From simple green algae to complex flowering plants: As a new study shows, three evolutionary steps were decisive for this path. They all involved a single family of genes. These produce the so-called PIN proteins, which transport growth hormones within the plant. The distribution strategy has become so refined in the course of evolution that the plants first acquired the ability to form roots and shoots, then inflorescences and finally the actual flowers that they use for reproduction.
Flowering plants evolved from the naked samers about 135 million years ago, which includes, for example, conifers. Today they comprise around 350,000 species and thus make up 90 percent of all land plants. The main reason for their success was the development of a completely new reproductive strategy: flowers. The sensitive seeds can mature in a well-protected place and, in due course, spread widely. But how did these complex and diverse plant organs develop? The distribution of growth hormones, so-called auxins, within the plant is relevant for this. Their local concentration determines how the plant develops.
Narrow wall as a model plant
A team led by Yuzhou Zhang from the Institute for Science and Technology (IST) in Klosterneuburg, Austria, has now investigated the function and evolution of the so-called PIN genes that the auxin transporters produce. The researchers used the thale cress (Arabidopsis thaliana) as a model plant. In addition to the wild type, there are various genetically modified variants of this plant in which individual PIN genes have been shut down. Depending on which gene has been switched off, the plants can no longer produce flowers, have difficulty aligning their roots with gravity, or grow with a stunted or more branched shoot.
When roots and shoots develop, the concentration of auxins usually determines the direction in which the plant grows. PIN proteins ensure that there is an auxin maximum on one side, which ensures greater growth at this point. This one-sided growth enables plants, for example, to orient their shoot towards the sun and their roots towards gravity. However, if the PIN proteins are missing, no auxin maximum can form, so this mechanism is disrupted.
To determine exactly which PIN protein has which function, the researchers administered one or more of the proteins that were missing to the genetically modified plants. It was found that, from the family of the PIN genes, PIN1 is apparently specifically responsible for the shape of the flowers. When the researchers added other PIN proteins, an inflorescence was able to form, but not a functioning flower. In contrast, all PIN proteins were able to compensate for genetic problems in root formation. Their function therefore overlaps for this process.
Three evolutionary steps
But how did the PIN genes evolve? To find out, the researchers replaced the missing PIN genes from the field cress mutants with related PIN genes from other plants – from green algae to mosses to conifers. While the PIN genes from green algae were not able to compensate for the defects, the genes from mosses were at least partially able to help in the formation of roots and shoots. “This suggests that the PIN genes, which are responsible for the growth of roots and shoots, developed in terrestrial plants after they separated from green algae through evolution,” the researchers conclude.
However, the genes from mosses were not enough for an inflorescence to form. The PIN genes from higher plants such as conifers, on the other hand, proved to be suitable. According to the researchers, this points to a further evolutionary step in which further development of the PIN genes differentiates the mosses from vascular plants. The actual flower, on the other hand, could only develop if the researchers brought in genes from other flowering plants – and the better, the more similar they were to the narrow walls of the fields.
On the one hand, the researchers have identified an important evolutionary step that separates the flowering plants from the naked samers. “In addition, the results indicate a micro-evolution within the PIN1 gene, which has led to the development of diverse flower shapes,” the researchers write. “The PIN genes have allowed flowering plants to develop many different fertilization variants and to adapt to different habitats as they conquer the earth.”
Source: Yuzhou Zhang (Institute of Science and Technology, Austria) et al., Science Advances, doi: 10.1126 / sciadv.abc8895