Ultrastructure of graniferous tracheary elements in the haustorium of Exocarpus bidwillii, a root hemi-parasite of the Santalaceae.

The xylem in the body of the haustorium of E. bidwillii has the shape of an inverted conical flask with the expanded portion being known as the vascular core. The tracheary elements of the vascular core are notable for the occurrence of numerous granules within their lumina and the presence of mostly imperforate walls. Elsewhere in the haustorium graniferous tracheary elements are absent and the cells are usually ordinary vessel elements. Thin sections for transmission electron microscopy, post-stained in potassium permanganate, show that the secondary wall thickenings of the graniferous tracheary elements consist of eccentric layers in which the microfibrils of each successive layer run alternately longitudinally and transversely. The granules of the tracheary elements average 2 micrometer in diameter and consist of a homogeneous matrix which shows a fine fibrillar structure on high resolution. The granules are naked and mostly remain as separate structures within the lumen of the cell, but occasionally they fuse into small groups or irregular masses. In some cells the granules become transformed into fibrillar material that disperses throughout the lumen. This dispersed material may accumulate in vessels of the interrupted zone proximal to the vascular core. Occasionally, the granules also change into compacted amorphous masses that adhere to the walls of the cell. Ultrastructural cytochemistry confirms that the granules are protein and not starch as was originally believed for the Santalaceae. The function of the vascular core and its graniferous tracheary elements is discussed and we suggest that it might help regulate the pressure and flow of xylem sap entering the parasite from the host. Graniferous tracheary elements in the Santalaceae and in root parasites of the Serophulariaceae are compared and it is concluded that they represent examples of convergent evolution.     

Graniferous tracheary elements in the haustorium ofAtkinsonia ligustrina,a root hemi-parasite of theLoranthaceae

Summary The xylem conducting cells that form the vascular core in the root haustorium ofAtkinsonia ligustrina are shown to be graniferous tracheary elements. Their luminal contents occur in the form of granules, amorphous bodies or finely dispersed material derived from the granules. Cells of this type are known for theSantalaceae, but have not previously been reported unequivocally in theLoranthaceae. The matrix of the granules consists of compacted fibrillar material often with the central region of the granule showing a crystalline substructure. The granules stain positively for protein. In some instances the matrix may become secondarily impregnated with phenolic compounds. The granules arise during differentiation of the tracheary element and develop as accumulations of matrix within dilated cisternae of endoplasmic reticulum. The function of graniferous tracheary elements inAtkinsonia may be to help regulate the flow of xylem sap from host to parasite, as has been suggested for theSantalaceae.     

Graniferous tracheary elements in haustoria of root parasitic angiosperms

Graniferous tracheary elements are unusual xylem conducting cells, characterized by having structural material in the lumen. They are known particularly from certain root parasitic angiosperms. The included material is usually granular but may also be amorphous or fibrillar, all having the same origin during differentiation of the tracheary element. Vessels and tracheids with such inclusions were first reported in 1895 by Heinricher inLathraea (Scrophulariaceae). During the early decades of this century graniferous tracheary elements were noted in a few other taxa by different workers but were largely forgotten until the early 1960’s. This paper reviews the early literature and the research carried out during the past twenty-five years on these peculiar cells. Graniferous tracheary elements are found typically in the body of the haustorium of the root parasite, especially in the expanded xylem tissue or “vascular core.” The cells are most widely documented for the hemi-parasitic Santalaceae and were first recorded there in 1910 by Benson. She named the cells “phloeotracheides,” believing they combined the functions of phloem and xylem conducting elements. Heinricher and Benson both considered the granules to be composed of amylodextrin starch and Benson also believed the cells contained an enucleated protoplast. Our work has demonstrated that the granules in the Santalaceae are proteinaceous and that the cells are dead at maturity. In 1978 we therefore renamed them “graniferous tracheary elements.” They occur in all species of the Santalaceae so far investigated and inAtkinsonia ligustrina andNuytsia floribunda of root parasitic Loranthaceae. In these two families graniferous tracheary elements have the same organization. Their occurrence in haustoria of root parasites from other families is also reviewed. Although few observations are yet available in the Olacaceae, the granules inXimenia americana are found to be starch grains, like those inLathraea, whereas those inOlax phyllanthi are protein. Such fundamentally different material in haustorial tracheary elements within the same taxonomic group naturally raises the question of relationships within the family. The function of graniferous tracheary elements has not been experimentally investigated but we have suggested that for some Santalaceae they might serve as a device for regulating the flow of xylem sap through the haustorium.     

