Biology Reference
In-Depth Information
IX. CYCADS
The order Cycadales is an ancient group of Gymnosperms that has existed for nearly 300 million
years. Commonly known as cycads, these are characteristic in possessing pinnately compound
leaves and giant cones. The major reasons for their long survival history are (i) their ability to survive
drought and fi re, (ii) their resistance to pathogens and predators and (iii) the production of protective,
secondary metabolites. Modern cycads comprise of 11 genera (Brenner
et al
., 2003).
Reinke in 1872 for the fi rst time reported the existence of endosymbionts in cycad roots. The
symbiotic association of cycads with nitrogen-fi xing cyanobacteria is restricted to the coralloid roots.
These roots fi rst begin to make their appearance as apogeotropic roots with papillose sheath. These
are also termed as precoralloid roots. The precoralloid roots are adventitious in nature and emerge
from the hypocotyls below the cotyledonary petiole. As the precoralloid roots mature, they acquire
the cyanobiont that generally occupies the cortical zone. Some other changes such as replacement of
sheath layer by a dermal layer with scattered lenticels are characteristic of the maturation process.
i)
Cyanobionts
:
Most commonly observed cyanobionts are species of
Nostoc
or
Anabaena
but other
members such as species of
Spirulina
,
Oscillatoria
,
Rivularia
and
Calothrix
have also been reported.
Species of
Nostoc
are mostly found in the coralloid roots of many cycads such as
Bowenia
,
Cycas
,
Dioon
,
Encephalartos
,
Macrozamia
,
Stangeria
and
Zamia
(Figs. 11 to 13). Species of
Anabaena
, i.e.
Anabaena
cycadeae
and
A
.
circinalis
and
N
.
ellipsosporum
are present in the coralloid roots of
Cycas
(
C
.
revoluta
). Additionally,
Nostoc
sp. is characteristic of the coralloid roots of
Macrozamia lucida
whereas species of
Calothrix
can infect the coralloid roots of
Encephalartos hildebrandtii
. Norstog and
Nicholls (1997) also mention the names of
Anabaena macrozamiae
,
Nostoc cycadeae
,
N
.
punctiforme
and
N
.
commune
as endophytes of cycads.
Endosymbionts of
Zamia
were fi rst recorded by Schacht in 1853 in
Z
.
pumila
. Since then coralloid
roots or roots with nodules harbouring cyanobionts have been reported in various species of
Zamia
such as
Z
.
fl oridana
(Watanabe, 1924; Chaudhuri and Akthar, 1931; Stewart and Rodgers, 1977;
Lindblad, 1984),
Z
.
furfuracea
(Neumann, 1977),
Z
.
lindenii
(Spratt, 1915) and
Z
.
skinneri
(Lindblad
et al
., 1985).
ii)
Infection process
:
The invasion of coralloid roots by the cyanobiont can occur at the base, the apex
or in an intercalary position. The exact mode of entry is not known but it is generally believed that it
is through wounds or natural openings that the hormogonia invade the roots (Nathanielsz and Staff,
1975a; Ow
et al
., 1999). Nathanielsz and Staff (1975b) observed that the cyanobionts
Nostoc
or
Anabaena
occur both intercellularly and intracellularly in the cells of inner and outer cortex of apogeotropic
roots of
Macrozamia communis
. Algal invasion is preceded by mucus secretion by cells of algal zone
in the cortex and that is deposited in the intercellular spaces of cortical parenchyma. During the
invasion, an actual algal invasion front of fi nely granular mucus material bypasses mucus already
deposited in the intercellular spaces fi lling much of the cell cavity. This is followed by the entry of
large number of algal symbionts. The entry of the cyanobiont is facilitated by the contribution of
other microorganisms in the infection process. It has been noted that the primary infection of the
coralloid roots of cycads by
Bacillus radicola
stimulated their development. This is followed by the
entry of
Azotobacter
and fi nally
Anabaena
(Spratt, 1915). McLuckie (1922) concluded that infection
by soil bacteria contributed to the growth of coralloid roots in
Macrozamia spiralis
. The distortions
caused by bacteria or bacteroids in the developing coralloid roots facilitated the entry of
Anabaena
.
On the contrary, the presence of other microorganisms had a negative effect on the establishment
of symbiosis in cycads (Wittman
et al.
, 1965; Grilli-Caiola, 1975a).
