Flower DevelopmentBIO 274-01Plant MorphologyFall 2006Discussion 9

Flowering Plants• Flowering plants (angiosperms) have two

growth phases

– vegetative growth, production of stems and leaves, occurs at the apical meristem, indeterminate growth

– flowering phase, production of organs for sexual reproduction, occurs at the floral meristem, determinate growth

From Vegetative to Reproductive Growth

• The above ground vegetative growth of a plant develops from the apical meristem

• The plant maintains a vegetative growth pattern until the apical meristemtransitions into an inflorescence meristem

• Inflorescence meristems produce small leaves and then produce floral meristems

• Flowers develop from the floral meristem

Flower Anatomy

•In general, angiosperms have a flower consisting of four floral organs arranged in four whorls with sepals on the outside and carpels in the innermost whorl

–Sepals–Petals–Stamens (produce pollen)–Carpels (contain ovary and ovule)

Flower Structure•The floral meristem differentiates into four groups of cells that form the four parts of the flower

•Whorl 1 – develop into sepals

•Whorl 2 – develop into petals

•Whorl 3 – develop into stamens; male reproductive organs

•Whorl 4 – develop into carpels; female reproductive organs

The Signal to Flower• In order for flowers to develop two important

genetic changes must occur1. Change from vegetative to floral state

• If this genetic change does not happen then flowering will not occur

2. Commitment to form flowers

• Signals that indicate its time to flower– Maturity of the plant– Temperature– Photoperiod (for most but not all plants)

•Regulated by internal and external factors that determine whether the meristem will produce vegetative or reproductive structures

•Signals that promote or inhibit flowering can move from the roots, cotyledons or leaves to the shoot apex

•Once the signal is received meristem competence determines if the plant will respond to the signal

Regulation of Vegetative to Reproductive Transition

Conversion of the Apical Meristem to a Floral Meristem

• Leaves produce a chemical signal called florigen

• This signal is transmitted to the apical meristem and the conversion to a floral meristem begins

• Have not completely identified the chemical nature of florigen

• One component is mRNA encoded by the gene FLOWERING LOCUS T (FT)

• FT protein translated from mRNA binds to and activates transcription factors in the nucleus of the meristem

• Activates LEAFY (LFY), which then turns on the expression of genes needed for flowering

Photoperiod•Photoperiod is detected in the leaves

•Even if only one leaf is exposed to the correct photoperiod the entire plant will bloom

•The plant in the example given below needs at least 8.5 hours of darkness to flower

Question 1

• Will a plant that has a mutation in its FT gene (no FT protein produced) flower? Explain your answer?

Genes Involved in Flower Development

• Divided into four classes

– Flowering time genes – mutations in these genes cause early or late flowering

– Meristem identity genes – specify flower meristem identity and maintain the inflorescence meristem

– Flower organ identity genes – determine fate of organ primordia; genes of ABC development model

– Late acting genes – control ovule development

Meristem Identity Genes

• This class includes

– genes that specify flower meristemidentity such as LEAFY, APETALA1and CAULIFLOWER

– genes that maintain infloresencemeristem identity such as the TERMINAL FLOWER gene

LEAFY (LFY) Mutants• mutations in LFY typically cause a partial to

complete transformation of flowers into shoots

• normal flowers are replaced with structures that look like inflorescence meristems and shoots

• early developing flower structures resemble inflorescence meristems

• late developing flower structures have sepal and carpel like structures but do not have petals or stamens

Question 2

• Studies and observations of LEAFY mutants have suggested that this gene has two main functions. What do you think these functions are?

APETALA1 (APL1) Mutants•Mutations in the APETALA1 gene do not affect plants as dramatically as mutations in LEAFY

•APETALA1 mutants have a partial a partial inflorescence meristemphenotype where secondary floral meristems appear in the axis region of the sepal

CAULIFLOWER (cal) Mutants•The CAULIFLOWER gene is very closely related to the APETALA1gene

•cal mutants look like the wild-type plant

•In order to see the effects of this mutation you must have a mutation in ap1 gene also

•Mutations in the CAL gene greatly enhance the phenotype of ap1 mutants resulting in the massive proliferation of inflorescence-like meristems (the “cauliflower”phenotype)

Question 3

• Both CAL and AP1 are MADS-box genes. What are MADS-box genes?

TERMINAL FLOWER (TFL) Mutaions

• Mutations in the TFL gene cause early flowering and premature conversion of the shoot meristeminto a flower meristem

Question 4

• Which gene do you think is most important in flower development, flo, LFY or TFL?