Bijugate phyllotaxis in Rhizophoreae (Rhizophoraceae)

Phyllotaxis in the mangrove genera of Rhizophoraceae is shown to be bijugate, the angle between orthostichies always less than 90 and in some species close to one half the Fibonacci angle (68.8). The same leaf arrangement occurs in both orthotropic shoots with extended internodes and in the crowded terminal rosettes of those branches which develop plagiotropy by apposition growth, and can be interpreted as an adaptive architectural feature in relation to mutual leaf shading.     

Restructuring plant types for developing tailor-made crops

Plants have adapted to different environmental niches by fine-tuning the developmental factors working together to regulate traits. Variations in the developmental factors result in a wide range of quantitative variations in these traits that helped plants survive better. The major developmental pathways affecting plant architecture are also under the control of such pathways. Most notable are the CLAVATA-WUSCHEL pathway regulating shoot apical meristem fate, GID1-DELLA module influencing plant height and tillering, LAZY1-TAC1 module controlling branch/tiller angle and the TFL1-FT determining the floral fate in plants. Allelic variants of these key regulators selected during domestication shaped the crops the way we know them today. There is immense yield potential in the ‘ideal plant architecture’ of a crop. With the available genome-editing techniques, possibilities are not restricted to naturally occurring variations. Using a transient reprogramming system, one can screen the effect of several developmental gene expressions in novel ecosystems to identify the best targets. We can use the plant's fine-tuning mechanism for customizing crops to specific environments. The process of crop domestication can be accelerated with a proper understanding of these developmental pathways. It is time to step forward towards the next-generation molecular breeding for restructuring plant types in crops ensuring yield stability.     

Application of Two Mathematical Models to the Araceae, a Family of Plants with Enigmatic Phyllotaxis

In this paper we show that two mathematical models can be ofgreat help in the analysis of observational data, in this casethe difficult and little studied phyllotactic phenomena thatoccur in the Araceae family. We apply the Fundamental Theoremof Phyllotaxis, together with an explanatory model of phyllotaxis,to plant specimens of this family, to obtain phyllotactic parametersand information that cannot be otherwise obtained. Most significantis the fact that the two models show evidence of regularitiesin the overwhelming diversity of the patterns observed in theAraceae (essentiallyDracontium and Anthurium) characterizedby discontinuous transitions. In particular, this work revealsthe regularity of the behaviour of the divergence angle in thespecimens analysed. Features of the inflorescences ofDracontium, especially the presence of whorls, are compared to those observedin inflorescences ofAnthurium (characterized by the absenceof whorls), and in the capitulae of Compositae (characterizedby continuous transition). We question the possible meaningat the genetic level of the diversity of patterns observed atthe macroscopic level. Copyright 2001 Annals of Botany Company Phyllotaxis, Araceae, mathematical models, inflorescence, development     

PHYLLOTAXIS AND VASCULAR ORGANIZATION OF THE CARPELS IN MICHELIA FUSCATA

Tucker , Shirley C. (U. Minnesota, Minneapolis.) Phyllotaxis and vascular organization of the carpels in Michelia fuscata. Amer. Jour. Bot. 48(1): 60–71. Illus. 1961.—Phyllotaxis pattern and vascular organization are closely related in the floral receptacle of Michelia fuscata (Magnoliaceae). The carpels arise in a spiral or helix. They are initiated alternately along each of 7, 8 or 10 helical parastichies according to a complex repetitive sequence. The pattern of the dorsal carpellary trace fusions is orderly for each of the 10 flowers investigated. The dorsal carpellary traces in each parastichy diverge from the same vascular sympodium. Among flowers one finds differing numbers of parastichies, differing angles of divergence, and varying sequences of parastichies which reflect the order of carpel initiation. The angle of divergence, although consistent for any 1 parastichy in a flower, can vary greatly between parastichies. The nature and importance of the organizers which determine appendage position at the apical meristem are considered. Changes in apical size, configuration, and activity are shown to be related to phyllotaxis.     

