Joanne M. Dannenhoffer

Temporal patterns of gene expression in developing maize endosperm identified through transcriptome sequencing

Significance In flowering plants, double fertilization gives rise to an embryo and the endosperm, an absorptive storage structure that supports embryogenesis and seedling germination. In cereal grains, endosperm comprises a large proportion of the mature seed, contains large amounts of carbohydrates and proteins, and is an important source of food, feed, and industrial raw materials. This study provides a comprehensive profile of the genes expressed in the early developing endosperm in maize. We also show how a series of temporal programs of gene expression correlate with progressive functional and cellular specializations. Endosperm is a filial structure resulting from a second fertilization event in angiosperms. As an absorptive storage organ, endosperm plays an essential role in support of embryo development and seedling germination. The accumulation of carbohydrate and protein storage products in cereal endosperm provides humanity with a major portion of its food, feed, and renewable resources. Little is known regarding the regulatory gene networks controlling endosperm proliferation and differentiation. As a first step toward understanding these networks, we profiled all mRNAs in the maize kernel and endosperm at eight successive stages during the first 12 d after pollination. Analysis of these gene sets identified temporal programs of gene expression, including hundreds of transcription-factor genes. We found a close correlation of the sequentially expressed gene sets with distinct cellular and metabolic programs in distinct compartments of the developing endosperm. The results constitute a preliminary atlas of spatiotemporal patterns of endosperm gene expression in support of future efforts for understanding the underlying mechanisms that control seed yield and quality.

Phloem-specific expression of the pumpkin fruit trypsin inhibitor

Abstract. Vascular exudates of Cucurbita maxima (Duch.) contain a group of highly conserved serine proteinase inhibitors collectively called Pumpkin Fruit Trypsin Inhibitors (PFTIs) that prevent proteolytic activity of trypsin or chymotrypsin. Polyclonal antibodies raised against PFTIs were used to immunolocalize these low-molecular-weight proteins within the phloem tissue and to study their developmental expression. The inhibitors were translocated throughout the transport phloem and were present in vascular exudates collected from both source and sink tissues throughout the plant. During the early stages of vascular development, PFTIs accumulated specifically in sieve element–companion cell complexes of the phloem tissue. Transcripts were initially detected by reverse transcription-polymerase chain reaction in seedlings 1 d after germination and the protein detected 24 h later. Pumpkin fruit trypsin inhibitors were present in both cell types of differentiating and translocating sieve element–companion cell complexes. The inhibitors were detected in the phloem of the bicollateral vascular bundles, but the protein was most consistently localized within the cortical and bundle-associated extrafascicular phloem.

DEVELOPMENT OF THE VASCULAR SYSTEM IN THE LEAF OF BARLEY (HORDEUM VULGARE L.)

The development of the vascular system in the leaf of barley, Hordeum vulgare L., was studied from its inception to maturity. Young primordia were examined with the scanning electron microscope, and distinctive early morphological stages of primordium development were correlated with the initiation of the longitudinal procambial strands. All longitudinal procambial strands originate in isolation, without continuity with the previously existing stem vasculature. The first-formed strands, which have large-bundle anatomy in both blade and sheath of the mature leaf, arise in the disk of insertion (the stem segment associated with the primordium) and from there develop acropetally into the free portion of the primordium and basipetally into the stem below. Subsequently formed strands arise in the primordium. These strands are interpolated between older strands across the width of the primordium in a precise order, and they appear to develop both acropetally and basipetally. By the time the young leaf is 4 mm long, all of its longitudinal procambial strands have been initiated; in addition, the ligule has been initiated, delimiting blade from sheath. The first protophloem and protoxylem elements of the large strands arise in the disk of insertion near the site of initiation of each strand, the protophloem in advance of the protoxylem. From the site of initiation, the protophloem and protoxylem develop acropetally into the primordium and basipetally into the stem, where, at a later stage of development, they interconnect with phloem and xylem in the nodal region of the leaf two nodes below. During further leaf growth, three successive regions can be identified within the blade: (1) a distal, mature region in which elongation has ceased and metaphloem and metaxylem elements are differentiating basipetally in all strands, (2) a region in which elongation is occurring and protophloem and protoxylem elements are mature or nearly mature but in which metaphloem and metaxylem elements have not yet begun to differentiate, (3) a region in which cell division is occurring and some protophloem and protoxylem elements are still differentiating.

