Collaboration of multiple pathways in making a compound leaf

SummaryThe variability in leaf form in nature is immense. Leaf patterning occurs by differential growth that occurs during a limited window of morphogenetic activity at the leaf marginal meristem. While many regulators have been implicated in the designation of the morphogenetic window and in leaf patterning, how these effectors interact to generate a particular form is still not well understood.We addressed the interaction among different effectors of tomato compound leaf development, using genetic and molecular analyses.Mutations in the tomato auxin response factor SlARF5/SlMP, which promotes leaflet formation, suppressed the increased leaf complexity of mutants with extended morphogenetic window. Impaired activity of the NAC/CUC transcription factor GOBLET (GOB), which specifies leaflet boundaries, also reduced leaf complexity in these backgrounds. Analysis of genetic interactions showed that the patterning factors SlMP, GOB and the MYB transcription factor LYRATE (LYR) act in parallel to promote leaflet formation.This work places an array of developmental regulators in a morphogenetic context. It reveals how organ-level differentiation rate and local growth are coordinated to sculpture an organ. These concepts and findings are applicable to other plant species and developmental processes that are regulated by patterning and differentiation.

Developmental sequence of the syncarpous gynoecium of Sarracenia purpurea (Sarraceniaceae) with flattened and broadened carpels and an umbrella-shaped style

The umbrella-shaped style of Sarracenia has a flattened and broadened distal half forming an umbrella canopy, and a slender cylindrical proximal half forming an umbrella stalk. The developmental sequence that gives rise to this unique structure has never been studied in detail. Data from light microscopy and scanning electron microscopy showed that the five carpels are initiated as discrete primordia, which then undergo congenital fusion and conduplicate folding and become a pentagonal syncarpous gynoecium. The distal region of the carpel then bends abaxially and undergoes significant expansion via a marginal meristem, forming the umbrella shape. Carpel closure is achieved via postgenital fusion at both transverse and longitudinal slits. Each of the five pollen tube transmitting tracts is enclosed by the adaxial surface of the carpel, and the inner epidermis of the umbrella canopy represents the expanded abaxial surface of the carpels, whereas the outer epidermis represents the expanded distal region of the fused carpellary margins. Epidermal trichomes develop first, then secretory glands and stomata appear later at the same stage on the umbrella canopy. This study provides insights into the evolution of the umbrella-shaped style utilizing both common and specialized carpel developmental programs with a novel spatial and temporal pattern.

Activation tagging of the LEAFY PETIOLE gene affects leaf petiole development in Arabidopsis thaliana

In a screen for leaf developmental mutants we have isolated an activator T-DNA-tagged mutant that produces leaves without a petiole. In addition to that leafy petiole phenotype this lettuce (let) mutant shows aberrant inflorescence branching and silique shape. The LEAFY PETIOLE (LEP) gene is located close to the right border of the T-DNA insert linked with these dominant phenotypes and encodes a protein with a domain with similarity to the DNA binding domain of members of the AP2/EREBP family of transcription factors. Introduction of the activation-tagged LEP gene in wild-type plants conferred all the phenotypic aberrations mentioned above. The leafy petiole phenotype consists of a conversion of the proximal part of the leaf from petiole into leaf blade, which means that leaf development in let is disturbed along the proximodistal axis. Therefore, LEP is involved in either cell division activity in the marginal meristem or patterning along the proximodistal axis.

Development of the sorus in tree ferns: Dicksoniaceae

Studies of soral development in the tree-fern family Dicksoniaceae in comparison with the Cyatheaceae led to (1) recognition of two basic patterns in the Dicksoniaceae, (2) clarification of marginal versus superficial sori and their indusia in tree ferns, and (3) phylogenetic interpretations. In Cibotium the sorus originates directly from the marginal initial file. The outer and inner indusia arise simultaneously, early in development, on the adaxial and abaxial sides of the receptacle, respectively. The receptacle in Dicksonia originates from a shifted segment of the marginal initial file. The outer indusium is initiated first, approximately at the same time as the receptacle. The initial cells of the marginal meristem give rise to the soral receptacle in both groups of dicksoniaceous genera. Preliminary studies of soral morphogenesis in some cyatheaceous genera indicate that abaxial derivatives originate the sorus. The Cyatheaceae have a single, abaxial indusium proximal to the sorus at maturity, or none. Consideration of these morphogenetic data in light of recent molecular phylogenies suggests that fundamental changes in the meristematic origin of tree-fern sori have taken place since the origin of the lineage that includes both Dicksoniaceae and Cyatheaceae.Key words: Cibotium, Dicksonia, Dicksoniaceae, sorus, tree ferns.

