Defense and senescence interplay in legume nodules.

Immunity and senescence play a crucial role in the functioning of the legume symbiotic nodules. The miss-regulation of one of these processes compromises the symbiosis leading to death of the endosymbiont and the arrest of the nodule functioning. The relationship between immunity and senescence has been extensively studied in plant organs where a synergistic response can be observed. However, the interplay between immunity and senescence in the symbiotic organ is poorly discussed in the literature and these phenomena are often mixed up. Recent studies revealed that the cooperation between immunity and senescence is not always observed in the nodule, suggesting complex interactions between these two processes within the symbiotic organ. Here, we discuss recent results on the interplay between immunity and senescence in the nodule and the specificities of this relationship during legume-rhizobium symbiosis.

The transcriptional co-regulators NBCL1 and NBCL2 redundantly coordinate aerial organ development and root nodule identity in legumes.

Medicago truncatula NODULE ROOT1 (MtNOOT1) and Pisum sativum COCHLEATA1 (PsCOCH1) are orthologous genes belonging to the NOOT-BOP-COCH-LIKE (NBCL) gene family which encodes key transcriptional co-regulators of plant development. In Mtnoot1 and Pscoch1 mutants, the development of stipules, flowers, and symbiotic nodules is altered. MtNOOT2 and PsCOCH2 represent the single paralogues of MtNOOT1 and PsCOCH1, respectively. In M. truncatula, MtNOOT1 and MtNOOT2 are both required for the establishment and maintenance of symbiotic nodule identity. In legumes, the role of NBCL2 in above-ground development is not known. To better understand the roles of NBCL genes in legumes, we used M. truncatula and P. sativum nbcl mutants, isolated a knockout mutant for the PsCOCH2 locus and generated Pscoch1coch2 double mutants in P. sativum. Our work shows that single Mtnoot2 and Pscoch2 mutants develop wild-type stipules, flowers, and symbiotic nodules. However, the number of flowers was increased and the pods and seeds were smaller compared to the wild type. Furthermore, in comparison to the corresponding nbcl1 single mutants, both the M. truncatula and P. sativum nbcl double mutants show a drastic alteration in stipule, inflorescence, flower, and nodule development. Remarkably, in both M. truncatula and P. sativum nbcl double mutants, stipules are transformed into a range of aberrant leaf-like structures.

Brachypodium distachyon UNICULME4 and LAXATUM-A are redundantly required for development.

In cultivated grasses, tillering, leaf, and inflorescence architecture, as well as abscission ability, are major agronomical traits. In barley (Hordeum vulgare), maize (Zea mays), rice (Oryza sativa), and brachypodium (Brachypodium distachyon), NOOT-BOP-COCH-LIKE (NBCL) genes are essential regulators of vegetative and reproductive development. Grass species usually possess 2-4 NBCL copies and until now a single study in O. sativa showed that the disruption of all NBCL genes strongly altered O. sativa leaf development. To improve our understanding of the role of NBCL genes in grasses, we extended the study of the two NBCL paralogs BdUNICULME4 (CUL4) and BdLAXATUM-A (LAXA) in the nondomesticated grass B. distachyon. For this, we applied reversed genetics and generated original B. distachyon single and double nbcl mutants by clustered regularly interspaced short palindromic repeats - CRISPR associated protein 9 (CRISPR-Cas9) approaches and genetic crossing between nbcl targeting induced local lesions in genomes (TILLING) mutants. Through the study of original single laxa CRISPR-Cas9 null alleles, we validated functions previously proposed for LAXA in tillering, leaf patterning, inflorescence, and flower development and also unveiled roles for these genes in seed yield. Furthermore, the characterization of cul4laxa double mutants revealed essential functions for nbcl genes in B. distachyon development, especially in the regulation of tillering, stem cell elongation and secondary cell wall composition as well as for the transition toward the reproductive phase. Our results also highlight recurrent antagonist interactions between NBCLs occurring in multiple aspects of B. distachyon development.

KARRIKIN INSENSITIVE2 regulates leaf development, root system architecture and arbuscular-mycorrhizal symbiosis in Brachypodium distachyon.

