Importance of Nitrogen to plants: Key Concepts Elements essential for life: C, H, O, N, S, P, and others in smaller quantities: K, Na, Mg, Mn, Fe, Mo, Cl, etc. N up to 2% of dried plant biomass Atmospheric N2 not available to organisms. Why? Lack of N availability a major limiting factor of plant growth 400Ma plants invaded land, likely with fungal (mycorhizal) symbionts Define: Biogeochemistry Immobilization Mineralization Diazotroph Atmospheric N2 Lightning Denitrification Pseudomonas,
etc. Excretion Urea CO(NH3)2 Soil bacteria Nitrate NO3- Herbivory Leaching* Assim ilatio o r hi Myc
*Stemflow, throughfall, litter Nutrients for mosses, etc. NO2- mo nifi cati on zae As s n Leaching Nitrifying bacteria:
Nitrobacter Am on imilati Biological N-Fixation (BNF) - Rhizobium / legume nodules - Spirochetes/Bacteroidales - Gram +: Clostridium, Bacillus - Frankia / Actinorhizal plant nodules - Cyanobacteria: Anabaena, Nostoc, Trichodesmium - etc. Ammonium NH4+
Nitrosifying bacteria: Nitrosomonas Ammonia-oxidizing Archaea Nitrification Volcanism Symbioses between plants and N-fixing bacteria Benefits for plant = fixed N Benefit for bacteria, carbon/food source, and sometimes protection from O2 A major force allowing plants to spread across land, invade new habitats N availablity is a major factor limiting plant growth in many habitats. Symbioses between plants and N-fixing bacteria Endophytic symbioses (bacteria live inside plant tissues) Legume nodules
Nodule-forming symbioses Rhizobium / Legumes Rhizobium / Parasponia (Cannabaceae) Frankia / Actinorhizal plants (8 families) Fagales (Betulaceae, Casuarinaceae, Myricaceae) Rosales (Rosaceae, Rhamnaceae, Eleaganceae) Cururbitales (Datiscaceae, Coriariaceae) Symbioses involving Cyanobacteria Nonvascular plants: Liverworts, hornworts, mosses Ferns: Azolla with Anabaena symbiont Cycads: Coralloid roots Angiosperms: Gunnera with Nostoc symbiont Associative symbioses (e.g., Poaceae) Cycad coralloid root Not present inside plant tissues; rather in rhizosphere Can significantly increase plant growth/biomass
Rhizobium Legume symbioses Rhizobium, an alpha-proteobacterium (Gram negative) Legumes: Family Fabaceae (beans, peas, Acacia, Lupinus, Acmispon, Bauhinia, etc.), one of the largest, most successful angiosperm families: 18,000 spp. Bauhinia (orchid tree) Acmispon glaber Infection and nodulation: Medicago nodules (pink due to legHemoglobin) 1. Free-living Rhizobium are attracted to root exudates; attach to root and multiply 2. Symbiotic genes activated in both plant and Rhizobium 3. Bacteria enter root, root cortex cells divide to form nodule. 4. Vascular system forms to supply photosynthates to bacteria, take up ammonium
Actinorhizal symbioses 220 spp incl. 8 families form endosymbioses with the actinomycete bacterium Frankia Rosales: Rosaceae, Eleanaceae, Rhamnaceae Cucurbitales: Datiscaceae, Coriariaceae Fagales: Betulaceae, Casaurinaceae, Myricaceae Actinomycetes (e.g., Frankia) Gram positive Mycelial growthbind soils in a netlike structure Frankia Very abundant in soils Very important in C and N cycling Fix N as free-living bacteria and in plant nodules (unlike Rhizobium) Metabolically more active in nodules Vesicles are sites of N-fixation, protect from O2 poisoning Produce Geosminsdistinctive smell of soils Warmth in compost piles Form nodules similar to those in Rhizobium-legume symbiosis Complex interactions involving many signaling compounds, modification of plant tissues (= modified lateral roots)
Actinorhizal symbioses 2 All actinorhizal plants belong to the Rosid I clade and share a common ancestor with Legumes 100 Mya ancestor evolved basis for evolution of RNS (root nodule symbiosis) This has evolved independently several times 50 60 mya (Doyle, 2011) Rosales: Rosaceae, Eleanaceae, Rhamnaceae Cucurbitales: Datiscaceae, Coriariaceae Fagales: Betulaceae, Casaurinaceae, Myricaceae Root nodule formation entails complex interactions involving many signaling compounds, modification of plant tissues (= modified lateral roots) Many actinorhizal plants also have mycorhizal symbioses and can grow in very N-poor soils Many are pioneer species and colonizers of disturbed areas (e.g., Alnus) Many are used in restoration, preventing desertification (e.g., Casuarina)
