Feedback from Lateral Organs Controls Shoot Apical Meristem Growth by Modulating Auxin Transport

1. Download high-res image (235KB)
2. Download full-size image

     

Cell Differentiation and Organ Initiation at the Shoot Apical Meristem

Plants continuously generate organs at the flanks of their shoot apical meristems (SAMs). The patterns in which these organs are initiated, also called patterns of phyllotaxis, are highly stereotypic and characteristic for a particular species or developmental stage. This stable, predictable behaviour of the meristem has led to the idea that organ initiation must be based on simple and robust mechanisms. This conclusion is less evident, however, if we consider the very dynamic behaviour of the individual cells. How dynamic cellular events are coordinated and how they are linked to the regular patterns of organ initiation is a major issue in plant developmental biology.     

植物茎尖分生组织分化调控机制研究进展

茎尖分生组织是位于植物顶端具有持续分化能力的组织,通过细胞分裂、分化产生茎、叶和花等器官,形成植株地上部分。茎尖分生组织在分化过程中受外界环境因素、内源激素水平和分子调控等影响,表现出明显变化。该文综合国内外近年来有关茎尖分生组织分化调控的研究进展,从茎尖分生组织的形态结构和环境影响因素,以及激素调控和分子调控等方面,对茎尖分生组织分化活动的研究进行综述,并对目前研究现状存在问题及未来研究方向进行了分析和展望。     

茎顶端分生组织在植物发育过程中的保持、转变和逆转

顶端分生组织(shoot apical meristems,SAM)为产生新的器官和组织而不断提供新的细胞,它的活性依赖于平衡分生组织细胞的增殖和器官发生之间关系的调控基因.来自不具备光合能力的顶端分生组织的细胞可形成具有光合能力的营养器官.在从营养生长到生殖发育的转变过程中,茎顶端分生组织,转变为花序分生组织,最终形成花分生组织.在进入开花决定状态以前,SAM的状态很大程度上受到环境信号和转录调控因子的影响.以模式植物拟南芥为主,对在顶端分生组织的保持和转变中复杂同时又有差异的基因调控网络进行讨论.在花和花序分生组织逆转过程中,SAM中的细胞也受到相关基因的调控,且表达方式存在明显的时空差异.因此,具有决定性的和未决定性双重特性的分生组织之间的转变和相互协调,对于器官发生和形态建成起到至关重要的作用.     

Control of Rice Embryo Development,Shoot Apical Meristem Maintenance,and Grain Yield by a Novel Cytochrome P450

Angiosperm seeds usually consist of two major parts： the embryo and the endosperm. However, the molec- ular mechanism（s） underlying embryo and endosperm development remains largely unknown, particularly in rice, the model cereal. Here, we report the identification and functional characterization of the rice GIANT EMBRYO （GE） gene. Mutation of GE resulted in a large embryo in the seed, which was caused by excessive expansion of scuteUum cells. Post-embryonic growth of ge seedling was severely inhibited due to defective shoot apical meristem （SAM） mainte- nance. Map-based cloning revealed that GE encodes a CYP78A subfamily P450 monooxygenase that is localized to the endoplasmic reticulum. GE is expressed predominantly in the scutellar epithelium, the interface region between embryo and endosperm. Overexpression of GE promoted cell proliferation and enhanced rice plant growth and grain yield, but reduced embryo size, suggesting that GE is critical for coordinating rice embryo and endosperm development. Moreover, transgenic Arabidopsis plants overexpressing AtCYP78AlO, a GE homolog, also produced bigger seeds, implying a con- served role for the CYP78A subfamily of P450s in regulating seed development. Taken together, our results indicate that GE plays critical roles in regulating embryo development and SAM maintenance.     

Lateral Root Initiation or the Birth of a New Meristem

Root branching happens through the formation of new meristems out of a limited number of pericycle cells inside the parent root. As opposed to shoot branching, the study of lateral root formation has been complicated due to its internal nature, and a lot of questions remain unanswered. However, due to the availability of new molecular tools and more complete genomic data in the model species Arabidopsis, the probability to find new and crucial elements in the lateral root formation pathway has increased. Increasingly more data are supporting the idea that lateral root founder cells become specified in young root parts before differentiation is accomplished. Next, pericycle founder cells undergo anticlinal asymmetric, divisions followed by an organized cell division pattern resulting in the formation of a new organ. The whole process of cell cycle progression and stimulation of the molecular pathway towards lateral root initiation is triggered by the plant hormone auxin. In this review, we aim to give an overview on the developmental events taking place from the very early specification of founder cells in the pericycle until the first anticlinal divisions by combining the knowledge originating from classical physiology studies with new insights from genetic-molecular analyses. Based on the current knowledge derived from recent genetic and developmental studies, we propose here a hypothetical model for LRI.     

Morphogenesis and Patterning at the Organ Boundaries in the Higher Plant Shoot Apex

Formation of lateral organ primordia from the shoot apical meristem creates boundaries that separate the primordium from surrounding tissue. Morphological and gene expression studies indicate the presence of a distinct set of cells that define the boundaries in the plant shoot apex. Cells at the boundary usually display reduced growth activity that results in separation of adjacent organs or tissues and this morphological boundary coincides with the border of different cell identities. Such morphogenetic and patterning events and their spatial coordination are controlled by a number of boundary-specific regulatory genes. The boundary may also act as a reference point for the generation of new meristems such as axillary meristems. Many of the genes involved in meristem initiation are expressed in the boundary. This review summarizes the cellular characters of the shoot organ boundary and the roles of regulatory genes that control different aspects of this unique region in plant development.     

Toward Molecular Understanding of In Vitro and In Planta Shoot Organogenesis

Shoot organogenesis, one of the in vitro plant regeneration processes that occur during in vitro micropropagation, is used in the study of plant development. Morphological, physiological, and molecular aspects of in vitro shoot organogenesis have been extensively studied for over 50 years. Because of the research progress in plant genetics and molecular biology, our understanding of in planta and in vitro shoot meristem development, the cell cycle and cytokinin signal transduction has advanced significantly. These research advances provide useful information as well as molecular tools to study further the genetic and molecular aspects of shoot organogenesis. A number of key molecular markers, genes, and pathways have been shown to play a critical role in the process of in vitro shoot organogenesis. Furthermore, these studies reveal that in vitro shoot organogenesis, as with in planta shoot development, is a complex, well-coordinated developmental process, given that the induction of a single molecular event is likely to be insufficient to induce the entire process. Continued study is required to identify additional molecular events that trigger dedifferentiation and act as developmental switches for de novo shoot development.