Maize early endosperm growth and development: From fertilization through cell type differentiation

UNLABELLED • PREMISE OF THE STUDY Given the worldwide economic importance of maize endosperm, it is surprising that its development is not the most comprehensively studied of the cereals. We present detailed morphometric and cytological descriptions of endosperm development in the maize inbred line B73, for which the genome has been sequenced, and compare its growth with four diverse Nested Association Mapping (NAM) founder lines.• METHODS The first 12 d of B73 endosperm development were described using semithin sections of plastic-embedded kernels and confocal microscopy. Longitudinal sections were used to compare endosperm length, thickness, and area.• KEY RESULTS Morphometric comparison between Arizona- and Michigan-grown B73 showed a common pattern. Early endosperm development was divided into four stages: coenocytic, cellularization through alveolation, cellularization through partitioning, and differentiation. We observed tightly synchronous nuclear divisions in the coenocyte, elucidated that the onset of cellularization was coincident with endosperm size, and identified a previously undefined cell type (basal intermediate zone, BIZ). NAM founders with small mature kernels had larger endosperms (0-6 d after pollination) than lines with large mature kernels.• CONCLUSIONS Our B73-specific model of early endosperm growth links developmental events to relative endosperm size, while accounting for diverse growing conditions. Maize endosperm cellularizes through alveolation, then random partitioning of the central vacuole. This unique cellularization feature of maize contrasts with the smaller endosperms of Arabidopsis, barley, and rice that strictly cellularize through repeated alveolation. NAM analysis revealed differences in endosperm size during early development, which potentially relates to differences in timing of cellularization across diverse lines of maize.

The Wood of Rellimia from the Middle Devonian of New York

Rellimia is one of the best known of the progymnosperms, although the anatomy of its wood and tree structure has yet to be fully described. The occurrence of Rellimia in Devonian strata makes it one of the earliest plants known to have developed wood. This study examines pyrite permineralizations from the Catskill region of New York and elucidates anatomical details of the secondary xylem using light and scanning electron microscopy. The wood is pycnoxylic with tall, narrow rays that are typically uniseriate, although biseriate and multiseriate rays can be present. Ray cells are slightly rectangular, and the rays are homocellular. All cells appear to have thin walls and appear to be ray parenchyma. Tracheids have crowded, circular‐bordered pits on all walls, including the long, tapering end walls. Bordered pits are in alternate spirals across the face walls and are often hexagonal in outline. Fractures through the bordered pit pair reveal details of the wall and pyrite casts that fill the pit canal and pit chamber during fossilization. Bordered pit pairs have crossed pit apertures, with the pit canal being longer than the shallow pit chamber. Preserved pit membranes and secondary walls show microfibrillar arrangement. Anatomical characteristics of the known wood of progymnosperm taxa are compared and discussed in light of the group’s position in lignophyte evolution.

The Primary Body of Rellimia thomsonii: Integrated Perspective Based on Organically Connected Specimens

The Blenheim‐Gilboa locality in New York contains Rellimia thomsonii specimens preserved as both compressions and permineralizations that offer an unusually complete perspective on the reproductive structures, primary body, and secondary anatomy of this Middle Devonian plant. Here we describe the vegetative appendages, axis and branch size relationships, and trace departure as well as primary anatomy, including phloem sieve cells of interconnected axis orders of vegetative and fertile specimens. At least five orders are present, and each axis order is approximately half the diameter of the parent axis. Three‐ribbed protosteles are evident in all axes. Protoxylem in transverse section is a continuous three‐ribbed sheet one or two tracheids wide, enveloped by several cell layers of metaxylem in turn surrounded by primary phloem. Sieve cells have vertically arranged sieve areas. Trace departure to axes is similar in vegetative and fertile axes. Traces are apparent proximally as radial elongations of the tip of the parent primary xylem rib. In distal sections, the rib tip is tangentially elongated; ultimately, the trace becomes three‐ribbed. Traces to vegetative appendages and fertile organs divide dichotomously. The new features described here allow us to expand and complement previous work and to discuss our whole‐plant concept of Rellimia.

Genetic Structure and Morphometric Variation among Fragmented Michigan Wild Rice Populations

Wild rice (Zizania spp.) has ecological and cultural importance in the Great Lakes region, but has been declining due to habitat loss and fragmentation. We investigated the potential impact of bed area and isolation by distance on genetic structure, reproductive effort, and morphometrics in populations of two wild rice species (Z. palustris and Z. aquatica) in Michigan. Leaves were analyzed with Amplified Fragment Length Polymorphisms (AFLPs) and stem height, leaf length, panicle length, and number of male and female flowers were recorded. Despite finding multiple genetic clusters in each species, we found no geographic pattern to the clusters or any isolation by distance. Correlations revealed no associations between bed area and AFLP band diversity, nor bed area and morphometric traits. The number of female flowers was positively correlated with AFLP band diversity, demonstrating a possible impact of genetic diversity on reproductive effort. The results of this study suggest that habitat loss is not yet affecting the genetic diversity of wild rice in Michigan, possibly because of long distance dispersal vectors facilitating gene flow. Nevertheless, the ongoing decline of populations shows their need for protection and restoration, such as through more seeding efforts.