MODULE 4 - Plant-Microbe Interactions Flashcards
what part of a plant hosts microbial communities?
every single part from leaves to roots and even the soil around it
what is the rhizosphere?
zone of root influence - extends less than 5mm from root
what is the rhizoplane?
root surface
outline the microbial community in the rhizosphere?
populations around plant roots usually 20-100x more than surrounding soil (called the rhizosphere effect)
home to a vast array of species but beta and alpha proteobacteria often predominate, their growth is dependent on root exudates
around the root tip is where many microbes accumulate sloughed off cells make this area nutrient rich
bacteria grow as microcolonies over about 5% of the root surface meaning it is mostly sterile
microbes prefer to colonise the gaps between plants cells and around the root tip
what are the bacterial communities in the rhizosphere strongly influenced by?
soil type and genetic constitution of host plant
what are the four main phylum of bacteria which dominate the rhizosphere of many plants?
actinobacteria, bacteroidetes, firmicutes and proteobacteria
of these, alpha and beta-proteobacteria are the two most dominant classes of bacteria
outline the multi-step process of how bacteria colonise plant roots?
bacteria swim up a gradient of exudates to reach root surface; especially root tip
primary attachment occurs (reversible) which is initially weak and mediated by hydrophobic and electrostatic forces
adhesion strengthened by something like flagella, fimbriae, adhesins or pili)
secondary attachment occurs (irreversible) where cellulose is produced further strengthening attachment
micro colonies form at site of adhesion and biofilms form through secretion of exopolysaccharides
what are the three groups which the rhizosphere microbiome can be classified in?
beneficial microbes
commensals (neutral) (however they do modulate ability of beneficial and pathogenic bacteria to colonise root through normal microbe-microbe interactions)
pathogens (can infect plant root or secrete compounds which harm plant root)
how might rhizosphere research help us reduce reliance on agrochemicals such as pesticides and fertilisers?
by tipping the balance of rhizosphere microbes in the favour of the beneficial ones
what are some examples of rhizosphere microbes that have harmful effects on plant growth?
pathogenic fungi, oomycetes, nematodes and bacteria
root rot fungi such as fusarium, pythium
phytophthora agathidicida causes kauri dieback
what are some of the harmful effects of pathogenic rhizosphere microbes other than the diseases they cause?
major problem for food production and ecosystem stability worldwide
e.g. pathogen resistance to applied agents, environmental impacts of pesticides, cost of pesticides, demand for pesticide-free food
what are beneficial rhizosphere bacteria called?
plant growth-promoting rhizobacteria (PGPR)
what are some of the direct beneficial effects of rhizosphere microbes like PGPR?
facilitate uptake of nutrients from enviro e.g. phosphate solubilisation, associative nitrogen fixation, siderophore production (iron acquisition)
synthesise compounds that affect plant growth e.g. plant growth regulators such as IAA which produces ACC deaminase that inactivates precursor of stress-hormone ethylene
what do auxin-producing bacteria do?
promote plant growth
e.g. Pseudomonas corrugata with radish plants, Azospirillum with maize
how do ACC deaminase-producing bacteria benefit plants?
halotolerant bacteria which protect plants from salt stress
what are some indirect effects of beneficial rhizosphere microbes such as PGPR?
lessen or prevent effects of plant pathogens
e.g. pseudomonas fluorescens can control root rots caused by fungal pathogens such as Ggt which causes take-all disease
how does Pseudomonas fluorescens prevent take-all disease from occurring in wheat and how was this discovered?
produces antibiotic (2,4-DAPG) which is toxic to Ggt, the take-all causing pathogen
this was sound out as many cases where wheat grown year after year in same soil and natural suppression of fungus occurred which is referred to as take-all decline (TAD)
TAD correlated with development of specific fluorescent Pseudomonas
important example of how bacteria control pathogens (biocontrol)
why is soil type important in shaping the rhizosphere bacterial community?
it is recruited from the microorganisms present in the soil, thus soil type important in shaping rhizosphere microbiome
why is plant genotype important in shaping the plant microbiome?
is a driving force for selection of specific elements from the bulk soil microbial community
when under stress/threat, evidence shows that plants can modify their rhizosphere microbiome to select for specific elements
what are the three main mechanisms of biocontrol of plant diseases by bacteria in the rhizosphere?
antibiosis
induced systematic resistance
competition for nutrients and niches
what is antibiosis?
bacterium colonises root system and delivers antibiotic molecules around the root thereby harming pathogens that approach the root
also commonly used to kill fungal pathogens which colonise same area as bacteria (gaps between plant cells)
what is induced systematic resistance (ISR)?
local root colonisation is sufficient to induce ISR. Many bacterial products induce systemic signalling via plant receptors which can result in protection of the whole plant against diseases caused by different organisms
how does biocontrol occur through competition for nutrients and niches?
biocontrol bacteria acting through this mechanism excel in fast chemotactic movement along the growing root in their efficient hunt for root exudate components, thereby outcompeting the pathogen in scavenging nutrients and in occupying niches on the root
basically just colonise quick as a form of niche exclusion
what is the phylloplane?
leaf surface