PIN1-Independent Leaf Initiation in Arabidopsis

Phyllotaxis, the regular arrangement of leaves and flowers around the stem, is a key feature of plant architecture. Current models propose that the spatiotemporal regulation of organ initiation is controlled by a positive feedback loop between the plant hormone auxin and its efflux carrier PIN-FORMED1 (PIN1). Consequently, pin1 mutants give rise to naked inflorescence stalks with few or no flowers, indicating that PIN1 plays a crucial role in organ initiation. However, pin1 mutants do produce leaves. In order to understand the regulatory mechanisms controlling leaf initiation in Arabidopsis (Arabidopsis thaliana) rosettes, we have characterized the vegetative pin1 phenotype in detail. We show that although the timing of leaf initiation in vegetative pin1 mutants is variable and divergence angles clearly deviate from the canonical 137° value, leaves are not positioned at random during early developmental stages. Our data further indicate that other PIN proteins are unlikely to explain the persistence of leaf initiation and positioning during pin1 vegetative development. Thus, phyllotaxis appears to be more complex than suggested by current mechanistic models.     

On the mystery of the golden angle in phyllotaxis

Phyllotaxis, the arrangement of leaves around a stem, shows in the vast majority of cases a regularity in the divergence angle of subsequent leaves which divide the whole circle into regular fractions. These are in most cases rational fractions of two Fibonacci numbers in an alternating series, converging towards the irrational limit of the golden section, corresponding to the golden divergence angle of 137.5 . . . degrees. This peculiarity was a long‐standing mystery. Here, it is related to the evolutionary pressure of optimal light capture for maximal photosynthetic activity. A model is established which relates minimal shadowing for the lower leaves to the divergence angle. Numerical results of this model agree well with semi‐empirical data on the dependence of light capture from the divergence angle. The basic shadow function of the model is also related with the demand of minimal shadowing for the angular separation of leaves and obtain, using elementary number theory, as solution the golden section. Further numerical studies show that the rational approach to the golden section (Schimper–Braun series) is related to the leaf width and the number of leaves of the plant.     

A Unified Model of Shoot Tropism in Plants: Photo-, Gravi- and Propio-ception

Land plants rely mainly on gravitropism and phototropism to control their posture and spatial orientation. In natural conditions, these two major tropisms act concurrently to create a photogravitropic equilibrium in the responsive organ. Recently, a parsimonious model was developed that accurately predicted the complete gravitropic and proprioceptive control over the movement of different organs in different species in response to gravitational stimuli. Here we show that the framework of this unifying graviproprioceptive model can be readily extended to include phototropism. The interaction between gravitropism and phototropism results in an alignment of the apical part of the organ toward a photogravitropic set-point angle. This angle is determined by a combination of the two directional stimuli, gravity and light, weighted by the ratio between the gravi- and photo-sensitivities of the plant organ. In the model, two dimensionless numbers, the graviproprioceptive number B and the photograviceptive number M, control the dynamics and the shapes of the movement. The extended model agrees well with two sets of detailed quantitative data on photogravitropic equilibrium in oat coleoptiles. It is demonstrated that the influence of light intensity I can be included in the model in a power-law-dependent relationship M(I). The numbers B and M and the related photograviceptive number D are all quantitative genetic traits that can be measured in a straightforward manner, opening the way to the phenotyping of molecular and mechanical aspects of shoot tropism.     

Competency for graviresponse in the leaf-sheath pulvinus of Avena sativa: onset to loss

The development of the leaf-sheath pulvinus of oat (Avena sativa L. cv. Victory) was studied in terms of its competency to respond to gravistimulation. Stages of onset of competency, maximum competency and loss of competency were identified, using the length of the supertending internode as a developmental marker. During the early phases in the onset of competency, the latency period between stimulus and graviresponse decreased and the steady state response rate increased significantly. When fully competent, the latency period remained constant as the plant continued to develop, suggesting that the latency period is relatively insensitive to quantitative changes (e.g., in carbohydrate or nutrient availability) at the cell level within the plant. In contrast, the response rate was found to increase with plant development, indicating that graviresponse rate is more strongly influenced by quantitative cellular changes. The total possible graviresponse of a single oat pulvinus was confirmed to be significantly less than the original presentation angle. This was shown to not result from a loss of competency, since the graviresponse could be reinitiated by increasing the presentation angle. As a result of the low overall graviresponse of individual pulvini, two or more pulvini are required to bring the plant apex to the vertical. This was determined to occur though the sequential, rather than simultaneous, action of successive pulvini, since a given pulvinus lost competency to gravirespond shortly after the next pulvinus became fully competent.     

