JXB Advance Access originally published online on May 19, 2006
Journal of Experimental Botany 2006 57(10):2227-2235; doi:10.1093/jxb/erj187
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
RESEARCH PAPER |
Root meristems in Medicago truncatula tissue culture arise from vascular-derived procambial-like cells in a process regulated by ethylene
1Australian Research Council Centre of Excellence for Integrative Legume Research, School of Environmental and Life Sciences, The University of Newcastle, University Drive, Callaghan, NSW 2308, Australia
2Australian Research Council Centre of Excellence for Integrative Legume Research, Genomics Interactions Group, The Australian National University, Canberra, ACT 0200, Australia
*To whom correspondence should be addressed. E-mail: rolfe{at}rsbs.anu.edu.au
Received 15 December 2005; Accepted 13 March 2006
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Leaf explants of Medicago truncatula were used to investigate the origins of auxin-induced root formation. On the application of auxin there is some callus formation (not the massive amount that occurs in response to auxin plus cytokinin) and roots appear shortly after the first visible callus. Histological examination reveals morphologically distinctive sheets of callus cells that emanate from the veins of the leaf explants and, within this cell type, root primordia are produced as well as some vascular tissue cells. What is suggested is that the vein-derived cells (VDCs) are procambial-like and function as pluripotent stem cells with a propensity to form root meristems or vascular tissues in response to added auxin. The development of root primordia from these pluripotent cells was clearly up-regulated by the use of the sickle (skl) mutant, which is a mutant impaired in ethylene signal transduction while the wild type and the sunn mutant, defective in auxin polar transport, produced similar numbers of roots. The skl mutant in generating many more roots concomitantly formed fewer vascular tissues. The root meristems differentiate similarly to normal roots producing a central cylinder of vascular tissue, which connects with the leaf explant veins. The VDCs appear to be derived from the cells of or near the phloem. The leaf observations suggest that a pool of stem cells exist in vascular tissue that, in combination with auxin and perhaps other factors, drive a diversity of plant development outcomes that is species specific. The way auxin interacts with other hormones is a key factor in determining the stem cell fate. The histological data in this study also assist in the interpretation of the molecular analysis of auxin-induced root formation in cultured leaves of M. truncatula.
Key words: Ethylene, Medicago truncatula, root meristems
|
Introduction |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
It has been known for almost 50 years that auxin can induce root formation in tissue culture. This was highlighted by the classic study of Skoog and Miller (1957) where it was shown that root and shoot formation can be regulated by the auxin:cytokinin ratio. Molecular investigations of development have been carried out using these strategies with root explants from Arabidopsis, focusing on shoot regeneration (Cary et al., 2002; Howell et al., 2003). Roots are also routinely induced in response to auxin in plant regeneration strategies in transformation and in rooting of cuttings (Callis, 2005), but less attention has been paid to their stages of morphogenesis.
Sachs (1993) described ‘auxin’ as a correlative signal, co-ordinating leaf development with vascular differentiation and other developmental processes throughout the plant. The importance of auxin flow as a determinant of the canalization of differentiation and vascular tissues has been a recurring theme of many of the papers of Tsvi Sachs (Sachs, 2000, and references therein). A recent review by Jiang and Feldman (2005) describes the establishment of the root apical meristem as being dependent upon the specification of a stem cell niche and the development of the quiescent centre. The ‘distribution of auxin and the establishment of auxin maxima are the early formative steps in niche specification that depend on the expression and distribution of auxin carriers’. The authors propose, ‘roots have evolved as part of an auxin homeostasis mechanism’. In plant development, auxin regulates lateral root formation from pericycle cells (Malamy and Benfey, 1997) and molecular investigations have implicated a number of cell cycle and developmental regulators in the control of this process (Himanen et al., 2002; Guo et al., 2005). Studies of lateral root (LR) formation in the model plant Arabidopsis showed that the lateral root primordia originate from a subset of pericycle founder cells (Casimiro et al., 2003). The authors also mapped the sites of the biosynthesis of auxin and its distribution during LR initiation and emergence, indicating the importance of the phytohormone to LR induction. Thus, LR formation can be divided into pericycle activation and meristem establishment. Auxin also plays a central role in adventitious root formation (Sorin et al., 2005). These latter studies have also suggested that different regulatory pathways control lateral root and adventitious root initiation, even though both root types initiate from pericycle cells (Sorin et al., 2005).
