The drop of mucilage at the tip of a 3-day-old axenically-grown corn root contains two population... more The drop of mucilage at the tip of a 3-day-old axenically-grown corn root contains two populations of cells detached from the cap by dissolution of the cementing portion of the middle-lamella. The spherical, densely cytoplasmic cells derive from the tip of the cap whereas the crescent-shaped and highly vacuolated cell derive from its sides. These detached cells are found in
In legumes, the formation of rhizobial and mycorrhizal root symbioses is a highly regulated proce... more In legumes, the formation of rhizobial and mycorrhizal root symbioses is a highly regulated process which requires close communication between plant and microorganism. Plant mutants that have difficulties establishing symbioses are valuable tools for unravelling the mechanisms by which these symbioses are formed and regulated. Here E151, a mutant of Pisum sativum cv. Sparkle, was examined to characterize its root growth and symbiotic defects. The symbioses in terms of colonization intensity, functionality of micro-symbionts, and organ dominance were compared between the mutant and wild type. The endogenous cytokinin (CK) and abscisic acid (ABA) levels and the effect of the exogenous application of these two hormones were determined. E151 was found to be a low and delayed nodulator, exhibiting defects in both the epidermal and cortical programmes though a few mature and functional nodules develop. Mycorrhizal colonization of E151 was intensified, although the fungal functionality was...
R50 (sym16) is a pea nodulation mutant with fewer and shorter lateral roots (LR), fewer nodules a... more R50 (sym16) is a pea nodulation mutant with fewer and shorter lateral roots (LR), fewer nodules and high levels of cytokinins (CK). Because a link exists between CK imbalance and abnormal vasculature, the vasculature of the primary root (PR) and LR of R50 was studied and it was compared with that of the wild-type 'Sparkle'. Also nodule vasculature was investigated to correlate R50 low nodulation phenotype with CK accumulation. PR and first-order LR were hand-sectioned transversely in different locations and at different ages. Vascular poles were counted and root and stele diameters measured. To evaluate LR primordia number, roots were cleared. Nodules obtained from inoculated plants were either fixed and sectioned or cleared; numbers of vascular strands and of tracheary elements in the strands were counted. 'Sparkle' PR is triarch, whereas that of R50 can be triarch, tetrarch or pentarch. Furthermore, as the R50 roots developed, supernumerary vascular strands appeare...
ABSTRACT Abstract The dry weight (0.1%) and water potential -7 kPa) of root-cap mucilage from 3-d... more ABSTRACT Abstract The dry weight (0.1%) and water potential -7 kPa) of root-cap mucilage from 3-d-old axenically grown maize seedlings have been determined. The results suggest strong gelling properties and weak water-holding capacity for the mucilage. Root tips from seedlings grown under low or high water stress were fixed by freeze-substitution. Micrographs showed that in both conditions, mucilage was secreted into the periplasmic space and extruded through the cell wall, though in dry conditions, the mucilage was tightly pressed against the root-cap surface. Histochemical and structural evidence is presented indicating chemical changes in the composition of the mucilage upon extrusion and a sharp increase in its hydration at increasing distance from the secretory cells. The possible functions of the root-cap mucilage in the rhizosphere are examined in light of these findings.
ABSTRACT Abstract Two populations of living detached cap cells are found within the drop of mucil... more ABSTRACT Abstract Two populations of living detached cap cells are found within the drop of mucilage at the root tip of a 3-d-old corn seedling. Axial cells, which pass through the length of the columella, are shed at the apex of the cap and lateral cells, which have spent less time or no time within the columella, are detached from the cap flanks. The two types of cells differ in shape, size and internal structure. Whereas the axial cells are more or less spherical and have a projected surface area of 1300μm2, the lateral cells are elongated and possess a projected surface area of 2000μm2. The axial cells are further distinguished by the association with a polyhedric net thought to be remnants of undigested middle lamellar material from the columellar tissue. Both populations of cells arc metabolically active and can be plasmolysed, but no evidence was found that they secrete mucilage after detachment. The lateral cells have been cultured on solid nutrient media and on nutrient-free agar. In both cases, they did not divide; however, they enlarged by one-third in their projected surface area over 15 d.
