Agriculture Reference
In-Depth Information
Fig. 2.2
Biosynthetic pathways of IAA in bacteria [Adapted from Patten and Glick (
1996
)]
Bradyrhizobium
spp. and
Azospirillum
spp., although the activity of the
corresponding enzymes is either negligible or not detectable. Indoleacetic acid
biosynthesis that involves tryptophan conversion into indole-3-acetonitrile is
found in plants,
Alcaligenes faecalis
, and possibly the cyanobacterium
Synechocystis
sp., and the tryptophan-independent pathway, more common in
plants, is also found in microorganisms (azospirilla and cyanobacteria). However,
the synthesis of IAA using this pathway is reported to be insignificant, and the
mechanisms are largely unknown. Many bacteria are known to synthesize auxins
using such pathways and help the plants to grow better. Bacteria in general form
maximum amount of IAA during the steady-state stage of their growth while
ammonium ions and glutamine inhibit IAA biosynthesis (Tsavkelova et al.
2006
).
The genes involved in IAA synthesis in bacterial strains may be plasmid or
chromosomal borne. For example, pathogenic bacteria contain Ti plasmids that
control the formation of the phytohormone, whereas in saprophytic microorgan-
isms, auxin biosynthesis is governed by chromosomal genes (Tsavkelova
et al.
2006
). It is reported that 80 % of microorganisms isolated from the rhizo-
sphere of various crops possess the ability to synthesize and release auxins as
secondary metabolites (Loper and Schroth
1986
). Of the various PGPR strains,
bacteria belonging to the genera
Azospirillum
,
Pseudomonas
,
Xanthomonas
and
Rhizobium
as well as
Alcaligenes
,
Enterobacter
,
Acetobacter
and
Bradyrhizobium
have been shown to produce auxins which help in stimulating plant growth
(Egamberdieva et al.
2007
; Wani et al.
2007c
; Kumar et al.
2008
; Poonguzhali
et al.
2008
). However, the extent of IAA production by bacterial strains could be
different due in part to the involvement of biosynthetic pathways, location of the
genes, regulatory sequences and the presence of enzymes to convert active free IAA
into conjugated forms. Moreover, the synthesis of IAA is also influenced by
environmental factors (Patten and Glick
1996
). Synthesis of IAA by
Rhizobium
spp. in the presence and absence of tryptophan has also been demonstrated (Wani
et al.
2007c
). In a similar study, Bent et al. (
2001
) reported that the concentration of
indole compounds by three different strains,
Paenibacillus polymyxa
(L6),
P. poly
myxa (Pw-2) and
Pseudomonas fluorescens
(M20), increased with increas-
ing rate of tryptophan (0-200 mg/ml) at different incubation interval.
