Nodule Lobes Research Articles

Seasonal patterns of root nodule growth, endophyte morphology, nitrogenase activity, and shoot development in Myrica gale

Myrica gale L. populations growing in central Massachusetts were observed throughout the ice-free season. Nitrogenase activity appeared in mid-May shortly after budbreak, was at its maximum between late June and mid-August, and disappeared in late October after all leaves had fallen. Growth of overwintering nodules began in early May and was largely complete by mid-July. Most nodules (88%) lived for 3 years or less and 61% of the nodule biomass present in autumn was produced during the current season.Colonizing hyphae of the Frankia sp. endophyte were seen throughout the year in partially expanded cortical cells near the nodule lobe apex. Vesicles first appeared in mature cortical cells coincident with the onset of nitrogenase activity in mid-May, occupied the bulk of the infected tissue during the summer, and disappeared as nitrogenase activity ceased in late October. Evidence is presented that the vesicles are the site of nitrogenase activity and are newly produced each season in freshly formed nodule lobe tissue. Sporangia frequently formed in mature infected cells in nodules at one site but were rare at another. The processes described here in M. gale are probably typical of winter-deciduous actinorhizal plants.

Fine structure of the root nodules of Dryas drummondii Richards (Rosaceae)

The root nodules of Dryas drummondii (common name: mountain avens; Family Rosaceae) possess numerous nodule lobes devoid of nodule roots and thus they exhibit the Alnus-type of nodule morphology. Each nodule lobe possesses a nodule meristem, infected cortical cells, and a central vascular cylinder. The endophyte is prokaryotic and has two forms: septate hyphae, 0.3–0.6 μm in diameter, and nonseptate vesicles, ca. 2 μm in diameter; thus, the endophyte appears to be an actinomycete similar to those of other Frankia-induced nodules. A polysaccharide capsule surrounds both forms of the endophyte and contains numerous electron-dense inclusions near the hyphae in host cells containing both forms of the endophyte.

Behavior in tissue culture of nitrogen-fixing root nodules of Ceanothus integerrimus

This study documents the in vitro growth of N2-fixing root nodules from the dicotyledonous shrub Ceanothus integerrimus (Rhamnaceae). Root nodule lobes were removed from cultivated plants, surface sterilized, and cultured on agar media containing varied amounts of nitrogen (N). Uninfected cells inside the cultured nodules divide and form a callus which is visible after 7 days of growth. Cells infected by the N2-fixing actinomycete do not divide, but rather degenerate. Host cell cytoplasm disappears and organelles and the plasma membrane are no longer seen under the electron microscope. Endophyte structure also deteriorates. Nucleoid areas within the terminal vesicles become diffuse. Cross walls separating vesicles from the hyphae disappear and cytoplasm of the hyphae withdraws from the actinomycete wall. Ceanothus nodules readily form callus in tissue culture. Callus proliferates from uninfected cortical parenchyma outside the infection zone. The callus is free of living endophyte and is incapable of fixing N2 as measured by acetylene reduction. Nodule tissue growth in vitro is more vigorous on media with high N than on N-deficient media.

Early Nodule Development in Myrica gale

Seedlings of Myrica gale were grown aeroponically and inoculated with suspensions prepared from mature nodules of M gale plants or with a homogenized preparation of a pure culture of the actinomycete isolated from Comptonia nodules. Morphological, anatomical, and cytological studies were made of early nodule development in these plants, and comparisons were made with similar stages in Comptonia and Casuarina Root hair infection was followed by prenodule formation in a manner similar to that in Comptonia In M. gale, most nodules originated with one or two primary nodule lobes. Thereafter, secondary nodule lobes formed sparsely and in a precisely ordered sequence. In 3-mo-old seedlings, nodules with up to five nodule lobes were observed, but the majority of nodules at this age were still one-to-three lobed In Comptonia the number of primary nodule lobes is much higher than in M. gale. In M. gale nodule roots developed from many but not all nodule lobes, usually in an ordered sequence.

Determinate development of nodule roots in actinomycete-induced root nodules of Myrica gale

Young seedlings of Myrica gale L. grown in water culture were inoculated with a nodule suspension containing the effective actinomycete which induced root nodule formation. Nodule development was followed from initiation to nodule lobe formation and nodule root development using living materials and fixed nodules sectioned for light microscopy. After root hair infection and prenodule formation, three stages were observed: nodule lobe formation, a transition or arrested state, and nodule root development. The primary nodule lobe meristem originates endogenously and its formation involves pericycle, endodermis, and cortical cell derivatives. The lobe develops slowly to about 2 mm in length while the cortical cells are invaded by the actinomycete endophyte. After a period of arrest of variable duration, from a few days to several weeks, the nodule lobe meristem begins altered development, forming the elongate nodule root which undergoes slow but continuous growth to about 3- to 4-cm final length. New nodule lobe primordia are initiated endogenously at the base of existing nodules lobes, ultimately forming a cluster of nodule roots. Each nodule root, which elongates at about 0.1–1.0 mm per day, has a terminal apical meristem with reduced root cap formation and produces a modified root structure possessing an elaborate cortical intercellular space system and a reduced central cylinder. Nodule root growth is distinctive in that it shows strong negative geotropism. The endophyte is restricted to cortical cells of the nodule lobe and is totally absent from tissues of the nodule root. A probable role for nodule roots is to facilitate gas diffusion to the nitrogen-fixing endophyte site in the nodule lobe when nodules occur under conditions of low oxygen tension.

