杨树试管苗嫩茎生根过程中内源IAA的免疫金银定位

生长素类物质在木本植物生根过程中发挥重要作用。杨树生根与生长素的关系及生根过程中内源激素的变化已有大量报道, 而生根过程中生长素的组织定位分析则尚未见报道。该文应用免疫化学分析方法对741杨(Populus alba × (P. davidiana × P. simonii) × P. tomentosa)嫩茎生根过程中内源IAA在组织中的分布进行了研究。结果显示, 741杨的嫩茎在无外源激素的1/2MS培养基上诱导10天后可生根, 14天后生根率达100%。诱导前, 嫩茎基部组织中几乎没有IAA信号; 诱导8天后, 嫩茎基部维管组织中有大量的IAA积累, 而且中部的维管组织中也有明显的IAA信号(主要分布在韧皮部和维管形成层); 10天后, 形成不定根原基, 此时IAA主要分布在根原基; 12天后, 根原基分化成不定根并突破表皮, IAA在不定根中的分布主要集中在根尖和中柱。该文对741杨的嫩茎生根过程中IAA的组织分布特点及运输途径进行了讨论。     

A role for IAA in the infection of Arabidopsis thaliana by Orobanche aegyptiaca

BACKGROUND: Vascular continuity is established between a host plant and the root parasite broomrape. It is generally accepted that the direction of vascular continuity results from polar flow of auxin. Our hypothesis was that chemical disruptions of auxin transport and activity could influence the infection of the host by the parasite. METHODS: A sterile system for the routine infection of Arabidopsis thaliana seedlings in Nunc cell culture plates by germinated seeds of Orobanche aegyptiaca was developed. This method permitted a quantitative assay of the rate of host infection. The three-dimensional structure of the vascular contacts was followed in cleared tissue. IAA (indole acetic acid) or substances that influence its activity and transport were applied locally to the host root. RESULTS: The orientation of the xylem contacts showed that broomrape grafts itself upon the host by acting hormonally as a root rather than a shoot. Local applications of IAA, PCIB (p-chlorophenoxyisobutyric acid) or NPA (naphthylphthalamic acid) all resulted in drastic reductions of Orobanche infection CONCLUSIONS: Broomrape manipulates the host by acting as a sink for auxin. Disruption of auxin action or auxin flow at the contact site could be a novel basis for controlling infection by Orobanche.     

A mathematical model of pattern formation in the vascular cambium of trees

The beautiful patterns apparent in wood grain have their origin in the alignment of fusiform initial cells in the vascular cambium of trees. We develop a mathematical model to describe the orientation of fusiform initial cells, and their interaction with the plant hormone indole-3-acetic acid (auxin). The model incorporates the following four assumptions: (1) auxin is actively transported parallel to the long axis of the initials, (2) auxin diffuses perpendicular to the long axis of the initials, (3) the initials tend to orient parallel to the flux of auxin through the cambium, and (4) adjacent initials tend to orient parallel to one another. Each assumption is justified on the basis of available evidence and cast in mathematical form. Our main result is a pair of nonlinear differential equations that describe the coupling between the distribution of auxin in the cambium and the orientation of fusiform initials. Numerical solutions to the equations show qualitative resemblance to the wood grain patterns observed at branch junctions, wounds and knots, and topological defects.     

Plant morphogenesis: long-distance coordination and local patterning

The overall morphology of a plant is largely determined by developmental decisions taken within or near the terminally positioned apical meristems of shoots and roots. The spatial separation of these developmental centers emphasizes the need for long-distance signaling. The same signaling events may simultaneously coordinate differentiation within meristems and in the connecting vascular tissues. Recent genetic and molecular analyses not only confirm the proposed role of auxin as a coordinating signal across the plant, but also implicate auxin as a patterning signal in embryo and meristem organization.     