Actinorhizal associations 3 Infection process and nodule development Intracellular infection (e.g., Fagales) 1. 2. 3. 4. 5. Root hairs deformed by Frankia signals Hyphae enmesh with root hairs, penetrate root Frankia nodule Penetration causes cell divisions in root forming a prenodule Nodule primordium arises from root pericycle Nodule is a modified lateral root. Vesicles form at tips of hyphae Intercellular infection (e.g., Rosales) 1. No root hair deformation 2. Frankia grow in middle lamella, spread through the apoplast
Mature Nodules are modified lateral roots from pericycle Multilobed, each lobe with vascular bundle Periderm, endodermis, expanded cortex. In Casuarina, plant cell wall lignification = O2 protection (no vesicles) Actinorhizal symbioses and Rhizobium-Legume symbiotic signalling mechanisms likely evolved from (are homologous with) mycorhizal signaling systems Arbuscular-Mycorhizal symbioses evolved 400 Ma (Remy et al 1994) RNS evolved repeatedly starting 60 70 Ma (Doyle 1998) Rhizobium Parasponia (Cannabaceae) Only non-legume plant known to form nodules with Rhizobium As in Rhizobium Legume symbioses, depends on Nod factors secrete from host plant Similar ontogeny to actinorhizal plants
As with Actinorhizal plants, nodules are modified lateral roots. Parasponia are pioneer species in N-poor soils and disturbed habitats. Plant Cyanobacterial Symbioses Cyanobacteria (blue-green algae) Arguably the most important organisms ever to appear on earth Andrew Knoll Invented oxygenic photosynthesis (= Oxygen Revolution) using H2O as an electron source rather than sulfide (H2S) Enslavement by a eukaryote led to establishment of plastids in Archaeplastida (Glaucophytes, Red Algae, Green Algae, Land Plants) Secondary and tertiary symbiogenesis in Stramenopiles (Brown algae, diatoms), Alveolates (Dinoflagellates), Excavates (Euglenids), Rhizaria (Chlorachniophytes), etc Very common in marine (Prochlorococcus, Synechococcus), freshwater, terrestrial habitats, in lichens, microbial mats, and in N-fixing symbioses w/ plants Prochlorococcus, a 0.6 m marine alga discovered in 1986, and likely the most abundant organism on the planet, producing ~50%
of all atmospheric O2 Plant Cyanobacterial Symbioses 2 Cyanobacteria involved in N-fixing symbioses with plants commonly belong to the order Nostocales: Specialized N-fixing cells (heterocysts), resting stage cells (akinetes) Akinete Heterocyst Anabaena filaments Short-lived gliding filaments called hormogonia are important for infection of host plant. Factors released from host plant under N starvation Increased frequency of heterocysts when in symbiosis with a plant Cyanobacteria associate with nonvascular plants (mosses, liverworts, hornworts), ferns, cycads, and angiosperms Plant Cyanobacterial Symbioses 3 Bryophytes (non-vascular plants): Marchantiophyta (liverworts): 2 spp
Anthoceratophyta (hornworts): All species Bryophyta: few spp. Endosymbiont filaments are housed in specialized cavities (auricles in liverworts; slime cavities in hornworts). Cavities continue to form as gametophyte grows Ferns: The aquatic fern Azolla holds Anabaena filaments in specialized cavities in leaves. Symbiosis may date to 130 Ma Cyanobiont permanently associated with host during all stages of lifecycle Used as fertilizer in rice paddies Azolla with Anabaena Plant Cyanobacterial Symbioses 3 Cycads: All of the approximately 150 spp of cycads harbor N-fixing cyanobacteria in coralloid roots Coralloid roots arise from lateral roots and subsequently are colonized by cyanobacterial filaments.
Cycad coralloid roots Plant Cyanobacterial Symbioses 4 Angiosperms Gunnera in the Gunneraceae Cyanobiont (Nostoc) enters the Gunnera stem through specialized glands that secrete polysaccharide mucilage Cyanobiont is held intracellularly (unlike in non-vascular plants, Azolla, and cycads), with filaments occupying most of the host cells (filaments surrounded by host cytoplasm) Gunnera Associative N-Fixation Rhizobacteria: Colonize the rhizosphere Those that have a positive impact on plant growth are called Plant Growth Promoting Rhizobacteria (PGPR) Many are attracted by root exudates, and adhere to roots, sometimes forming biofilms Less complex than nodule symbioses, but still require molecular signalling
Occur in several plant groups including Poaceae Many soil-dwelling diazotrophic bacteria (alpha- and betaproteobacteria) identified as rhizobacteria: Acetobacter, Azotobacter, Pseudomonas, etc. PGPR significantly increase plant height, biomass in wheat, rice, corn In some PGPR, production of phytohormones may also influence plant growth May increase P and Fe availability
