Apical and basal auxin sources pattern shoot branching in a moss

Shoot branching mechanisms where branches arise in association with leaves – referred to as lateral or axillary branching – evolved by convergence in the sporophyte of vascular plants and the gametophyte of bryophytes, and accompanied independent events of plant architectural diversification. Previously, we showed that three hormonal cues, including auxin, have been recruited independently to co-ordinate branch patterning in flowering plant leafy shoots and moss gametophores (Coudert, Palubicki et al., 2015). Moreover, auxin-mediated apical dominance, which relies on local auxin production, has been proposed as a unifying molecular regulatory mechanism of branch development across land plants. Whilst our previous work in the moss Physcomitrium patens has gathered indirect evidence supporting the notion that auxin synthesized in gametophore apices regulates branch formation at a distance, direct genetic evidence for a role of auxin biosynthesis in gametophore branching control is still lacking. Here, we show that gametophore apex decapitation promotes branch emergence through massive and rapid transcriptional reprogramming of auxin-responsive genes and altering auxin biosynthesis gene activity. Specifically, we identify a subset of P. patens TRYPTOPHAN AMINO-TRANSFERASE (TAR) and YUCCA FLAVIN MONOOXYGENASE-LIKE (YUC) auxin biosynthesis genes expressed in apical and basal regions of the gametophore, and show that they are essential for branch initiation and outgrowth control. Our results demonstrate that local auxin biosynthesis coordinates branch patterning in moss and thus constitutes a shared and ancient feature of shoot architecture control in land plants.

The Role of Plant Hormones in the Interaction of Colletotrichum Species with Their Host Plants

Colletotrichum is a plant pathogenic fungus which is able to infect virtually every economically important plant species. Up to now no common infection mechanism has been identified comparing different plant and Colletotrichum species. Plant hormones play a crucial role in plant-pathogen interactions regardless whether they are symbiotic or pathogenic. In this review we analyze the role of ethylene, abscisic acid, jasmonic acid, auxin and salicylic acid during Colletotrichum infections. Different Colletotrichum strains are capable of auxin production and this might contribute to virulence. In this review the role of different plant hormones in plant—Colletotrichum interactions will be discussed and thereby auxin biosynthetic pathways in Colletotrichum spp. will be proposed.

Wheat–Fusarium graminearum Interactions Under Sitobion avenae Influence: From Nutrients and Hormone Signals

The English grain aphid Sitobion avenae and phytopathogen Fusarium graminearum are wheat spike colonizers. “Synergistic” effects of the coexistence of S. avenae and F. graminearum on the wheat spikes have been shown in agroecosystems. To develop genetic resistance in diverse wheat cultivars, an important question is how to discover wheat–F. graminearum interactions under S. avenae influence. In recent decades, extensive studies have typically focused on the unraveling of more details on the relationship between wheat-aphids and wheat-pathogens that has greatly contributed to the understanding of these tripartite interactions at the ecological level. Based on the scientific production available, the working hypotheses were synthesized from the aspects of environmental nutrients, auxin production, hormone signals, and their potential roles related to the tripartite interaction S. avenae–wheat–F. graminearum. In addition, this review highlights the relevance of preexposure to the herbivore S. avenae to trigger the accumulation of mycotoxins, which stimulates the infection process of F. graminearum and epidemic of Fusarium head blight (FHB) in the agroecosystems.

Phenotype Switching in Metal-Tolerant Bacteria Isolated from a Hyperaccumulator Plant

As an adaptation to unfavorable conditions, microorganisms may represent different phenotypes. Azolla filiculoides L. is a hyperaccumulator of pollutants, but the functions of its microbiome have not been well recognized to date. We aimed to reveal the potential of the microbiome for degradation of organic compounds, as well as its potential to promote plant growth in the presence of heavy metals. We applied the BiologTM Phenotypic Microarrays platform to study the potential of the microbiome for the degradation of 96 carbon compounds and stress factors and assayed the hydrolytic potential and auxin production by the microorganisms in the presence of Pb, Cd, Cr (VI), Ni, Ag, and Au. We found various phenotype changes depending on the stress factor, suggesting a possible dual function of the studied microorganisms, i.e., in bioremediation and as a biofertilizer for plant growth promotion. Delftia sp., Staphylococcus sp. and Microbacterium sp. exhibited high efficacy in metabolizing organic compounds. Delftia sp., Achromobacter sp. and Agrobacterium sp. were efficient in enzymatic responses and were characterized by metal tolerant. Since each strain exhibited individual phenotype changes due to the studied stresses, they may all be beneficial as both biofertilizers and bioremediation agents, especially when combined in one biopreparation.

