Biology Reference
In-Depth Information
Although these observations suggest that vegetative phase change and
floral induction may have little to do with each other, they are readily
explained by the architecture of the regulatory pathways that control these
transitions. Genetic analyses of flowering time in
Arabidopsis
have shown that
floral induction is regulated by multiple inputs (“pathways”) that regulate
the expression of genes involved in the transformation of the vegetative
meristem into an inflorescence meristem (reviewed in (
Amasino, 2010;
Wellmer & Riechmann, 2010
)). In
Arabidopsis
, these inputs include photo-
period, prolonged cold temperature (vernalization), ambient temperature,
gibberellic acid (GA), sugar, the autonomous pathway, and the vegetative
phase change pathway. The ability of a plant to respond to any one of these
pathways depends on the state of the other pathways. A good example is the
interaction between the vernalization and photoperiod pathways. Under
laboratory conditions—but not necessarily in the field (
Wilczek et al.,
2009
)—plants that contain functional alleles of
FRI
and
FLC
require several
weeks of exposure to cold in order to respond to floral inductive long-day
(LD) conditions; that is, in the absence of a cold treatment, the vernalization
pathway overrules the photoperiod pathway. On the other hand, vernalized
plants only flower early if they are grown under LD, demonstrating that ver-
nalization by itself does not induce flowering. Rather, it creates a permissive
state in which other factors can operate.
The vegetative phase change pathway resembles the vernalization path-
way in the sense that it regulates the competence of the shoot to respond to
various conditions that promote flowering rather than by directly mediating
flower production. High levels of miR156 presumably set a threshold that
buffers fluctuation in the abundance of SPL transcripts, preventing prema-
ture floral induction. Evidence that this pathway impacts reproductive com-
petence was initially obtained in maize, where it was found that
Teopod2
—a
hypermorphic mutation of miR156—delays the photosensitive period for
floral induction (
Bassiri, Irish, & Poethig, 1992
). In
Arabidopsis
, over-
expressing miR156 delays flowering, whereas reduced levels of miR156
accelerate flowering under LD (
Schwab et al., 2005; Schwarz et al., 2008;
Wang et al., 2009
). However, plants with reduced levels of miR156 do
not flower earlier than normal in a noninductive short-day (SD) photope-
riod (
Wang et al., 2009
). These results suggest that miR156 represses
flowering early in development when it is highly expressed, and that the
decline in its expression creates a permissive state for floral induction. During
the adult phase, floral induction is dependent on other factors—such as pho-
toperiod, vernalization or GA signaling—which regulate the transcription of
