TRENDS IN BIOSCIENCES 6-1, 2013 EDITION

30 Trends in Biosciences 6 (1), 2013
totipotency of plant cells confer flexibility to their
organogenetic programs. Observed that cells have to either
die (apotosis) or to differentiate and divide to undergo
morphogenesis patterns such as embryogenesis. It is now
assumed that the plant developmental program can give rise
to alternative pathways have detected the concept of
totipotent cells versus pluripotent cells.
In date palm callus cultures, three phases of
embryogenesis can be determined (i) the acquisition phase,
which is mostly under the influence of a strong auxin like 2,4-
D (100 mg/l), mostly in high concentration (Kackar, et al.,
1989; Yadav, et al., 2001). But also there are reports where low
level of 2,4-D (2 mg/l) were found to be equally effective
(Bhargava, et al., 2003; Zouine, and El-Hadrami, 2007; Taha,
et al., 2007). In majority of these reports BAP / Kn was one of
the cytokinin; (ii) the determination phase for embryogenic
pathway mostly depending on an auxin like NAA / IAA with
or without a cytokinin (2.5-10 mg/l, Bhargava, et al., 2003;
Zouine and El-Hadrami, 2007; Taha, et al., 2007), (iii)
differentiating phase where maturation and germination occurs
under the influence of a cytokinin/ auxin or only ½MS medium.
In present investigation, it looks not less than 15 days
when primary callus was produced under the influence of 2,4-
D + BA at places it was composed of meristematic zones
surrounded by parenchymatous cortex. However, uptill this
stage it was difficult to ascertain that whether these
meristematic structures would develop into shoots or
embryos. However, the presence of meristematic zones in
primary callus indicated the acquisition of competence, the
first stage of embryogenesis. Although in the callus induction
medium BAP (3 mg/l) was present but dominating role of 2,4-
D (10-100 mg/l) was evident. Auxins have many effects and
can modulated diverse processes and tropic responses.
Exogenous auxins are assumed to orient developmental
pathways and to favour either callogenesis or rhizogenesis.
In our experiments when primary competent callus was
transferred on to maturation medium, globular embryogenic
masses and embryos were observed within 20-25 days (Fig.
2d) due to hard well developed vascular supply. Absence of
phytohormones has been reported to favour embryo
maturation in various date palm tissue culture studies (Kackar,
et al., 1989 and Yadav, et al., 2001).
It is well established that 2,4-D plays a role in the
induction of somatic embryogenesis which is presumably
mediated by a signal cascade triggered by this exogenous
auxin (Zouine and El Hadrami, 2007).
Increasing the proportion of cytokinin in the hormonal
balance is thought to promote the expression of somatic
embryogenesis and later development of embryos. In present
investigation low level of cytokinin (BA, 3 mg/l) was more
effective in maturation of somatic embryos. Bhargava, et al.,
2003 have observed that both the level of 2,4-D in the callus
induction medium and level of cytokinin (0.25 mg/l) was very
important for normal somatic embryos induction. More high
was the level of these hormones, more was the number of
abnormal embryos.
Sane, et al., 2006 have also observed that application of
cytokinin (BA) promoted the appearance of meristematic
territories in the tissues whose successive divisions led to
polarization of the pro-embryo within 3-4 weeks of culture.
They undertook a detailed study of histological events during
somatic embryogenesis in date palm and has described
different stages: the division of perivascular cells on primary
explants, the secondary caulogenesis in primary, the
unicellular or pluricellular origin of the embryos and the
development of embryos into complete explants. All these
stages were observed in present study as well. Abnormal
embryos and precocious germination of embryos was also
observed in present study. This could be due to precocious
cell reactivation as suggested by Gueye, et al., 2009. As for
many monocotyledons, callogenesis is a prerequisite for the
initiation of somatic embryos and requires the presence of an
auxin in the medium (Davoodi, et al., 2002).
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Recieved on 04-12-2012 Accepted on 18-11-2012