Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

With respect to the latter, bacterial populations in upper layers of the soil can contain
as many as 10 9 cells per gram of soil [15]. Most of these cells are unculturable. The
fraction of the cells making up soil microbial biomass that have been cultured and
studied in detail is negligible and often comprises less than 5% of the total population
[16,17].
Stimulation of microbial growth around plant roots by the release of different
organic compounds is known as the rhizospheric effect. The ability to secrete a vast
array of compounds into the rhizosphere is one of the most remarkable metabolic
features of plant roots, with nearly 5–21% of all photosynthetically fixed carbon
being transferred to rhizosphere through root exudates [18]. The nature of root
exudates is chemically diverse and can be grouped as low and high molecular weight
compounds (Table 5.1).
The microbial population in and around the roots includes bacteria, fungi,
yeasts and protozoa. Some are free living while others form symbiotic associations
with various plants. Rhizosphere microbial populations could be regarded as a
stable community around a particular plant species in a specific soil, or alternatively,
as a succession of populations. The interaction between these microorganisms
and the roots of the plant may be beneficial, harmful or neutral for the plant,
and sometimes the effect of microorganisms may vary as a consequence of soil
conditions [21].
Table 5.1 Compounds and enzymes identified in plant root exudates. a
Class of compounds Type of compounds
Amino acids Alanine, a-aminoadipic acid, g-aminobutyric acid, arginine, asparagine,
aspartic acid, cysteine, cystine, glutamic acid, glutamine, glycine,
histidine, homoserine, isoleucine, leucine, lysine, methionine,
ornithine, phenylalanine, proline, serine, therionine, tryptophan,
tyrosine, valine
Organic acids Acetic acid, aconitic acid, aldonic acid, butyric acid, citric acid,
erythronic acid, formic acid, fumaric acid, glutaric acid, glycolic acid,
lactic acid, malic acid, malonic acid, oxalic acid, piscidic acid, propionic
acid, pyruvic acid, succinic acid, tartaric acid, tartronic acid, valeric acid
Sugars Arabinose, deoxyribose, fructose, galactose, glucose, maltose,
oligosaccharides, raffinose, rhamnose, ribose, sucrose, xylose
Vitamins p-Aminobenzoic acid, biotin, choline, n-methylnicotinic acid, niacin,
panthothenate, pyridoxine, riboflavin, thiamine
Purines/pyrimidines Adenine, guanine, uridine, cytidine
Enzymes Amylase, invertase, phosphatase, polygalactouronase, proteases
Inorganic
ions/gaseous
molecules
HCO3 ,OH ,H þ ,CO2, H2
Miscellaneous Auxins, flavonones, glycosides, saponin, scopoletin
a Partially adopted from Refs [19,20].
5.2 Rhizosphere and Bacterial Diversityj83