E107 (brz) is a pleiotropic mutant of pea (Pisum sativum L. cv Sparkle) characterized by low nodu... more E107 (brz) is a pleiotropic mutant of pea (Pisum sativum L. cv Sparkle) characterized by low nodulation, leaf necrosis, excessive ion accumulation, and decreased plant size. The defective nodulation of E107 was studied by light microscopy of lateral roots. The number of infections per centimeter of lateral root was only a third that of Sparkle. Moreover, most of the infections were aborted early; i.e. in only 14% of the infections did the infection thread penetrate beyond the epidermis. Nodulation of E107 was partly restored by treating the plant with the ethylene inhibitors aminoethoxyvinylglycine (AVG) or Ag+. Treatment with Ag+ did not increase the number of infections, but half of the infections went to completion. Ag+ and AVG did not alter the size of the mutant, the accumulation of cations in its shoots, nor the leaf necrosis. Thus, in E107, nodule development can be uncoupled from other pleiotropic characteristics.
We compared nodule initiation in lateral roots of Pisum sativum (L.) cv Sparkle and in a low-nodu... more We compared nodule initiation in lateral roots of Pisum sativum (L.) cv Sparkle and in a low-nodulating mutant E2 (sym 5). In Sparkle, about 25% of the infections terminated in the epidermis, a similar number stopped in the cortex, and 50% resulted in the formation of a nodule meristem or an emerged nodule. The mutant E2 (sym 5) was infected as often as was the parent, and it formed a normal infection thread. In the mutant, cell divisions rarely occurred in advance of the infection thread, and few nodule primordia were produced. Growing the mutant at a low root temperature or adding Ag+ to the substrate increased the number of cell divisions and nodule primordia. We conclude that, in the E2 line, the infection process is arrested in the cortex, at the stage of initial cell divisions before the establishment of a nodule primordium.
E107 is a pleiotropic mutant of Pisum sativum (pea) characterized by a decreased ability to form ... more E107 is a pleiotropic mutant of Pisum sativum (pea) characterized by a decreased ability to form nodules. Its colonization by the arbuscular mycorrhizal fungus Glomus aggregatum is reported, and the mycorrhizal phenotype compared with the nodulation phenotype. • Four cleared lateral roots from 21-d-old plants were chosen randomly, and their lengths scanned for any fungal structures. Colonization success was determined by counting the numbers of extraradical hyphae, and calculating epidermal entry and cortical entry. Graft experiments were performed to establish which organ regulates mycorrhiza-formation.
Little is known about the role of phytohormones in the formation of arbuscular mycorrhizas (AM). ... more Little is known about the role of phytohormones in the formation of arbuscular mycorrhizas (AM). Although the involvement of ethylene in AM formation is unclear, it is considered very important for several aspects of plant growth and development. The effect of a suspected inhibitory level of ethylene was investigated to help elucidate its role in regulating the formation of AM. In particular, the morphology of AM fungal structures at various stages of the colonization process was documented. Exogenous application of 5.5 ppm ethylene to the substrate resulted in typical morphological changes to Pisum sativum and a significant reduction in the colonization of roots by the AM fungus Glomus aggregatum. Elevated substrate-ethylene did not affect the number of appressoria formed; however, it did result in the formation of abnormal appressoria, which appeared swollen and highly branched. Deformation of appressoria was correlated with a reduction of AM fungal entry into the root tissue, resulting in less colonization by intraradical hyphae and arbuscules. Colonization generally proceeded normally provided the fungal hyphae breached the epidermis, although the extension of colonization units was restricted.
Ethylene is involved in the nodulation phenotype of Pisum sativum R50 (sym 16), a pleiotropic mut... more Ethylene is involved in the nodulation phenotype of Pisum sativum R50 (sym 16), a pleiotropic mutant that nodulates poorly and has pale green leaves Fré dé rique Abstract 1988) and genetic analysis showed that its abnormal nodulation was conditioned by a single recessive gene.
