The peripheral xylem of grapevine (Vitis vinifera) berries. 2. Anatomy and development

It has been hypothesized that the substantial reductions in xylemic water flow occurring at veraison are due to physical disruption (breaking) of the xylem as a result of renewed berry growth. In a companion paper, evidence was presented that the vast majority of xylem tracheary elements remained intact despite the growth of the berry, and it was proposed that existing tracheary elements stretch to accommodate growth and that additional elements may also differentiate after veraison. Measurements of the intergyre distance of tracheary elements in macerated tissue were used to test for stretching, and the numbers of tracheary elements per vascular bundle and of branch points of the peripheral xylem network were analysed to test for continued differentiation from 18 to 120 d after anthesis in Chardonnay berries. The distance between the epidermis and the vasculature increased substantially from pre- to post-veraison, potentially increasing the amount of skin available for analysis of compounds important for winemaking. Tracheary elements continued to differentiate within the existing vascular bundles throughout berry development. Additional vascular bundles also appeared until after veraison, thereby increasing the complexity of the peripheral vascular network. The results also confirmed that tracheary elements stretched by approximately 20%, but this was not as much as that predicted based on the growth of the vascular diameter (40%). These results complete a comprehensive evaluation of grape berry peripheral xylem during its development and show that tracheary development continues further into berry maturation than previously thought.     

植物管状细胞栓塞后的重新充注研究进展

在植物吸收水分以后 ,水分运输对于植物正常的生长发育是非常重要的。在干旱和冬季反复冻融循环以后 ,植物体内的管状细胞容易充满水蒸气和空气 ,形成腔隙和栓塞。腔隙和栓塞的形成对水分在植物体内的运输造成了很大的障碍 ,从而影响了植物的生长与发育。当植物重新获得水分时 ,已形成腔隙和栓塞的管状细胞的重新充注能使一部分管状细胞的输水功能得到恢复 ,从而保证了一些器官的生理功能的正常进行。近些年来 ,人们对植物管状细胞的重新充注涉及到的许多植物组织和生理过程进行深入的研究 ,并提出了各种机理。鉴于植物管状细胞形成栓塞后重新充注对植物水分运输的重要生理作用 ,本文对重新充注的许多机理进行了综合评述     

Characteristics of the Inhibitory Effect of 5-Fluorodeoxyuridine on Cytodifferentiation into Tracheary Elements of Isolated Mesophyll Cells of Zinnia elegans

The inhibitory effect of 5-fluorodeoxyuridine (FdU) on the differentiationinto tracheary elements was characterized in isolated mesophyllcells of Zinnia elegans. Both thymidine and uridine counteracted the inhibitory effectof FdU on the differentiation into tracheary elements, whileonly thymidine was effective in counteracting the effect ofFdU on cell division. Higher concentrations of thymidine wereneeded for the restoration of the differentiation that was blockedby FdU than for the restoration of cell division. These resultssuggest that FdU prevents the differentiation via a mechanismthat is different from the inhibition of thymidylate (dTMP)synthase by fluorodeoxyuridine monophosphate (FdUMP), derivedfrom FdU, to which the blockage of cell division by FdU shouldbe attributable. The differentiation into tracheary elements was prevented whenFdU was added earlier than the 36th hour of culture, and thymidineovercame the inhibitory effect of FdU only when added withinthe first 4 h of culture. Pretreatment with FdU before applicationof 6-benzyladenine (BA) and 1-naphthaleneacetic acid (NAA),which are essential for the formation of tracheary elements,also inhibited the differentiation. Thus, the aspect of thedifferentiation that is the target of inhibition by FdU appearsto occur between the 4th hour and the 36th hour of culture andto begin even in the absence of exogenous plant growth regulators. (Received April 3, 1989; Accepted October 27, 1989)     

Programmed cell death during vascular system formation

Vascular plants have vessels and tracheids composed of dead tracheary elements. Differentiation of procambial or cambial cells to tracheary elements is a typical example of programmed cell death in higher plants. Recent studies on tracheary element differentiation, in particular with an in vitro differentiation system, have revealed a unique cell death process which differs from apoptosis. Herein I summarize the present knowledge about the induction of cell death, the morphological features of cell death, and the mechanism of autolysis, including the involvement of a DNase and cysteine proteases, during tracheary element differentiation.     