KARRIKIN INSENSITIVE2 (KAI2) is an α/ß-hydrolase required for plant responses to karrikins, which are abiotic butenolides that can influence seed germination and seedling growth. Although represented by four angiosperm species, loss-of-function kai2 mutants are phenotypically inconsistent and incompletely characterised, resulting in uncertainties about the core functions of KAI2 in plant development. Here we characterised the developmental functions of KAI2 in the grass Brachypodium distachyon using molecular, physiological and biochemical approaches. Bdkai2 mutants exhibit increased internode elongation and reduced leaf chlorophyll levels, but only a modest increase in water loss from detached leaves. Bdkai2 shows increased numbers of lateral roots and reduced root hair growth, and fails to support normal root colonisation by arbuscular-mycorrhizal (AM) fungi. The karrikins KAR1 and KAR2 , and the strigolactone (SL) analogue rac-GR24, each elicit overlapping but distinct changes to the shoot transcriptome via BdKAI2. Finally, we show that BdKAI2 exhibits a clear ligand preference for desmethyl butenolides and weak responses to methyl-substituted SL analogues such as GR24. Our findings suggest that KAI2 has multiple roles in shoot development, root system development and transcriptional regulation in grasses. Although KAI2-dependent AM symbiosis is likely conserved within monocots, the magnitude of the effect of KAI2 on water relations may vary across angiosperms.

Overexpression of a Cytochrome P450 Monooxygenase Involved in Orobanchol Biosynthesis Increases Susceptibility to Fusarium Head Blight.

Fusarium Head Blight (FHB) is a cereal disease caused primarily by the ascomycete fungus Fusarium graminearum with public health issues due to the production of mycotoxins including deoxynivalenol (DON). Genetic resistance is an efficient protection means and numerous quantitative trait loci have been identified, some of them related to the production of resistance metabolites. In this study, we have functionally characterized the Brachypodium distachyon BdCYP711A29 gene encoding a cytochrome P450 monooxygenase (CYP). We showed that BdCYP711A29 belongs to an oligogenic family of five members. However, following infection by F. graminearum, BdCYP711A29 is the only copy strongly transcriptionally induced in a DON-dependent manner. The BdCYP711A29 protein is homologous to the Arabidopsis thaliana MAX1 and Oryza sativa MAX1-like CYPs representing key components of the strigolactone biosynthesis. We show that BdCYP711A29 is likely involved in orobanchol biosynthesis. Alteration of the BdCYP711A29 sequence or expression alone does not modify plant architecture, most likely because of functional redundancy with the other copies. B. distachyon lines overexpressing BdCYP711A29 exhibit an increased susceptibility to F. graminearum, although no significant changes in defense gene expression were detected. We demonstrate that both orobanchol and exudates of Bd711A29 overexpressing lines stimulate the germination of F. graminearum macroconidia. We therefore hypothesize that orobanchol is a susceptibility factor to FHB.

The BOP-type co-transcriptional regulator NODULE ROOT1 promotes stem secondary growth of the tropical Cannabaceae tree Parasponia andersonii.

Tree stems undergo a massive secondary growth in which secondary xylem and phloem tissues arise from the vascular cambium. Vascular cambium activity is driven by endogenous developmental signalling cues and environmental stimuli. Current knowledge regarding the genetic regulation of cambium activity and secondary growth is still far from complete. The tropical Cannabaceae tree Parasponia andersonii is a non-legume research model of nitrogen-fixing root nodulation. Parasponia andersonii can be transformed efficiently, making it amenable for CRISPR-Cas9-mediated reverse genetics. We considered whether P. andersonii also could be used as a complementary research system to investigate tree-related traits, including secondary growth. We established a developmental map of stem secondary growth in P. andersonii plantlets. Subsequently, we showed that the expression of the co-transcriptional regulator PanNODULE ROOT1 (PanNOOT1) is essential for controlling this process. PanNOOT1 is orthologous to Arabidopsis thaliana BLADE-ON-PETIOLE1 (AtBOP1) and AtBOP2, which are involved in the meristem-to-organ-boundary maintenance. Moreover, in species forming nitrogen-fixing root nodules, NOOT1 is known to function as a key nodule identity gene. Parasponia andersonii CRISPR-Cas9 loss-of-function Pannoot1 mutants are altered in the development of the xylem and phloem tissues without apparent disturbance of the cambium organization and size. Transcriptomic analysis showed that the expression of key secondary growth-related genes is significantly down-regulated in Pannoot1 mutants. This allows us to conclude that PanNOOT1 positively contributes to the regulation of stem secondary growth. Our work also demonstrates that P. andersonii can serve as a tree research system.

