The shape of leaves is a very noticeable trait. Leaf shape is controlled by environmental
and genetic programs as well as hormones. Some species such as tomato contain compound
leaves, while others such asArabidopsiscontain simple, nonlobed leaves. Cell death within
leaf primordia in plants such as philodendron produce “holes” in leaves. Some leaves such
as pea also contain tendrils that function in “grasping” surrounding structures in the
environment and facilitate directional growth. Corn leaves contain specialized domains
called thesheath,blade, andligule, which also facilitate growth by providing a way to
change the position of the leaf surface, ensuring that photosynthetic tissues get maximal
exposure to light.
Maize has been especially useful as a model plant to study leaf development. Theknotted
(KN1) gene, which is the related to theshoot meristemless gene(stm), mentioned in the dis-
cussion on shoot apical meristem development, was first identified in corn mutants that con-
tained knots of tissue on their leaves. TheseKN1mutants were defective in the normal
regulation of theKN1gene, which would normally be confined to the apical meristem.
Instead,KN1mutants containKN1expression in the leaves, which results in an aberrant
mass or knot of tissue. The cornKN1gene was ectopically expressed in transgenic tomato
plants to investigate the role of this homeodomain transcription factor in dicot leaf develop-
ment. The results were transgenic tomato plants containing an increase in leaf complexity.
Recall that most tomato species contain compound leaves with several leaflets. Ectopic
expression of the cornKN1gene caused a large increase in the number of leaflets per leaf,
suggesting that in dicots,KN1can alter leaf complexity specification (Fleming 2006).
4.5 Flower Development
4.5.1 Floral Evocation
Flowersare plant’s most obvious and aesthetically pleasing organ. In general, all flowers
are specified in a similar manner. For flower development to occur, vegetative meristems
must first undergo a transition to produce theinflorescence meristem. These meristems
are self-renewing and also give rise to the floral meristems that produce flowers. The
termfloral evocationrefers to the process of inflorescence meristem commitment. This is
controlled by many factors, including plant size, whether a cold season has passed (vernal-
ization), environmental stress, and daylength. For example, short-day plants such as cockle-
bur and Christmas cactus require a minimum light period (,15h) to flower. Only one
inductive period of light is needed to block flowering in many short-day plants. In contrast,
long-day plants, such asArabidopsis, require a longer period of light (usually 12–16h) to
flower.Arabidopsisis also considered to be a long-day facultative plant, as it can flower in
short-day conditions but will flower much faster if placed under long-day conditions.
Daylength-neutral plants, such as tomato, are not as affected by the photoperiod.
After floral evocation has taken place, a plant can be moved to noninductive conditions
and still flower. Many historical studies have suggested that a hormonal factor, termedflori-
gen, is produced elsewhere in the plant, such as the leaves, and then stimulates floral evoca-
tion in the meristem. Trying to determine the identity of florigen has been a focus in plant
biology for years because of its importance in agriculture. Flowers are the precursor of fruit,
and if flowering can be controlled, plants can be manipulated to remain in a vegetative or
flowering state. Accelerated flowering could lead to a much shorter growing season, which
would be advantageous for growers.
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