CHANGES IN EPILOBIUM PHYLLOTAXY INDUCED BY N-1-NAPHTHYLPHTHALAMIC ACID AND α-4-CHLOROPHENOXYISOBUTYRIC ACID

Preliminary studies establishing relationships between leaf plastochron index and Epilobium hirsutum L. shoot growth provide a method for rigorous selection of plants utilized in experiments designed to test the working hypothesis that endogenous auxin gradient interactions are factors of phyllotactic control in this species. Application of N-1-naphthylphthalamic acid (NPA), an auxin transport inhibitor, to one of the youngest bijugate primordia on the shoot meristem results in increased growth of the treated primordium. Fasciation between the treated primordium and one of the next primordia to be initiated alters relative vertical spacing of primordia. Angular shifts between subsequent primordia result in spiral transformation of Epilobium bijugate phyllotaxy. Application of α-4-chlorophenoxyisobutyric acid (CPIB), an auxin antagonist, to one of the youngest bijugate primordia on the shoot meristem results in decreased growth of the treated primordium that alters both radial and vertical spacing of primordia. This is followed by angular shifts between subsequent primordia resulting in spiral transformation of the bijugate phyllotaxy. Changes in the growth parameters of NPA- and CPIB-treated shoots are similar. Relative plastochron rates of radial and vertical shoot growth of induced spiral shoots are about half those of lanolin paste control shoots, as are the plastochrons and relative plastochron rates of leaf elongation. Treated shoot meristems have eccentricities of 0.5 as compared to bijugate control meristem eccentricities of 0.7. No significant difference is apparent between basal transverse areas of treated and control shoot meristems. The relative chronological rates of growth of treated shoots are not significantly different from those rates of control shoots. Spiral transformation results from changes in relative positions of leaf primordia insertion on the shoot meristem, not from changes in growth of treated shoots. These changes are accompanied by an increased rate of leaf initiation on a more circular shoot meristem. Existing theoretical models of phyllotaxy are discussed in relation to these chemically induced changes of Epilobium leaf arrangement.     

Studies of aberrant phyllotaxy1 Mutants of Maize Indicate Complex Interactions between Auxin and Cytokinin Signaling in the Shoot Apical Meristem

One of the most fascinating aspects of plant morphology is the regular geometric arrangement of leaves and flowers, called phyllotaxy. The shoot apical meristem (SAM) determines these patterns, which vary depending on species and developmental stage. Auxin acts as an instructive signal in leaf initiation, and its transport has been implicated in phyllotaxy regulation in Arabidopsis (Arabidopsis thaliana). Altered phyllotactic patterns are observed in a maize (Zea mays) mutant, aberrant phyllotaxy1 (abph1, also known as abphyl1), and ABPH1 encodes a cytokinin-inducible type A response regulator, suggesting that cytokinin signals are also involved in the mechanism by which phyllotactic patterns are established. Therefore, we investigated the interaction between auxin and cytokinin signaling in phyllotaxy. Treatment of maize shoots with a polar auxin transport inhibitor, 1-naphthylphthalamic acid, strongly reduced ABPH1 expression, suggesting that auxin or its polar transport is required for ABPH1 expression. Immunolocalization of the PINFORMED1 (PIN1) polar auxin transporter revealed that PIN1 expression marks leaf primordia in maize, similarly to Arabidopsis. Interestingly, maize PIN1 expression at the incipient leaf primordium was greatly reduced in abph1 mutants. Consistently, auxin levels were reduced in abph1, and the maize PIN1 homolog was induced not only by auxin but also by cytokinin treatments. Our results indicate distinct roles for ABPH1 as a negative regulator of SAM size and a positive regulator of PIN1 expression. These studies highlight a complex interaction between auxin and cytokinin signaling in the specification of phyllotactic patterns and suggest an alternative model for the generation of altered phyllotactic patterns in abph1 mutants. We propose that reduced auxin levels and PIN1 expression in abph1 mutant SAMs delay leaf initiation, contributing to the enlarged SAM and altered phyllotaxy of these mutants.     

