Abstract
Many plants can establish symbioses with nitrogen-fixing bacteria, some of which lead to nodulation, including legumes. Indeed, in the rhizobium/legume symbiosis, new root organs, called nodules, are formed by the plant in order to host the rhizobia in protective conditions, optimized for nitrogen fixation. In this way, these plants can benefit from the reduction of atmospheric dinitrogen into ammonia by the hosted bacteria, and in exchange the plant provides the rhizobia with a carbon source. Since this symbiosis is costly for the plant it is highly regulated. Both legume nodule and lateral root organogenesis involve divisions of the root inner tissues, and both developmental programs are tightly controlled by plant hormones. In fact, most of the major plant hormones, such as auxin, cytokinins, abscisic acid, and strigolactones, control both lateral root formation and nodule organogenesis, but often in an opposite manner. This suggests that the sensitivity of legume plants to some phytohormones could be linked to the antagonism that exists between the processes of nodulation and lateral root formation. Here, we will review the implication of some major phytohormones in lateral root formation in legumes, compare them with their roles in nodulation, and discuss specificities and divergences from non-legume eudicot plants such as Arabidopsis thaliana.
Highlights
Many plants can establish symbioses with nitrogen-fixing bacteria, some of which lead to nodulation, including legumes
Liang and Harris [5] have studied the ability of various legumes and non-legumes plants to respond to the phytohormone abscisic acid (ABA) by increasing their lateral root (LR) density and hypothesized that the common predisposition of plants to form nodules may be linked to a difference in ABA sensitivity
The bsh mutant shows reduced expression of the SGL biosynthetic gene PsCCD8, revealing a positive action of auxin on SGL biosynthesis [98]. Both genetic and pharmacological data suggests that SGL, in legumes as in Arabidopsis, are closely linked to auxin responses and that SGL are overall negative regulators of both LR formation (LRF) and nodulation in legumes, their action may depend on the overall phosphate status of the plant
Summary
Nitrogen is indispensable for life and is required in many biochemical processes such as nucleic acid and amino acid biosynthesis. (clover), produce indeterminate nodules whose meristems persist over the entire nodule lifespan while other legumes, like Lotus japonicus (Lotus) or Glycine max (soybean), produce determinate nodules, whose meristematic activity stops early on during nodule development giving rise to small, round shaped nodules. In legume species, both the onset of nodulation and rhizobial entry into the root are dependent on a specific molecular dialog involving the production by the rhizobia of Nodulation Factors (NF) which control notably host specificity [2].
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