In vitro rhizogenesis: histoanatomy of Cedrela odorata (Meliaceae) microcuttings

Cedrela odorata (Meliaceae) is considered as one of the most valuable forest tree in the tropics. Clonal propagation of this species provide an alternative method to propagate superior genotypes, being the production of good quality adventitious roots one of the most important steps in micropropagation techniques. The sequence of anatomical changes that takes place during the formation of adventitious roots in shoots of Cedrela odorata cultured in vitro is described in this study. Eigth-week-old shoots, from multiplication cultures, were rooted in Murashige and Skoog's medium (1962) with half-strength macronutrients and with 0 or 1 mg/l indole-3-butyric acid (IBA). Between 12 and 24h after the start of rooting, some cambium, phloem and interfascicular parenchyma cells became dense cytoplasm, nuclei with prominent nucleoli and the first cell divisions were observed, especially in shoots treated with auxin (dedifferentiation phase). After 3-4 days, the number of dedifferentiated cells and mitotic divisions increased considerably, and the formation of groups of some 30-40 meristematic cells (meristemoids) was observed (induction phase). The first primordial roots developed from the 4th-5th day. The vascular tissues of these primordia connected to those of the explant, and roots began to emerge from the base by day 6. Development of the primordial roots was similar in the control shoots and shoots treated with 1 mg/l IBA, although there were more roots per explant in the latter.     

The position of regenerating cambia: auxin/sucrose ratio and the gradient induction hypothesis.

To account for the positions in which vascular cambia regenerate in wound callus, a gradient induction hypothesis was proposed in 1961 in terms of gradients in 'some factor as yet unknown'. It now seems likely that the gradient is based on morphogen diffusion between source and sink on opposite sides of existing cambia, with morphogen diffusing into the adjoining wound callus. It is specifically proposed that there are two morphogens, auxin diffusing centrifugally and sucrose diffusing centripetally. The cambium then regenerates along a path where the ratio of auxin to sucrose concentration is similar to that at the original cambium, and its orientation (as regards xylem and phloem formation) is determined by the direction of the gradient in this ratio. These proposals are supported by published evidence on auxin and sucrose concentration gradients across the cambium, and on their sources, movements, and known effects on vascular differentiation. Simulations of the proposed positional control system predict patterns of cambial regeneration and orientation corresponding to those observed in four different types of wound and graft.     

Polarity and the Induction of Organized Vascular Tissues

This work deals with those properties of plant tissues whichare responsible for the organization of vascular cells in orderedstrands. It is shown that auxin alone is sufficient to causethe differentiation of strands of xylem cells in the parenchymaof pea roots. An artificially induced strand, once it is formed,attracts towards itself newly induced vascular strands, andthis attraction results in the union of old and new strands.It is also shown that the application of auxin to natural vasculartissues prevents their being joined by newly induced vascularstrands. It is proved that this is dependent on a directionaleffect and not simply on a local accumulation of auxin. To understand these results, it must be assumed that the polarityin terms of auxin transport is increased during the processof vascular tissue induction. The same polarity, once established,is maintained by the presence of auxin, so that the differentiationof strands perpendicular to the axis of this polarity is prevented.These characteristics of plant tissues concerning auxin transportexplain the basic phenomena of the organization of vascularcells in defined and ordered strands.     

Epidermal growth factor stimulates type-V collagen synthesis in cultured murine palatal shelves

Abstract. The problem studied was whether treatments that reorient vascular differentiation have a similar effect on the polarity of auxin transport. Hypocotyls of Phaseolus vulgaris L. were cut so that a transverse bridge connected the shoot and root directions. Within three days these bridges of tissue regenerated both vessels and sieve tubes along the new orientation, at 90° to the original axis. Experiments involving organ removal, wounds, and hormone application confirm previous suggestions that this differentiation follows the expected flow of the hormone auxin in the direction of the roots. Transport of (³H) indoleacetic acid through sections in which vascular reorientation occurred was polar: it was at least twice as great in the new direction of the roots than in the opposite direction. This new polarity of transport, at right angles to the original axis of the plant, can be readily understood if there is a positive feed-back between the differentiation of tissue polarity and auxin transport.     

