植物根毛生长发育及分子调控机理

植物根毛是植物吸收营养的主要器官, 了解根毛的发生、发育及遗传规律, 能对植物的养分吸收研究提供有利依据。文章旨在介绍植物根毛形态发生特性、发育生长过程及分子调控机理的研究进展, 利用比较基因组学方法研究农作物根毛形态和功能, 及有目的性的对根生长发育进行调控提供参考。研究发现植物根毛发育有反馈侧向抑制(lateral inhibition with feedback)和位置决定模式(position-dependent pattern of cell differentiation)两种方式。拟南芥根表皮细胞是以位置方式决定毛或非毛细胞发育类型, 已成为研究植物细胞命运和分化的模型。目前, 已经鉴定出控制根毛发育的基因, 包括一些转录因子如MYB家族蛋白TRIPTYCHON(TRY)、CAPRICE(CPC)和basic Helix-Loop-Helix (bHLH)蛋白GLABRA3、ENHANCER OF GLABRA3(EGL3)及WD-repeat蛋白等基因。最后针对根毛研究前景提出展望。     

WEREWOLF, a MYB-related protein in Arabidopsis, is a position-dependent regulator of epidermal cell patterning

The formation of the root epidermis of Arabidopsis provides a simple and elegant model for the analysis of cell patterning. A novel gene, WEREWOLF (WER), is described here that is required for position-dependent patterning of the epidermal cell types. The WER gene encodes a MYB-type protein and is preferentially expressed within cells destined to adopt the non-hair fate. Furthermore, WER is shown to regulate the position-dependent expression of the GLABRA2 homeobox gene, to interact with a bHLH protein, and to act in opposition to the CAPRICE MYB. These results suggest a simple model to explain the specification of the two root epidermal cell types, and they provide insight into the molecular mechanisms used to control cell patterning.     

A novel regulatory circuit specifies cell fate in the Arabidopsis root epidermis

The specification of distinct cell fates in multicellular organisms is a fundamental process in developmental biology. The Arabidopsis root epidermis, which consists of root-hair cells and non-hair cells, provides a useful model system for studying cell fate specification. In this tissue, the cell fates are determined by their relative position to the underlying cortical cells, and many genes have been identified that regulate this position-dependent cell fate specification. Recent studies using genetic, molecular, and biochemical approaches have shed new light on this process and revealed a complex network of interacting and interdependent components. In particular, a novel regulatory circuit has recently been identified, which includes a lateral inhibition pathway and a feedback loop that enables intercellular communication and ensures that two distinct cell types arise in an appropriate pattern. This regulatory circuit is also influenced by a positional signaling pathway which includes the SCRAMBLED leucine-rich repeat receptor kinase. The studies of cell fate specification in the Arabidopsis root epidermis provide new insights into the molecular strategies used to define distinct cell types in plants.     

The expression patterns of arabinogalactan-protein AtAGP30 and GLABRA2 reveal a role for abscisic acid in the early stages of root epidermal patterning

In the Arabidopsis root, patterning of the epidermal cell types is position-dependent. The epidermal cell pattern arises early during root development, and can be visualized using reporter genes driven by the GLABRA (GL)2 promoter as markers. The GL2 gene is preferentially expressed in the differentiating hairless cells (atrichoblasts) during a period in which epidermal cell identity is believed to be established. We show that AtAGP30 is also expressed in atrichoblasts. This gene encodes an arabinogalactan-protein (AGP) that is known to play a role in root regeneration and increases abscisic acid (ABA)-response rates. Although the expression level of this gene is regulated by the plant growth factors ABA and ethylene, only ABA was found to affect the tissue-specific pattern of expression. ABA also disrupts the expression pattern of the GL2::GUS (beta-glucuronidase) reporter gene. Our results indicate that ABA regulates epidermal cell-type-specific gene expression in the meristematic zone of the Arabidopsis root, while ethylene is known to act at later stages of epidermal differentiation. Despite its effects on the early stages of root epidermal patterning, ABA does not affect root hair formation on mature wild-type epidermal cells, suggesting that other developmental cues, like positional information, can progressively over-ride the ABA-mediated disruption of early epidermal patterning.     

