Abstract
Cytokinins are phytohormones that regulate many developmental and environmental responses. The Medicago truncatula cytokinin receptor MtCRE1 (Cytokinin Response 1) is required for the nitrogen-fixing symbiosis with rhizobia. As several cytokinin signaling genes are modulated in roots depending on different biotic and abiotic conditions, we assessed potential involvement of this pathway in various root environmental responses. Phenotyping of cre1 mutant roots infected by the Gigaspora margarita arbuscular mycorrhizal (AM) symbiotic fungus, the Aphanomyces euteiches root oomycete, or subjected to an abiotic stress (salt), were carried out. Detailed histological analysis and quantification of cre1 mycorrhized roots did not reveal any detrimental phenotype, suggesting that MtCRE1 does not belong to the ancestral common symbiotic pathway shared by rhizobial and AM symbioses. cre1 mutants formed an increased number of emerged lateral roots compared to wild-type plants, a phenotype which was also observed under non-stressed conditions. In response to A. euteiches, cre1 mutants showed reduced disease symptoms and an increased plant survival rate, correlated to an enhanced formation of lateral roots, a feature previously linked to Aphanomyces resistance. Overall, we showed that the cytokinin CRE1 pathway is not only required for symbiotic nodule organogenesis but also affects both root development and resistance to abiotic and biotic environmental stresses.
Highlights
Cytokinins control many aspects of plant development as well as response to environmental stresses [1,2,3]
Cytokinin signaling gene transcriptional regulations revealed six main clusters: (1) genes mainly repressed in the various conditions analysed (18 probes corresponding to 12 genes); (2) genes mainly weakly repressed; (3) genes weakly or not regulated across the various selected conditions; (4) genes mainly weakly induced; (5) genes showing a differential expression between the different stresses; and (6) one gene mainly strongly induced
As MtCRE1 is required for symbiotic nodule formation [18], we wanted to determine if this pathway was involved in the mycorrhizal symbiotic interaction
Summary
Cytokinins control many aspects of plant development as well as response to environmental stresses [1,2,3]. Cytokinins affect Arabidopsis thaliana vascular bundle patterning, regulating negatively protoxylem differentiation depending on the CRE1/AHK4 (Cytokinin Response 1 / Authentic Histidine Kinase 4) cytokinin receptor [7]. Cytokinins negatively control Arabidopsis lateral root formation, blocking initial pericycle cell divisions and affecting patterning of lateral root primordia [9]. This regulation of lateral root formation involves, depending on studies, combinations of AHK2, AHK3 and/or CRE1/AHK4 cytokinin receptors [10,11,12]. Potential roles of phytohormones in these interactions, including cytokinins, are poorly documented [21]
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