The huge variety of flowers we know has made them seem very complex, but there may be a relatively simple reason why they look so different.
A team led by Yuzhou Zhang of the Institute of Science and Technology Austria has found that a single family of genes, known as PIN-FORMED (PIN) auxin transporters, is important in helping multiple plant organs grow – and could be largely responsible for the origin of flowering plants.
These genes appear to have experienced three major evolutionary events, which likely contributed to the vast array of modern plant shapes, they explain in a paper in the journal Science Advances.
Determining this was a multi-step process. The team first introduced mutant genes into Arabidopsis thaliana, a plant commonly used in research. They found that the genes regulated the growth of the stem, stalk and flower clusters and, in particular, the formation of flowers. Plants suffered without them.
To check exactly how this happened, they traced the amount of auxin – an important plant hormone – in plants with and without PIN genes. They found that the PIN proteins helped move auxin to specific parts of plant organs, such as the root tip, to help them grow best.
Arabidopsis is an angiosperm: one of a group of flowering plants that evolved from non-flowering plants about 135 million years ago and now accounts for about 80% of living plants. Until recently, how and why these plants started flowering was relatively unknown.
To understand this, Zhang and colleagues put Arabidopsis PIN genes into other plants to see if they could still function.
First, they established that root PIN genes worked in almost every plant group except for algae. This meant that the genes were old and had occurred very early in evolution when plants began evolving to live on land.
Then they traced which plants could use PIN genes to form the inflorescence: the cluster of branches on which flowers or seeds will grow. These genes worked in vascular plants, which have a xylem to transport water from their roots up to their leaves. Vascular plants include plants that grow flowers and cones.
Last, they found that the flower-controlling PINs worked only in angiosperms to control the shape and number of petals and pollen-making organs.
Hence three potential evolutionary events gave PIN genes three functions. First, they evolved to help land plants grow roots. Second, they evolved a function in vascular plants to influence the arrangement of flowers and seeds. Finally, they evolved a third function to change the shape of flowers.
Altogether, this means that PIN genes may have been a major reason why we see so many different roots, stems, petals and flower arrangements. PIN genes in angiosperms had all three organ development functions, but other plants had only one or two.
“Therefore, we proposed that this extensive diversification of PIN molecular properties and their different expression patterns enables the PIN gene family to acquire a variety of developmental roles in flowering plants, and thus contributes to the establishment of their complex architecture,” the authors write in their paper.
However, further genomic and bioinformatic data is needed to determine the exact evolutionary history of this diverse gene family.
Dr Deborah Devis is a science journalist at The Royal Institution of Australia.
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