Lecture 3 Flashcards
benefits of bacterial colonisation of plants
Plant growth promoted by a diversity of mechanisms:
1. Increase nutrient accessibility, facilitate mineral
and nutrient uptake, decrease soil toxicity.
2. Release growth stimulating phytohormones, modulate
hormone production by the plant, supply nitrogen and
phosphate via symbioses, or enhance the effects of symbioses
3.Plant growth-promoting (PGP) bacteria inhabit the plant
rhizosphere and/or rhizoplane, beneficial phyllosphere
and endophytic bacteria have been isolated
improving crop yield.
4. PGP bacteria can be important partners of pioneer plants for
revegetation and reforestation of barren or contaminated soils
Summary of interactions between beneficial microorganism and plants for defence.
(i) Selected commensal bacteria and fungi act as direct antagonists or growth promoters of soil microorganisms due to production of primary and secondary metabolites.
(ii) They can enhance plant root colonisation, in part through suppression of plant defence responses. Also, plant growth and local defence responses are enhanced through induced systemic resistance via direct root contact and/or endophytic growth. The induced defences include changes in cell wall composition and expression of defence related genes.
(iii) Pathogen resistance in distal plant parts is enhanced by some bacteria, and this is accompanied by strain-specific changes in plant gene expression levels.
Improved plant nutrition and health induced systematic resistance
- The signal transduction pathway is in most cases controlled by the sequential sensitization of jasmonic acid (JA) and ethylene (ET) at the site of application of the inducing agent.
- As in the case of the systemic resistance induced by incompatible pathogens (SAR), rhizobacteria-mediated ISR required the positive regulator protein NPR1 that is activated following unknown systemic signalization events downstream to ET perception. This results in a priming effect meaning that the onset of the phenomenon is not usually accompanied by substantial transcriptional reprogramming in the host plant before the pathogen attack.
- It is likely that no direct transcriptional stimulation of the defence-related genes occurs upon non-pathogenic rhizobacteria recognition but rather the host resistance capacity is enhanced once the infection process is started.
- Alternatively, some non-pathogenic rhizobacteria were reported to trigger a salicylic acid (SA)-dependent signalling pathway that leads to a state of induced resistance resembling pathogen-dependent SAR
PHYTOHORMONES
. Many rhizosphere microbes produce and secrete phytohormones or mimics thereof and directly modulate plant growth. A diverse group of Grampositive bacteria, including Arthrobacter, Micrococcus, Bacillus, Rhodococcus, Mycobacterium, Microbacterium, Streptomyces and Corynebacterium species, are capable of producing auxins that might stimulate nutrient uptake and root proliferation.
IAA
Inoculation of different terrestrial and epiphytic orchid seeds with indole-3-acetic acid (IAA)-producing Bacillus pumilus improved seed germination and stimulated orchid development.
acc
Plant-associated Rhodococcus spp. and B. pumilus often display 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity and use ACC as a nitrogen source by hydrolyzing it to ammonium and a-ketobutyrate. The ACC concentration in plants associated with these bacteria is lowered and, consequently, stress-induced ethylene accumulation is reduced. Hence, bacterial ACC deaminase activity has the potential to sustain and improve plant growth under unfavourable environmental conditions. Arthrobacter and a Bacillus species, positive for IAA production and ACC deaminase activity, increased the ability of pepper plants to cope with abiotic stress (drought).
cytokinin
Inoculation of lettuce plants with cytokinin producing Bacillus subtilis had a beneficial effect on plant growth under moderate drought stress and led to accumulation of this hormone in the plant tissues and to an increased biomass Paenibacillus polymyxa improves plant growth through the production of cytokinin and auxin. Finally, ABA (plant hormone abscisic acid) and jasmonic acid produced by endophytic B. pumilis strains isolated from sunflower and gibberellin secreted by several plant-associated Bacillus, Micrococcus, Arthrobacter and Clostridium species promote plant growth .
p
In soil, organic phosphorus is stored mainly as insoluble myo-inositol hexaphosphate or phytate. Many rhizosphere bacteria can solubilize phosphorus from phytate by secreting active phytases. The most efficient are Bacillus, Brevibacterium, Sarcina, Paenibacillus, Corynebacterium and Micrococcus strains . Other activities include release phosphorus from rock sediments through the secretion of organic acids.
k
Bacillus mucilaginosus promotes growth of tobacco by dissolving potassium from feldspar and phosphorus from calcium phosphate; introduction of the phyA gene of Aspergillus fumigatus resulted in a transgenic strain NKTS-3 with increased soil-improving properties and a superior PGP.