Meristem characteristics of genetically modified pea (Pisum sativum) leaf primordia

Meristem characteristics of normal, afila (af), acacia (tl), reduced stipule (st), and combinations of these leaf phenotypes were investigated in pea (Pisum sativum L.). The multiple tendrils of the afila leaf are formed from numerous secondary branches on the leaflet primordia. Adaxial and marginal meristems are absent in afila leaflets. The tendril-like morphology of the terminal and secondary branches of the afila leaflets is derived from a radial marginal meristem, which is characteristic of normal tendril development. The small terminal leaflet lamina on tendrils of the acacia leaf is produced by adaxial and marginal meristems which become apparent in the distal portion of the tendril late in leaf ontogeny. The reduced stipules of the reduced stipule leaf result from early loss of abaxial and adaxial stipule marginal meristems. Combinations of the af, tl, and st genes apparently have no modifying influences on their mutual expression with one exception; the aftlst mature reduced stipule is significantly wider than stipules in st, afst, and tlst phenotypes. The greater final width of triple recessive stipules is attributed to the persistence of the adaxial stipular marginal meristem in this phenotype.

Morphogenesis of the spikelets and inflorescence of Andropogon gerardii Vit. (Gramineae) and the relationship between form, information theory, and thermodynamics

The spikelets of Andropogon gerardii occur in pairs, one sessile and one pedicellate. The first glume of the sessile spikelet is bikeeled. The fertile lemma of the sessile spikelet is awned and the awn develops after the lemma has been initiated. The paleas of both spikelets initiate at two positions, are bikeeled, and, on occasion, are two-parted as a result of an interrupted zone of initiation. Each functional lodicule of A. gerardii is developmentally similar to one keel of the palea that has become thickened as a result of activity of an adaxial meristem. The spikelet pairs develop from one primordium. At early stages, spikelet pair primordia about each other along the inflorescence axis and the spikelets of a pair are not separated by a pedicel. The pedicel and the axis of the inflorescence develop through intercalary growth. Differences between appendages in the spikelet of A. gerardii can be viewed as the result of differing amounts of developmental activity (apical growth, marginal meristem, adaxial meristem) common to phyllomic structures. These common developmental activities are, in turn, the result of certain patterns of cell division and cell growth. The evolution of form thus results from alteration of common developmental events. When viewed in such a manner, the evolution of form is seen to be the modification of the informational entropy in an organism. With evolution, there are increases and decreases in informational entropy but, generally speaking, more complex organisms have higher entropy.

Salvinia leaves. II. Morphogenesis of the floating leaf

The floating leaves of Salvinia arise in a manner not previously described in plants. Leaf morphogenesis is the result of meristematic activity in the leaf apical cell and two abaxial meristems. These abaxial meristems originate in the dorsal sectors of the primordium and are separated from one another by a notch which runs the length of the primordial blade region. Each meristem consists of a single longitudinal file of cells which increases the width of the blade panel by strict anticlinal divisions. These anticlinal derivatives divide periclinally to establish the cell layers of the lamina. Unlike most dorsiventral leaves in which the blade is produced by a marginal meristem. Salvinia floating leaf blade panels increase radially by the action of these abaxial meristems. Thus, the leaf surface exposed to the air is morphologically the abaxial surface and that in contact with the water is the adaxial surface. Leaf differentiation and maturation are acropetal in the longitudinal direction and from the midvein to the margins in the horizontal direction.

Initiation and development of leaves in Douglas fir

The initiation and development of the leaves of Douglas fir is described in detail. Leaf initiation is similar to that of other foliar organs and involves both protodermal and peripheral cells of the apex. Apical and subapical initials are present but active for only a short time. Most enlargement is a result of intercalary cell division and enlargement. A limited marginal meristem is present. No differentiation of tissues occurs before bud dormancy in the fall. Growth following dormancy shows the various tissues to mature at different rates and all tissues are fully mature when the leaf becomes dormant in the fall. Growth periodicity of the shoot apex and the length of the growing season are discussed. Initiation and development of leaves in relation to all other foliar organs in Douglas fir are compared.