E107 is a mutant of pea (Pisum sativum L. cv. Sparkle) characterized by necrotic spots on its lea... more E107 is a mutant of pea (Pisum sativum L. cv. Sparkle) characterized by necrotic spots on its leaves, high Fe accumulation in its shoot, and decreased nodulation. Fresh, cleared, and fixed tissues of the mutant and its parent were observed by light and electron microscopy. E107 has cell wall appositions in its basal leaflets and in the endodermis of its lateral roots. The wall deposits in the leaves are layered and contain callose and phenols. The appositional walls in the lateral root endodermis, made of lignin or of phenols, are not organised. We hypothesize that the appositions are stress responses to metal toxicity. Key words: pea mutant, wall appositions, histochemistry, iron toxicity.
We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both proce... more We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encou...
There are very few studies of hormonal regulation of arbuscular mycorrhiza formation that include... more There are very few studies of hormonal regulation of arbuscular mycorrhiza formation that include the gaseous hormone ethylene. Ethylene is considered inhibitory to the formation of arbuscular mycorrhizae; however, very low concentrations may promote their formation. We used an improved method of exogenous ethylene application to determine whether ethylene concentration dependent changes in colonization occur in the leek (Allium porrum L. cv. Giant Musselburgh) Glomus aggregatum Schenck & Smith emend. Koske system. This improved method allowed for a continuous flow of constant concentration of the gas to be applied to a substrate. The 0.6 µL/L substrateethylene treatment reduced both root and leaf length and resulted in significantly lower arbuscular colonization compared with controls, whereas the 0.3 µL/L treatment reduced root length only and did not significantly affect colonization levels. Despite continuous application of exogenous ethylene, the amount of ethylene detected ...
Ethylene inhibits nodulation in various legumes. In order to investigate strategies employed by R... more Ethylene inhibits nodulation in various legumes. In order to investigate strategies employed by Rhizobium to regulate nodulation, the 1-aminocyclopropane-1-carboxylate (ACC) deaminase gene was isolated and characterized from one of the ACC deaminase-producing rhizobia, Rhizobium leguminosarum bv. viciae 128C53K. ACC deaminase degrades ACC, the immediate precursor of ethylene in higher plants. Through the action of this enzyme, ACC deaminase-containing bacteria can reduce ethylene biosynthesis in plants. Insertion mutants with mutations in the rhizobial ACC deaminase gene (acdS) and its regulatory gene, a leucine-responsive regulatory protein-like gene (lrpL), were constructed and tested to determine their abilities to nodulate Pisum sativum L. cv. Sparkle (pea). Both mutants, neither of which synthesized ACC deaminase, showed decreased nodulation efficiency compared to that of the parental strain. Our results suggest that ACC deaminase in R. leguminosarum bv. viciae 128C53K enhances...
We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both proce... more We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encou...
R50 (sym16) is a pleiotropic mutant of pea (Pisum sativum L.) which develops few, pale nodules an... more R50 (sym16) is a pleiotropic mutant of pea (Pisum sativum L.) which develops few, pale nodules and has pale young leaves. This phenotype coincides with elevated cytokinin content in vegetative organs, especially mature shoots. Because cytokinin content is known to be tightly regulated by the catabolic action of cytokinin dehydrogenase (CKX), this study focuses on whether CKX-mediated regulation of cytokinin content is involved in the R50 phenotype. Thus, we analyzed the biochemical activity of this enzyme in vitro and found that R50 displayed an aberrant activity profile. During development, PsCKX activity was significantly reduced when compared to wild-type (WT); this was observed in many tissues, specifically in mature shoots and nodules where decrease in activity correlated with elevated cytokinin content. To further address this issue, a full-length cDNA corresponding to CKX1 from pea (PsCKX1) was obtained via RACE-PCR. Although sequencing the entire PsCKX1 cDNA from R50 did not reveal any significant mutations that could have linked PsCKX1 to the sym16 mutation, relative transcript levels of PsCKX1 and of another PsCKX homolog (PsCKX2) were compared between R50 and WT using semiquantitative reverse transcriptase PCR. Interestingly, transcription of these homologs was upregulated in the tissues of R50 displaying the most aberrant phenotype, namely, the mature shoots and nodules. We propose that the R50 phenotype is linked to elevated cytokinin content as a result of deficient PsCKX activity and that transcription of two PsCKX homologs is upregulated as a means to compensate for the biochemical deficiency of this enzyme in R50 mutants.