植物管状细胞栓塞后的重新充注研究进展

在植物吸收水分以后,水分运输对于植物正常的生长发育是非常重要的。在干旱和冬季反复冻融循环以后,植物体内的管状细胞容易充满水蒸气和空气,形成腔 隙和栓塞。腔隙和栓塞的形成对水分在植物体内的运输造成了很大的障碍,从而影响了植物的生长与发育。当植物重新获得水分时,已形成腔隙和栓塞的管状细胞的重新充注能使一部分管状细胞的输水功能得到恢复,从而保证了一些器官的生理功能的正常进行。近些年来,人们对植物管状细胞的重新充注涉及到的许多植物组织和生理过程进行深入的研究,并提出了各种机理。鉴于植物管状细胞形成栓塞后重新充注对植物水分运输的重要生理作用,本文对重新充注的许多机理进行了综合评述。     

Direct Evidence for Cytodifferentiation to Tracheary Elements without Intervening Mitosis in a Culture of Single Cells Isolated from the Mesophyll of Zinnia elegans

A serial observation of the process of tracheary element differentiation from single cells isolated from the mesophyll of Zinnia elegans L. cv. Canary bird provided the first direct evidence for the cytodifferentiation without intervening mitosis. Percentage of the tracheary elements formed without cell division was about 60% of total tracheary elements formed on the 4th day of culture. The number of tracheary elements formed without intervening mitosis was not reduced in the presence of colchicine at the concentrations blocking cell division. These facts clearly indicate that cell division is not a prerequisite for tracheary element differentiation in this system.     

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.     

In vitro induction of secondary xylem-like tracheary elements in calli of hybrid poplar (Populus sieboldii × P. grandidentata)

The formation of tracheary elements was induced in calli derived from petioles of hybrid poplar (Populus sieboldii × P. grandidentata) after 10 days of culture on medium that lacked auxin but contained 1 μM brassinolide. Some differentiated cells formed broad regions of cell walls and bordered pits, which are typical features of tracheary elements of secondary xylem. Other differentiated cells resembled tracheary elements of primary xylem, with spiral or reticulate thickening of cell walls. The tracheary elements that developed in calli were formed within cell clusters. This induction system provides a new model for studies of the mechanism of differentiation of secondary xylem cells in vitro.     

Brassinosteroid levels increase drastically prior to morphogenesis of tracheary elements

As the first step toward understanding the involvement of endogenous brassinosteroids (BRs) in cytodifferentiation, we analyzed biosynthetic activities of BRs in zinnia (Zinnia elegans L. cv Canary Bird) cells differentiating into tracheary elements. The results of feeding experiments suggested that both the early and late C6-oxidation pathways occur during tracheary element differentiation. Gas chromatography-mass spectrometry analysis revealed that five BRs, castasterone, typhasterol, 6-deoxocastasterone, 6-deoxotyphasterol, and 6-deoxoteasterone, actually existed in cultured zinnia cells and culture medium. Quantification of endogenous BRs in each stage of tracheary element differentiation by gas chromatography-mass spectrometry exhibited that they increased dramatically prior to the morphogenesis, which was consistent with the idea that BRs are necessary for the initiation of the final stage of tracheary element differentiation. Moreover, the proportion of each BR in culture medium was quite different from that in cells, suggesting that specific BRs are selectively secreted into medium and may function outside the cells.     

Tracheary Element Formation from Single Isolated Cells in Culture

Friable callus tissue of Centaurea cyanus L. was grown on a solidified synthetic nutrient medium (EBM-1) to produce a tissue with a low frequency of differentiated tracheary elements. Tissues were then suspended in liquid nutrient medium with agitation to produce a suspension which was filtered and the single-cell suspension resulting was used as inoculum for either cell suspension cultures or for plating of cells into solidified medium in Petri plates. Media for the suspension cultures were selected to favor cytodifferentiation of tracheary elements. Differentiated tracheary elements formed as early as 10 days and numbers of tracheary elements increased with time roughly in relation to the increase in total cell number. From plating experiments it was shown conclusively that single isolated parenchyma cells differentiated directly into single isolated tracheary elements, although this event was rare. More usual was the division of isolated cells to form small colonies and then the differentiation of one, several or all of the cells into tracheary elements. Comparisons are made between results with cell plating experiments and cell suspension cultures. Optimism is expressed for finding a cell suspension culture system for studying cytodifferentiation.     