In cell cultures and suspensions, there is differentiation of individual vascular cells, but it is only in the whole plant that vascular strands are formed, which link up with each other to form a vascular network (Lyndon, 1990). Sachs (1981) has extensively investigated the induction of new vascular strands in relation to the existing strands. In the plant, vascular strands differentiate into or from young developing leaves, which are thought to be sources of auxin. Fukuda (2004) describes how plant vascular cells originate from procambial cells, which are vascular stem cells, and that the body plan of plants is controlled by a combination of clonal fate and positional information that is provided by local signals. The most important correlative signal is auxin, which co-ordinates leaf development with vascular differentiation and other developmental processes throughout the plant (Sachs, 1993). This means that cells must be sensitive to the gradients or the flux of correlative signals (Sachs, 1981, 1991). Vascular cells usually differentiate at predicted positions and at a predicted time to form a specific vascular pattern, however, the arrangement of the vascular network can be altered by local signals or in response to environmental stimuli (Fukuda, 2004).
Studies on the cell cycle in plants have shown that sensitivity to auxin occurs at specific points, potentially implicating auxin as a central mediator of cell division (Teale et al., 2005). Collectively, the many studies show that, in parallel to auxin functions in cell cycle control, and the tropic cell-elongation response, auxin can also have a morphogenic effect able to control plant organogenesis (Teale et al., 2005).
Medicago truncatula is a model legume that can be regenerated from tissue culture via somatic embryogenesis using highly regenerable genotypes such as Jemalong 2HA (Rose et al., 1999) and added auxin and cytokinin. Leaf explants were found to induce root formation in response to auxin in both wild type and 2HA genotypes (Nolan et al., 2003). As part of an investigation into the cellular and molecular mechanisms of morphogenesis of somatic embryos and root formation, a histological approach was used to investigate auxin-induced root formation in leaf explants. Specific mutants of M. truncatula, altered in their signal transduction pathways, were used to examine auxin-induced root formation further. These data are discussed in the context of auxin-induced organogenesis and the value of this system for molecular dissection of organogenesis and stem cell biology using this model legume.
|
Materials and methods |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Plant material
The initial investigation of auxin-induced root growth was carried out on the highly regenerable seed line Jemalong 2HA (Rose et al., 1999) and its progenitor Jemalong. Seeds of M. truncatula cv. Jemalong were obtained from the National Medicago Collection (Northfield Research Laboratories, Adelaide, South Australia). The great advantage of the leaf explant system used in this work is that pieces of leaves can be placed into tissue culture and incubated, and they will form calli and then, under the appropriate hormone conditions, will form roots or embryos. This enables the investigation of effects of specific plant mutants on a particular developmental pathway.
The sunn and the sickle (skl) mutants of Jemalong A17 were kindly provided by Professor Doug Cook, UC Davis. The sunn mutant was chosen because it had an alteration in its polar auxin transport capacity, showing an increased amount of auxin transfer from the shoot to the root compared with its parent line Jemalong A17 (van Noorden, 2006). The skl mutant has a defect in ethylene perception, is ethylene-insensitive, and has been mapped and sequenced and shown to be an orthologue of the EIN2 gene of Arabidopsis, which is a nuclear membrane protein involved in the transmission of the ethylene signalling pathway (Penmetsa and Cook, 1997; D Cook, personal communication). Plants were grown under controlled glasshouse conditions with a day temperature of 23 °C and a night temperature of 19 °C.
Tissue culture
The tissue culture procedure using leaf explants has been described previously (Nolan et al., 2003). Leaf explants of approximately 2 mmx4 mm with the midvein in the centre were placed abaxial side down on the agar in 9 cm plastic Petri dishes. The tissue was grown on P4 basal medium (Thomas et al., 1990) and 10 µM NAA (P4:10). The early callus emerges from the margin of the cut explant tissue.
