Biology Reference
In-Depth Information
on the species, GA promotes or suppresses vegetative phase change and
flowering (
Zimmerman et al., 1985
). For example, exogenous GA promotes
rejuvenation in English ivy (
Rogler &Hackett, 1975
) and
Acacia melanoxylon
(
Borchert, 1965
), but accelerates vegetative phase change and flowering in
maize (
Evans & Poethig, 1995
) and
Arabidopsis
(
Telfer et al., 1997; Wilson
et al., 1992
). GA has no effect on the expression miR156 in
Arabidopsis
,
but promotes the expression of some
SPL
genes (
Galvao, Horrer,
Kuttner, & Schmid, 2012; Jung et al., 2012; Wang et al., 2009; Yu et al.,
2012
). This response contributes to GA-mediated flowering under SD con-
ditions because plants overexpressing miR156 (i.e., plants in which
SPL
expression is suppressed) are less sensitive to GA than wild-type plants
(
Yu et al., 2012
). The extent to which GA promotes vegetative phase change
via its effect on
SPL
expression is less clear because this hormone has nearly
the same effect on vegetative phase change in wild-type plants, plants over-
expressing miR156, and plants doubly mutant for
spl9
and
spl15
(
Schwarz
et al., 2008
). In this respect, it is interesting that GA levels do not increase in
the SAM within the first 2 weeks after germination (
Eriksson, Bohlenius,
Moritz, & Nilsson, 2006
); GA would be expected to increase during this
period if it played a major role in vegetative phase change.
Evidence for the involvement of carbohydrates in vegetative phase
change has been accumulating for over 100 years. Goebel (
Goebel, 1900
)
was the first to propose that nutrients play a crucial role in this transition,
and he performed many experiments to test this hypothesis (
Goebel,
1908
). A particularly comprehensive analysis of the effect of specific nutrients
on leaf morphology was conducted by Allsopp using the water fern
Marsilea
drummondii
(
Allsopp, 1954, 1963
). Allsopp showed that plants grown in the
absence of exogenous sugar produced only simple, juvenile leaves (
Allsopp,
1952
), and found that supplementing the growthmediumwith any of several
metabolizable sugars accelerated the production of adult leaves (
Allsopp,
1953b
). Adult shoots transferred to a medium lacking exogenous carbohy-
drates reverted to the juvenile form (
Allsopp, 1953a
). Similar results have
been obtained with isolated shoot apices (
Feldman & Cutter, 1970;
Njoku, 1971
), and isolated leaf primordia (
Sussex & Clutter, 1960
) cultured
on media containing varying amounts of sugar.
Recent studies in
Arabidopsis
suggest that the striking effect of sugar on
heteroblastic development is attributable to its effect on the expression of
miR156 (
Yang et al., 2013; Yu et al., 2013
). Mutations in the chlorophyll
biosynthetic gene,
ch1
, as well as several other yellow-green mutations, pro-
long the juvenile phase (
R¨bbelen, 1957; Yang et al., 2013; Yu et al., 2013
).