Following the initiation and development of individual leaf primordia at the level of the shoot apical meristem: the case of distichous phyllotaxis in Begonia

BACKGROUND AND AIMS: By using the technique of replicas of a developing apex it is possible to obtain a direct measure of phyllotactic parameters (plastochrone and platochronic ratio) involved in the initiation of two successive primordia at the level of the SAM. The goal of this study is to compare, in a real time setting, the value of phyllotactic parameters in distichous systems using Begonia as a case study, with the value of the same parameters in spiral phyllotactic systems. METHODS: To determine the real-time sequence of events at the level of the SAM, replicas were made of the developing apex at different intervals using previously described techniques. Impression moulds were made at 24-h intervals. The following phyllotactic parameters were measured: plastochrone, angle of divergence, plastochrone ratio and ratio between the diameter of the leaf and the apex. RESULTS: The time between the appearance of two successive leaves is 15-20 d. The average value of the plastochrone ratio (R) is 1.3, and the ratio of the leaf to the diameter of the apex (Gamma) is 2.5. The angle of divergence varies from 165 masculine to 180 masculine. The speed of advection of the primordium from the apex, varies from 0.28 to 0.37 microm d(-1). CONCLUSIONS: The speed of advection of primordia in Begonia is lower than that of Anagalis. This is not in accordance with theoretical simulations that predict the opposite. In Begonia, the plastochrone ratio does not reflect the real time of appearance of two successive primordia. The time separating the appearance of two primordia is not directly related to the distance of these two primordia from the centre of the apex but is related instead to the enlargement of leaves.     

Model for the role of auxin polar transport in patterning of the leaf adaxial–abaxial axis

Leaf adaxial–abaxial polarity refers to the two leaf faces, which have different types of cells performing distinct biological functions. In 1951, Ian Sussex reported that when an incipient leaf primordium was surgically isolated by an incision across the vegetative shoot apical meristem (SAM), a radialized structure without an adaxial domain would form. This led to the proposal that a signal, now called the Sussex signal, is transported from the SAM to emerging primordia to direct leaf adaxial–abaxial patterning. It was recently proposed that instead of the Sussex signal, polar transport of the plant hormone auxin is critical in leaf polarity formation. However, how auxin polar transport functions in the process is unknown. Through live imaging, we established a profile of auxin polar transport in and around young leaf primordia. Here we show that auxin polar transport in lateral regions of an incipient primordium forms auxin convergence points. We demonstrated that blocking auxin polar transport in the lateral regions of the incipient primordium by incisions abolished the auxin convergence points and caused abaxialized leaves to form. The lateral incisions also blocked the formation of leaf middle domain and margins and disrupted expression of the middle domain/margin‐associated marker gene WUSCHEL‐RELATED HOMEOBOX 1 (SlWOX1). Based on these results we propose that the auxin convergence points are required for the formation of leaf middle domain and margins, and the functional middle domain and margins ensure leaf adaxial–abaxial polarity. How middle domain and margins function in the process is discussed.     

A stochastic approach to phyllotactic patterns analysis

A statistical method is presented to characterize the degree of order in phyllotactic systems. We developed equations allowing the theoretical estimation of the number of leaves regularly distributed (spiral or verticillate) in a partially random phyllotactic system. The equations are simple and accurate enough to make quantitative predictions concerning the organization of different phyllotactic patterns (verticillate, distichous, spiral and random). This method can bring out patterns that are not visible a priori on a planar representation of the shoot apex. As a case study, the method was applied to the quantitative analysis of the sho mutants recently produced by Itoh et al. [2000. SHOOT ORGANIZATION genes regulate shoot apical meristem organization and the pattern of leaf primordium initiation in Rice. Plant Cell 12, 2161-2174]. By using our method, it was possible to predict the number of leaves distributed in distichous or random patterns on these phyllotactic mutants.     

Do Fibonacci numbers reveal the involvement of geometrical imperatives or biological interactions in phyllotaxis?

