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 encounter a regulatory level of ethylene at the epidermis outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants.Key words: AM, epidermis, evolution, pea, rhizobia, sym mutant.
Lotus japonicus har1 mutants respond to inoculation with Mesorhizobium loti by forming an excessive number of nodules due to genetic lesions in the HAR1 autoregulatory receptor kinase gene. In order to expand the repertoire of mutants available for the genetic dissection of the root nodule symbiosis (RNS), a screen for suppressors of the L. japonicus har1-1 hypernodulation phenotype was performed. Of 150,000 M2 plants analyzed, 61 stable L. japonicus double-mutant lines were isolated. In the context of the har1-1 mutation, 26 mutant lines were unable to form RNS, whereas the remaining 35 mutant lines carried more subtle symbiotic phenotypes, either forming white ineffective nodules or showing reduced nodulation capacity. When challenged with Glomus intraradices, 18 of the 61 suppressor lines were unable to establish a symbiosis with this arbuscular mycorrhiza fungus. Using a combined approach of genetic mapping, targeting induced local lesions in genomics, and sequencing, all non-nodulating mutant lines were characterized and shown to represent new alleles of at least nine independent symbiotic loci. The class of mutants with reduced nodulation capacity was of particular interest because some of them may specify novel plant functions that regulate nodule development in L. japonicus. To facilitate mapping of the latter class of mutants, an introgression line, in which the har1-1 allele was introduced into a polymorphic background of L. japonicus ecotype MG20, was constructed.
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.
Summary• 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.• Two blocks to infection were obvious, at the root surface, and at the interface between the epidermis and the outermost cortical cell layer. Once the fungus breached this interface, it spread within the cortex and formed normal arbuscules. The mutant phenotype was controlled by the shoot.• The E107 mycorrhizal phenotype was designated as app + , low pen, low coi. The development of the fungal infection was similar to that of rhizobial infection and under the same shoot control. The phenotype of the mutant E107 strongly supports the hypothesis that mycorrhiza formation and nodulation are regulated by the same processes.
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 in inoculated substrates was reduced to near zero 20 days after inoculation. This decrease may be either due to an increased capacity for ethylene oxidation by arbuscular mycorrhizal roots or because arbuscular mycorrhizal fungi (or other microbes in the pot-cultured inoculum) are capable of metabolizing ethylene. The present study highlights the need for investigations into arbuscular mycorrhizal fungal physiology and the mechanisms by which ethylene regulates arbuscular mycorrhiza formation.Key words: arbuscular mycorrhiza, colonization, exogenous ethylene, monocot.
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