Histology
Transverse slices of 5x8 mm from 2–3-week-old calli were fixed on ice in 4% (v:v) glutaraldehyde and 2% (v:v) paraformaldehyde in 100 mM sodium cacodylate buffer, pH 7.2 for 4 h. Following 3x10 min rinses in the same buffer, the tissue was dehydrated on ice through a 10% (v:v) step-graded series of ethanol for 30 min per step. Once in 100% ethanol, the solution was changed after 30 min and the tissue left overnight. The next step was infiltration, which was carried out on a rotator at room temperature. Infiltration used a graded series of LR White resin (Proscitech, Kirwan, Qld, Australia) in ethanol with 10%, 20%, and 40% (v:v) for 2 h each and left overnight in 60% (v:v). Infiltration was continued in 80% (w:v) for 2.5 h, 90% (w:v) for 4 h then 100% of LRWhite. At the 100% LRWhite stage, the solution was changed three times over 3 d. The tissue was then polymerized at 60 °C overnight.
Sections of 0.5–1 µm were cut using glass knives (LKB Knife maker) on a Reichert–Jung Ultracut E microtome and stained with 1.0% (w:v) Toluidine Blue and 0.5% Azur II in 1% borax at pH 9. Micrographs were taken using bright field optics on a Zeiss Axiophot Photomicroscope (D-7082 Oberkochen, West Germany). For colour images, Kodak Ektachrome Elite (160 ASA) was used.
|
Results |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
A leaf explant consists of major and minor veins, epidermal and mesophyll cells. On the application of auxin there is some callus formation (not the massive amounts that occur in response to auxin plus cytokinin) and then root primordia start to appear in both Jemalong and 2HA (Fig. 1A, B). What was observed histologically were morphologically distinctive sheets of callus cells that arise from the veins of the leaf explants and, within this cell type, root primordia were produced as well as some vascular tissue (Fig. 2A–C).
|
|
Leaf explants treated with auxin to form calli that produce roots is a classic auxin response. Little attention has been given to the progenitor cells from which these organs form. Superficially, the answer appears simply that the roots form from calli cells that, in turn, are derived from dedifferentiation of leaf cells. The leaf explant, however, consists of mesophyll, vascular, and epidermal cells that could contribute cells to the callus and have different developmental fates. Histological investigations revealed an unexpected order to the callus and the cells from which the root meristems form. Figure 2A shows a longitudinal section through a leaf explant and the callus that has developed from it. What can clearly be visualized is a sheet of cells that can be traced back to cells of the vascular tissue consistent with having been derived by cell divisions from vascular tissue cells (Fig. 2A, B). These latter cells, which have been called vein-derived cells (VDCs), are rectangular-shaped and, in the tissue they form, there is little intercellular space. Adjacent to these cells are larger, more spherical cells, which are typical of callus-like cells and derived from mesophyll cells. These cells are much more loosely packed and are surrounded by large intercellular spaces.
The VDCs are the site of the developing root meristems (Fig. 2C) and appear to be the progenitor cells of the meristems (Fig. 1C–E). These VDCs are also the source of the tracheids that differentiate from these cells (Fig. 1C, D). Figure 1 C–E shows the development of the densely stained, cytoplasmically rich cells of the developing root primordia. Sometimes a VDC gives rise to multiple meristems. The VDCs are derived from the abaxial side of the leaf (Fig. 3A) from cells more likely associated with the phloem parenchyma (Fig. 3B). This is because the phloem is located on the abaxial spongy mesophyll side of the leaf and the xylem on the adaxial side of the leaf (Fig. 3B). The root meristems that form from the VDCs develop into roots with normal morphology when viewed in longitudinal (Fig. 4A) and transverse section (Fig. 4B–D). The root is, however, more rounded at the tip and the root widens out more away from the tip than a seedling root.