Xylogenesis in tissue culture: Taxol effects on microtubule reorientation and lateral association in differentiating cells

Summary InZinnia elegans tissue cultures, cortical microtubules reorient from longitudinal to transverse arrays as the culture age increases and before differentiation of tracheary elements is visible. The orientation of microtubules, in the period just before visible differentiation, determines the direction of the secondary wall bands in forming tracheary elements. Taxol, applied early in culture, stabilizes the microtubules of most cells in the longitudinal direction. Tracheary elements differentiating in these taxol treated cultures show secondary wall bands parallel to the long axis of the cell while those differentiating in control cultures always have wall bands transverse to the long axis of the cell.It is proposed that, in untreatedZinnia cultures, microtubules are reoriented by a gradual shift from longitudinal to transverse and this reorientation normally occurs before differentiation becomes visible. Once initiated, tracheary element differentiation involves lateral association of microtubules to form the discrete bands typical of secondary wall patterns.     

Xylem changes in the correlative inhibition of lateral bud growth in flax (Linum usitatissimum L.)

Xylem or tracheary changes at the base of the cotyledonary buds of flax seedlings (Linum usitatissimum L.), released from inhibition by decapitation of the main apex were studied. The differentiation of xylem strands and/or tracheary elements was correlated with the growth in length of the lateral buds, especially 48–72 hr after the removal of the main apex. The xylem strands, connected to the hypocotylary stele or not, and the tracheary elements increased with age within and outside the strands of both non-decapitated and decapitated seedlings. In the latter, the differentiation of these structures, however, occurred much earlier and in greater abundance in the same regions. The early growth in length of lateral buds, 1 or 2 hr after decapitation, was correlated with the early development of tracheary perforations in the xylem strands. The xylary strands with perforated elements are known to be more efficient than those without them. Therefore, it is suggested that the inhibition of lateral-bud growth was due, in fact, to a lack of appropriate tracheary perforations in the bud xylem strands that were connected with the hypocotylary stele of flax seedlings.     

Secondary wall deposition in tracheary elements of cucumber grown in vitro

It is a matter of controversy whether secondary wall deposition is dependent on lignification during the development of tracheary elements. To understand this, tracheary element differentiation was studied in the homogeneous calli obtained from the cotyledonary explants of Cucumis sativus subsequent to treatment with plant growth regulators, such as naphthalene acetic acid (NAA) and benzylamino purine (BAP), which are necessary for the induction of tracheary elements, along with metabolic blockers such as 2-aminoindan-2-phosphonic acid (AIP), 2,3,5-triiodobenzoic acid (TIBA) and nifedipine. Calli treated with AIP, a potential inhibitor of L-phenylalanine ammonia-lyase (PAL), have no PAL activity at any time during the culture period. There was a complete inhibition of lignification although secondary wall deposition was unaltered. Similar results were obtained using TIBA, an inhibitor of auxin transport, and nifedipine, a known calcium channel blocker. Thus the present study suggests that secondary wall deposition in the course of tracheary element differentiation need not to be dependent on lignification. This revised version was published online in July 2006 with corrections to the Cover Date.     

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     

Determination of Tracheary Element Differentiation in Lettuce Pith Explants

Transfer experiments with lettuce pith explants revealed thatprovision of inductive concentrations of both cytokinin andauxin in the culture medium for the first 3 and 5 d respectivelywas sufficient to cause determination of tracheary elementsas shown by their subsequent differentiation in substantialnumbers in explants removed from the inductive hormonal stimuli. Determination, differentiation, tissue culture, IAA, zeatin, tracheary elements, lettuce, Lactuca     

Effect of high salinity on tracheary element differentiation in light-grown callus of Mesembryanthemum crystallinum

This study investigated the inhibitory effects of NaCl on tracheary element (TE) differentiation in light-grown callus of ice plant Mesembryanthemum crystallinum L., a halophyte which adaptes well to saline environments. When ice plant callus was grown in a modified Linsmaier-Bednar and Skoog culture medium containing no NaCl (control medium), up to 20% of ice plant cells differentiated into tracheary elements during in vitro culture. Close examination of callus tissues stained with potassium permanganate revealed that tracheary elements were aggregated as discrete nodules. Some strikingly elongated tracheary elements were found in the macerated tissues. Experimental results indicated that adding 200 mM NaCl to the control medium reversibly inhibited the formation of tracheary element in the halophytic cells. The rate of tracheary element formation increased accordingly as the rate of cell growth in control medium. In the presence of high salt, the degree of tracheary element differentation remained low through the growth cycle. The inhibitory effect of salt on tracheary element differentiation was overcome by adding 10 mg l⁻¹ salicylic acid, a known signaling compound that induces a diverse group of defense-related genes, including genes involved in reinforcing the host cell wall. Furthermore, microscopic examination revealed that most tracheary elements formed under this treatment (200 mM NaCl plus 10 mg l⁻¹ salicylic acid) were round shaped. The results suggest that high salt inhibits both the biosynthesis of secondary wall components and cell elongation ice plant in vitro culture. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Activity of cell-wall degradation associated with differentiation of isolated mesophyll cells of Zinnia elegans into tracheary elements