Species differences in ligand specificity of auxin-controlled elongation and auxin transport: comparing Zea and Vigna

The plant hormone auxin affects cell elongation in both roots and shoots. In roots, the predominant action of auxin is to inhibit cell elongation while in shoots auxin, at normal physiological levels, stimulates elongation. The question of whether the primary receptor for auxin is the same in roots and shoots has not been resolved. In addition to its action on cell elongation in roots and shoots, auxin is transported in a polar fashion in both organs. Although auxin transport is well characterized in both roots and shoots, there is relatively little information on the connection, if any, between auxin transport and its action on elongation. In particular, it is not clear whether the protein mediating polar auxin movement is separate from the protein mediating auxin action on cell elongation or whether these two processes might be mediated by one and the same receptor. We examined the identity of the auxin growth receptor in roots and shoots by comparing the response of roots and shoots of the grass Zea mays L. and the legume Vigna mungo L. to indole-3-acetic acid, 2-naphthoxyacetic acid, 4,6-dichloroindoleacetic acid, and 4,7-dichloroindoleacetic acid. We also studied whether or not a single protein might mediate both auxin transport and auxin action by comparing the polar transport of indole-3-acetic acid and 2-naphthoxyacetic acid through segments from Vigna hypocotyls and maize coleoptiles. For all of the assays performed (root elongation, shoot elongation, and polar transport) the action and transport of the auxin derivatives was much greater in the dicots than in the grass species. The preservation of ligand specificity between roots and shoots and the parallels in ligand specificity between auxin transport and auxin action on growth are consistent with the hypothesis that the auxin receptor is the same in roots and shoots and that this protein may mediate auxin efflux as well as auxin action in both organ types.     

The role of auxin stored in scots pine trunk during spring activation of cambial activity

In a 9-year-old pine girdled during the winter cambial activity was observed below the girdle in the next spring. This indicates that cambial activity was initiated without auxin produced in the spring by buds. The auxin produced in apical shoots successively flows down the stem, where as a result of periodic restriction in transport it remains over the winter till the next year. This auxin of apical origin but locally stored over the winter in the stem is responsible for the activation of cambium before the new flow of auxin produced in the apical meristems arrives. Calculations based on seasonal changes in auxin levels can explain both, earlier spring activation of cambium in the crown and the temporary cambial divisions below the girdle, without assumption of direct auxin synthesis in the lateral meristems.     

Histological study of initiation and development in vitro of adventitious roots in minicuttings of apple rootstocks of M 26 and EMLA 9

Apple rootstocks M 26 and EMLA 9 'COST' shoots propagated in vitro were used for the histological study of initiation and development of adventitious roots after a brief induction pretreatment. The results show that there are differences in mode and timing of initiation and development of adventitious roots between the two varieties. In M 26, adventitious roots were directly initiated from the derivatives of the cambium, some of which were immediately transformed into meristemoids in situ 36 h after pretreatment. Subsequently, meristemoids differentiated into root primordia. Development of adventitious roots were completed when they emerged at the surface of stem bases 10 days after pretreatment. In EMLA 9, before the meristemoids formed, internal cell files were formed by continuous divisions of cambial cells. The cells were regularly arranged in files external to the cambium. On the fourth day after pretreatment, some cells in the outermost layers of these files became meristematic, started to divide and turned into meristemoids, which differentiated into root primordia. The cells of the files between the root primordium and the cambium were transformed into vascular tissues which connected the vascular systems of the adventitious roots and stems.     

Lateral meristems of higher plants: Phytohormonal and genetic control

Lateral meristems (pericycle, procambium and cambium, phellogen) are positioned in parallel to the lateral surface of the organ, where they are present, and produce concentric layers of undifferentiated cells. Primary lateral meristems, procambium and pericycle, arise during embryogenesis; secondary lateral meristems, cambium and phellogen, — during post embryonic development. Pericycle is most pluripotent plant meristem, as it may give rise to a variety of other types of meristems: lateral meristems (cambium, phellogen), apical meristems of lateral roots, and also shoot meristems during plant in vitro regeneration. Procambium and cambium developing from it give rise to the vascular tissues of the stems and roots, ensuring their thickening. The review considers the genetic control of lateral meristem development and the role of phytohormones in the control of their activities.     