The drop of mucilage at the tip of a 3-day-old axenically-grown corn root contains two population... more The drop of mucilage at the tip of a 3-day-old axenically-grown corn root contains two populations of cells detached from the cap by dissolution of the cementing portion of the middle-lamella. The spherical, densely cytoplasmic cells derive from the tip of the cap whereas the crescent-shaped and highly vacuolated cell derive from its sides. These detached cells are found in
In legumes, the formation of rhizobial and mycorrhizal root symbioses is a highly regulated proce... more In legumes, the formation of rhizobial and mycorrhizal root symbioses is a highly regulated process which requires close communication between plant and microorganism. Plant mutants that have difficulties establishing symbioses are valuable tools for unravelling the mechanisms by which these symbioses are formed and regulated. Here E151, a mutant of Pisum sativum cv. Sparkle, was examined to characterize its root growth and symbiotic defects. The symbioses in terms of colonization intensity, functionality of micro-symbionts, and organ dominance were compared between the mutant and wild type. The endogenous cytokinin (CK) and abscisic acid (ABA) levels and the effect of the exogenous application of these two hormones were determined. E151 was found to be a low and delayed nodulator, exhibiting defects in both the epidermal and cortical programmes though a few mature and functional nodules develop. Mycorrhizal colonization of E151 was intensified, although the fungal functionality was...
R50 (sym16) is a pea nodulation mutant with fewer and shorter lateral roots (LR), fewer nodules a... more R50 (sym16) is a pea nodulation mutant with fewer and shorter lateral roots (LR), fewer nodules and high levels of cytokinins (CK). Because a link exists between CK imbalance and abnormal vasculature, the vasculature of the primary root (PR) and LR of R50 was studied and it was compared with that of the wild-type 'Sparkle'. Also nodule vasculature was investigated to correlate R50 low nodulation phenotype with CK accumulation. PR and first-order LR were hand-sectioned transversely in different locations and at different ages. Vascular poles were counted and root and stele diameters measured. To evaluate LR primordia number, roots were cleared. Nodules obtained from inoculated plants were either fixed and sectioned or cleared; numbers of vascular strands and of tracheary elements in the strands were counted. 'Sparkle' PR is triarch, whereas that of R50 can be triarch, tetrarch or pentarch. Furthermore, as the R50 roots developed, supernumerary vascular strands appeare...
ABSTRACT Abstract The dry weight (0.1%) and water potential -7 kPa) of root-cap mucilage from 3-d... more ABSTRACT Abstract The dry weight (0.1%) and water potential -7 kPa) of root-cap mucilage from 3-d-old axenically grown maize seedlings have been determined. The results suggest strong gelling properties and weak water-holding capacity for the mucilage. Root tips from seedlings grown under low or high water stress were fixed by freeze-substitution. Micrographs showed that in both conditions, mucilage was secreted into the periplasmic space and extruded through the cell wall, though in dry conditions, the mucilage was tightly pressed against the root-cap surface. Histochemical and structural evidence is presented indicating chemical changes in the composition of the mucilage upon extrusion and a sharp increase in its hydration at increasing distance from the secretory cells. The possible functions of the root-cap mucilage in the rhizosphere are examined in light of these findings.
ABSTRACT Abstract Two populations of living detached cap cells are found within the drop of mucil... more ABSTRACT Abstract Two populations of living detached cap cells are found within the drop of mucilage at the root tip of a 3-d-old corn seedling. Axial cells, which pass through the length of the columella, are shed at the apex of the cap and lateral cells, which have spent less time or no time within the columella, are detached from the cap flanks. The two types of cells differ in shape, size and internal structure. Whereas the axial cells are more or less spherical and have a projected surface area of 1300μm2, the lateral cells are elongated and possess a projected surface area of 2000μm2. The axial cells are further distinguished by the association with a polyhedric net thought to be remnants of undigested middle lamellar material from the columellar tissue. Both populations of cells arc metabolically active and can be plasmolysed, but no evidence was found that they secrete mucilage after detachment. The lateral cells have been cultured on solid nutrient media and on nutrient-free agar. In both cases, they did not divide; however, they enlarged by one-third in their projected surface area over 15 d.