Complex biological patterns are often governed by simple mathematical rules. A favourite botanical example is the apparent relationship between phyllotaxis (i.e. the arrangements of leaf homologues such as foliage leaves and floral organs on shoot axes) and the intriguing Fibonacci number sequence (1, 2, 3, 5, 8, 13 . . .). It is frequently alleged that leaf primordia adopt Fibonacci-related patterns in response to a universal geometrical imperative for optimal packing that is supposedly inherent in most animate and inanimate structures. This paper reviews the fundamental properties of number sequences, and discusses the under-appreciated limitations of the Fibonacci sequence for describing phyllotactic patterns. The evidence presented here shows that phyllotactic whorls of leaf homologues are not positioned in Fibonacci patterns. Insofar as developmental transitions in spiral phyllotaxis follow discernible Fibonacci formulae, phyllotactic spirals are therefore interpreted as being arranged in genuine Fibonacci patterns. Nonetheless, a simple modelling exercise argues that the most common spiral phyllotaxes do not exhibit optimal packing. Instead, the consensus starting to emerge from different subdisciplines in the phyllotaxis literature supports the alternative perspective that phyllotactic patterns arise from local inhibitory interactions among the existing primordia already positioned at the shoot apex, as opposed to the imposition of a global imperative of optimal packing.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 150 , 3–24.     

Geometrical relationships specifying the phyllotactic pattern of aquatic plants

The complete range of various phyllotaxes exemplified in aquatic plants provide an opportunity to characterize the fundamental geometrical relationships operating in leaf patterning. A new polar-coordinate model was used to characterize the correlation between the shapes of shoot meristems and the arrangements of young leaf primordia arising on those meristems. In aquatic plants, the primary geometrical relationship specifying spiral vs. whorled phyllotaxis is primordial position: primordia arising on the apical dome (as defined by displacement angles θ ≤ 90° during maximal phase) are often positioned in spiral patterns, whereas primordia arising on the subtending axis (as defined by displacement angles of θ ≥ 90° during maximal phase) are arranged in whorled patterns. A secondary geometrical relationship derived from the literature shows an inverse correlation between the primordial size?:?available space ratio and the magnitude of the Fibonacci numbers in spiral phyllotaxis or the number of leaves per whorl in whorled phyllotaxis. The data available for terrestrial plants suggest that their phyllotactic patterning may also be specified by these same geometrical relationships. Major exceptions to these correlations are attributable to persistent embryonic patterning, leaflike structures arising from stipules, congenital splitting of young primordia, and/or non-uniform elongating of internodes. The geometrical analysis described in this paper provides the morphological context for interpreting the phenotypes of phyllotaxis mutants and for constructing realistic models of the underlying mechanisms responsible for generating phyllotactic patterns.     

Microsurgical and laser ablation analysis of leaf positioning and dorsoventral patterning in tomato

Leaves are arranged according to regular patterns, a phenomenon referred to as phyllotaxis. Important determinants of phyllotaxis are the divergence angle between successive leaves, and the size of the leaves relative to the shoot axis. Young leaf primordia are thought to provide positional information to the meristem, thereby influencing the positioning of new primordia and hence the divergence angle. On the contrary, the meristem signals to the primordia to establish their dorsoventral polarity, which is a prerequisite for the formation of a leaf blade. These concepts originate from classical microsurgical studies carried out between the 1920s and the 1970s. Even though these techniques have been abandoned in favor of genetic analysis, the resulting insights remain a cornerstone of plant developmental biology. Here, we employ new microsurgical techniques to reassess and extend the classical studies on phyllotaxis and leaf polarity. Previous experiments have indicated that the isolation of an incipient primordium by a tangential incision caused a change of divergence angle between the two subsequent primordia, indicating that pre-existing primordia influence further phyllotaxis. Here, we repeat these experiments and compare them with the results of laser ablation of incipient primordia. Furthermore, we explore to what extent the different pre-existing primordia influence the size and position of new organs, and hence phyllotaxis. We propose that the two youngest primordia (P1 and P2) are sufficient for the approximate positioning of the incipient primordium (I1), and therefore for the perpetuation of the generative spiral, whereas the direct contact neighbours of I1 (P2 and P3) control its delimitation and hence its exact size and position. Finally, we report L1-specific cell ablation experiments suggesting that the meristem L1 layer is essential for the dorsoventral patterning of leaf primordia.     

