Agriculture Reference
In-Depth Information
Non-legume-Frankia Symbiosis
Frankia a genus of actinomycetes, is a free, gram's positive nitrogen fixing bac-
terium that lives in soil and develops symbiotic interaction with various trees and
shrubs forming symbiotic nodules (Verghese and Misra
2002
). There are about 264
species belonging to 25 genera which take part in Frankia symbiosis. The Frankia
is of fundamental and ecological interests for diverse reasons that include its wide
distribution, its ability to fix nitrogen, differentiate specialized cell for nitrogen
fixation (Verghese 2002). These specialized cells are called sporangium and ves-
icles and in addition to it can nodulate non-leguminous trees by forming root nod-
ules, such
as Casuarina
,
Alnus, Dansea, Myrica, Elaeagnus
(Dawson et al.
2005
;
Franche et al.
2009
). In wastelands fertility of soil can be improved by growing
such non-leguminous plants in nitrogen deficient soils. In the process of nodula-
tion,
Frankia
develops as little lateral swelling on roots and subsequently develops
into new lobes at their apices forming cluster coralloid structure (Duhoux et al.
2001
). Inoculation of
Frankia
enhances growth, nodulation, nitrogenase activity of
nodule and nodule dry weight of
Casuarina
and
Alnus
plants. They live in the soil
and have a symbiotic relationship with certain woody angiosperms, called actino-
rhizal plants.
Frankia sp.
produces three types of cells: sporangiospores, hyphae,
and diazo-vesicles (Tjepkema et al.
1980
), these diazo-vesicles are spherical, thick
walled, lipid-enveloped cellular structures responsible for providing sufficient ni-
trogen to the host plant during symbiosis.
Frankia
enter into plants by root hair
infection, nodules formed on lateral roots with cortical cylinder of vascular tissue
(Ganesh et al.
1994
).
Frankia
supplies almost total nitrogen needed by host plant
and thus can establish a nitrogen-fixing symbiosis with host plants where nitrogen
is the limiting factor for plant development. Therefore, actinorhizal plants colonize
and often prosper in soils that are low in combined nitrogen (Benson and Silvester
1993
). Symbiotic interaction of this category adds a large quantity of new nitrogen
to numerous ecosystems such as temperate forests, dry chaparral, sand dunes, mine
wastes etc. They also assist in creating and transporting certain root hormones, con-
trolling pathogens and nematodes, water retention, mineral uptake, root exploration
and resource sharing (Benson and Silvester
1993
).
Frankia
specifically fixes nitro-
gen in the air and produces molecules that other plants can use.
Frankia
is said to be
responsible for 15 % of the biologically fixed nitrogen in the world (Trujillo 2008).
Plant Growth Promoting Rhizobacteria
An assemblage of rhizobacteria (bacteria on rhizosphere) to facilitate beneficial
effect on plant growth is referred to as plant growth promoting rhizobacteria or
PGPR (Schroth and Hacock
1981
). PGPR belong to several genera, e.g.,
Alcalige-
nes
,
Agrobacterium Arthrobacter
, Azotobacter, Actinoplanes,
Bacillus
,
Bradyrhizo-
bium
,
Amorphosporangium
,
Pseudomonas
sp.,
Enterobacter
,
Rhizobium
,
Erwinia
,
Cellulomonas
,
Streptomyces
Flavobacterium
, and
Xanthomonas
(Weller
1988
). In
a recent study it was found that PGPR covers a wide range of plant species. In all
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