|
|
The two mutants sunn and skl were used to investigate controls to root formation in tissue culture. Only the skl mutant had an increased number of roots (Fig. 5A–D) compared with wild type. The skl calli develop a ‘porcupine’ appearance (Fig. 5C) and again it was shown that the roots were derived from VDCs (Fig. 6A). Moreover, there were fewer tracheids formed. In skl leaf explants the VDCs appeared to undergo less proliferation and the roots were formed closer to the vein (Fig. 6A) and more of the VDCs produced roots, often forming them into small clusters. Figure 6A also shows varying numbers of VDCs associated with different veins and again derived from the phloem side of the vein. In Fig. 6B there is another clear illustration of the derivation of a root meristem from VDCs. The sickle mutant also provides some clear examples of the way separate vascular tissue traces are able to interconnect. As differentiation procedes back from the meristem a central vascular cylinder is formed (Fig. 6C, D), which can then rejoin the vascular tissue of the original leaf explant (Fig. 6D). The various developmental steps involved in root formation are summarized in Fig. 7A. The proposed influence of the sickle mutant is to release the early conversion of procambial like cells into tracheid development. In the wild type (Fig. 7B), both tracheids and cells forming a more organized rooting structure are elaborated from the procambial cells. Ehylene (as shown by the use of the skl mutant, Fig. 7C) influences an early step of commitment, and converts a number of cells traveling along a tracheid developmental pathway into one which gives rise to the more organized structures seen in a functional root.
|
|
|
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
The histological investigation of auxin-induced root formation in the Medicago leaf explants has provided an improved understanding of the stages of morphogenesis of the roots generated in this system, as well as a new perspective into stem cell biology. It seems clear that cells in the vein are stimulated into division by the added auxin forming VDCs that grow out into the callus and that it is from these cells that the root meristems are formed. A morphologically distinctive lineage of cells can be traced from the veins out into the callus. Meristems can be visualized amongst the VDCs (Fig. 2A, C) and differentiation proceeds from the meristems to produce the distinctive tissues of the root. The vascular tissue differentiated from the root meristem then joins the vascular tissue of the leaf explant (Fig. 6B–D).
What was observed in this study has analogies with roots that develop from somatic cells such as lateral roots and adventitious roots. There is a group of cells initially stimulated into division which then go on to produce root meristems. Auxin can stimulate the division of pericycle cells without lateral initiation (Grant and Fuller, 1970) as well as the initiation of lateral roots (Scott and Norris, 1970). The pericycle is close to the vascular tissue and the vascular tissues of the lateral root eventually become connected to the vascular tissue of the primary root (Esau, 1977). Adventitious roots are similar in their ontogeny to lateral roots where they arise close to vascular tissue in a variety of organs (Esau, 1977). In Arabidopsis hypocotyls the origin cells are referred to as pericycle, whereas in the older literature there are examples of roots being apparently derived from phloem parenchyma in tomato stems (Esau, 1977).
What are the cells that produced the VDCs in this study? This question cannot at this stage be answered unequivocally, but the VDCs appear to be derived from the cells of or near the phloem (Fig. 3B). The candidates are phloem parenchyma cells, the sheath cells surrounding the vein, or procambium cells that would normally be recruited for vein formation in the growing leaf. Procambium usually moves ahead of vascular tissue and the phloem and xylem cells are derived from these cells (Esau, 1977). However, it is possible that there is a reservoir of procambium cells that remain in the primary vein. There is support for the idea that veins are a rich source of stem cells. For example, wounding studies show that severed veins can rejoin (Sachs, 1989, 1993). It is suggested that the procambium cells are pluripotent stem cells, which can be readily switched into the root differentiation pathway. What appears to be occurring in the case of M. truncatula leaves is the production of procambial-like cells that are pluripotent, producing both vascular tissue and root meristems. What has been observed in these studies reinforces classical auxin and root biology that has served practical use in cuttings and plant regeneration (Callis, 2005). Adventitious rooting can occur in many organs, including leaves (Esau, 1977; Rolfe and McIver, 1996) and the auxin-induced root formation in Medicago leaf explants is a very clear demonstration of the involvement of cells of the veins, closely associated with the phloem of the leaf tissue. This phenomenon has received little, if any, attention and has a number of interesting implications in the context of stem cell biology, and organogenesis in legumes and assists in the analysis of in vitro morphogenesis in Medicago truncatula.
