Vegetative Phase Change and Shoot Maturation in Plants - Current Topics in Developmental Biology - page 133

slowly over several weeks in plants grown in short days to delay flowering
( Wahl et al., 2013; Wang et al., 2009 ). If quantitative variation in the abun-
dance of miR156 produces corresponding variation in SPL gene expression,
it is not difficult to imagine how continuous variation in the expression of
this miRNA could lead to multiple, apparently discrete leaf types. Prelim-
inary evidence for this scenario is provided by Acacia colei , where the abun-
dance miR156 and miR157 have been measured in individual juvenile,
transition, and adult leaves ( Fig. 5.1 ). In this species, miR156 and
miR157 are present at high levels in juvenile leaves, at intermediate levels
in transition leaves, and at low levels in adult leaves ( Wang et al., 2011 ).
In other species, miR156 expression has only been measured in entire plants,
in shoot apices containing multiple leaves, or in completely juvenile or adult
leaves, so it is unknown how tightly its expression is correlated with the
expression of the morphological traits that are used to define vegetative
phases. The precise relationship between miR156 levels, target gene expres-
sion, and leaf morphology will need to be assessed in order to determine if
this regulatory system is capable of generating the diversity of heteroblastic
patterns seen in nature. It is of particular interest to determine how this sys-
tem operates to specify intermediate developmental states. At present, the
juvenile phase is probably best defined as the period during which
miR156 expression is sufficiently high to completely, or nearly completely
suppress the production of SPL proteins.
6.2. The relationship between vegetative and
reproductive maturation
One of the most confusing issues in shoot maturation is the relationship
between vegetative and reproductive aspects of this process ( Jones, 1999 ).
Most plants only flower when they are in an adult vegetative phase, but the
timing of flower production varies widely relative to the timing of vegetative
phase change: some plants flower immediately after this transition, others
remain in an adult vegetative phase for a long time before flowering, and some
species routinely flower in a juvenile vegetative phase ( Brown, Ariati,
Murphy, Miller, & Ladiges, 2006; Hopper & Maslin, 1978; Potts &
Wiltshire, 1997; Wiltshire, Potts, & Reid, 1991; Zimmerman, Hackett, &
Pharis, 1985 ). Furthermore, genetic analyses of vegetative phase change and
floral induction indicate that these developmental transitions are inherited
independently in Eucalyptus ( Jordan, Potts, & Wiltshire, 1999; Wiltshire,
Potts, & Reid, 1998 ), Pisum ( Wiltshire, Murfet, & Reid, 1994 ), maize
( Abedon, Revilla, & Tracy, 1996 ), and Arabidopsis ( Telfer et al., 1997 ).