Molecular Genetics and Developmental Physiology: Implications for Designing Better Forest Crops

Current tree biology related to tree genetics and breeding has two important developments that have not well been integrated in the literature. The first is the physiological and biochemical dissection of plant yield, whereas the second is the genetic mapping based on molecular markers, such as RFLPs, RAPDs, AFLPs, and microsatellites. Genetic mapping has revolutionized traditional quantitative genetic analysis by which the genetic variation of a character is described in terms of its mean and (co)variance without the knowledge of the underlying genes. By integrating physiological and developmental studies of yield traits, genetic mapping can provide a unique means for detecting key QTL that play important roles in affecting tree growth and metabolism. The incorporation of these QTL into commercial populations through gene transformation or marker-assisted selection will move current breeding programs strictly based on an empirism to an approach that is mechanistically oriented. In this review, we discuss how plant physiology and development are merged with genetic mapping to formulate the strategy of molecular breeding in which superior forest crops are selected at the gene level. It is anticipated that this novel breeding strategy can potentially provide major breakthroughs for tree breeding.     

Quantitative genetic variation and developmental clocks

It is well-known that most genetic variation affects quantitative traits, and natural or artificial selection can act to change quantitative features of organisms more rapidly than qualitative ones. Surprisingly, variability is not confined to outbred species, but also occurs in inbred mice at a much higher rate than expected from known mutation rates. The size and shape of organisms and their constituent parts are, at least in part, controlled by the number of cell divisions, and there is published evidence for the existence of developmental clocks, which may count cell divisions. A molecular model for a developmental clock was previously proposed. It depends on the DNA methylation of repeated sequences of DNA, where the methylation of each additional sequence is tied to DNA synthesis and therefore cell division. The number of repeats specifies the number of divisions which will occur before a signal is produced which can activate or inactivate one or more genes. It is known that crossing over occurs between sister chromatids, and where tandemly repeated sequences occur unequal exchange can generate a larger or smaller number of repeats. An example of this is seen in the well-known variability of "minisatellite" sequences in human DNA. Unequal sister chromatid exchange can occur in mitotic and meiotic cells in the germ line, and in the case of developmental clock sequences could generate variation in clock length which in turn would directly affect quantitative traits. These events can be regarded as a special case of molecular drive during evolution.     

Between the sheets: inter-cell-layer communication in plant development

The cells of plant meristems and embryos are arranged in an organized, and sometimes extremely beautiful, layered pattern. This pattern is maintained by the controlled orientation of cell divisions within layers. However, despite this layered structure, cell behaviour during plant development is not lineage dependent, and does not occur in a mosaic fashion. Many studies, both classical and recent, have shown that plant cell identity can be re-specified according to position, allowing plants to show remarkable developmental plasticity. However, the layered structure of meristems and the implications of this during plant development, remain subjects of some speculation. Of particular interest is the question of how cell layers communicate, and how communication between cell layers could allow coordinated developmental processes to take place. Recent research has uncovered several examples both of the molecular mechanisms by which cell layers can communicate, and of how this communication can infringe on developmental processes. A range of examples is used to illustrate the diversity of mechanisms potentially implicated in cell-layer communication during plant development.     