Vascular tissue has a remarkable potential for regeneration and gives rise to cambial stem cells needed for secondary vascular growth (Scheres, 2005). Fukuda (2004), however, argues that ‘plant vascular cells originate from procambial cells, which are vascular stem cells’ that are produced continuously from the apical meristems of roots and shoots in growing plants and give rise to xylem and phloem precursor cells. Xylem precursor cells give rise to tracheary elements, which together form the xylem, the tracheary cells lose all their contents and develop secondary cell walls of annular, spiral or reticulate wall thickenings (Fukuda, 2004). These steps of morphological events of patterned secondary-wall formation and programmed cell death are the most distinctive processes of tracheid development. Sachs (1981) showed that auxin was the limiting and controlling factor in the regeneration of vascular tissues and that polar auxin flow is required for continuous vascular pattern formation. However, little is still known about procambial cells and their differentiation in the root while the xylem cell poles start from procambial cells next to the quiescent centre. The initiation of lateral and adventitious roots has a close association with vascular tissue, as do root nodules (Mathesius et al., 1998, 2000a, b). In all these cases there is an auxin involvement in the regulation (Mathesius et al., 1998; Ferguson and Mathesius, 2003; Mulder et al., 2005). When leaf explants from the Jemalong 2HA line are placed on auxin-alone medium the calli clearly form both roots and tracheid vascular tissues. By contrast, the ethylene-insensitive skl mutant produces a marked number of roots per leaf explant calli, but only a few tracheids were found with these same calli. The early formation of root primordia proliferating in the leaf explants has been observed, which it is believed resulted from the auxin stimulation of the division of procambial-like cells within the vascular traces of the veins of the leaf pieces. These cells then form new stem cell niches and, subsequently, the meristem of a future root of the leaf explant calli (Fig. 7). Further development of this meristem produces the initial files of cells for the development of the vascular bundle and the growing root with its full vascular system.
This study's observations suggest that a pool of procambial or stem cells exist in the vascular tissues of the leaf explants that can be stimulated into pluripotency by auxin and, possibly, other factors and drive a diversity of plant development and architecture such as leaf veins, adventitious roots, and responses to leaf wounding. An important question is whether these stem cell precursors are actually a group of cells held by the adjacent leaf cells in a developmental check that is released upon the making of cut explant tissues. Alternatively, these may be a set of cells capable of responding more rapidly and thus having their developmental progression reset by new signals. The first possibility would require a form of ‘positional information’ control of differentiation within the leaf, as occurs in animal embryology where cells respond as though they appreciate their spatial positions in the embryo. Perhaps in the leaf vascular bundle consisting of parenchyma sheath cells, xylem and phloem cells the proposed procambium cells are held in check by their neighbours and position in the bundle. The way auxin interacts with other hormones is a key factor in determining the stem cell fate. In M. truncatula, if cytokinin is added with auxin, root production is blocked as expected for cytokinin (Skoog and Miller, 1957) and embryos are produced and many more vascular tissues are produced throughout the callus.
Ethylene is known to inhibit the growth of roots (Guzman and Ecker, 1990) and the early stages of lateral root and nodule formation (Penmetsa and Cook, 1997; Penmetsa et al., 2003; J Prayitno, BG Rolfe, U Mathesius, unpublished results). That is, it inhibits the progress of organogenesis of root outgrowths. In the situation here, the loss of ethylene sensitivity, in the skl mutant, enables a marked increase in successful root formation and little tracheid formation. The enhanced auxin-stimulated root formation in the skl mutant suggests that blocking ethylene transduction in this mutant enhances root meristem initiation by enabling more stem cells to differentiate in this direction; rather than producing vascular (tracheid) tissue (Fig. 7B, C). The simplest explanation is that ethylene (via the skl gene product, or processes affected by it) acts as a negative regulator of commitment to the root developmental pathway. It is proposed that the ethylene inhibition is most likely to affect the stage of development where the formation of the initial files of early vascular tissues occurs. Associated with this event is the concomitant loss of growth of the rootlet meristem. Thus, these initial vascular cells could form tracheids but they would not be associated with root formation.
