2. Plant Growth-Promoting Microbes
• PGPM are soil and rhizosphere-inhabiting microorganisms that can
colonize plant roots in significant numbers (105–107 CFU per gram of
fresh root) and influence plant growth in a positive manner.
• These soil microorganisms with beneficial activities assist in plant
growth and health.
• In general, the rhizospheric region is a hot spot for microbial activities
contributed mainly by indigenous bacteria and fungi.
3. PGPM can be divided into two main groups:
• Plant growth-promoting rhizobacteria (PGPR).
• Plant growth-promoting fungi (PGPF).
4. •PGPR were first defined by Kloepper and
Schroth (1978) to describe soil bacteria that
colonize the roots of plants and in a mutualistic
manner enhance the plant growth.
5. Plant Growth-Promoting Rhizobacteria
• PGPR are soil bacteria that stimulate plant growth by various means, often
in association with plant roots, sometimes on leaves and/or within plant
tissues.
• A vast array of PGPR including species of Pseudomonas, Rhizobium,
Azospirillum, Azotobacter, Klebsiella, Enterobacter, Alcaligenes,
Arthrobacter, Burkholderia, Bacillus, and Serratia have been reported to
enhance plant growth.
• These microbes directly assist in several fundamental processes required
for plant growth, for example, fixation of atmospheric nitrogen (N),
solubilization of inorganic phosphate, sequestration of iron (Fe), and
synthesis of phytohormones.
• PGPR indirectly help in plant growth promotion by preventing it from the
deleterious phytopathogens.
6.
7. • Biological nitrogen fixation (BNF) is the conversion of atmospheric N into
ammonia by symbiotic, associative, and free-living bacteria and being
considered as relevant to the environment and to world agriculture.
• Among all N-fixing microbes, bacteria-forming root nodules, commonly
known as rhizobia, are of most importance and show obligate symbiotic
association with legumes. These bacteria colonize the host plant’s root
system and cause the roots to form nodules, which are a storehouse of
fixed nitrogen.
• Symbiotic N-fixing PGPR (Rhizobium, Bradyrhizobium, Sinorhizobium,
Mesorhizobium, and Azorhizobium) increase N content in legumes in field
conditions.
8.
9. • Phosphate-solubilizing bacteria (PSB), and their use as inoculants, are
known to increase phosphate uptake by plants. Pseudomonas,
Bacillus, and Rhizobium are dominant PSB used for commercial
application.
• PGPR also play a significant role in Fe bioavailability to plants by
secretion of siderophores. Siderophores are Fe-binding extracellular
compounds with low molecular weight (2 kDa) and high affinity for
ferric (Fe3+) form of Fe.
• Siderophores chelate Fe in a reversible. Siderophores first bind with
Fe+3 tightly and then the siderophore-Fe complex moves into the cell
through the cell membrane receptors.
10.
11. Plant Growth-Promoting Fungi
• Among the PGPF, species of Phoma, Penicillium, Aspergillus,
Fusarium, Trichoderma, and arbuscular mycorrhizal fungus (AMF)
have gained attention due to their effective role in plant growth
activities and disease suppression.
• Mechanisms, stimulating plant growth by PGPF, involve production of
plant hormones, decomposing organic matter, solubilization of
unavailable soil bound nutrient elements, and protection of plants
from biotic and abiotic stresses.
• Indirect growth promotion by PGPF occurs via niche exclusion,
antibiosis, predation, mycoparasitism, and ISR.
12.
13.
14. Roles of PGPM in Agriculture Sustainability
• Improving Soil Fertility
• Macro- and Micronutrient facilitators
• Suppressing Phytopathogens
• Improving Food Quality: Biofortification
• Roles in Environmental Sustainability
• Rhizoremediation
15. • The term “rhizoremediation” involves the elimination of the
contaminants by the microbes present in the rhizosphere (Segura et
al. 2009).
• The technique is developing as a prominent method of removing
pollutants from contaminated sites by utilizing the combined
degradative potential of plants and their rhizospheric
microorganisms.
• Rhizospheric microorganisms accelerate the degradation process by
producing a wide range of hydrolytic enzymes and help in
ecorestoration of polluted sites.
• Various processes are involved in biotransformation, degradation,
and removal of the pollutants from the soil by plants associated
with rhizospheric microbes.
