Phosphorus, in its orthophosphate form (P i ), is one of the most limiting macronutrients in soils for plant growth and development. However, the whole-genome molecular mechanisms contributing to plant acclimation to P i deficiency remain largely unknown. White lupin (Lupinus albus) has evolved unique adaptations for growth in P i -deficient soils, including the development of cluster roots to increase root surface area. In this study, we utilized RNA-Seq technology to assess global gene expression in white lupin cluster roots, normal roots, and leaves in response to P i supply. We de novo assembled 277,224,180 Illumina reads from 12 complementary DNA libraries to build what is to our knowledge the first white lupin gene index (LAGI 1.0). This index contains 125,821 unique sequences with an average length of 1,155 bp. Of these sequences, 50,734 were transcriptionally active (reads per kilobase per million reads $ 3), representing approximately 7.8% of the white lupin genome, using the predicted genome size of Lupinus angustifolius as a reference. We identified a total of 2,128 sequences differentially expressed in response to P i deficiency with a 2-fold or greater change and P # 0.05. Twelve sequences were consistently differentially expressed due to P i deficiency stress in three species, Arabidopsis (Arabidopsis thaliana), potato (Solanum tuberosum), and white lupin, making them ideal candidates to monitor the P i status of plants. Additionally, classic physiological experiments were coupled with RNA-Seq data to examine the role of cytokinin and gibberellic acid in P i deficiency-induced cluster root development. This global gene expression analysis provides new insights into the biochemical and molecular mechanisms involved in the acclimation to P i deficiency.

Plants associate with beneficial arbuscular mycorrhizal fungi facilitating nutrient acquisition. Arbuscular mycorrhizal fungi produce chitooligosaccharides (COs) and lipo-chitooligosaccharides (LCOs), that promote symbiosis signalling with resultant oscillations in nuclear-associated calcium. The activation of symbiosis signalling must be balanced with activation of immunity signalling, which in fungal interactions is promoted by COs resulting from the chitinaceous fungal cell wall. Here we demonstrate that COs ranging from CO4-CO8 can induce symbiosis signalling in Medicago truncatula. CO perception is a function of the receptor-like kinases MtCERK1 and LYR4, that activate both immunity and symbiosis signalling. A combination of LCOs and COs act synergistically to enhance symbiosis signalling and suppress immunity signalling and receptors involved in both CO and LCO perception are necessary for mycorrhizal establishment. We conclude that LCOs, when present in a mix with COs, drive a symbiotic outcome and this mix of signals is essential for arbuscular mycorrhizal establishment.

The ability of root cells to distinguish mutualistic microbes from pathogens is crucial for plants that allow symbiotic microorganisms to infect and colonize their internal root tissues. Here we show that and possess very similar LysM pattern-recognition receptors, LYS6/LYK9 and LYR4, enabling root cells to separate the perception of chitin oligomeric microbe-associated molecular patterns from the perception of lipochitin oligosaccharide by theNFR1/LYK3 and NFR5/NFP receptors triggering symbiosis. Inactivation of chitin-receptor genes in ,, and mutants eliminates early reactive oxygen species responses and induction of defense-response genes in roots., , and mutants were also more susceptible to fungal and bacterial pathogens, while infection and colonization by rhizobia and arbuscular mycorrhizal fungi was maintained. Biochemical binding studies with purified LYS6 ectodomains further showed that at least six GlcNAc moieties (CO6) are required for optimal binding efficiency. The 2.3-Å crystal structure of theLYS6 ectodomain reveals three LysM motifs similar to other LysM proteins and a conserved chitin-binding site. These results show that distinct receptor sets in legume roots respond to chitin and lipochitin oligosaccharides found in the heterogeneous mixture of chitinaceous compounds originating from soil microbes. This establishes a foundation for genetic and biochemical dissection of the perception and the downstream responses separating defense from symbiosis in the roots of the 80-90% of land plants able to develop rhizobial and/or mycorrhizal endosymbiosis.

Plants evolved lysine motif (LysM) receptors to recognize and parse microbial elicitors and drive intracellular signaling to limit or facilitate microbial colonization. We investigated how chitin and nodulation (Nod) factor receptors of Lotus japonicus initiate differential signaling of immunity or root nodule symbiosis. Two motifs in the LysM1 domains of these receptors determine specific recognition of ligands and discriminate between their in planta functions. These motifs define the ligand-binding site and make up the most structurally divergent regions in cognate Nod factor receptors. An adjacent motif modulates the specificity for Nod factor recognition and determines the selection of compatible rhizobial symbionts in legumes. We also identified how binding specificities in LysM receptors can be altered to facilitate Nod factor recognition and signaling from a chitin receptor, advancing the prospects of engineering rhizobial symbiosis into nonlegumes.

Recognition of Nod factors by LysM receptors is crucial for nitrogen-fixing symbiosis in most legumes. The large families of LysM receptors in legumes suggest concerted functions, yet only NFR1 and NFR5 and their closest homologs are known to be required. Here we show that an epidermal LysM receptor (NFRe), ensures robust signalling in L. japonicus. Mutants of Nfre react to Nod factors with increased calcium spiking interval, reduced transcriptional response and fewer nodules in the presence of rhizobia. NFRe has an active kinase capable of phosphorylating NFR5, which in turn, controls NFRe downstream signalling. Our findings provide evidence for a more complex Nod factor signalling mechanism than previously anticipated. The spatio-temporal interplay between Nfre and Nfr1, and their divergent signalling through distinct kinases suggests the presence of an NFRe-mediated idling state keeping the epidermal cells of the expanding root system attuned to rhizobia.