Heteroblasty in the moss,Aphanoregma patens (Physcomitrella patens), results from progressive modulation of a single fundamental leaf developmental programme

Abstract

Leaves at the apex of a mature Aphanoregma patens (Hedw.) Lindb. (Physcomitrella patens (Hedw.) Bruch Schimp. in B.S.G.) gametophore differ markedly in size and form from those at its base. To determine how these differences are produced during development, we first examined qualitative and quantitative differences between successive leaves along the stem and among leaves at different developmental stages. Differences between successive leaves were slight and cumulative. Local changes in cell number and size combined to produce a regularly shaped and approximately bilaterally symmetrical leaf suggesting that cell division and cell expansion are regionally regulated and coordinated at the organ level. The midrib and marginal teeth are discrete characters, which were prefigured by changes in cell shape in leaves that lacked these characters. In leaf primordia, cell proliferation was responsible for most of the changes in leaf form and size early in development and may have continued as cell expansion took over as the primary contributor to leaf growth and morphogenesis. Thus, leaf heteroblasty in Physcomitrella probably results from modulation of a single developmental programme by external and/or internal forces, which alter progressively in intensity as a gametophore grows. We applied exogenous cytokinin and auxin separately to growing cultures to explore their effects on leaf growth. Cytokinin and auxin stimulated leaf cell division and leaf cell elongation, respectively. Also, young upper leaves of gametophores exposed to exogenous auxin closely resembled basal leaves of untreated plants. Therefore, endogenous cytokinins and auxins may be among the modulating internal forces involved in leaf morphogenesis and the establishment of leaf heteroblasty.     

Patterns of auxin transport and gene expression during primordium development revealed by live imaging of the Arabidopsis inflorescence meristem

BACKGROUND: Plants produce leaf and flower primordia from a specialized tissue called the shoot apical meristem (SAM). Genetic studies have identified a large number of genes that affect various aspects of primordium development including positioning, growth, and differentiation. So far, however, a detailed understanding of the spatio-temporal sequence of events leading to primordium development has not been established. RESULTS: We use confocal imaging of green fluorescent protein (GFP) reporter genes in living plants to monitor the expression patterns of multiple proteins and genes involved in flower primordial developmental processes. By monitoring the expression and polarity of PINFORMED1 (PIN1), the auxin efflux facilitator, and the expression of the auxin-responsive reporter DR5, we reveal stereotypical PIN1 polarity changes which, together with auxin induction experiments, suggest that cycles of auxin build-up and depletion accompany, and may direct, different stages of primordium development. Imaging of multiple GFP-protein fusions shows that these dynamics also correlate with the specification of primordial boundary domains, organ polarity axes, and the sites of floral meristem initiation. CONCLUSIONS: These results provide new insight into auxin transport dynamics during primordial positioning and suggest a role for auxin transport in influencing primordial cell type.     

The 35S promoter‐driven mDII auxin control sensor is uniformly distributed in leaf primordia

<正>The DII auxin sensor has been an invaluable tool for mapping the spatiotemporal auxin response and distribution in the model plant Arabidopsis thaliana.The DII sensor and the m DII control sensor are driven by the widely used constitutive 35S promoter. Recently, however, the reliability of the DII sensor has been questioned (Bhatia et al. 2019).     

Pea compound leaf architecture is regulated by interactions among the genes UNIFOLIATA, cochleata, afila, and tendril-lessn

The compound leaf primordium of pea represents a marginal blastozone that initiates organ primordia, in an acropetal manner, from its growing distal region. The UNIFOLIATA (UNI) gene is important in marginal blastozone maintenance because loss or reduction of its function results in uni mutant leaves of reduced complexity. In this study, we show that UNI is expressed in the leaf blastozone over the period in which organ primordia are initiated and is downregulated at the time of leaf primordium determination. Prolonged UNI expression was associated with increased blastozone activity in the complex leaves of afila (af), cochleata (coch), and afila tendril-less (af tl) mutant plants. Our analysis suggests that UNI expression is negatively regulated by COCH in stipule primordia, by AF in proximal leaflet primordia, and by AF and TL in distal and terminal tendril primordia. We propose that the control of UNI expression by AF, TL, and COCH is important in the regulation of blastozone activity and pattern formation in the compound leaf primordium of the pea.     

