Question

Describe the various ways flowering is triggered in plants, how different parts of the flower are formed, and how the flower eventually dies and goes away.

Answer 1

Genetic analysis have identified two classes of genes that regulate floral development:

floral meristem identity and floral organ identity genes

1) The transition from shoot vegetative meristem to floral meristem requires floral meristem identity genes that both specify the floral organs and cause the termination of the production of stem cells. This group of genes include LEAFY (LFY), APETALA(AP1), and CAULIFLOWER (CAL).

2) floral organ identity genes directly control floral identity. The proteins encoded by these genes are transcription factors that lightly control the expression of other genes, whose products are involved in the formation and function of floral organs.

In ABC model, discovered in Arabidopsis specifies the molecular basis of organ development vin flowers. It includes 5 genes APETALA1 (AP1), APETALA2 (AP2), APETALA3 (AP3), PISTILLATA (PI) and AGAMOUS (AG).

The A class genes- AP1, AP2 specifies the sepals and petals development.

The B class genes- AP3 and PI specifies the petal and stamen development.

The C class gene- AG specifies stamens and carpel development.

Figure- ABC model of flower development.

Also the transition to flowering is regulated by multiple signals and multiple pathways which includes photo period, autonomous, vernalization gibberellin. All these pathways converts to regulate the meristem identity genes LFY and AP1.

The photo period and vernalization pathways mediate the response to environmental cues and autonomous and the gibberellic acid pathway act largely independently from the external signals. The CONSTANS (CO) gene plays an important role in photoperiod pathway. Both light and internal clock precisely regulate the accumulation of product of CO gene.

A central gene in the autonomous and vernalization pathways is FLOWERING LOCUS (FLC), which represses several locci size that promotes flowering. FLC repression of flowering is relieved by epigenetic switch that replaces the expression of FLC itself via chromatin remodeling. FLC repression can occur after plants experience and extended period of cold ( vernalization) or, alternatively, by the autonomic pathway, which senses plant age and ambient temperature.

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