Abstract
Cytokinins are essential for legume plants to establish a nitrogen-fixing symbiosis with rhizobia. Recently, the expression level of cytokinin biosynthesis IPTs (ISOPENTENYLTRANSFERASES) genes was shown to be increased in response to rhizobial inoculation in Lotus japonicus, Medicago truncatula and Pisum sativum. In addition to its well-established positive role in nodule primordium initiation in root cortex, cytokinin negatively regulates infection processes in the epidermis. Moreover, it was reported that shoot-derived cytokinin inhibits the subsequent nodule formation through AON (autoregulation of nodulation) pathway. In L. japonicus, LjIPT3 gene was shown to be activated in the shoot phloem via the components of AON system, negatively affecting nodulation. However, in M. truncatula, the detailed analysis of MtIPTs expression, both in roots and shoots, in response to nodulation has not been performed yet, and the link between IPTs and AON has not been studied so far. In this study, we performed an extensive analysis of MtIPTs expression levels in different organs, focusing on the possible role of MtIPTs in nodule development. MtIPTs expression dynamics in inoculated roots suggest that besides its early established role in the nodule primordia development, cytokinin may be also important for later stages of nodulation. According to expression analysis, MtIPT3, MtIPT4, and MtIPT5 are activated in the shoots in response to inoculation. Among these genes, MtIPT3 is the only one the induction of which was not observed in leaves of the sunn-3 mutant defective in CLV1-like kinase, the key component of AON, suggesting that MtIPT3 is activated in the shoots in an AON-dependent manner. Taken together, our findings suggest that MtIPTs are involved in the nodule development at different stages, both locally in inoculated roots and systemically in shoots, where their expression can be activated in an AON-dependent manner.
Highlights
Cytokinins (CKs) are involved in different aspects of nodule development, playing a dual role in nodulation, depending on the time and place of their action
Cytokinins have been previously shown to regulate different aspects of nodulation, including rhizobial infection during which CKs act as negative regulators and nodule primordia development and nitrogen fixation for which CKs act as positive regulators (Tirichine et al, 2007; Held et al, 2014; Boivin et al, 2016; Jardinaud et al, 2016)
In M. truncatula, accumulation of cytokinins was first observed at 3 h after NF exposure and this occurs in MtCCaMK-dependent manner
Summary
Cytokinins (CKs) are involved in different aspects of nodule development, playing a dual role in nodulation, depending on the time and place of their action. It was shown that the Mtsunn mutant had increased amount of auxin transported from the shoot to the root This indicates that the activation of CLV1-like kinase in the shoot leads to a reduction of auxin transport to developing nodules, thereby to a reduction in FIGURE 1 | Phylogenetic tree for Medicago truncatula, Arabidopsis thaliana, and Lotus japonicas IPT genes constructed based on their nucleotide sequences. The activation of LjIPT3 in the shoot phloem, downstream of CLV1-like kinase, suggests that the shoot-to-root transport of both auxin and cytokinin is targeted by CLV1-like kinase, so that in roots the auxin amount is reduced and the cytokinin level is increased to restrict subsequent nodule formation Other hormones such as jasmonic acid (JA) have been implicated in the shoot-to-root communication during AON (Kinkema and Gresshoff, 2008; Reid et al, 2012). We analyzed the expression of MtIPTs in the sunn-3 mutant to identify MtIPTs potentially involved in the systemic control of nodulation in a SUNN-dependent manner
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