Cell walls were prepared from cultured mesophyll cells of Zinnia elegans L. that were transdifferentiating into tracheary elements and incubated in a buffer to undergo autolysis. The rate of autolysis of cell walls was determined by measuring the amount of carbohydrate released from the cell walls into the buffer during incubation. During the course of culture of mesophyll cells, the autolysis rate increased markedly at the time when thickenings of secondary cell walls characteristic of tracheary elements became visible (after 48-72 h of culture), and thereafter the rate remained at a high level. Comparative studies on the autolysis rate of cell walls using various control cultures, in which tracheary element differentiation did not take place, revealed a close relationship between the autolysis rate around the 60th hour of culture and differentiation. Sugar analysis by colorimetric assays and gas chromatography of carbohydrates released from the cell walls detected uronic acid, arabinose, galactose, glucose, xylose, rhamnose, fucose, and mannose. Among these sugars, uronic acid was the most abundant, and accounted for approximately half of the total released sugars. The decrease of acidic polysaccharides in the primary cell walls during tracheary element differentiation was visualized by staining cultured cells with alcian blue at pH 2.5. These results suggest that active degradation of components of primary cell walls, including pectin, is integrated into the program of tracheary element differentiation.     

Transition of G1 to early S phase may be required for zinnia mesophyll cells to trans-differentiate to tracheary elements

We have used the Zinnia elegans mesophyll cell system, in which single isolated leaf mesophyll cells can be induced to trans-differentiate into tracheary elements in vitro, to study the relationship between the cell division cycle and cell differentiation. Almost all cells go through several rounds of division before characteristic features of tracheary element formation are observed. The addition of aphidicolin, a DNA synthesis inhibitor, blocks cell division but not cell differentiation in the zinnia system. Low concentrations of aphidicolin, which possibly delay cells in the early S phase, can significantly enhance levels of tracheary element formation. In contrast, roscovitine, an inhibitor of cyclin-dependent kinase activity, decelerates the cell division cycle and inhibits tracheary element formation with similar dose responses. Cells blocked in S phase and then transferred to roscovitine-containing medium can divide once, indicating that roscovitine may target the G1/S transition, but do not differentiate. Cells inhibited in G1/S in roscovitine-containing medium that are subsequently blocked in S phase by transfer to aphidicolin-containing medium, do not divide but do differentiate. Taken together, our results indicate that cells may be required to transit the G1/S checkpoint and enter early S phase to acquire competence to trans-differentiate to tracheary elements.     

Tracheary element differentiation is correlated with inhibition of cell expansion in xylogenic mesophyll suspension cultures.

To test the hypothesis that xylogenesis is coupled to cell growth suppression, cell expansion in Zinnia elegans L. var. Envy mesophyll suspension cultures was manipulated by varying the extracellular osmolarity and the effect on xylogenesis was examined. Cell expansion and tracheary element differentiation were inversely related along a gradient of extracellular osmolarity ranging from 200 to 400 mOsm, supporting the hypothesis that tracheary element differentiation is coupled to cessation of cell expansion. Above 300 mOsm, reduction in the number of cells that differentiated into tracheary elements coincided with an increase in the number of plasmolyzed cells as extracellular osmolarity was increased, indicating that plasmolysis inhibits tracheary element differentiation, although not specifically. Using the plasmolysis method we showed that cellular osmolarity within populations of isolated Zinnia mesophyll cells ranges from 250 to 600 mOsm with a mean of 425 mOsm. The broad range in cellular osmolarity within Zinnia mesophyll cell populations, coupled with inhibition of differentiation in the low range due to cell expansion and in the high range due to plasmolysis, may help explain why tracheary element differentiation in Zinnia suspension cultures is never complete nor perfectly synchronous and enable further optimization of this culture system.