Transcriptomics differentiate two novel bioactive strains of Paenibacillus sp. isolated from the perennial ryegrass seed microbiome

Paenibacillus species are Gram-positive bacteria that have been isolated from a diverse array of plant species and soils, with some species exhibiting plant growth-promoting (PGP) activities. Here we report two strains (S02 and S25) of a novel Paenibacillus sp. that were isolated from perennial ryegrass (Lolium perenne) seeds. Comparative genomics analyses showed this novel species was closely related to P. polymyxa. Genomic analyses revealed that strains S02 and S25 possess PGP genes associated with biological nitrogen fixation, phosphate solubilisation and assimilation, as well as auxin production and transportation. Moreover, secondary metabolite gene cluster analyses identified 13 clusters that are shared by both strains and three clusters unique to S25. In vitro assays demonstrated strong bioprotection activity against phytopathogens (Colletotrichum graminicola and Fusarium verticillioides), particularly for strain S02. A transcriptomics analysis evaluating nitrogen fixation activity showed both strains carry an expressed nif operon, but strain S02 was more active than strain S25 in nitrogen-free media. Another transcriptomics analysis evaluating the interaction of strains with F. verticillioides showed strain S02 had increased expression of core genes of secondary metabolite clusters (fusaricidin, paenilan, tridecaptin and polymyxin) when F. verticillioides was present and absent, compared to S25. Such bioactivities make strain S02 a promising candidate to be developed as a combined biofertiliser/bioprotectant.

Production of Indole Auxins by Enterobacter sp. Strain P-36 under Submerged Conditions

Bioactive compounds produced by plant growth-promoting bacteria through a fermentation process can be valuable for developing innovative second-generation plant biostimulants. The purpose of this study is to investigate the biotechnological potential of Enterobacter on the production of auxin—a hormone with multiple roles in plant growth and development. The experiments were carried in Erlenmeyer flasks and a 2-L fermenter under batch operating mode. The auxin production by Enterobacter sp. strain P-36 can be doubled by replacing casein with vegetable peptone in the culture medium. Cultivation of strain P36 in the benchtop fermenter indicates that by increasing the inoculum size 2-fold, it is possible to reduce the fermentation time from 72 (shake flask cultivation) to 24 h (bioreactor cultivation) and increase the auxin volumetric productivity from 6.4 to 17.2 mg [IAAequ]/L/h. Finally, an efficient storage procedure to preserve the bacterial auxin was developed. It is noteworthy that by sterilizing the clarified fermentation broth by filtration and storing the filtrated samples at +4 °C, the level of auxin remains unchanged for at least three months.

Metabolomics and Dual RNA-Sequencing on Root Nodules Revealed New Cellular Functions Controlled by Paraburkholderia phymatum NifA

Paraburkholderia phymatum STM815 is a nitrogen-fixing endosymbiont that nodulate the agriculturally important Phaseolus vulgaris and several other host plants. We previously showed that the nodules induced by a STM815 mutant of the gene encoding the master regulator of nitrogen fixation NifA showed no nitrogenase activity (Fix−) and increased in number compared to P. vulgaris plants infected with the wild-type strain. To further investigate the role of NifA during symbiosis, nodules from P. phymatum wild-type and nifA mutants were collected and analyzed by metabolomics and dual RNA-Sequencing, allowing us to investigate both host and symbiont transcriptome. Using this approach, several metabolites’ changes could be assigned to bacterial or plant responses. While the amount of the C4-dicarboxylic acid succinate and of several amino acids was lower in Fix− nodules, the level of indole-acetamide (IAM) and brassinosteroids increased. Transcriptome analysis identified P. phymatum genes involved in transport of C4-dicarboxylic acids, carbon metabolism, auxin metabolism and stress response to be differentially expressed in absence of NifA. Furthermore, P. vulgaris genes involved in autoregulation of nodulation (AON) are repressed in nodules in absence of NifA potentially explaining the hypernodulation phenotype of the nifA mutant. These results and additional validation experiments suggest that P. phymatum STM815 NifA is not only important to control expression of nitrogenase and related enzymes but is also involved in regulating its own auxin production and stress response. Finally, our data indicate that P. vulgaris does sanction the nifA nodules by depleting the local carbon allocation rather than by mounting a strong systemic immune response to the Fix− rhizobia.

Effect of Exogenous Auxin Treatment on Cell Wall Polymers of Strawberry Fruit

The role of auxin in the fruit-ripening process during the early developmental stages of commercial strawberry fruits (Fragaria x ananassa) has been previously described, with auxin production occurring in achenes and moving to the receptacle. Additionally, fruit softening is a consequence of the depolymerization and solubilization of cell wall components produced by the action of a group of proteins and enzymes. The aim of this study was to compare the effect of exogenous auxin treatment on the physiological properties of the cell wall-associated polysaccharide contents of strawberry fruits. We combined thermogravimetric (TG) analysis with analyses of the mRNA abundance, enzymatic activity, and physiological characteristics related to the cell wall. The samples did not show a change in fruit firmness at 48 h post-treatment; by contrast, we showed changes in the cell wall stability based on TG and differential thermogravimetric (DTG) analysis curves. Less degradation of the cell wall polymers was observed after auxin treatment at 48 h post-treatment. The results of our study indicate that auxin treatment delays the cell wall disassembly process in strawberries.