TIME‐LAPSE PHOTOGRAPHIC OBSERVATIONS OF MORPHOGENESIS IN ROOT NODULES OF COMPTONIA PEREGRINA (MYRICACEAE)

Seedlings of the sweet fern Comptonia peregrina (L.) Coult. were grown aeroponically with their roots bathed in a nutrient mist lacking nitrogen except for 10 ppm N at the outset. The initiation and early development of root nodules capable of fixing atmospheric nitrogen were recorded with time‐lapse photography through early development to the establishment of highly branched, roughly spherical nodules. In Comptonia multiple primary nodule lobes are formed at or near the site of infection with as many as 10 primary lobes occurring together. On the shoulders of the swollen primary lobes new primordia develop, forming secondary nodule lobes, which may persist without nodule root elongation, giving a coralloid appearance. The tips of the lobes may elongate, forming nodule roots which grow vertically upward, or, if disturbed, in random orientation. Nodule roots occasionally form lateral roots. The root axis upon which the nodule forms undergoes secondary thickening on the proximal side of the nodule attachment; the distal portion of the root shows no secondary thickening and later atrophies. Thus, nodules are perennial structures on a woody root system. The endophyte infects and occupies the basal cortical tissues of the primary nodule lobes and successive nodule lobes as they are formed, being restricted to the swollen bases and not infecting the elongate nodule roots. Development of the nodule is interpreted in terms of complex host‐endophyte interactions involving the initiation of multiple primordia forming nodule lobes, the active inhibition of nodule lobes and finally nodule root elongation. Anatomical evidence for the endogenous origin of nodule primordium formation substantiates the view obtained from time‐lapse photomacrography.

INITIATION AND DEVELOPMENT OF ROOT NODULES OF CASUARINA (CASUARINACEAE)

Roots of seedlings of the “beefwood” tree, Casuarina cunninghamiana Miq. grown in nitrogen‐free nutrient solution were inoculated with a suspension prepared from crashed root nodules taken from mature plants. Marked deformation of root hairs was evident but no infection threads were observed in root hairs. The mode of infection remains undetermined. Root nodules were initiated within three weeks and thereafter numerous upward‐growing nodule roots developed from each nodule. Nodules in this symbiotic nitrogen‐fixing plant resulted from an infection caused by an unidentified actinomycete‐like soil microorganism. Anatomical analysis of nodule formation showed that nodules are the result of repeated endogenous lateral root initiations, one placed upon another in a complexly branched and truncated root system. The endophyte‐infected cortical tissues derived from successive root primordia form the swollen nodular mass. Nodule roots develop from nodule lobes after escaping from the initial inhibitory effects of the endophyte. Included is a discussion of the anatomical similarities between nodules of Casuarina which produce nodule roots and those of Alnus which form coralloid nodules usually lacking nodule roots.

The Development and Significance of the Root Nodules of Casuarina

Abstract Young plants of Casuarina cunninghamiana and of C. equisetifolia growing in water culture developed macroscopic nodules in 26 to 35 days from inoculation, the nodulation being most successful at pH near to neutrality and falling off much more rapidly at lower pH than in other non-legume nodule-forming genera, confirming the distinctiveness of the Casuarina organism. The roots springing from the nodule lobes are shown to be characterized by upward growth and in this to resemble remarkably closely the corresponding roots of Myrica. Nodulated plants of Casuarina are able to grow vigorously in culture solution free of combined nitrogen, showing that fixation of atmospheric nitrogen occurs, amounting in C. cunninghamiana to 50 mg. per plant during 6 months of active growth. The evidence indicates that the fixation occurs in the nodules, and that these have exactly the same functional significance as those of legumes.

A Feature of the Root Nodules of Casuarina

IN the course of an investigation into the development and physiological activity of the root nodules of Casuarina, considerable numbers of nodulated plants of C. cunninghamiana and C. equisetifolia are being grown in water culture. Nodulation was secured by inoculating the roots of young plants with a suspension of crushed Casuarina nodules obtained from abroad through the assistance of Dr. H. R. Fletcher, of the Royal Botanic Garden, Edinburgh, and commenced three to four weeks after such inoculation. McLuckie1 noted a resemblance between Casuarina and Myrica root nodules in that both show a lobed structure as the result of repeated branching (actually this is true of most non-legume and some legume nodules) and in that a root grows out from each nodule lobe.