How petals shape up

Have you ever marveled at a summer meadow in full bloom? And maybe even wondered how it is that flowers and their petals come in so many beautiful and different shapes?

Well, new findings published in PLOS Biology reveal a hidden map within growing petals that directs how they are shaped as they develop. And what’s really intriguing about this work is that a similar hidden map is known to also direct how leaves take shape, revealing that plants use a similar strategy to shape two  plant organs with very different functions  – one attracts pollinators, while the other is the main site for photosynthesis and respiration.

Enrico Coen and colleagues have been investigating for several years now how it is that plant buds form the diverse flower and leaf shapes that we see around us.  In an earlier study, this team uncovered the existence of a so-called polarity field in the developing leaves of the plant Arabidopsis. This field, they discovered,  converges towards the tip of the leaf and determines rates of cell growth both parallel and perpendicular to this field. In essence, it orients patterns of growth so that they give rise to the characteristic shape of Arabidopsis leaves: elliptical and narrow towards the tip.

Arabidopsis petals by contrast aren’t pointed at their tips but are rounded in shape rather like a paddle. To find out how petals acquire this shape, Coen and his team turned to both experimental approaches and computer modelling. First, they measured and analyzed petal shape and the patterns of growth in developing petals. They then compared different computer models to see which would most accurately predict the patterns of growth and shapes they’d seen in real petals. The first simulation they tried used a polarity pattern similar to that found in the leaf; however this model (the “convergent model”) didn’t produce virtual petal shapes that matched real ones.  So instead, the researchers tested a different polarity pattern – the “divergent model” – in which the polarity field fans outward. This model produced virtual petals that looked much more like the petals of Arabidopsis flowers.

In both organs, what helps to form this hidden polarity map is the plant hormone auxin, which regulates plant growth and is transported around plant cells in specific directions. This directional movement of auxin, called polar auxin transport, is determined by where PIN proteins  – which move auxin out of plant cells – are located in cell membranes. In their PLOS Biology study, Coen and colleagues found that the location of PIN1 in petal epidermal cells indicates that auxin is transported in many directions in a petal and not only towards the tip, as occurs in leaves – this contributes to a different pattern of growth and ultimately to a different shape. They also show how a protein called Jagged has a key role in promoting increased rates of growth at the tips of petals and how it also influences the extent of this divergent polarity field.

So together these findings show that plants can create differently shaped organs by altering a common, auxin-based map that determines where and in what direction different rates of growth occur. And that in turn helps to answer the larger question of how it is that plants shape their different tissues despite their cells being unable to escape the constraints of the plant cell wall and move around to create new shapes (as happens in animal cells). It partly comes down to growth – where it occurs and how quickly – and how that growth can be influenced by an underlying polarity field to sculpt the complex shapes that attract the bee and please the human eye.

Susanna Sauret-Güeto, Katharina Schiessl, Andrew Bangham, Robert Sablowski, Enrico Coen (2013). JAGGED Controls Arabidopsis Petal Growth and Shape by Interacting with a Divergent Polarity Field PLOS Biology, 11 (4) : 10.1371/journal.pbio.1001550