Quantitative genetic analysis and mapping of leaf angle in durum wheat

The leaf erectness profile has been used to optimize plant architecture since erect leaves can enhance photosynthesis and dry matter production by greater sunlight capture. Brassinosteroid is a recent class of phytohormones that has been related to a more erect profile. There are no reports in the literature of the genetic variability of leaf angle in doubled haploid durum wheat populations; most studies on leaf angle have focused on the inheritance. Our aim was to study the genetic variation in flag and penultimate leaf angle in a durum wheat doubled haploid mapping population, identifying and mapping quantitative trait loci influencing leaf angle. An F₁-derived doubled haploid population of 89 lines from the cross Strongfield/Blackbird was used to construct a genetic map using 423 molecular marker loci. Two greenhouse experiments and one field test were conducted using an alpha lattice in a randomized complete block design with three replicates. The leaf angle was measured on flag and penultimate leaf with a protractor at three different growth stages. The results indicated poor to moderate correlations between the position of the leaf angle and the growth stage. Transgressive segregation beyond Strongfield and Blackbird of leaf angle was observed for all environments. Putative trait loci were identified on chromosomes 2A, 2B, 3A, 3B, 4B, 5B and 7A. This work helps to understand the genetics of leaf angle in durum wheat.     

Conservation and divergence of signalling pathways between roots and soil microbes – the Rhizobium-legume symbiosis compared to the development of lateral roots, mycorrhizal interactions and nematode-induced galls

This review compares endophytic symbiotic and pathogenic root–microbe interactions and examines how the development of root structures elicited by various micro-organisms could have evolved by recruitment of existing plant developmental pathways. Plants are exposed to a multitude of soil micro-organisms which affect root development and performance. Their interactions can be of symbiotic and pathogenic nature, both of which can result in the formation of new root structures – how does the plant regulate the different outcomes of interactions with microbes? The idea that pathways activated in plant by micro-organisms could have been `hijacked' from plant developmental pathways is not new, it was essentially proposed by P. S. Nutman in 1948, but at that time, the molecular evidence to support that hypothesis was missing. Genetic evidence for overlaps between different plant–microbe interactions have previously been examined. This review compares the physiological and molecular plant responses to symbiotic rhizobia with those to arbuscular mycorrhizal fungi, pathogenic nematodes and the development of lateral roots and summarises evidence from both molecular and cellular studies for substantial overlaps in the signalling pathways underlying root–micro-organism interactions. A more difficult question has been why plant responses to micro-organisms are so similar, even though the outcomes are very different. Possible hypotheses for divergence of signalling pathways and future approaches to test these ideas are presented.     

Genetic and evolutionary perspectives on the interplay between plant immunity and development

There is now ample evidence that plant development, responses to abiotic environments, and immune responses are tightly intertwined in their physiology. Thus optimization of the immune system during evolution will occur in coordination with that of plant development. Two alternative and possibly complementary forces are at play: genetic constraints due to the pleiotropic action of players in both systems, and coevolution, if developmental changes modulate the cost-benefit balance of immunity. A current challenge is to elucidate the ecological forces driving evolution of quantitative variation for defense at molecular level. The analysis of natural co-variation for developmental and immunity traits in Arabidopsis thaliana promises to bring important insights.     

植物重力反应的分子调控机制

重力是调节植物生长发育和形态建成的重要环境因子。植物感受到重力刺激后可以通过重力反应来协调自身各个器官的生长方向与重力方向之间的最适角度。植物重力反应过程分为重力信号的感受、重力信号的转导、生长素不对称分布的形成和重力反应器官的弯曲生长4个阶段。近年来,随着大量重力反应缺陷突变体的鉴定及其控制基因的功能解析,重力信号的感受和生长素不对称分布的分子机制等方面的研究取得了重要进展。作为植物适应环境变化的重要手段之一,重力反应还可以通过调节水稻(Oryza sativa L.)的分蘖角度实现对水稻株型和产量的调控。因此,研究植物的重力反应,不仅有助于解析植物生长发育的调控机制,对于作物株型的改良也具有重要的指导意义。然而,重力反应的分子机制及其调控网络仍不清楚。本文综述了近年来植物重力反应的调控机理及其调控水稻分蘖角度的作用机制,并对该领域未来的研究方向和热点进行了展望。     

Leaf phyllotaxis: Does it really affect light capture?