Ultrastructure of differentiating graniferous tracheary elements in the haustorium ofExocarpus bidwillii (Santalaceae)

Summary The haustorium of the root hemi-parasiteExocarpus bidwillii has tracheary elements that contain protein granules suspended within the lumen of the cell. The differentiation of these graniferous tracheary elements has been studied by transmission electron microscopy based mainly on tracheary elements formed during secondary growth of the haustorium. The vascular cambium of the haustorium is unusual in differentiating tracheary elements and some parenchyma centripetally and a few parenchyma cells centrifugally but no phloem. The cambial initials contain the usual complement of organelles and in the active state vacuoles are small and the groundplasm of the cell is rather opaque. Differentiating tracheary elements are distinguished from developing parenchyma by the early appearance of granules within the cytoplasm and the presence of small vacuoles and only a few lipid bodies. The granules arise amid local masses of endoplasmic reticulum (ER) and are initiated as small swellings of the cisternae in which the matrix material of the granule accumulates. Continuity between the membrane sac of the granule and ER is often maintained as small tubular connections. By the stage the cell is fully expanded the granules are well developed and during the subsequent phase of secondary wall formation they undergo only a small amount of growth. The secondary wall is initiated on the primary wall as low ridges that soon expand circumferentially into the stalked bands of the mature cell. Lignification begins early and spreads progressively centrifugally throughout the band. Microtubules are closely associated with the developing bands and dictyosomes are usually also common in the vicinity. Once the secondary wall thickenings are developed the cell enters a phase of senescence and the components of the protoplast, with the exception of the granules, become smaller and eventually disappear. Disintegration of cell contents occurs rapidly on disappearance of the tonoplast and the release of the contents of the vacuole into the cytoplasm. The granules remain unchanged throughout senescence and on death of the cell they persist as naked structures in the lumen.Sabbatical visitor 1977.     

Anatomy of the haustorium of Jodina rhombifolia (Santalaceae)

The structure of the haustorium of Jodina rhombifolia in material from Argentina was analysed. Self-haustoria and interespeciñc haustoria, the latter on Celtis tala and Scutia buxifolia , were detected. Different developmental stages of the haustorium were found, since immature to mature ones. Features such as the gland, the interrupted zone and the sucker were described. The gland was a constant feature of J. rhombifolia haustorium. A glandular activity was also observed in the clasping folds and in groups of meristematic cells disposed in the body of the haustorium. The presence of graniferous tracheary elements was confirmed under SEM. The interrupted zone was represented by few parenchyma cells as well as by large parenchyma cells. Self-haustoria were larger than interespeciñc ones. In the former, graft-like unions occurred at more mature stages.     

Morphology and Anatomy of the Haustorium of the Root Hemiparasite Olax phyllanthi (Olacaceae), with Special Reference to the Haustorial Interface

Gross morphology and internal structure of haustoria of Olaxphyllanthi are described in parasitism with a range of hosts,including roots of woody and herbaceous dicotyledons and certainmonocotyledons, and occasional instances of autoparasitism andhaustorial formation on monocotyledon rhizomes. Successful penetrationto xylem occurs on virtually all hosts across broad diameters,ages and anatomies of host root, but anatomical impedimentsto haustorial establishment and penetration are recorded forcertain host taxa. Each haustorium is a comparatively simpleand ephemeral structure. Its developing sucker (endophytic regionof the haustorium) spreads laterally around the surface of thehost xylem, yet never completely encircles the host stele. Damageto hosts is minimal and secondary thickening (of hosts) continueson the side of a host root opposite to a haustorium. The haustorialsucker lacks phloem and its interface with host xylem is comprisedalmost entirely (more than 98.7%) of parenchyma. The few terminatingtracheids at an interface lie in very close proximity to oroccasionally directly against exposed xylem vessels, but lumento lumen continuity between tracheary elements of the partnersis not achieved. Three dimensional reconstructions based onserial transverse sectioning indicate that well defined filesof tracheids connect back from an interface to the core of graniferoustracheary elements in the external body of the haustorium, andthence to the xylem of the parent parasite root. The findingsare discussed in relation to existing studies on haustorialanatomy. Root parasite, Olacaceae, haustorial anatomy, host specificity     

Nucellus Cultures of two Cucurbits--Luffa cylindrica and Trichosanthes anguina

Nucellus cultures of Luffa cylindrica and Trichosanthes anguina(Cucurbitaceae) have been established. Tracheary differentiationoccurred in cultures of both taxa. Supplementing the culturemedium with 2,4-D caused limited and diffuse differentiationof tracheary elements. In the absence of 2,4-D, tracheary differentiationwas profuse and the tracheary elements formed compact nodules;the differentiation of tracheary nodules could be traced towell-developed meristemoids. Our results show that the formationof tracheary nodules is an ultimate event in morphogenesis andbears no causal relation to organogenesis. The suitability ofthe nucellus as a system for studies on tracheary differentiationis emphasized.     

