How Does a Flower Come to be?

This is a re-post of a piece I wrote for the All Under One Leaf, the blog of the UK Plant Science Federation in August 2016. It is reposted here with permission.

Death Valley, California super bloom, spring 2016. Photo by Nat Prunet.

A flowering world

There are around 369,000 known flowering plant species on Earth today, by far the most numerous group of plants living on Earth by an order of magnitude. The next largest group is the ~15,000 species of bryophytes. Humans are 100% reliant on flowering plants for food, medicines, wood, air, culture, and our environment.

What makes a plant decide to flower?

There is a network of genes at the center of when a plant flowers. Much of what we know is from studying the reference plant Arabidopsis thaliana. As more plant genomes are sequenced, genes similar to those in Arabidopsis have been found. Part of the answer to the astonishing diversity of plants is that while the blueprint is similar, many different shapes can be created. That blueprint has also had 140 million years of copying itself with slight changes occurring each time molded by specific environments. As Dr. Caroline Dean, Professor at the John Innes Centre– recently named a Dame of the British Empire– told me by email about some specific components of the flower time program:

There are MADS box proteins and FT homologues across all species studied so far – but they appear to function in somewhat different ways“.

The blooming vote

As multicellular, complex organisms, flowering is a systemic decision for plants. Plant cells communicate with neighboring cells and through mobile signals such as proteins, RNA, and small molecule hormones to other parts of the plant. Recent research has started exploring what flowering time genes are expressed in roots, concluding that roots seem to contribute to the flowering decision by communicating sugar/nutrition status.  

Plant cells file a vote based on what they see as their environment– and each cell does see a slightly different environment. If a leaf cell interprets cues it gets as good for flowering, it activates and transmits a signal called florigen (the protein product the FT gene) that moves to the shoot apical meristem(s)- a small group of cells that buds off the visible parts of a plant like leaves– that it is time to stop making vegetative tissue and flower. I asked Dr. Dean about the voting analogy applied to the vernalization/cold treatment period some plants need to flower that silences the MADS box family protein FLC, a repressor of FT and therefore flowering:

“My guess is yes – each cell switches FT on (as FLC is shut off) and as that level quantitatively increases throughout the plant the chances of the meristem switching to a floral fate increases”.

Shoot meristem(s) are the ballot box. The flowering decision of enough cells transitions the meristem from vegetative (leaf production) to an inflorescence meristem (ready to produce flowers) that then generates floral meristems that produce the actual flowers of the plant.

The result of the vote, an Arabidopsis inflorescence meristem (middle) with various stage floral meristems budding (peripheral buds with the fluorescent colors representing 2 genes turned on during flower development. Image by Nat Prunet. Source.

Complex Biological Switches

Flowering time timing involves epigenetics, the selective regulation of genes via how their DNA is packaged making the gene more or less accessible to cellular machinery that expresses genes. After germinating, plants don’t enter into flowering straight away (though some flowering plants do flower quickly– see the super bloom photo above) and plants have evolved ways of temporarily silencing genes that promote flowering program. Based on cues from developmental stage, day length, temperature, nutrient status, etc., reversing the epigenetic silencing activates FT and promotes flowering while the flowering repressor FLC is silenced. It is remarkable that plants coordinate themselves so well, as Dr. Dean told me when I asked if she had a favorite flowering natural history:

Just how synchronously plants from the same species flower in spring – each species has an optimal time which depends on that year’s environment”.

Flowering is an amazing process and scientists still have questions about how plants decide. One is that plants can sense cold, though just how is not fully understood. Dr. Dean told me one frontier in the flowering time field is

“How plant cells are registering they have had enough cold”.

Flowers are an evolutionary success story in a fiercely competitive natural world. Think how incredible it is that each species of flowering plant goes through this decision to be able to reproduce to create the seeds of the next generation.

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