Auxin polar transport flanking incipient primordium initiates leaf adaxial‐abaxial polarity patterning

The leaves of most higher plants are polar along their adaxial‐abaxial axis, and the development of the adaxial domain (upper side) and the abaxial domain (lower side) makes the leaf a highly efficient photosynthetic organ. It has been proposed that a hypothetical signal transported from the shoot apical meristem (SAM) to the incipient leaf primordium, or conversely, the plant hormone auxin transported from the leaf primordium to the SAM, initiates leaf adaxial‐abaxial patterning. This hypothetical signal has been referred to as the Sussex signal, because the research of Ian Sussex published in 1951 was the first to imply its existence. Recent results, however, have shown that auxin polar transport flanking the incipient leaf primordium, but not the Sussex signal, is the key to initiate leaf polarity. Here, we review the new findings and integrate them with other recently published results in the field of leaf development, mainly focusing on the early steps of leaf polarity establishment.     

Asymmetric auxin response precedes asymmetric growth and differentiation of asymmetric leaf1 and asymmetric leaf2 Arabidopsis leaves

We have analyzed the development of leaf shape and vascular pattern in leaves mutant for ASYMMETRIC LEAVES1 (AS1) or AS2 and compared the timing of developmental landmarks to cellular response to auxin, as measured by expression of the DR5:beta-glucuronidase (GUS) transgene and to cell division, as measured by expression of the cycB1:GUS transgene. We found that the earliest visible defect in both as1 and as2 first leaves is the asymmetric placement of auxin response at the distal leaf tip. This precedes visible changes in leaf morphology, asymmetric placement of the distal margin gap, formation of margin gaps along the leaf border, asymmetric distribution of marginal auxin, and asymmetry in cell division patterns. Moreover, treatment of developing leaves with either exogenous auxin or an auxin transport inhibitor eliminates asymmetric auxin response and subsequent asymmetric leaf development. We propose that the initial asymmetric placement of auxin at the leaf tip gives rise to later asymmetries in the internal auxin sources, which subsequently result in asymmetrical cell differentiation and division patterns.     

Inflorescence and flower development in the Hedychieae (Zingiberaceae): Hedychium coccineum Smith

The inflorescence of Hedychium coccineum Smith is thyrse, and the primary bracts are initiated in a spiral phyllotactic pattern on the sides of the inflorescence dome. Cincinnus primordia are initiated on the flank of the inflorescence apex, in the axils of primary bracts. This primordium subsequently develops a bract and a floral primordium. Then, the floral primordium enlarges, flattens apically, and becomes rounded. Sepals are initiated sequentially from the rounded corner of the primordium ring sepal initiation, and the floral primordium continues to enlarge and produces a ring primordium. Later, this ring primordium separates three common primordia surrounding a central cavity. The adaxial common primordium is the first separation. This primordium produces the posterior petal and the fertile stamen. The remaining two common primordia separate and produce respectively a petal and a petaloid, the inner androecial member. As the flower enlarges, the cavity of the floral cup becomes a rounded–triangular apex; these apices are the sites of outer androecial primordium initiation. The abaxial outer androecial member slightly forms before the two adaxial members develop. But this primordium ceases growth soon after initiation, while the two posterior primordia continue growth to produce the lateral petaloid staminodes. During this stage, gynoecial initiates in the floral cup and continues to grow until extending beyond the labellum.     

Inflorescence and Flower Development of Globba barthei ( Zingiberaceae)

Inflorescence of Globba barthei is a thyrse . Primary bracts are initiated in a spiral phyllotactic pattern on the inflorescence apex . Cincinnus primordia are initiated in the axils of primary bracts . These promordia develop secondarybracts and floral primordia . The floral primordium continues to enlarge and produce a ring primordium . Sepals are initiated sequentially from the rounded corner of the primordium . The ring primordium separates three common primordium surrounding a central cavity . The adaxial common primordium is the first to separate . This primordium divides transversely and producespetal and fertile stamen . The remaining two common primordium transversely separate and produce respectively a petal and a petaloid . As the flower developing , the cavity of the floral cup becomes triangular . The angles of this triangle are the sites of outer androecial primordium . The abaxial androecia forms slightly earlier than the two adaxial ones, and then this primordium ceases growth soon . The two posterior primordia continue growth to produce the lateral petaloid staminodes . During this stage , gynoecia initiate from the floral cup and continue to fuse and develop into style and stigma. In addition ,Initiation of the bulbil primordium is observed at base of inflorescence axis during the early floral development . The bulbil primordium initiates in the axil of primary bract . The evolutionary significance of six androecia is discussed .     