Fine Structure of Hydrolysed Primary Walls in Tracheary Elements of Petiolar Xylem in Eucalyptus delegatensis

The hydrolysed lateral primary walls of tracheary elements ofthe petiolar xylem of Eucalyptus delegatensis were examinedby electron microscopy. Vessel-vessel and vessel—tracheidhydrolysed walls were strikingly different in appearance fromtracheid—tracheid walls. The difference seemed to be inthe degree to which the primary walls were hydrolysed. The observationssuggest the wall hydrolysis to be an ordered and controlledprocess. Eucalyptus delegatensis, hydrolysed wall, petiolar xylem, tracheary elements     

The Initiation of the Secondary Haustorium in Alectra vogelii Benth

Anatomical studies were carried out on initiation of the secondaryhaustorium in Alectra vogelii, a root parasite of leguminouscrops in Nigeria. In both the normal and self-haustorium, theformation of the haustorial initial on the parasite root soonafter initial contact between the host and parasite roots isfollowed by the penetration of the host root by the haustorium.Specialized penetrating cells (intrusive cells) at the haustorialfront prise apart and loosen the host root cortical cells, whichlater become digested. Through the same processes, a few ofthese columnar intrusive cells at the haustorial front piercethe endodermis to make contact with the xylem of the host root.Thereafter, a true conductive bridge consisting of short, isodiametric,reticulate vessel elements is established between the parasiteand host roots through the secondary haustorium. No pholem tissuewas observed in the connection. There is a close similaritybetween the mode of initiation of the secondary haustorium ofAlectra vogelii and that previously described for its primaryhaustorium. Alectra vogelii Benth, haustorium, self-haustorium, root parasite, hemiparasitism, Vigna unguiculata, Arachis hypogaea     

Naturally Occurring Regenerative Differentiation of Xylem Around Adventitious Roots in Luffa cylindrica Seedlings

Regeneration of xylem induced by adventitious root formationin the hypocotyl of Luff a cylindrica Roem. seedlings is described.This naturally occurring form of xylem regeneration involvesthe formation of a bypass of regenerated tracheary elementsaround a root without external severance of the vascular strands.The regeneration of xylem around an adventitious root is polarand is very similar in its developmental pattern to the well-knownxylem regeneration induced by wounding vascular strands. Adventitious root formation, Luffa cylindrica Roem, regenerated tracheary elements, vascular differentiation, xylem regeneration.     

Naturally Occurring Regenerative Differentiation of Xylem Around Adventitious Roots in Luffa cylindrica Seedlings

Regeneration of xylem induced by adventitious root formationin the hypocotyl of Luffa cylindrica Roem. seedlings is described.This naturally occurring form of xylem regeneration involvesthe formation of a bypass of regenerated tracheary elementsaround a root without external severance of the vascular strands.The regeneration of xylem around an adventitious root is polarand is very similar in its developmental pattern to the well-knownxylem regeneration induced by wounding vascular strands. Adventitious root formation, Luffa cylindrica Roem, regenerated tracheary elements, vascular differentiation, xylem regeneration     

Tracheary Element Differentiation Induced in Isolated Cylinders of Lettuce Pith: a Bipolar Gradient Technique

To study the influence of morphogenetic gradients on vasculardifferentiation patterns, a new technique was developed whichallows different substances to be applied at opposite ends ofa tissue block. It yielded information on the mobility of particularmorphogens and on the dependence of callus formation and trachearyelement differentiation on their presence. Application of indol-3ylacetic acid (1AA) (10 mg l^–1), zeatin (0.1 mg l^–1)and sucrose (3 per cent, w/v) in various combinations to theends of cylindrical explants of lettuce pith (Lactuca sativaL.) showed that (a) callus formation was stimulated by IAA,whereas induction of tracheary elements required both IAA andzeatin; (b) callus was confined to a few millimetres at theends of the explants, and tracheary elements occurred mainlywithin the callus; (c) sucrose or its metabolic products diffusedthe 10 mm length of the explants, while IAA and zeatin wereeffective only close to the application site; and (d) some callusand tracheary elements formed when no sucrose was applied, butboth increased with sucrose application, though inhibition oftracheary elements formation occurred with high sucrose concentrations. differentiation, pith explant, tissue culture, xylogenesis, indol-3yl acetic acid, sucrose, zeatin, lettuce, Lactuca sativa     

