, and bars denote negative regulation.

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