New clues to organ size control in plants

Plant growth has unparalleled importance for human civilization, yet we are only starting to gain an understanding of its mechanisms. The growth rate and final size of plant organs is determined by both genetic constraints and environmental factors. Regulatory inputs act at two control points: on proliferation; and on the transition between proliferation and differentiation. Cell-autonomous and short-range growth signals act within meristematic domains, whereas diffusible signals from differentiated parts to proliferating cells provide measures of geometry and size and channel environmental inputs.     

Plant TOR signaling components

Cell growth is a process that needs to be tightly regulated. Cells must be able to sense environmental factors like nutrient abundance, the energy level or stress signals and coordinate growth accordingly. The Target Of Rapamycin (TOR) pathway is a major controller of growth-related processes in all eukaryotes. If environmental conditions are favorable, the TOR pathway promotes cell and organ growth and restrains catabolic processes like autophagy. Rapamycin is a specific inhibitor of the TOR kinase and acts as a potent inhibitor of TOR signaling. As a consequence, interfering with TOR signaling has a strong impact on plant development. This review summarizes the progress in the understanding of the biological significance and the functional analysis of the TOR pathway in plants.     

The role of a class III gibberellin 2‐oxidase in tomato internode elongation

A network of environmental inputs and internal signaling controls plant growth, development and organ elongation. In particular, the growth‐promoting hormone gibberellin (GA) has been shown to play a significant role in organ elongation. The use of tomato as a model organism to study elongation presents an opportunity to study the genetic control of internode‐specific elongation in a eudicot species with a sympodial growth habit and substantial internodes that can and do respond to external stimuli. To investigate internode elongation, a mutant with an elongated hypocotyl and internodes but wild‐type petioles was identified through a forward genetic screen. In addition to stem‐specific elongation, this mutant, named tomato internode elongated ‐1 (tie‐1) is more sensitive to the GA biosynthetic inhibitor paclobutrazol and has altered levels of intermediate and bioactive GAs compared with wild‐type plants. The mutation responsible for the internode elongation phenotype was mapped to GA2oxidase 7, a class III GA 2‐oxidase in the GA biosynthetic pathway, through a bulked segregant analysis and bioinformatic pipeline, and confirmed by transgenic complementation. Furthermore, bacterially expressed recombinant TIE protein was shown to have bona fide GA 2‐oxidase activity. These results define a critical role for this gene in internode elongation and are significant because they further the understanding of the role of GA biosynthetic genes in organ‐specific elongation.     

控制植物器官大小的分子机理

植物器官大小是植物形态的一个重要特征并受严格的遗传调控。器官大小与两个不同的过程有关：细胞扩张和细胞分裂。分子遗传分析已经鉴定了许多基因,这些基因通过作用于其中一个或两个过程来影响器官的最终大小。某种植物个体间器官大小的差异是由控制该器官特征的基因表达水平变化引起的,通过拟南芥的遗传分析显示这些基因是如何受控制或被修饰的。以上这些资料阐明了植物如何确定继续或停止生长,同时也提供了改变植物积累生物量的方法。     

The plant-specific G protein γ subunit AGG3 influences organ size and shape in Arabidopsis thaliana

? Control of organ size and shape by cell proliferation and cell expansion is a fundamental developmental process, but the mechanisms that set the size and shape of determinate organs are largely unknown in plants. ? Molecular, genetic, cytological and biochemical approaches were used to characterize the roles of the Arabidopsis thaliana G protein γ subunit (AGG3) gene in organ growth. ? Here, we describe A. thaliana AGG3, which promotes petal growth by increasing the period of cell proliferation. Both the N-terminal region and the C-terminal domains of AGG3 are necessary for the function of AGG3. By contrast, analysis of a series of AGG3 derivatives with deletions in specific domains showed that the deletion of any of these domains cannot completely abolish the function of AGG3. The GFP-AGG3 fusion protein is localized to the plasma membrane. The predicted transmembrane domain plays an important role in the plasma membrane localization of AGG3. Genetic analyses revealed that AGG3 action requires a functional G protein α subunit (GPA1) and G protein β subunit (AGB1). ? Our findings demonstrate that AGG3, GPA1 and AGB1 act in the same genetic pathway to influence organ size and shape in A. thaliana.     

Size Control in Plants—Lessons from Leaves and Flowers

To achieve optimal functionality, plant organs like leaves and petals have to grow to a certain size. Beginning with a limited number of undifferentiated cells, the final size of an organ is attained by a complex interplay of cell proliferation and subsequent cell expansion. Regulatory mechanisms that integrate intrinsic growth signals and environmental cues are required to enable optimal leaf and flower development. This review focuses on plant-specific principles of growth reaching from the cellular to the organ level. The currently known genetic pathways underlying these principles are summarized and network connections are highlighted. Putative non–cell autonomously acting mechanisms that might coordinate plant-cell growth are discussed.Over millions of years, plant leaves and flowers evolved into an enormous range of shapes and sizes. Likely reflecting adaptations to changing environmental conditions, even closely related species often differ dramatically in their organ sizes (Mizukami 2001). Although interspecies diversity is remarkably high, species-specific leaf and petal characteristics are often highly uniform between individuals grown under constant conditions. This suggests that tight genetic control is used to integrate intrinsic growth signals and environmental cues to enable organ growth to a defined size. This review summarizes the current knowledge of the regulatory networks of plant size control at the cellular and at the organ level. We will focus on the regulation of determinate growth of lateral plant organs, such as simple leaves and petals.     