The haustorium of the root parasite Triphysaria (Scrophulariaceae), with special reference to xylem bridge ultrastructure

Haustoria of Triphysaria pusilla and T. versicolor subsp. faucibarbata from a natural habitat were analyzed by light and electron microscopy. Secretory trichomes (root hairs) participate in securing the haustorium to the surface of the host root. The keel-shaped intrusive part of the secondary haustorium penetrates to the depth of the vascular tissue of the host. Some of the epidermal interface cells differentiate into xylem elements. A significant number of haustoria do not differentiate further, but in most haustoria one to five of the epidermal xylem elements terminate a similar number of xylem strands. The strands mostly consist of vessel members and they connect host xylem or occasionally host parenchyma to the plate xylem adjacent to the stele of the parasite root. Each strand of this xylem bridge is accompanied by highly protoplasmic parenchyma cells with supposed transfer cell function. Increased surface area of the plasmalemma occurs in these cells as it does in interface parenchyma cells. Graniferous tracheary elements are restricted to the haustorium and occur most frequently in the plate xylem. The plate xylem is also accompanied by highly protoplasmic parenchyma cells. Hyphae of mycorrhizal fungi of the host root occasionally penetrate into the distal part of the xylem bridge. We combine structural observations and physiological facts into a hypothesis for translocation of water and nutrients between host and parasite. Some evolutionary aspects related to endogeny/exogeny of haustoria are discussed, and it is argued that the Triphysaria haustorium represents a greatly advanced and/or reduced condition within Scrophulariaceae.     

Effects of Auxin Concentration on the Dimensions and Patterns of Tracheary Elements Differentiating in Pith Explants

The optimal concentration of IAA (0.03 mM) for tracheary elementdifferentiation in lettuce pith explants was about ten timesgreater than the optimal concentration for callus proliferation.Related to this, the mean volume per tracheary element increasedwith increasing IAA concentration, 18-fold between 0.001 mMand 0.3 mM IAA. At the highest concentrations, some pith cellsappeared to differentiate directly into tracheary elements,without cell division, resulting in especially large trachearyelements. Tracheary strands developed at intermediate concentrationsof IAA, and led to a small increase in the mean length/breadthratio of tracheary elements. For tracheary elements differentiating from stem cambial derivatives,a reassessment of previous studies indicates that increase inauxin concentration brings greater tracheary element size atconcentrations up to the 0.03 mM optimum. Above this optimum,however, further increase in auxin concentration brings progressivelysmaller tracheary elements, as the high auxin curtails enlargementof the differentiating cells. This contrasts with the pith explants,in which tracheary element size increases with IAA concentrationmost markedly above the optimum concentration. The interpretationof these relations requires an understanding of the effectsof auxin concentration on interacting quantities such as initialsize of cells, rate of enlargement, and rate of differentiation. Lactuca sativa, lettuce, IAA concentration, pith explants, tracheary element dimensions     

Establishment, Structure and Morphology of the Tuber of Balanophora

During germination of the ‘seed’ of Balanophora,endosperm cells at the radicular pole grow out as tubular structuresand anchor the ‘seed’ to the host rootlet. The radiculartier of cells of the embryo elongate as primary haustorial tubesand establish contact with the host root vasculature. A secondaryhaustorium arises from a meristem adjoining the primary haustorium.The remainder of the embryo contributes to the tuber proper. Host parenchyma in the immediate vicinity of the primary haustoriumreverts to meristematic activity. Some of the derivatives matureas perforate tracheary cells. The remainder, retaining meristematicactivity, squeeze themselves between secondary haustorial cellsand together initiate a composite conducting strand, which repeatedlydichotomizes as the tuber grows. The conducting strand of Balanophora is looked upon as the equivalentof combined adventitious root system of parasite and host. Theremaining part of the tuber is equivalent to the shoot. Balanophora, tuber, morphology, host-parasite relations, parasite     

Structure and Development of the Mature Secondary Haustorium in Alectra vogelii Benth

