Helping plants take root at molecular level

By researching what makes plants easy to root at the molecular level, researchers in Sweden are hoping to discover new ways of helping plants grow better. Professor Catherine Bellini from the Umeå Plant Science Centre at Umeå University explains the possible impact unravelling molecular mechanisms that regulate adventitious root formation will have.

Professor Catherine Bellini and her group from the Umeå Plant Science Centre have been attempting to dissect the molecular mechanisms that control the initiation and development of adventitious roots, a limiting step in plant vegetative propagation, using the plant model system Arabidopsis thaliana.

They hope to then be able to transfer this knowledge to better understand what differentiates easy-to-root from difficult-to-root plant genotypes, which is something that is barely understood currently and is of great interest to those using vegetative propagation (a form of asexual reproduction) for commercial purposes.

“My interest in the subject began back when I was working on the regeneration of protoplasts”, says Bellini. “These were plant cells that had been isolated from leaves of plants such as tobacco and tomatoes, and I was quite fascinated by the plasticity of the cells and the fact that we could regenerate an entire plant from just one of them.”

“I noticed at this time, however, that this regeneration process was not the same every time – in some species there were genotypes that were regenerating much easier than others and it was difficult to understand why this was happening.

That is when I started my work in developmental biology with Arabidopsis thaliana, and later on focussed on the developmental processes involved in the creation of adventitious roots.”

Adventitious roots are a specialised type of root that develop from the shoot rather than from the primary roots, and can serve a variety of different roles in plants.

In some species, such as strawberries and blackberries, vegetative propagation naturally occurs from the stems, and it is adventitious roots that regenerate to anchor the new plants to the ground. The ability of plants to multiply vegetatively is exploited commercially, often to speed up the selection process of favourable traits.

Breeding programs in particular for long lives species such as eucalyptus and poplar trees rely on these methods. The study of adventitious roots also provides a good model for studying the de-differentiation of cells, which is the process by which specialised (or differentiated) cells become less specialised or reprogram themselves, and the redifferentiation, which is when the cells acquire a new fate after having returned to their original undifferentiated state.

“While studying Arabidopsis, we managed to isolate some mutant lines that showed defects in the development of adventitious roots, and which also had an altered response to growth factors such as phytohormones and so we became very interested in identifying these genes so as to dissect the molecular mechanisms involved in the regulation of some of these dvelopmental processes.”

Arabidopsis mutant plants often display extreme phenotypes, with some mutants creating these adventitious roots much more easily than others, and so it is an excellent plant model to use for trying to identify the regulatory network that underlines the development of these roots. There are still a lot of gaps in the understanding of how different factors affect this developmental process, so this work is contributing greatly to a field where a lot is unknown.

By performing proteomic analyses of Arabidopsis mutants that are affected in their ability to develop these adventitious roots, Bellini is able to identify associated molecular markers that can be used to select genotypes for their rooting ability as well as gain a deeper insight into the molecular mechanisms that control the rooting. One of these mutants, for example, is altered in the expression of a gene and this causes hypersensitivity to light and destabilises the plant’s auxin homeostasis, a phytohormone known to affect the formation of adventitious roots. The overall effect of this is that these mutants are barely able to form the roots. However, this is only one of many genes that Bellini believes affects the development of the roots, and there are still many that have not been identified.

Adventitious rooting is an essential step in the vegetative propagation of economically important plants, as it allows clonal propagation and rapid fixation of superior genotypes prior to their introduction into breeding programs. Consequently, any problems that occur when rooting cuttings can incur serious losses to companies involved in this field, which makes Bellini’s work of considerable interest to them.

Catherine explains what impact she hopes her research will have on some practical applications. “I think that a few years down the line this work will almost certainly have an effect on budding strategies in the forestry sector, and also particularly for fruit trees and horticultural plants that are vegetatively propagated.”

However, this is one of those projects in which every answer ends up asking at least two new questions, and so we are still trying to add pieces to the puzzle.”

An example of this is the understanding of the role of the phytohormone auxin, which is used to induce the development of adventitious roots. Bellini and her team have, through careful analysis, managed to uncover part of the process by which this happens. Auxin acts to inhibit another molecule, that itself has an inhibitory effect on the rooting, thus allowing the development of the root to occur. But the signalling pathway is still not completely understood, and Bellini estimates it will be another few years before they have all the data.

As for the future, Bellini believes that her and her team will be working on the subject for the next 10 years to try and piece together the remaining molecular mechanisms that are not yet fully understood. Hopefully this will allow for a greater comprehension of this aspect of botany and provide greater reliability and open new opportunities in the relevant fields.

Click here to access the Umeå Plant Science Centre website.

Published: Tuesday, 8th November 2011