Roles of BdUNICULME4 and BdLAXATUM-A in the non-domesticated grass Brachypodium distachyon.

In cultivated grasses, tillering, spike architecture and seed shattering represent major agronomical traits. In barley, maize and rice, the NOOT-BOP-COCH-LIKE (NBCL) genes play important roles in development, especially in ligule development, tillering and flower identity. However, compared with dicots, the role of grass NBCL genes is underinvestigated. To better understand the role of grass NBCLs and to overcome any effects of domestication that might conceal their original functions, we studied TILLING nbcl mutants in the non-domesticated grass Brachypodium distachyon. In B. distachyon, the NBCL genes BdUNICULME4 (CUL4) and BdLAXATUM-A (LAXA) are orthologous, respectively, to the barley HvUniculme4 and HvLaxatum-a, to the maize Zmtassels replace upper ears1 and Zmtassels replace upper ears2 and to the rice OsBLADE-ON-PETIOLE1 and OsBLADE-ON-PETIOLE2/3. In B. distachyon, our reverse genetics study shows that CUL4 is not essential for the establishment of the blade-sheath boundary but is necessary for the development of the ligule and auricles. We report that CUL4 also exerts a positive role in tillering and a negative role in spikelet meristem activity. On the other hand, we demonstrate that LAXA plays a negative role in tillering, positively participates in spikelet development and contributes to the control of floral organ number and identity. In this work, we functionally characterized two new NBCL genes in a context of non-domesticated grass and highlighted original roles for grass NBCL genes that are related to important agronomical traits.

Author Correction: Amaryllidaceae alkaloids: identification and partial characterization of montanine production in Rhodophiala bifida plant.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Amaryllidaceae alkaloids: identification and partial characterization of montanine production in Rhodophiala bifida plant.

Rhodophiala bifida (R. bifida) is a representative of the Amaryllidaceae plant family and is rich in montanine, an alkaloid with high pharmaceutical potential. Despite the interest in these compounds, many steps of the biosynthetic pathway have not been elucidated. In this study, we identified the alkaloids produced in different organs of R. bifida under different growth conditions, set up the conditions for in vitro R. bifida regeneration and initiated the molecular characterization of two R. bifida genes involved in alkaloids biosynthesis: the Norbelladine 4'-O-Methyltransferase (RbN4OMT) and the Cytochrome P450 (RbCYP96T). We show that montanine is the main alkaloid produced in the different R. bifida organs and developed a direct organogenesis regeneration protocol, using twin-scale explants cultivated on media enriched with naphthalene acetic acid and benzyladenine. Finally, we analyzed the RbN4OMT and RbCYP96T gene expressions in different organs and culture conditions and compared them to alkaloid production. In different organs of R. bifida young, adult and regenerated plants, as well as under various growing conditions, the transcripts accumulation was correlated with the production of alkaloids. This work provides new tools to improve the production of this important pharmaceutical compound and for future biotechnological studies.

MtNODULE ROOT1 and MtNODULE ROOT2 Are Essential for Indeterminate Nodule Identity.