Anatomical investigations were carried out on the structureand development of the mature secondary haustorium in Alectravogelii growing on Arachis hypogaea or Vigna unguiculata. Followingthe formation of the young secondary haustorium, both the cambiumand pericycle of the host root directly opposite the young secondaryhaustorium are stimulated to divide and form new tissues andorgans including haustorial roots. Further proliferations ofthe host root pericycle and the haustorial cortex give riseto a large, tuberous and complex mature secondary haustoriumwithin which the tissues of the host and parasite remain inintimate contact forming a perfect graft union with a wide zoneof contact. Apart from the haustorial axial xylcm strand whichnormally connects the xylem of the parasite secondary root withthat of the host, direct xylary connections are also establishedbetween the axial xylem of the haustorium and the xylem of thehaustorial roots. The entire surface of the mature secondaryhaustorium of Alectrais covered with these haustorial rootsas was previously observed in its mature primary haustorium. Alectra vogelii Benth, secondary haustorium, haustorium, haustorial roots, root parasite, hemiparasitism, Arachis hypogaea, Vigna unguiculata     

Cell Wall Thickening of the Xylem Tracheary Elements in Different Zones of the Adventitious Root of Allium cepa L.

Cell wall thickness of the xylem tracheary elements was measuredin the proto- and metaxylem of the Allium cepa L. adventitiousroot. Measurements were taken in root fragments of known age(1, 3, 5, 7 and 9 d) located in either the basal or medio-apicalzone. Tracheary elements in the protoxylem matured within ashorter period of time than those in the metaxylem. Final cellwall thickness was greater in metaxylem than in protoxylem components.The cell wall thickening in the tracheary elements in both proto-and metaxylem was more rapid in the basal zone of the root thanin the medio-apical zone. Additionally, cell walls of the maturetracheary elements were thicker in the basal zone than in areasfurther from the bulb. Allium cepa, onion, root, cell wall, xylem maturation     

The Endophytic System of Arceuthobium minutissimum, the Indian Dwarf Mistletoe

The Indian dwarf mistletoe, Arceuthobium minutissimum Hook f.is the most diminutive dicotyledonous stem parasite on Pinusexcelsa. The endophytic system is well developed, having a largenumber of anastomosing strands in the cortex and sinkers penetratingthe medullary rays in wood. The cortical strand is protostelicwith the central tracheary elements, the vessels, surroundedby paren-chymatous cells. An earlier report of absence of vesselsseems to be erroneous. The growth of the cortical strands iseffected by an apical cell. The sinkers typically associatedwith the rays of host, are composed of parenchymatous cellsand tracheary elements including vessels. They make contactswith the cells of the ray through pits present in the trachearyelements. The sinkers cause hypertrophy and even fusion of twoor more rays to form a composite medullary ray. The tracheidsof the host tissue also become stunted and contorted in shape.These observations are in agreement with those of other investigatorson American host species for Arceuthobium.     

Characterization of Mitotic Cycles Preceding Xylogenesis in Cultured Explants of Helianthus tuberosus L.

The number of mitotic cycles intervening between the transferof dormant Helianthus tuberosus (Jerusalem artichoke) tuberexplants to culture medium and the differentiation of the firsttracheary elements at 48 h was investigated by a pulse-labellingtechnique employing quantitative autoradiography. Silver-graincounts indicated that differentiation was preceded by threemitotic cycles Duration of the cell cycle phases were estimatedby a pulse-labelling method. From calculation of the phase durationsit was estimated that the first visible signs of tracheary elementdifferentiation occurred from 7–10 h after the last mitosis. Helianthus tuberosus L., Jerusalem artichoke, tracheary elements, cultured explants, tissue culture, mitotic cycles, cell cycle, tritiated thymidine, autoradiography     

Dilated Endoplasmic Reticulum Cisternae in Differentiating Xylem of Minor Veins of Mimosa pudica L. Leaf

The differentiating tracheary elements in the xylem of minorveins in Mimosa pudica L. contain, as is usual, complete nucleateprotoplasts. Within the latter, dictyosomes with associatedvesicles and endoplasmic reticulum (ER) are prominent components.The ER cisternae show an uncommon feature of developing voluminousdilations accumulating finely fibrous material. Similar dilatedER cisternae occur in parenchyma cells associated with the trachearyelements. Most of the dilated cisternae are elongated, taperingat both ends, and almost circular in transections. They varyin size. The largest measured was 10 µm long and 3 µmwide. In the tracheary elements the cisternae break down asthe protoplast disintegrates. For a time, mature cells containfibrous material, apparently the product of the dilated cisternae,at least in part. In the parenchyma cells the dilated cisternaeare released into the vacuoles after the associated trachearyelements reach maturity. They become structurally modified anddisintegrate. The timing in the appearance and disintegrationof the dilated ER cisternae suggests that these structures havesome function with reference to the differentiation of trachearyelements in the xylem.