E107 (brz) is a pleiotropic mutant of pea (Pisum sativum L. cv Sparkle) characterized by low nodu... more E107 (brz) is a pleiotropic mutant of pea (Pisum sativum L. cv Sparkle) characterized by low nodulation, leaf necrosis, excessive ion accumulation, and decreased plant size. The defective nodulation of E107 was studied by light microscopy of lateral roots. The number of infections per centimeter of lateral root was only a third that of Sparkle. Moreover, most of the infections were aborted early; i.e. in only 14% of the infections did the infection thread penetrate beyond the epidermis. Nodulation of E107 was partly restored by treating the plant with the ethylene inhibitors aminoethoxyvinylglycine (AVG) or Ag+. Treatment with Ag+ did not increase the number of infections, but half of the infections went to completion. Ag+ and AVG did not alter the size of the mutant, the accumulation of cations in its shoots, nor the leaf necrosis. Thus, in E107, nodule development can be uncoupled from other pleiotropic characteristics.
We compared nodule initiation in lateral roots of Pisum sativum (L.) cv Sparkle and in a low-nodu... more We compared nodule initiation in lateral roots of Pisum sativum (L.) cv Sparkle and in a low-nodulating mutant E2 (sym 5). In Sparkle, about 25% of the infections terminated in the epidermis, a similar number stopped in the cortex, and 50% resulted in the formation of a nodule meristem or an emerged nodule. The mutant E2 (sym 5) was infected as often as was the parent, and it formed a normal infection thread. In the mutant, cell divisions rarely occurred in advance of the infection thread, and few nodule primordia were produced. Growing the mutant at a low root temperature or adding Ag+ to the substrate increased the number of cell divisions and nodule primordia. We conclude that, in the E2 line, the infection process is arrested in the cortex, at the stage of initial cell divisions before the establishment of a nodule primordium.
E107 is a pleiotropic mutant of Pisum sativum (pea) characterized by a decreased ability to form ... more E107 is a pleiotropic mutant of Pisum sativum (pea) characterized by a decreased ability to form nodules. Its colonization by the arbuscular mycorrhizal fungus Glomus aggregatum is reported, and the mycorrhizal phenotype compared with the nodulation phenotype. • Four cleared lateral roots from 21-d-old plants were chosen randomly, and their lengths scanned for any fungal structures. Colonization success was determined by counting the numbers of extraradical hyphae, and calculating epidermal entry and cortical entry. Graft experiments were performed to establish which organ regulates mycorrhiza-formation.
Little is known about the role of phytohormones in the formation of arbuscular mycorrhizas (AM). ... more Little is known about the role of phytohormones in the formation of arbuscular mycorrhizas (AM). Although the involvement of ethylene in AM formation is unclear, it is considered very important for several aspects of plant growth and development. The effect of a suspected inhibitory level of ethylene was investigated to help elucidate its role in regulating the formation of AM. In particular, the morphology of AM fungal structures at various stages of the colonization process was documented. Exogenous application of 5.5 ppm ethylene to the substrate resulted in typical morphological changes to Pisum sativum and a significant reduction in the colonization of roots by the AM fungus Glomus aggregatum. Elevated substrate-ethylene did not affect the number of appressoria formed; however, it did result in the formation of abnormal appressoria, which appeared swollen and highly branched. Deformation of appressoria was correlated with a reduction of AM fungal entry into the root tissue, resulting in less colonization by intraradical hyphae and arbuscules. Colonization generally proceeded normally provided the fungal hyphae breached the epidermis, although the extension of colonization units was restricted.
Ethylene is involved in the nodulation phenotype of Pisum sativum R50 (sym 16), a pleiotropic mut... more Ethylene is involved in the nodulation phenotype of Pisum sativum R50 (sym 16), a pleiotropic mutant that nodulates poorly and has pale green leaves Fré dé rique Abstract 1988) and genetic analysis showed that its abnormal nodulation was conditioned by a single recessive gene.