Epidermal Patterning Genes Impose Non-cell Autonomous Cell Size Determination and have Additional Roles in Root Meristem Size Control

The regulation of cellular growth is of vital importance for embryonic and postembryonic patterning. Growth regulation in the epidermis has importance for organ growth rates in roots and shoots, proposing epidermal cells as an interesting model for cellular growth regulation. Here we assessed whether the root epidermis is a suitable model system to address cell size determination. In Arabidopsis thaliana L., root epidermal cells are regularly spaced in neighbouring tricho- (root hair) and atrichoblast (non-hair) cells, showing already distinct cell size regulation in the root meristem. We determined cell sizes in the root meristem and at the onset of cellular elongation, revealing that not only division rates but also cellular shape is distinct in tricho- and atrichoblasts. Intriguingly, epidermal-patterning mutants, failing to define differential vacuolization in neighbouring epidermal cell files, also display non-differential growth. Using these epidermal-patterning mutants, we show that polarized growth behaviour of epidermal tricho- and atrichoblast is interdependent, suggesting non-cell autonomous signals to integrate tissue expansion. Besides the interweaved cell-type-dependent growth mechanism, we reveal an additional role for epidermal patterning genes in root meristem size and organ growth regulation. We conclude that epidermal cells represent a suitable model system to study cell size determination and interdependent tissue growth.     

Hormones act downstream of TTG and GL2 to promote root hair outgrowth during epidermis development in the Arabidopsis root.

The Arabidopsis root produces a position-dependent pattern of hair-bearing and hairless cell types during epidermis development. Five loci (TRANSPARENT TESTA GLABRA [TTG], GLABRA2 [GL2], ROOT HAIR DEFECTIVE6 [RHD6], CONSTITUTIVE TRIPLE RESPONSE1 [CTR1], and AUXIN RESISTANT2 [AXR2]) and the plant hormones ethylene and auxin have been reported to affect the production of root hair and hairless cells in the Arabidopsis root. In this study, genetic, molecular, and physiological tests were employed to define the roles of these loci and hormones. Epistasis tests and reporter gene studies indicated that the hairless cell-promoting genes TTG and GL2 are likely to act early to negatively regulate the ethylene and auxin pathways. Studies of the developmental timing of the hormone effects indicated that ethylene and auxin pathways promote root hair outgrowth after cell-type differentiation has been initiated. The genetic analysis of ethylene-and auxin-related mutations showed that root hair formation is influenced by a network of hormone pathways, including a partially redundant ethylene signaling pathway. A model is proposed in which the patterning of root epidermal cells in Arabidopsis is regulated by the cell position-dependent action of the TTG/GL2 pathway, and the ethylene and auxin hormone pathways act to promote root hair outgrowth at a relatively late stage of differentiation.     

Tissue organization of fasciated lateral roots of Arabidopsis mutants suggestive of the robust nature of outer layer patterning

In three temperature-sensitive mutants of Arabidopsis, root redifferentiation 1 (rrd1), rrd2, and root initiation defective 4 (rid4), formation of fasciated lateral roots was previously observed under high temperature conditions of 28°C. When lateral roots were induced from explants of very young seedlings of these mutants by culture with exogenously supplied auxin at 28°C, expansion of lateral root primordia leading to lateral root fasciation occurred reproducibly and semi-synchronously with a high frequency. This experimental system allowed us to examine how radial organization of root tissues is altered in association with expansion of primordia. Analysis with various tissue-specific reporter genes indicated that in the fasciated lateral roots, cell files of the stele are increased markedly while the numbers of cortical and epidermal cell layers are not changed. This suggests that radial organization during root primordium development involves a mechanism that makes outer layer patterning more robust than inner layer patterning against unusual enlargement of the morphogenetic field.     

Epidermal cell patterning and differentiation throughout the apical-basal axis of the seedling

The idea of common pathways guiding different fates is an emerging concept in plant development, and epidermal cell-fate specification in Arabidopsis thaliana is an excellent example to illustrate it. In the root epidermis, both hair patterning and differentiation depend on a complex interaction between both negative (WER, TTG, GL3, EGL3, and GL2) and positive (CPC, TRY, and ETC1) regulators of hair cell fate. These regulators pattern and differentiate hairs through a bi-directional signalling mechanism. The same molecular components (WER, TTG, GL3, EGL3, and GL2) seem to be involved in the patterning of stomata in the embryonic stem. However, the possible role of CPC, TRY, and ETC1 on stomatal patterning and/or differentiation has not been studied, questioning whether they, and the underlying bi-directional mechanism, guide patterning formation and differentiation in the hypocotyl.