毛舞花姜花器官的发生与发育

通过扫描电镜观察了毛舞花姜(Globba barthei Gagne p.)的花序及花器官的发生与发育。3枚萼片原基首先于花顶连续发生,随后花顶的中心凹陷形成环状原基,环状原基进一步分化形成三枚花瓣—雄蕊共同原基,并在花顶的中心形成花杯。共同原基分化形成花瓣和三枚内轮雄蕊,紧接着外轮雄蕊在花杯的顶点发生。远轴的两枚内轮雄蕊延伸生长并相互融合形成了唇瓣,近轴的一枚形成了可育雄蕊;近轴的两枚外轮雄蕊发育形成了成熟花结构中的侧生退化雄蕊,而远轴的一枚缺失。近轴的两枚外轮雄蕊原基起始的同时,3枚心皮原基也在中心花杯的内侧发生而后与外轮雄蕊相间排列。对毛舞花姜花序的发生和发育的观察发现,在花序轴的头几片初级苞片中产生的是珠芽原基而非蝎尾状小花序原基,其形态特征类似于早期的蝎尾状小花序原基,由此推测珠芽很可能是蝎尾状小花序的变异。     

CORKSCREW1 defines a novel mechanism of domain specification in the maize shoot

In higher plants, determinate leaf primordia arise in regular patterns on the flanks of the indeterminate shoot apical meristem (SAM). The acquisition of leaf form is then a gradual process, involving the specification and growth of distinct domains within the three leaf axes. The recessive corkscrew1 (cks1) mutation of maize (Zea mays) disrupts both leaf initiation patterns in the SAM and domain specification within the mediolateral and proximodistal leaf axes. Specifically, cks1 mutant leaves exhibit multiple midribs and leaf sheath tissue differentiates in the blade domain. Such perturbations are a common feature of maize mutants that ectopically accumulate KNOTTED1-like homeobox (KNOX) proteins in leaf tissue. Consistent with this observation, at least two knox genes are ectopically expressed in cks1 mutant leaves. However, ectopic KNOX proteins cannot be detected. We therefore propose that CKS1 primarily functions within the SAM to establish boundaries between meristematic and leaf zones. Loss of gene function disrupts boundary formation, impacts phyllotactic patterns, and leads to aspects of indeterminate growth within leaf primordia. Because these perturbations arise independently of ectopic KNOX activity, the cks1 mutation defines a novel component of the developmental machinery that facilitates leaf-versus-shoot development in maize.     

A Role for Auxin in Triggering Lamina Outgrowth of Unifacial Leaves

A common morphological feature of typical angiosperms is the patterning of lateral organs along primary axes of asymmetry—a proximodistal, a mediolateral, and an adaxial–abaxial axis. Angiosperm leaves usually have distinct adaxial–abaxial identity, which is required for the development of a flat shape. By contrast, many unifacial leaves, consisting of only the abaxial side, show a flattened morphology. This implicates a unique mechanism that allows leaf flattening independent of adaxial–abaxial identity. In this study, we report a role for auxin in outgrowth of unifacial leaves. In two closely related unifacial-leaved species of Juncaceae, Juncus prismatocarpus with flattened leaves, and Juncus wallichianus with transversally radialized leaves, the auxin-responsive gene GLYCOSIDE HYDROLASE3 displayed spatially different expression patterns within leaf primordia. Treatment of J. prismatocarpus seedlings with exogenous auxin or auxin transport inhibitors, which disturb endogenous auxin distribution, eliminated leaf flatness, resulting in a transversally radialized morphology. These treatments did not affect the radialized morphology of leaves of J. wallichianus. Moreover, elimination of leaf flatness by these treatments accompanied dysregulated expression of genetic factors needed to specify the leaf central-marginal polarity in J. prismatocarpus. The findings imply that lamina outgrowth of unifacial leaves relies on proper placement of auxin, which might induce initial leaf flattening and subsequently act to specify leaf polarity, promoting further flattening growth of leaves.

Lamina outgrowth of unifacial leaves, which lack adaxial identity, relies on proper localization of auxin, which might induce initial leaf flattening and subsequently act to specify leaf polarity, promoting further flattening growth of leaves.