E107 is a mutant of pea (Pisum sativum L. cv. Sparkle) characterized by necrotic spots on its lea... more E107 is a mutant of pea (Pisum sativum L. cv. Sparkle) characterized by necrotic spots on its leaves, high Fe accumulation in its shoot, and decreased nodulation. Fresh, cleared, and fixed tissues of the mutant and its parent were observed by light and electron microscopy. E107 has cell wall appositions in its basal leaflets and in the endodermis of its lateral roots. The wall deposits in the leaves are layered and contain callose and phenols. The appositional walls in the lateral root endodermis, made of lignin or of phenols, are not organised. We hypothesize that the appositions are stress responses to metal toxicity. Key words: pea mutant, wall appositions, histochemistry, iron toxicity.
We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both proce... more We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encou...
There are very few studies of hormonal regulation of arbuscular mycorrhiza formation that include... more There are very few studies of hormonal regulation of arbuscular mycorrhiza formation that include the gaseous hormone ethylene. Ethylene is considered inhibitory to the formation of arbuscular mycorrhizae; however, very low concentrations may promote their formation. We used an improved method of exogenous ethylene application to determine whether ethylene concentration dependent changes in colonization occur in the leek (Allium porrum L. cv. Giant Musselburgh) Glomus aggregatum Schenck & Smith emend. Koske system. This improved method allowed for a continuous flow of constant concentration of the gas to be applied to a substrate. The 0.6 µL/L substrateethylene treatment reduced both root and leaf length and resulted in significantly lower arbuscular colonization compared with controls, whereas the 0.3 µL/L treatment reduced root length only and did not significantly affect colonization levels. Despite continuous application of exogenous ethylene, the amount of ethylene detected ...
Ethylene inhibits nodulation in various legumes. In order to investigate strategies employed by R... more Ethylene inhibits nodulation in various legumes. In order to investigate strategies employed by Rhizobium to regulate nodulation, the 1-aminocyclopropane-1-carboxylate (ACC) deaminase gene was isolated and characterized from one of the ACC deaminase-producing rhizobia, Rhizobium leguminosarum bv. viciae 128C53K. ACC deaminase degrades ACC, the immediate precursor of ethylene in higher plants. Through the action of this enzyme, ACC deaminase-containing bacteria can reduce ethylene biosynthesis in plants. Insertion mutants with mutations in the rhizobial ACC deaminase gene (acdS) and its regulatory gene, a leucine-responsive regulatory protein-like gene (lrpL), were constructed and tested to determine their abilities to nodulate Pisum sativum L. cv. Sparkle (pea). Both mutants, neither of which synthesized ACC deaminase, showed decreased nodulation efficiency compared to that of the parental strain. Our results suggest that ACC deaminase in R. leguminosarum bv. viciae 128C53K enhances...
We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both proce... more We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encou...
R50 (sym16) is a pleiotropic mutant of pea (Pisum sativum L.) which develops few, pale nodules an... more R50 (sym16) is a pleiotropic mutant of pea (Pisum sativum L.) which develops few, pale nodules and has pale young leaves. This phenotype coincides with elevated cytokinin content in vegetative organs, especially mature shoots. Because cytokinin content is known to be tightly regulated by the catabolic action of cytokinin dehydrogenase (CKX), this study focuses on whether CKX-mediated regulation of cytokinin content is involved in the R50 phenotype. Thus, we analyzed the biochemical activity of this enzyme in vitro and found that R50 displayed an aberrant activity profile. During development, PsCKX activity was significantly reduced when compared to wild-type (WT); this was observed in many tissues, specifically in mature shoots and nodules where decrease in activity correlated with elevated cytokinin content. To further address this issue, a full-length cDNA corresponding to CKX1 from pea (PsCKX1) was obtained via RACE-PCR. Although sequencing the entire PsCKX1 cDNA from R50 did not reveal any significant mutations that could have linked PsCKX1 to the sym16 mutation, relative transcript levels of PsCKX1 and of another PsCKX homolog (PsCKX2) were compared between R50 and WT using semiquantitative reverse transcriptase PCR. Interestingly, transcription of these homologs was upregulated in the tissues of R50 displaying the most aberrant phenotype, namely, the mature shoots and nodules. We propose that the R50 phenotype is linked to elevated cytokinin content as a result of deficient PsCKX activity and that transcription of two PsCKX homologs is upregulated as a means to compensate for the biochemical deficiency of this enzyme in R50 mutants.
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Papers by F. Guinel