Genetic control of floral size and proportions

Floral size is an ecologically important trait related to pollination success and genetic fitness. Independently of the sexual reproduction strategy, in many plants, floral size seems to be controlled by several genetic programs that are to some extent independent of vegetative growth. Flower size seems to be governed by at least two independent mechanisms, one controlling floral architecture that affects organ number and a second one controlling floral organ size. Different organ-dependent growth control may account for the final proportions of a flower as a whole. Genes controlling floral organ identity, floral symmetry and organ polarity as well as auxin and gibberellin response, also play a role in establishing the final size and architecture of the flower. The final size of an organ seems to be controlled by a systemic signal that might in some cases overcome transgenic modifications of cell division and expansion. Nevertheless, modification of basic processes like cell wall deposition might produce important changes in the floral organs. The coordination of the direction of cell division and expansion by unknown mechanisms poses a challenge for future research.     

Morphogens, nutrients, and the basis of organ scaling

The regulation of organ size is a long-standing problem in animal development. Studies in this area have shown that organ-intrinsic patterning morphogens influence organ size, guiding growth in accordance with positional information. However, organ-extrinsic humoral factors such as insulin also affect organ size, synchronizing growth with nutrient levels. Proliferating cells must integrate instructions from morphogens with those from nutrition so that growth proceeds as a function of both inputs. Coordinating cell proliferation with morphogens and nutrients ensures organs scale appropriately with body size, but the basis of this coordination is unclear. Here, the problem is illustrated using the Drosophila wing--a paradigm for organ growth and size control--and a potential solution suggested.     

植物器官大小相关基因研究进展

器官大小是植物形态的一个重要特征,而且具有严格的种属特异性。植物器官大小虽然受到外在的环境因素（如光照、营养等）的影响,但它由内在特有的细胞数目和细胞大小决定。许多通过转录调节、蛋白合成、激素调节或松弛细胞壁等途径作用于植物细胞繁殖和/或细胞扩张的基因已经被鉴定,它们的过表达或缺失表达能促进植物器官大小和加快植物生长。尽管如此,这些基因通过相对独立的途径起作用,在植物中难以阐明一个相对整合的器官大小基因调控网络,这也是该研究领域的亟待需要解决的问题。目前,一些器官大小相关基因已经应用农作物育种,并培育出显著增大的农作物品种,这也证实了利用器官大小基因进行植物品种选育的可行性。因此,通过研究药用植物器官大小的基因,人为地在分子水平上有目的的调控器官的大小和形态,是缓解当前许多药用植物面临的资源紧缺、枯竭濒危困境的可考虑途径之一。     

Control of plant organ size by KLUH/CYP78A5-dependent intercellular signaling

Plant organs grow to characteristic sizes that are genetically controlled. In animals, signaling by mobile growth factors is thought to be an effective mechanism for measuring primordium size, yet how plants gauge organ size is unclear. Here, we identify the Arabidopsis cytochrome P450 KLUH (KLU)/CYP78A5 as a stimulator of plant organ growth. While klu loss-of-function mutants form smaller organs because of a premature arrest of cell proliferation, KLU overexpression leads to larger organs with more cells. KLU promotes organ growth in a non-cell-autonomous manner, yet it does not appear to modulate the levels of known phytohormones. We therefore propose that KLU is involved in generating a mobile growth signal distinct from the classical phytohormones. The expression dynamics of KLU suggest a model of how the arrest of cell proliferation is coupled to the attainment of a certain primordium size, implying a common principle of size measurement in plants and animals.     

Do antheridiogens act via gametophyte size? A study of Woodwardia radicans (Blechnaceae)

For many plants, sex is not fixed by genotype but determined by environmental conditions during development. In homosporous pteridophytes, sex is environmentally determined by the presence or absence of antheridiogens, maleness-inducing pheromones. It has been proposed that antheridiogens primarily reduce growth rate, with small gametophyte size responsible for maleness. To test this hypothesis, the effects of antheridiogen and intergametophytic competition on gender expression and gametophyte size were studied in a culture experiment with Woodwardia radicans. We found that (1) antheridiogen inhibited growth of gametophytes; and (2) slow growth favored maleness, whereas fast growth favored femaleness, irrespective of the presence or absence of antheridiogen. Both conclusions are consistent with the hypothesis that, in W. radicans, antheridiogen effect is mediated by size. They also agree with the "size-advantage" hypothesis in which energetic limitations associated with relatively small individual size impose a less severe limitation for male reproductive success than for female reproductive success. The results are also discussed with regard to a genetic sex-determining pathway that has recently been identified.     

The genetic control of organ growth: insights from Drosophila

The genetic control of growth ensures that animals grow to reproducible sizes and that tumourous growth is rare. This year, the regulation of organ growth has been studied extensively in Drosophila imaginal discs, and a signalling pathway that regulates organ growth and size has been identified. Furthermore, the role of Drosophila homologues to human tumour suppressor genes and oncogenes in imaginal disc growth has been investigated.