The intriguing mathematical properties of leaf phyllotaxis still attract scientific attention after centuries of research. Phyllotaxis, and in particular the divergence angle between successive leaves, have been frequently interpreted in terms of maximization of light capture, although certain model simulations of light capture by vertical shoots revealed minor effects of phyllotaxis in comparison with the effect of other morphological features of the plant. However, these simulations assumed a number of simplifications, did not take into account diffuse light, and were not based on real plants with their natural range of morphological variation. This study was aimed at filling these gaps by examining the influence on light harvesting of shoot architecture and divergence angle in four species with spiral phyllotaxis (Quercus ilex, Arbutus unedo, Heteromeles arbutifolia and Daphne gnidium) with a realistic 3-D model (Y-plant). A wide range of divergence angles (from 100° to 154°) was observed within each species, with 144° being the most frequent one. These different divergence angles rendered very different vertical projections of the shoot due to contrasting patterns of leaf overlap as seen from above, but they rendered indistinguishable light interception efficiencies (Ea). Setting the leaves with an opposite-decussate phyllotaxis led, however, to a 40–50% decrease of Ea. The interplay of internode length, leaf size and shape, and leaf elevation angle led to significant species differences in Ea. Thus, only particular phyllotaxis (e.g., decussate) might be functionally inefficient under certain combinations of the various morphological variables that influence light capture of a shoot. This revised version was published online in June 2006 with corrections to the Cover Date.     

Kinetics of carrot somatic embryo development in suspension culture

Somatic embryos in liquid culture can serve as a mass cloning system in a plant propagation program. A quantitative formulation of embryo development obtained from cell suspension cultures is used to develop a segregated kinetic model. The model is based on standard classification schemes as previously developed by plant physiologists. Dependent variables include carbohydrate concentrations (sucrose, fructose, and glucose) and biomass apportioned among the inoculum (free single cells, cell clusters), normal developmental stages, and aberrant cell and embryo types. Good agreement between the model and experimental results is indicated and allows for a rigorous approach to media optimization and reactor scaleup for embryo formation.     

Phyllotaxis--a new chapter in an old tale about beauty and magic numbers

Phyllotaxis, the regular arrangement of leaves and flowers around the stem, is one of the most fascinating patterning phenomena in biology. Numerous theoretical models, that are based on biochemical, biophysical and other principles, have been proposed to explain the development of the patterns. Recently, auxin has been identified as the inducer of organ formation. An emerging model for phyllotaxis states that polar auxin transport in the plant apex generates local peaks in auxin concentration that determine the site of organ formation and thereby the different phyllotactic patterns found in nature. The PIN proteins play a primary role in auxin transport. These proteins are localized in a polar fashion, reflecting the directionality of polar auxin transport. Recent evidence shows that most aspects of phyllotaxis can be explained by the expression pattern and the dynamic subcellular localization of PIN1.     

The plant cell cycle

Molecular controls of the plant cell cycle must integrate environmental signals within developmental contexts. Recent advances highlight the fundamental conservation of underlying cell cycle mechanisms between animals and plants, overlaid by a rich molecular and regulatory diversity that is specific to plant systems. Here we review plant cell cycle regulators and their control.     

Small RNAs in development - insights from plants

microRNAs (miRNAs) and small interfering RNAs (siRNAs), which constitute two major classes of endogenous small RNAs in plants, impact a multitude of developmental and physiological processes by imparting sequence specificity to gene and genome regulation. Although lacking the third major class of small RNAs found in animals, Piwi-interacting RNAs (piRNAs), plants have expanded their repertoire of endogenous siRNAs, some of which fulfill similar molecular and developmental functions as piRNAs in animals. Research on plant miRNAs and siRNAs has contributed invaluable insights into small RNA biology, thanks to the highly conserved molecular logic behind the biogenesis and actions of small RNAs. Here, I review progress in the plant small RNA field in the past two years, with an emphasis on recent findings related to plant development. I do not recount the numerous developmental processes regulated by small RNAs; instead, I focus on major principles that have been derived from recent studies and draw parallels, when applicable, between plants and animals.