Symbiotic interactions between legume plants and rhizobia result in the formation of nitrogen-fixing nodules, but the molecular actors and the mechanisms allowing for the maintenance of nodule identity are poorly understood. Medicago truncatula NODULE ROOT1 (MtNOOT1), Pisum sativum COCHLEATA1 (PsCOCH1), and Lotus japonicus NOOT-BOP-COCH-LIKE1 (LjNBCL1) are orthologs of Arabidopsis (Arabidopsis thaliana) AtBLADE-ON-PETIOLE1/2 and are members of the NBCL gene family, which has conserved roles in plant development and is essential for indeterminate and determinate nodule identity in legumes. The loss of function of MtNOOT1, PsCOCH1, and LjNBCL1 triggers a partial loss of nodule identity characterized by the development of ectopic roots arising from nodule vascular meristems. Here, we report the identification and characterization of a second gene involved in regulating indeterminate nodule identity in M. truncatula, MtNOOT2MtNOOT2 is the paralog of MtNOOT1 and belongs to a second legume-specific NBCL subclade, the NBCL2 clade. MtNOOT2 expression was induced during early nodule formation, and it was expressed primarily in the nodule central meristem. Mtnoot2 mutants did not present any particular symbiotic phenotype; however, the loss of function of both MtNOOT1 and MtNOOT2 resulted in the complete loss of nodule identity and was accompanied by drastic changes in the expression of symbiotic, defense, and root apical meristem marker genes. Mtnoot1 noot2 double mutants developed only nonfixing root-like structures that were no longer able to host symbiotic rhizobia. This study provides original insights into the molecular basis underlying nodule identity in legumes forming indeterminate nodules.

Lotus japonicus NOOT-BOP-COCH-LIKE1 is essential for nodule, nectary, leaf and flower development.

The NOOT-BOP-COCH-LIKE (NBCL) genes are orthologs of Arabidopsis thaliana BLADE-ON-PETIOLE1/2. The NBCLs are developmental regulators essential for plant shaping, mainly through the regulation of organ boundaries, the promotion of lateral organ differentiation and the acquisition of organ identity. In addition to their roles in leaf, stipule and flower development, NBCLs are required for maintaining the identity of indeterminate nitrogen-fixing nodules with persistent meristems in legumes. In legumes forming determinate nodules, without persistent meristem, the roles of NBCL genes are not known. We thus investigated the role of Lotus japonicus NOOT-BOP-COCH-LIKE1 (LjNBCL1) in determinate nodule identity and studied its functions in aerial organ development using LORE1 insertional mutants and RNA interference-mediated silencing approaches. In Lotus, LjNBCL1 is involved in leaf patterning and participates in the regulation of axillary outgrowth. Wild-type Lotus leaves are composed of five leaflets and possess a pair of nectaries at the leaf axil. Legumes such as pea and Medicago have a pair of stipules, rather than nectaries, at the base of their leaves. In Ljnbcl1, nectary development is abolished, demonstrating that nectaries and stipules share a common evolutionary origin. In addition, ectopic roots arising from nodule vascular meristems and reorganization of the nodule vascular bundle vessels were observed on Ljnbcl1 nodules. This demonstrates that NBCL functions are conserved in both indeterminate and determinate nodules through the maintenance of nodule vascular bundle identity. In contrast to its role in floral patterning described in other plants, LjNBCL1 appears essential for the development of both secondary inflorescence meristem and floral meristem.

The legume NOOT-BOP-COCH-LIKE genes are conserved regulators of abscission, a major agronomical trait in cultivated crops.

Plants are able to lose organs selectively through a process called abscission. This process relies on the differentiation of specialized territories at the junction between organs and the plant body that are called abscission zones (AZ). Several genes control the formation or functioning of these AZ. We have characterized BLADE-ON-PETIOLE (BOP) orthologues from several legume plants and studied their roles in the abscission process using a mutant approach. Here, we show that the Medicago truncatula NODULE ROOT (NOOT), the Pisum sativum COCHLEATA (COCH) and their orthologue in Lotus japonicus are strictly necessary for the abscission of not only petals, but also leaflets, leaves and fruits. We also showed that the expression pattern of the M. truncatula pNOOT::GUS fusion is associated with functional and vestigial AZs when expressed in Arabidopsis. In addition, we show that the stip mutant from Lupinus angustifolius, defective in stipule formation and leaf abscission, is mutated in a BOP orthologue. In conclusion, this study shows that this clade of proteins plays an important conserved role in promoting abscission of all aerial organs studied so far.