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 meristem transitions into an inflorescence meristem Inflorescence meristems produce small leaves and then produce floral meristems Flowers develop from the floral meristem

The Signal to Flower In order for flowers to develop two important genetic changes must occur 1. 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)

Regulation of Vegetative to Reproductive Transition 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

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 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

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

Genes Involved in Flower Development Divided into three classes – 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 meristem identity such as LEAFY, APETALA1 and CAULIFLOWER in A. thaliana FLORICAULA (FLO) and SQUAMOSA (SQUA) in A. majus LFY and AP1 promotes floral meristem fate and repress inflorescence meristem – genes that maintain inflorescence meristem identity such as the TERMINAL FLOWER gene in A. thaliana and CENTRORADIALIS (CEN) and AGL24 in A. majus

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

APETALA1 (APL1) Mutants Mutations in the APETALA1 gene do not affect plants as dramatically as mutations in LEAFY APETALA1 mutants have a partial inflorescence meristem phenotype where secondary floral meristems appear in the axis region of the sepal

CAULIFLOWER (cal) Mutants The CAULIFLOWER gene is very closely related to the APETALA1 gene 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)

TERMINAL FLOWER (TFL) Mutations Mutations in the TFL gene cause early flowering and premature conversion of the shoot meristem into a flower meristem

Floral organ patterning Homeotic mutations and ABC model Floral organ identity genes activate small sets of genes to specify floral organ identity Originally identified in A. thaliana and A. majus on the basis of mutant phenotypes

a-Wild type, b- ap2 mutant having carpels in place of sepals and stamens in place in petals, c- ap3 pi mutant having homeotic transformation of petal into sepals and stamens in to carpels, d- ag mutant having petals in place of stamens and sepal in place of carpels, indeterminate flowers continue to add new whorls of sepals and petals e- mutant flower for four SEPALATA genes (sep1sep2sep3sep4), look like whorls of leaf like organs, impaired A, B,C functions

ABC model Genetic analysis of A. thaliana homeotic mutants and in A. majus, led to the formulation of ABC model in 1991 ABC model postulates three regulatory genes functions- A, B and C work in combination to confer organ identity to each whorl A function- conferred by class A homeotic genes, specify sepal identity in first whorl A function combined with B, conferred by class B genes, specify petal identity in 2 nd whorl B function combined with C, conferred by class C homeotic genes specify stamen identity in whorl 3 C function alone specify carpels identity in whorl 4

A. majusA. thalianaP. hybrida Function A SQUAMOSA (SQUA) APETALA2 (AP2) APETALA1 (AP1) PhAP2A, PhAP2C, PhAP2C Function B DEFICIENS (DEF) GLOBOSA (GLO) APETALA3 (AP3) PISTILLATA (PI) PhGLO1, PhGLO2 (=PMADS2 o FBP3), PhDEF, PhTM6 Function C PLENA (PLE) FARINELLI (FAR) AGAMOUS (AG) pMADS3, FBP6

ABCE model Class E floral homeotic genes In A. thaliana class E SEP gene encodes related proteins required to specify petals, stamens and carpels as triple mutant contain only sepals (sep1 sep2 sep3) In sep quadruple mutants show conversion of all four floral organs in to leaf like structures with some carpeloid character FBP2, FBP5 in P. hybrida specify petal, stamens and carpels identity

Ovule identity genes: A further class of floral D function genes- confers ovule identity on tissues Co-suppression of FBP7 and FBP11 genes causes the replacement of ovules with carpel like structures in P. hybrida In A. thaliana ovule identity is conferred by a clade of four closely related genes- AG, SEEDSTICK (STK), SHATTERPROOF 1 and 2 (SHP1, SHP2) Ovules are converted into leaf like or carpel like organs in stk shp1 shp2 mutants

ABCE Model

Mechanism of floral- organ identity gene function Floral organ identity genes encode transcription factors AP2/LIP1/2 are members of plant specific Ap2/ERF (Ethylene responsive element binding factor) family of transcription factors All other genes including SEP, STK, SHP encode MADS domain that is conserved among eukaryotes The floral MADS domain proteins form dimers to bind to their ‘CArG (CC(A/T)6 GG) box’ DNA target sequence The class B proteins AP3/DEF and PI/GLO bind DNA only as heterodimers, and both components are required to maintain transcription from their own promoters

Quartet model for formation of MADS domain protein regulatory complexes