The formation of nitrogen-fixing nodules in legumes is tightly controlled by a long-distance signaling system in which nodulating roots signal to shoot tissues to suppress further nodulation. A screen for supernodulating Medicago truncatula mutants defective in this regulatory behavior yielded loss-of-function alleles of a gene designated ROOT DETERMINED NODULATION1 (RDN1). Grafting experiments demonstrated that RDN1 regulatory function occurs in the roots, not the shoots, and is essential for normal nodule number regulation. The RDN1 gene, Medtr5g089520, was identified by genetic mapping, transcript profiling, and phenotypic rescue by expression of the wild-type gene in rdn1 mutants. A mutation in a putative RDN1 ortholog was also identified in the supernodulating nod3 mutant of pea (Pisum sativum). RDN1 is predicted to encode a 357-amino acid protein of unknown function. The RDN1 promoter drives expression in the vascular cylinder, suggesting RDN1 may be involved in initiating, responding to, or transporting vascular signals. RDN1 is a member of a small, uncharacterized, highly conserved gene family unique to green plants, including algae, that we have named the RDN family.
Plant synthetic biology promises immense technological benefits, including the potential development of a sustainable bio-based economy through the predictive design of synthetic gene circuits. Such circuits are built from quantitatively characterized genetic parts; however, this characterization is a significant obstacle in work with plants because of the time required for stable transformation. We describe a method for rapid quantitative characterization of genetic plant parts using transient expression in protoplasts and dual luciferase outputs. We observed experimental variability in transient-expression assays and developed a mathematical model to describe, as well as statistical normalization methods to account for, this variability, which allowed us to extract quantitative parameters. We characterized >120 synthetic parts in Arabidopsis and validated our method by comparing transient expression with expression in stably transformed plants. We also tested >100 synthetic parts in sorghum (Sorghum bicolor) protoplasts, and the results showed that our method works in diverse plant groups. Our approach enables the construction of tunable gene circuits in complex eukaryotic organisms.
35 36The combinatorial interaction of a receptor kinase and a modified CLE peptide is 37 involved in several developmental processes in plants, including Autoregulation of 38 Nodulation (AON), which allows legumes to limit the number of root nodules formed 39 based on available nitrogen and previous rhizobial colonization. Evidence The symbiosis involves extensive signaling within the plant as well as between the two 65 partners in order to establish and maintain the relationship. Reviewed in (Oldroyd, G. E. 66 and Dixon, R. 2014), the establishment of the symbiosis requires both positive and 67 negative regulation by the plant. While the benefit to the plant of nitrogen from the 68 atmosphere is high, the cost of the symbiosis to the plant (estimated at 12g of carbon per 69 g of nitrogen fixed (Crawford, N. M. et al. 2000)) makes the symbiosis only "cost-70effective" under nitrogen limiting conditions. Nitrogen limiting conditions are detected 71 systemically by the plant through C-terminally Encoded Peptides (CEPs) sent from the 72 root to the shoot to interact with receptors such as CRA2 (Huault, 73 E. et al. 2014) and initiate nitrogen-acquiring responses such as nodulation and 74 upregulation of nitrate transporters, while acting locally to control lateral root initiation 75 . nodule. When the infection thread reaches these dividing cells, rhizobia are released into 82 membrane bound compartments called symbiosomes, where they fix nitrogen for the 83 plant . 84Autoregulation of nodulation (AON) allows the plant to limit the number of 85 nodules formed based on available nitrogen and previous rhizobial colonization. Grafting 86 experiments with mutant plants demonstrated nodule regulation in the roots was 87 controlled from the shoot , and subsequent research has shown 88 that factors in both the root and the shoot, as well as the transmission of signals between 89 these parts of the plant, contribute to the ability to regulate nodule number (reviewed in 90 (Shabala, S. et al. 2016). 91In Medicago truncatula, a model legume for studying indeterminant nodulation, 92 in which the meristem continues to grow throughout the lifespan of the nodule, gene 93 products demonstrated by mutation or overexpression to regulate nodule number include 94 SICKLE, an EIN2 ortholog regulating from the root (Penmetsa, R. V. et al. 2008), 95 SUNN, a CLV1-like receptor kinase expressed throughout the plant that acts from the 96 shoot to control AON (Penmetsa, R. V. et al. 2003), CRA2, a 97 leucine rich-repeat receptor regulating systemically from the shoot and locally within the 98 root (Huault, E. et al. 2014) and peptides MtCLE12p and MtCLE13p affecting AON 99 through expression in the root ). These peptides are likely 100 transported to the shoot ) and have been shown to require SUNN 101 for their effects . Also demonstrated to be involved in AON by 102 mutation is RDN1, expressed throughout the plant vasculature but shown by grafting to 103 modulate AON from the root . The AON signal in M. 5 truncatul...
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