The morphogenetic events observed in this study will assist in the interpretation of the molecular data obtained in the system used here to investigate in vitro embryo and root formation (Nolan et al., 2003; Imin et al., 2005). Both root-forming and embryo-forming cultures express MtSERK1 and the transcription factor BabyBoom (MtBBM) and the information obtained in this study showing proliferation of procambium-like cells suggests that this may be the reason for the expression of these auxin-induced genes in root-forming cultures. The MtSERK1 expression not only marks stem cells destined to produce embryos (Nolan et al., 2003), but also cells destined to form roots.
|
Acknowledgements |
|---|
We thank Jeremy Weinman for critical reading of the manuscript and help in the presentation of the figures; Professor Douglas Cook for supplying seed for the sunn and skl mutants and providing us with unpublished recent data on the skl mutant. The study was supported by a grant from the ARC Centre of Excellence for Integrative Legume Research, No. CEO348212.
|
References |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
|---|
Callis J. (2005) Auxin action. Nature 435:436–437.[Medline]
Cary AJ, Che P, Howell SH. (2002) Developmental events and shoot apical meristem gene expression patterns during shoot development in Arabidopsis thaliana. The Plant Journal 32:867–877.[CrossRef][Web of Science][Medline]
Casimiro I, Beeckman T, Graham N, Bhalerao R, Zhang H, Casero P, Sandberg G, Bennett M. (2003) Dissecting Arabidopsis lateral root development. Trends in Plant Science 8:165–171.[CrossRef][Web of Science][Medline]
Esau K. (1977) Anatomy of seed plants 2nd edn (John Wiley and Sons, New York).
Ferguson BJ and Mathesius U. (2003) Signaling interactions during nodule development. Journal of Plant Growth Regulation 22:47–72.[CrossRef][Web of Science]
Fukuda H. (2004) Signals that control plant vascular cell differentiation. Nature Reviews, Molecular Cell Biology 5:379–391.[CrossRef][Web of Science][Medline]
Grant M and Fuller KW. (1970) Biochemical changes associated with the growth of root tips of Vicia faba in vitro, and the effect of 2,4-dichlorophenoxyacetic acid. Journal of Experimental Botany 22:49–59.
Guo H-S, Xie Q, Fei J-F, Chua N-H. (2005) MicroRNA directs mRNA cleavage of the transcription factor NAC1 to downregulate auxin signals for Arabidopsis lateral root development. The Plant Cell 17:1376–1386.
Guzman P and Ecker JR. (1990) Exploiting the triple response of Arabidopsis to identify ethylene-induced mutants. The Plant Cell 2:513–523.
Himanen K, Boucheron E, Vanneste S, Engler J de A, Inzé D, Beeckman T. (2002) Auxin-mediated cell cycle activation during lateral root initiation. The Plant Cell 14:2339–2351.
Howell SH, Lall S, Che P. (2003) Cytokinins and shoot development. Trends in Plant Science 8:453–459.[CrossRef][Web of Science][Medline]
Imin N, Nizamidin M, Daniher D, Nolan KE, Rose RJ, Rolfe BG. (2005) Molecular analysis of Medicago truncatula explant cultures grown under 6-benzylaminopurine and 1-naphthaleneacetic acid treatments. Plant Physiology 137:1250–1260.
Jiang K and Feldman LJ. (2005) Regulation of root apical meristem development. Annual Reviews in Cell and Developmental Biology 21:485–509.
Lyndon RF. (1990) Plant development. The cellular basis (Unwin Hyman, London).
Malamy JE and Benfey PN. (1997) Organization and cell differentiation in lateral roots of Arabidopsis thaliana. Development 121:3303–3310.
Mathesius U, Schlaman HRM, Spaink HP, Sautter C, Rolfe BG, Djordjevic MA. (1998) Auxin transport inhibition precedes root nodule formation in white clover roots and is regulated by flavonoids and derivatives of chitin oligosaccharides. The Plant Journal 14:23–34.[CrossRef][Web of Science][Medline]
Mathesius U, Charon C, Rolfe BG, Kondorosi A, Crespi M. (2000a) Temporal and spatial order of events during the induction of cortical cell divisions in white clover by Rhizobium leguminosarum bv. trifolii inoculation or localized cytokinin addition. Molecular Plant–Microbe Interactions 13:617–628.