Fe
The prevailing form of iron in the soil, Fe3+ relatively insoluble compared with the more reduced Fe2+ ions that are readily taken up. Several bacteria can reduce metals, increasing bioavailability of iron. Plants themselves secrete siderophores or chelators to facilitate iron uptake, some plant species recognize and use siderophores synthesized by rhizospheric bacteria.
N
Atmospheric dinitrogen is inaccessible to plants and, consequently, nitrogen can be a limiting factor for growth. The ability to fix atmospheric dinitrogen into ammonium is restricted to diazotrophic bacteria that possess the nitrogenase enzyme complex. The best-described nitrogen fixing symbioses are those between rhizobia and legume crops but tripartite relationships with Gram-positive bacteria may stimulate nodulation. Retama sphaerocarpa, a drought-adapted legume, is used for the prevention of erosion and desertification in semi-arid and arid areas.
H20
Water availability can be improved by increased surface area due to mycelia of endophytic fungi – the mycorrhizal group and perhaps some filamentous bacteria (the streptomycetes).
endophytic bacteria in root zone
- Endophytic bacteria have been found in virtually every plant studied, where they colonize the internal tissues of their host plant and can form a range of different relationships including symbiotic, mutualistic, commensalistic and trophobiotic.
- Most endophytes appear to originate from the rhizosphere or phyllosphere; however, some may be transmitted through the seed.
- Endophytic bacteria can promote plant growth and yield and can act as biocontrol agents.
- Endophytes can also be beneficial to their host by producing a range of natural products that could be harnessed for potential use in medicine, agriculture or industry. In addition, it has been shown that they have the potential to remove soil contaminants by enhancing phytoremediation and may play a role in soil fertility through phosphate solubilization and nitrogen fixation.
- There is increasing interest in developing the potential biotechnological applications of endophytes for improving phytoremediation and the sustainable production of nonfood crops for biomass and biofuel production
Various microorganisms in baticular bacteria and fungi can colonise the internal tissues of the plant.
Endophytic fungi, represented here as arbuscular mycorrhizal fungi (AMF) (lilac), might form specialized structures, called arbuscules, where plant- derived carbon sources, mainly sucrose (Su), are exchanged for fungus-provided phosphate (Pi), nitrogen (NH4 ), and potassium (K ) elements (blue). Plant cytoplasmic sucrose is transported to the periarbuscular space, where it is converted to hexose (HEX) to be assimilated by the fungus. Hexose is finally converted to glycogen (G) for long-distance transport. Phosphate and nitrogen are transported inside the fungal cytoplasm as polyphosphate granules (Poly-P), which are converted to Pi and arginine (Arg) in the arbuscule. Pi is transported to the host cytoplasm, whereas Arg is initially converted to urea (Ur) and then to ammonium (NH4 ). \
Fungal and bacterial plant hormones, such as auxins (IAA), gibberellins (GAs), cytokinins (CKs), volatile organic compounds (VOCs), and polyamines (Poly-NH2), as well as secondary metabolites (SMs), are transferred to the host (violet).
Various bacterial structures, such as flagella, pili, secretion system machineries (e.g., TIV SS and SEC), and lipopolysaccharides, as well as bacterium-derived proteins and molecules, such as effectors (EF), autoinducers, and antibiotics, are detected by the host cells and trigger the induced systemic resistance (ISR) response (red). ACC, the direct precursor of ethylene (ET), is metabolized by bacteria via the enzyme ACC deaminase (ACCd), thus ameliorating abiotic stress (light green).
A range of reactive oxygen species detoxification (ROS detox) enzymes might also ameliorate the plant-induced stress (orange). Diazotrophic bacterial endophytes are capable of fixing atmospheric nitrogen (N2) and might actively transport NH4 and nitrate (NO3 ) to the host (dark green).
Bacterial processes of siderophore production (Sid) and uptake (Fe) that are involved in plant growth promotion, biocontrol, and phytoremediation are shown in brown.
Hypothesis for Rhizobium sp. infecting legumes
Flavonoid is apigenin- activates NodD1 (protein lock) Nod factor allows bacterial entry lock 2 Components of the symbiosis (locks and keys in 3) enable infection thread development. Flavonoid inducible TTSS produced by some rhizobia additional keys- secreted proteins – Nops (nodulation outer proteins)
