Biology Reference
In-Depth Information
is amenable for assay of nitrogenase activity by ARAs conducted by the incubation of intact stolons
and nodes in an atmosphere of 10% v/v of acetylene in air. Such experiments have been helpful in
assessing
15
N as percentage of all N atoms, µg
15
N g
-1
dry weight of plant tissue for understanding
the distribution of the isotope and
15
N% as a percentage of all the
15
N taken up in a particular organ.
Rapid translocation of recently fi xed nitrogen from mature regions to the apex of
G
.
monoica
has been
demonstrated by
15
N pulse-chase experiments. Additionally, stem-girdling experiments provided
evidence for the translocation of recently fi xed nitrogen through phloem. It means that the fl ow of
fi xed nitrogen via phloem must be counter to the fl ow of carbohydrates into the glands (Stock and
Silvester, 1994).
XI. PHYLOGENY OF
NOSTOC
STRAINS ACROSS ALL SYMBIOSES
In most of the cyanobacterial symbioses the cyanobiont is a strain of
Nostoc
. In the different
symbioses described in this Chapter, the diversity and phylogeny of
Nostoc
strains have been
discussed. The molecular markers selected for this purpose varied greatly as also the hosts from
which the cyanobionts have been derived. These include comparison of the sequences of tRNA
Leu
(UAA) intron (Lindblad
et al
., 1989; Paulsrud and Lindblad, 1998; Paulsrud
et al
., 1998, 2000; Costa
et al
., 2001, 2002, 2004; Wirtz
et al
., 2003; Summerfi eld and Eaton-Rye, 2006; Rikkinen and Virtanen,
2008), 16S rDNA (Oksanen
et al
., 2004; Myllys
et al
., 2007; Gehringer
et al.
, 2010), 16S rRNA-23S
rRNA ITS region (West and Adams, 1997), 16S rDNA and tRNA
Leu
(UAA) intron (Rikkinen
et al
.,
2002; Summerfi edld
et al
., 2002; Stenroos
et al
., 2006) and gene locus
rbcLXS
(O'Brien
et al
., 2005;
Stenroos
et al
., 2006; Myllys
et al
., 2007; Otálora
et al
., 2010). The other molecular markers include
PCR fi ngerprinting of STRR sequences (Zheng
et al
., 2002; Guevara
et al
., 2002; Costa
et al
., 2004)
STRR and LTRR sequences (Rasmussen and Svenning, 1998; Nilsson
et al.
, 2000) and RFLP patterns
of
nifK
and
glnA
(Lindblad
et al
., 1989) and
nifH
and
glnA
genes (Zimmerman and Bergman, 1990).
Either a high degree of specifi city or a great diversity in the strains of
Nostoc
has been reported. The
study of Costa
et al
. (2002) consisted of 54 symbiotic strains of
Nostoc
derived from different hosts
such as
Peltigera
(18 species),
Nephroma
(7 species),
Blasia
(6 species)
A
.
fusiformis
(4 species) and few
cycads (
C
.
circinalis
,
C
.
rumphii
,
E
.
lebomboensis
and
Z
.
pumila
). They compared the stem-loop (P6b)
of tRNA
Leu
(UAA) intron sequences that possesses degenerate heptapeptide repeats. According to
them there is a high degree of similarity and the
Nostoc
strains shared high degree of conserved
intron sequence. Oksanen
et al
. (2004) questioned the validity of tRNA
Leu
(UAA) intron sequence
comparisons for deriving the phylogeny of
Nostoc
strains. According to few workers the locus of
rbcLXS
is quite suitable and reliable molecular marker that can provide the degree of variation
needed to unravel the specifi city of
Nostoc
strains. Moreover, the results from
rbcLXS
locus are very
much comparable to the multilocus sequence typing approach (O'Brien
et al
., 2005; Stenroos
et al
.,
2006; Myllys
et al
., 2007; Otálora
et al
., 2010) but which is not the case with 16S rDNA sequences
(Costa
et al
., 2002; Oksanen
et al
., 2004; Rikkinen
et al
., 2004; Stenroos
et al
., 2006). The observations
of Rikkinen
et al
. (2002) revealed (i) the presence of
Nostoc
strains that are specifi c to the species of
Peltigera
(“Peltigera guild”) and
Nephroma
(“
Nephroma
guild”) corresponding to clade I and clade II,
respectively; and (ii) the cyanobiont selection very much depended on a community scale depending
on the habitat. The existence of the two clades, clade I and clade II has further been substantiated
by other workers (Lohtander
et al
., 2003; Rikkinen
et al
., 2003, 2004; Oksanen
et al
., 2004; Stenroos
et
al
., 2006; Myllys
et al
., 2007). Though clade I consisted of a homogeneous sequences of
Nostoc
strains
from terricolous lichens, the clade II has been found be heterogeneous with sequences of
Nostoc