Mathesius U, Weinman JJ, Rolfe BG, Djordjevic MA. (2000b) Rhizobia can induce nodules in white clover by ‘hijacking’ mature cortical cells activated during lateral root development. Molecular Plant–Microbe Interactions 13:170–182.[CrossRef]
Mulder L, Hogg B, Bersoult A, Cullimore J. (2005) Integration of signaling pathways in the establishment of the legume–rhizobia symbiosis. Physiologia Plantarum 123:207–218.
Nolan KE, Irwanto RR, Rose RJ. (2003) Auxin up-regulates MtSERK1 expression in both root-forming and embryogenic cultures. Plant Physiology 133:218–230.
Penmetsa RV and Cook DR. (1997) A legume ethylene-insensitive mutant hyperinfected by its rhizobial symbiont. Science 275:527–530.
Penmetsa RV, Frugoli JA, Smith LS, Long SR, Cook DR. (2003) Dual genetic pathways controlling nodule number in Medicago truncatula. Plant Physiology 131:998–1008.
Rolfe BG and McIver J. (1996) Single-leaf plantlet bioassays for the study of root morphogenesis and Rhizobium–legume nodulation. Australian Journal of Plant Physiology 23:271–283.
Rose RJ, Nolan KE, Bicego L. (1999) The development of the highly regenerable seed line Jemalong 2HA for transformation of Medicago truncatula: implications for regenerability via somatic embryogenesis. Journal of Plant Physiology 155:788–791.[Web of Science]
Sachs T. (1981) The control of patterned differentiation of vascular tissues. Advances in Botanical Research 9:151–162.[CrossRef]
Sachs T. (1989) The development of vascular networks during leaf development. Current topics in Plant Biochemistry and Physiology 8:168–183.
Sachs T. (1991) Pattern formation in plant tissues (Cambridge University Press, Cambridge).
Sachs T. (1993) The role of auxin in the polar organisation of apical meristem. Australian Journal of Plant Physiology 20:541–553.[Web of Science]
Sachs T. (2000) Integrating cellular and organismic aspects of vascular differentiation. Plant and Cell Physiology 41:649–656.
Scheres B. (2005) Stem cells: a plant biology perspective. Cell 122:499–504.[CrossRef][Web of Science][Medline]
Scott PC and Norris LA. (1970) Separation of auxin and ethylene in pea roots. Nature 227:1366–1367.
Skoog F and Miller CO. (1957) Chemical regulation of growth and organ formation in plant tissue cultured in vitro. Symposia of the Society for Experimental Biology 11:118–131.
Sorin C, Bussell JD, Camus I, et al. (2005) Auxin and light control of adventitious rooting in Arabidopsis require ARGONAUTE1. The Plant Cell 17:1343–1359.
Teale WD, Paponov IA, Ditengou F, Palme K. (2005) Auxin and the development root of Arabidopsis thaliana. Physiologia Plantarum 123:130–138.[CrossRef]
Thomas MR, Johnson LB, White FF. (1990) Selection of interspecific somatic hybrids of Medicago by using Agrobacterium-transformed tissues. Plant Science 69:189–198.[CrossRef][Web of Science]
van Noorden GE, Ross JJ, Reid JB, Rolfe BG, Mathesius U. (2006) Defective long-distance auxin transport regulations in the Medicago truncatula super numeric nodules mutant. Plant Physiology 140:1494–1506.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  F. R. Mantiri, S. Kurdyukov, D. P. Lohar, N. Sharopova, N. A. Saeed, X.-D. Wang, K. A. VandenBosch, and R. J. Rose The Transcription Factor MtSERF1 of the ERF Subfamily Identified by Transcriptional Profiling Is Required for Somatic Embryogenesis Induced by Auxin Plus Cytokinin in Medicago truncatula Plant Physiology, April 1, 2008; 146(4): 1622 - 1636. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 N. Imin, M. Nizamidin, T. Wu, and B. G. Rolfe Factors involved in root formation in Medicago truncatula J. Exp. Bot., February 1, 2007; 58(3): 439 - 451. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||