Have you ever wondered why the petals fall off your tulips after a week in a vase? Why trees drop their leaves in the autumn? Why fruit fall to the ground when ripe? And why farmers suffer losses when seeds are shed before harvesting time? My research group at the Department of Biosciences, University of Oslo may have found the answer. For more than fifteen years, my group and I have studied processes leading to detachment of plant organs, and the genes controlling these abscission processes.
Our main discoveries have come from studies of floral organ abscission in the plant mouse-ear cress (Arabidopsis thaliana among scientists). All over the world Arabidopsis, a small weed of the mustard family, is the favourite model for scientists studying plant development. All its genes are known, and mutants are available for most of them, which make it possible to discover their functions. We have recently used the knowledge we have acquired from research on organ shedding in Arabidopsis to show that the corresponding genes are present in most plants, including a large number of important crops.
Plants produce new organs like branches, leaves, or flowers regularly, but the shedding of organs, including fruits and seeds, is also part of a normal life cycle. An Arabidopsis plant loses its petals and other floral organs after they have served their purpose in reproduction: pollination has taken place (Figure 1A). But I discovered an Arabidopsis mutant that never loses its floral organs – (Figure 1B); even after the plants were completely dry the sepal, petals and stamen remained attached. To understand why, we have to look at the plant on a cellular level.
Whenever a plant loses an organ – be it a petal, a leaf, a seed or a whole flower – there has to be a separation of cells. There are specialised cells (abscission zone, AZ, cells) at the base of the organs to be shed (Figure 1C), and degradation of the matrix connecting cell walls of adjacent AZ cell layers leads to organ separation.
The mutant we had found defected in cell separation, and it was named inflorescence deficient in abscission, abbreviated ida (Figure 1B). My team identified the gene that was mutated and found that it encoded a small peptide that triggers the abscission process by turning on genes involved in the cell separation process (Figure 2). So why are peptides important? Over the last decade, scientists have found that secreted peptides act as messengers between neighbouring cells.
Peptides secreted from one layer of cells are recognized by receptors of adjacent cell layers, and this will trigger reactions that turn on sets of genes. In the case of IDA signalling, such genes encode enzymes responsible for cell separation. In general peptides are involved in the regulation of cell to cell communication related to all aspects of a cell’s life in a multicellular organism: cell division, cell expansion, cell differentiation, cell separation and cell death. Because peptide signalling is important in all aspects of plant life, my lab took the initiative to organise the first European Workshop on Peptide Signalling in Plants in Oslo in 2013. This initiative was welcomed and has been followed by a 2nd and 3rd workshop in Regensburg in 2014 and Ghent in 2015.
Agriculture faces major challenges to develop sustainable and affordable crop production in a world with growing populations and threatening climatic changes. Insights into the molecular details of cell to cell communication by peptides gained from Arabidopsis can likely be generalised. More specifically, our research suggests that IDA signalling is involved in cell separation processes in other plant species, although they may shed other organs than Arabidopsis. My group has found that IDA peptides and their signalling partners have been around for 175 million years almost unchanged, and today can be found expressed in falling autumn leaves of aspen, as well as in fruit, flowers or leaves of important crops like different kinds of beans, cabbage species, tomatoes, wine grapes, palm trees and citrus trees. Thus the IDA signalling system is ancient and has been preserved virtually unchanged during evolution.
In domesticated cereals, genes involved in seeds shattering are mutated. However, in many cultivated species uncoordinated or untimely abscission of fruits and seeds results in severe (10-25%) yield losses. I am therefore excited about my team’s recent findings. I started to study a pretty mutant. Now I aim to extend my research on peptide signalling and abscission to important crops. Our findings can help shape the future development of sustainable agriculture and horticulture.
Reidunn Birgitta Aalen
Department of Biosciences University of Oslo, Norway
Tel: +47 2285 4437