Ethanol in the stems of trees

Acetaldehyde and ethanol are usually thought to be produced in plant tissues as a mechanism to tolerate hypoxic conditions. We have found acetaldehyde and ethanol to be common in the vascular cambium and in the transpiration stream of trees. In nonflooded trees, acetaldehyde and ethanol concentrations averaged 130 and 40 μ M in the cambium and 130 and 50 μ M in the xylem sap, respectively. Ethanol concentrations in the transpiration stream and the cambium increased to as much as 5 m M upon flooding. Ethanol concentrations in the vascular cambium of Populus deltoides could not be eliminated by placing logs from nonflooded trees in a pure oxygen environment for as long as 96 h, but increased by almost 3 orders of magnitude when logs were exposed to low external partial pressures of O₂. These results suggest that the vascular cambium was not hypoxic, despite the presence of acetaldehyde, ethanol and the enzymes for their synthesis.     

Cellular Modelling of Secondary Radial Growth in Conifer Trees: Application to Pinus Radiata (D. Don)

The radial growth of conifer trees proceeds from the dynamics of a merismatic tissue called vascular cambium or cambium. Cambium is a thin layer of active proliferating cells. The purpose of this paper was to model the main characteristics of cambial activity and its consecutive radial growth. Cell growth is under the control of the auxin hormone indole-3-acetic. The model is composed of a discrete part, which accounts for cellular proliferation, and a continuous part involving the transport of auxin. Cambium is modeled in a two-dimensional cross-section by a cellular automaton that describes the set of all its constitutive cells. Proliferation is defined as growth and division of cambial cells under neighbouring constraints, which can eliminate some cells from the cambium. The cell-growth rate is determined from auxin concentration, calculated with the continuous model. We studied the integration of each elementary cambial cell activity into the global coherent movement of macroscopic morphogenesis. Cases of normal and abnormal growth of Pinus radiata (D. Don) are modelled. Abnormal growth includes deformed trees where gravity influences auxin transport, producing heterogeneous radial growth. Cross-sectional microscopic views are also provided to validate the model's hypothesis and results.     

How canalization can make loops: a new model of reticulated leaf vascular pattern formation

Formation of the vascular system in plant leaves can be explained by the canalization hypothesis which states that veins are formed in an initially homogeneous field by a self-organizing process between the plant hormone auxin and auxin carrier proteins. Previous models of canalization can generate vein patterns with branching but fail to generate vein patterns with closed loops. However, closed vein loops are commonly observed in plant leaves and are important in making them robust to herbivore attacks and physical damage. Here we propose a new model which generates a vein system with closed loops. We postulate that the "flux bifurcator" level is enhanced in cells with a high auxin flux and that it causes reallocation of auxin carriers toward neighbouring cells also having a high bifurcator level. This causes the auxin flux to bifurcate, allowing vein tips to attach to other veins creating vein loops. We explore several alternative functional forms for the flux bifurcator affecting the reallocation of efflux carriers and examine parameter dependence of the resulting vein pattern.     

The anatomy of the chi-chi of <Emphasis Type="Italic">Ginkgo biloba</Emphasis> suggests a mode of elongation growth that is an alternative to growth driven by an apical meristem

The chi-chi of Ginkgo biloba L. are cylindrical woody structures that grow downwards from the branches and trunks of old trees, eventually entering the soil where they give rise to adventitious shoots and roots. Examination of segments of young chi-chi taken from a mature ginkgo tree revealed an internal woody portion with irregular growth rings of tracheid-containing secondary xylem covered by a vascular cambium and bark. The cambium was composed of both fusiform cells and parenchymatous ray cells. Near the tip of the chi-chi, these two types of cambial cells had orientations ranging between axial, radial and circumferential with respect to the cylindrical form of the chi-chi. The xylem rays and tracheids that derived from the cambium showed correspondingly variable orientations. Towards the base of the chi-chi, the fusiform cells and young tracheids were aligned parallel to the axis, indicating that the orientation of the cambial cells in basal regions of the chi-chi gradually became normalised as the tip of the chi-chi extended forwards. Nevertheless, in such basal sites, tracheids near the centre of the chi-chi showed variable orientations in accordance with their mode of formation during the early stages of chi-chi development. The initiation of a chi-chi is proposed to derive from a localised hyperactivity of vascular cambial-cell production in the supporting stem. The chi-chi elongates by tip growth, but it does so in a manner different from organ growth driven by an apical meristem. It is suggested that the chi-chi of Ginkgo is an “evolutionary experiment” that makes use of the vascular cambium, not only for its widening growth but also for its elongation.