108
C. Dileep et al.
have common property of antagonism against Gram-positive and Gram-negative
pathogenic bacteria and this gives the “double-action” of PGPR—they control the
pathogens and thus control disease, besides adding to growth and yield. This antagonistic activity is exerted usually only when the iron is deficient, as only under
conditions of iron limitation the PGPR produce the siderophores that chelate the
iron. Thus, the term PGPR is a misnomer because their action is by antagonism of
deleterious bacteria or “minor pathogens” rather than as plant growth promotor as
such. What happens is the removal of growth inhibition, rather than growth promotion. Hence, something is to work against as infrequently or over-cropped potato
soils where about 50% of Pseudomonas isolates observed deleterious, and not in
sterile soils or when the DRB are absent. Thus the modus operandi for plant growth
promotion is competition for iron supported by the following in vitro experiments
(a) addition of dissolved iron (Fe
3+ ) to the environment abolishes in vitro antagonism
and also prohibits the plant growth promotion by PGPR strains and (b)siderophorenegative mutants, obtained by exposure of wild type of PGPR strains to UV light or
mutagenic chemicals or obtained by transposon mutagenesis, also lost their PGPR
activities, although they colonized roots as well as similar to the wild type PGPR
(Bakker et al. 1987).
5.3 Iron Deficiency and Siderophore Production
Iron is an essential element for living organisms under its two stable valences that
act as cofactors in various oxidative-reductive enzymatic reactions. Although iron
is abundant, comprising 4–5% of the average soil, iron deficiency of crop plants is
common in calcareous soils that represent over one-third of the world’s land surface
area. In well-aerated soils with a high pH, the concentration of Fe
2+ and Fe
3+ becomes
negligible. The concentration of chelated iron required for optimal growth is of the
order of 10
−6 –10
−5 M. Cultivars that grow in alkaline soils without developing symptoms of lime-induced chlorosis are called “iron efficient” while those that become
“chlorotic” are iron-defficient.
Fe is reflected by the number of mechanisms developed by plants and microorganisms for its acquisition. Plants undergo in iron deficiency overcome by several
mechanisms: (i) secretion of protons by roots (Marschner et al. 1974) (ii) secretion
of reducing compounds by roots and (iii) secretion of phytosiderophores (Suguira
and Nomato 1984). Although, phytosiderophores are widely distributed in higher
plants (Rippenger and Schreiber 1982), they have only been isolated from root
washings of graminaceous plants. These are amino hydroxyl carboxylates with high
affinity for Fe
3+ (Kf 10s) although not as high as those possessed by bacterial and
fungal siderophores. Besides, plants’ supporting fluorescent pseudomonad flora is a
mechanism to make iron available to itself (Dileep and Dileepkumar 2000).
Microorganisms have evolved efficient uptake systems to obtain sufficient
amounts of iron. Most aerobic and facultative aerobic bacteria possess a highaffinity iron-transport system in which siderophores are excreted and the consequent
C. Dileep et al.
have common property of antagonism against Gram-positive and Gram-negative
pathogenic bacteria and this gives the “double-action” of PGPR—they control the
pathogens and thus control disease, besides adding to growth and yield. This antagonistic activity is exerted usually only when the iron is deficient, as only under
conditions of iron limitation the PGPR produce the siderophores that chelate the
iron. Thus, the term PGPR is a misnomer because their action is by antagonism of
deleterious bacteria or “minor pathogens” rather than as plant growth promotor as
such. What happens is the removal of growth inhibition, rather than growth promotion. Hence, something is to work against as infrequently or over-cropped potato
soils where about 50% of Pseudomonas isolates observed deleterious, and not in
sterile soils or when the DRB are absent. Thus the modus operandi for plant growth
promotion is competition for iron supported by the following in vitro experiments
(a) addition of dissolved iron (Fe
3+ ) to the environment abolishes in vitro antagonism
and also prohibits the plant growth promotion by PGPR strains and (b)siderophorenegative mutants, obtained by exposure of wild type of PGPR strains to UV light or
mutagenic chemicals or obtained by transposon mutagenesis, also lost their PGPR
activities, although they colonized roots as well as similar to the wild type PGPR
(Bakker et al. 1987).
5.3 Iron Deficiency and Siderophore Production
Iron is an essential element for living organisms under its two stable valences that
act as cofactors in various oxidative-reductive enzymatic reactions. Although iron
is abundant, comprising 4–5% of the average soil, iron deficiency of crop plants is
common in calcareous soils that represent over one-third of the world’s land surface
area. In well-aerated soils with a high pH, the concentration of Fe
2+ and Fe
3+ becomes
negligible. The concentration of chelated iron required for optimal growth is of the
order of 10
−6 –10
−5 M. Cultivars that grow in alkaline soils without developing symptoms of lime-induced chlorosis are called “iron efficient” while those that become
“chlorotic” are iron-defficient.
Fe is reflected by the number of mechanisms developed by plants and microorganisms for its acquisition. Plants undergo in iron deficiency overcome by several
mechanisms: (i) secretion of protons by roots (Marschner et al. 1974) (ii) secretion
of reducing compounds by roots and (iii) secretion of phytosiderophores (Suguira
and Nomato 1984). Although, phytosiderophores are widely distributed in higher
plants (Rippenger and Schreiber 1982), they have only been isolated from root
washings of graminaceous plants. These are amino hydroxyl carboxylates with high
affinity for Fe
3+ (Kf 10s) although not as high as those possessed by bacterial and
fungal siderophores. Besides, plants’ supporting fluorescent pseudomonad flora is a
mechanism to make iron available to itself (Dileep and Dileepkumar 2000).
Microorganisms have evolved efficient uptake systems to obtain sufficient
amounts of iron. Most aerobic and facultative aerobic bacteria possess a highaffinity iron-transport system in which siderophores are excreted and the consequent