James L. Seago

Organization of the root apical meristem in angiosperms.

Although flowers, leaves, and stems of the angiosperms have understandably received more attention than roots, the growing root tips, or root apical meristems (RAMs), are organs that could provide insight into angiosperm evolution. We studied RAM organization across a broad spectrum of angiosperms (45 orders and 132 families of basal angiosperms, monocots, and eudicots) to characterize angiosperm RAMs and cortex development related to RAMs. Types of RAM organization in root tips of flowering plants include open RAMs without boundaries between some tissues in the growing tip and closed RAMs with distinct boundaries between apical regions. Epidermis origin is associated with the cortex in some basal angiosperms and monocots and with the lateral rootcap in eudicots and other basal angiosperms. In most angiosperm RAMs, initials for the central region of the rootcap, or columella, are distinct from the lateral rootcap and its initials. Slightly more angiosperm families have exclusively closed RAMs than exclusively open RAMs, but many families have representatives with both open and closed RAMs. Root tips with open RAMs are generally found in angiosperm families considered sister to other families; certain open RAMs may be ancestral in angiosperms.

Anatomical aspects of angiosperm root evolution.

BACKGROUND AND AIMS Anatomy had been one of the foundations in our understanding of plant evolutionary trends and, although recent evo-devo concepts are mostly based on molecular genetics, classical structural information remains useful as ever. Of the various plant organs, the roots have been the least studied, primarily because of the difficulty in obtaining materials, particularly from large woody species. Therefore, this review aims to provide an overview of the information that has accumulated on the anatomy of angiosperm roots and to present possible evolutionary trends between representatives of the major angiosperm clades. SCOPE This review covers an overview of the various aspects of the evolutionary origin of the root. The results and discussion focus on angiosperm root anatomy and evolution covering representatives from basal angiosperms, magnoliids, monocots and eudicots. We use information from the literature as well as new data from our own research. KEY FINDINGS The organization of the root apical meristem (RAM) of Nymphaeales allows for the ground meristem and protoderm to be derived from the same group of initials, similar to those of the monocots, whereas in Amborellales, magnoliids and eudicots, it is their protoderm and lateral rootcap which are derived from the same group of initials. Most members of Nymphaeales are similar to monocots in having ephemeral primary roots and so adventitious roots predominate, whereas Amborellales, Austrobaileyales, magnoliids and eudicots are generally characterized by having primary roots that give rise to a taproot system. Nymphaeales and monocots often have polyarch (heptarch or more) steles, whereas the rest of the basal angiosperms, magnoliids and eudicots usually have diarch to hexarch steles. CONCLUSIONS Angiosperms exhibit highly varied structural patterns in RAM organization; cortex, epidermis and rootcap origins; and stele patterns. Generally, however, Amborellales, magnoliids and, possibly, Austrobaileyales are more similar to eudicots, and the Nymphaeales are strongly structurally associated with the monocots, especially the Acorales.

Anatomy and Histochemistry of Roots and Shoots in Wild Rice (Zizania latifolia Griseb.)

Wild rice (Zizania latifolia Griseb.) is a famous, perennial, emergent vegetable in China. The current work explores the anatomy and histochemistry of roots, stems, and leaves and the permeability of apoplastic barriers of wild rice. The adventitious roots in wild rice have suberized and lignified endodermis and adjacent, thick-walled cortical layers and suberized and lignified hypodermis, composed of a uniseriate sclerenchyma layer (SC) underlying uniseriate exodermis; they also have lysigenous aerenchyma. Stems have a thickened epidermal cuticle, a narrow peripheral mechanical ring (PMR), an outer ring of vascular bundles, and an inner ring of vascular bundles embedded in a multiseriate sclerenchyma ring (SCR). There is evidence of suberin in stem SCR and PMR sclerenchyma cells. Sheathing leaves are characterized by thick cuticles and fibrous bundle sheath extensions. Air spaces in stems and leaves consist of mostly lysigenous aerenchyma and pith cavities in stems. Apoplastic barriers are found in roots and stems.

Selected Structural Features of the Riverine Plants, Trapa natans (Lythraceae) and Justicia americana (Acanthaceae)

Aquatic, stoloniferous plantlets with floating leaves and nodal, adventitious roots of Trapa natans and emergent, stoloniferous, rooted plants of Justicia americana from the Oswego River, New York, USA, were investigated to determine root and shoot structures. These riverine eudicots have typical root anatomy with aerenchyma, endodermis, and exodermis. Both species contain stem tissues with endodermis which encircles one stele in T. natans and polysteles in J. americana. The aquatic stolon of T. natans has circumferentially spaced primary xylem with primary phloem. Erect stems of J. americana normally lack an endodermis in aerial portions above the water line. Extensive collenchyma is a conspicuous feature under the epidermis in stems of both species. Large stolon cortical cavities are characteristic of T. natans, but they are not found in the hypocotyls; in J. americana aerenchyma lacunae occur throughout ground tissue. The peduncle of T. natans has a stele with a ring of vascular tissue of primary xylem with phloem exterior to the xylem, surrounded by endodermis and air cavities, and pith aerenchyma. Leaves of both species do not contain barrier layers. Petioles of T. natans usually contain subepidermal collenchyma and aerenchyma with enlarged lacunae in inflated bladders.

Root development and structure in seedlings of Ginkgo biloba

PREMISE OF THE STUDY The popular, highly recognizable, well-known gymnosperm, Ginkgo biloba, was studied to document selected developmental features, which are little known in its primary root system from root tips to cotyledonary node following seed germination. METHODS Using seedlings grown in soil, vermiculite, or a mixture, we examined sections at various distances from the root cap to capture a developmental sequence of anatomical structures by using standard brightfield, epifluorescence, and confocal microscopic techniques. KEY RESULTS The vascular cylinder is usually a diarch stele, although modified diarchy and triarchy are found. Between exarch protoxylem poles, metaxylem usually develops into a complete disc, except near the transition region, which has irregularly arranged tracheary cells. The disc of primary xylem undergoes secondary growth on its metaxylem flanks with many tracheids added radially within a few weeks. Production of fibers in secondary phloem also accompanies secondary growth. In the cortex, endodermis produces Casparian bands early in development and continues into the upper transition region. Phi cells with phi-thickenings (bands of lignified walls) of a layer of inner cortex are often evident before endodermis, and then adjoining, additional layers of cortex develop phi cells; phi cells do not occur in the upper transition region or stem. An exodermis is produced early in root development and is continuous into the transition region and cotyledonary node. CONCLUSIONS Seedling root axes of Ginkgo biloba are more complex than the literature suggests, and our findings contribute to our knowledge of root structure of this ancient gymnosperm.