Abstract
KNOX transcription factors (TFs) regulate different aspects of plant development essentially through their effects on phytohormone metabolism. In particular, KNOX TF SHOOTMERISTEMLESS activates the cytokinin biosynthesis ISOPENTENYL TRANSFERASE (IPT) genes in the shoot apical meristem. However, the role of KNOX TFs in symbiotic nodule development and their possible effects on phytohormone metabolism during nodulation have not been studied to date. Cytokinin is a well-known regulator of nodule development, playing the key role in the regulation of cell division during nodule primordium formation. Recently, the activation of IPT genes was shown to take place during nodulation. Therefore, it was hypothesized that KNOX TFs may regulate nodule development and activate cytokinin biosynthesis upon nodulation. This study analysed the expression of different KNOX genes in Medicago truncatula Gaertn. and Pisum sativum L. Among them, the KNOX3 gene was upregulated in response to rhizobial inoculation in both species. pKNOX3::GUS activity was observed in developing nodule primordium. KNOX3 ectopic expression caused the formation of nodule-like structures on transgenic root without bacterial inoculation, a phenotype similar to one described previously for legumes with constitutive activation of the cytokinin receptor. Furthermore, in transgenic roots with MtKNOX3 knockdown, downregulation of A-type cytokinin response genes was found, as well as the MtIPT3 and LONELYGUY2 (MtLOG2) gene being involved in cytokinin activation. Taken together, these findings suggest that KNOX3 gene is involved in symbiotic nodule development and may regulate cytokinin biosynthesis/activation upon nodule development in legume plants.
Highlights
Interaction between legume plants and soil rhizobia leads to the development of new organs on roots, symbiotic nodules, where nitrogen fixation takes place
These findings suggest that KNOX3 gene is involved in symbiotic nodule development and may regulate cytokinin biosynthesis/activation upon nodule development in legume plants
This study focused on the Medtr2g022140 gene, which was shown to be induced quite early after Nod factor treatment (1–3 h), and the Medicago homologues of the LjIPT1 and LjIPT3 genes, which are induced during nodule development in L. japonicus (Chen et al, 2013)
Summary
Interaction between legume plants and soil rhizobia leads to the development of new organs on roots, symbiotic nodules, where nitrogen fixation takes place. It was shown that the expression levels of genes involved in cytokinin biosynthesis were increased early in response to rhizobial signals. Nod factor treatment was shown to induce accumulation of cytokinins, including trans-zeatin and isopentenyl adenine, in the root susceptible zone within 3 h (van Zeijl et al, 2015). Taken together, these data suggest that cytokinin biosynthesis and activation are induced in legume roots in response to rhizobial signals
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