The hormonal bond in Plants: Auxin supply from Mother to Offspring

Fertilized plants increase the supply of auxin, a plant hormone, to seeds, allowing the growth of embryos   


                      

Localized Auxin production (green), transport (blue), and signaling (red) in the Arabidopsis embryo 2 

by Dr. Hélène Robert (Cover image: Robert et al. 2015)



Auxin: Source of Life


During pregnancy, the bodies of the mother and the infant in her womb are intertwined. Biological factors like enzymes and hormones released by the mother crucially influence the infant’s development - a well-known fact about animal reproduction (mainly mammals).


Some scientists wonder: what about plants?

Do plants also supply hormones

to their offspring besides nutrients, similarly to mammals?


Over the past decades, plant scientists identified five major plant hormones produced in plant cells: auxin, cytokinin, abscisic acid, gibberellins, and ethylene. Among them, auxin plays a major role in plant growth.


Produced in the stem, bud, and root tips, auxin travels throughout the plant to stimulate plant growth via cell elongation and division. It also directs the plant to grow towards the direction of sunlight 4.


In short, auxin is fundamental to the plant’s survival.


Zygote inside a seed  (Photo by Dr. Hélène Robert)


Only recently, scientists noticed the presence of auxin in the seeds (maternal tissue) for the development of the plant embryos (offspring), the parts inside of fertilized plant ovules (seeds) that grow into new plants.


Similar to an animal embryo, a plant embryo starts as a single fertilized cell (zygote) and grows up by dividing itself, thus forming the future plant body 5, 6.

By testing on Arabidopsis thaliana, a type of plant similar to mustard and cabbage, past studies recorded that auxin is present in plant embryos throughout the patterning process 6, 7, 8.

However: what triggers the maternal auxin supply? How does it flow from the maternal plant, the seeds, to the embryos? And how important is this maternal auxin to plant embryo growth? These questions remain mainly unanswered.


Dr. Hélène Robert and her research team from CEITEC have been collaborating with plant scientists from Germany and Austria in search of those answers.

After a relentless effort, they find out that the act of fertilization triggers an increase of auxin production in the embryo attachment region of the seed coat, a protective seed shell covering the embryo.

This maternally produced auxin enters then the embryo. Moreover, auxin of maternal origin is particularly crucial to early embryo development 1.


                      

Dr. Hélène Robert and her favorite Arabidopsis plants (Photography by Emil Gallík)



Auxin In The Embryo: How Does It Get Here?


To identify the trigger of maternal auxin production, researchers observe auxin-sensitive reporters (that monitor the levels of auxin in the tissue) inside fertilized seeds.

It is shown that these reporters give out strong signals from the moment the seeds are fertilized.

Noticeably, these signals are observed from self-pollinating plants, confirming that it is not external factors such as pollination by wind and bugs, but the act of fertilization itself that triggers an increase in maternal auxin production.


Direction of maternal auxin produced by TAA1 and YUC (green) enzymes to the embryo (blue) (Source: Dr. Hélène Robert)



The reporters give out strong signals in 2 opposite regions of the seed coat: the chalazal region connecting the seed to the fruit; and the embryo attachment region (green part), where the embryo (blue part) connects to the maternal seed coat.


Moreover, in an effort to pinpoint the source of the increasing auxin found in the seed coat and the embryo attachment region, plants with location-specific defective auxin pathways are examined.


The reporters give no signals in both the chalazal region and the embryo attachment regions, confirming that auxin detected in these 2 regions after fertilization is originally produced from the maternal seed coat.



A Mother Cannot Be Replaced


The connection between the maternal seed coat and the embryo can also help to answer these questions:


Will an embryo grow with auxin of alternative sources? In other words, is maternal auxin indispensable?  


By cutting off the maternal auxin source, the embryos with no import of maternal auxin become either defective or impoverished. What’s more, when the researchers executed crosses between plants so that only the embryo- but not the mother- synthesize auxin, similar embryological defects are witnessed. Collectively, the results reveal that maternal auxin is essential to the growth of a healthy plant embryo.


.....



The Growing Importance of Plant Research


Although the result of this study clarifies the role of auxin in seed development shortly after fertilization, it also raises more follow-up questions.

For example, while embryos become deformed or underdeveloped without maternal auxin, they still somehow manage to grow a little- what are these unknown factors contributing to the development?


Dr. Robert is currently working on identifying them -

a task that is very challenging: interfering with one factor

can affect the others, causing a domino effect

capable of killing the plant in the process.



The result of this study builds a foundation for Dr. Robert and her team to understand the impacts of abiotic stressors, such as extreme temperature, on plant reproduction.


The unseen stress on plants (Credit: Adrien King on Unsplash)


Similar to pregnancy in animals, stress from the surrounding environment can influence the quality, as well as the quantity, of maternal hormones supplied to the plant embryos.

Unlike human and animals, plants cannot escape when the weather turns unfavorable. This is why drastic environmental changes (e.g. global warming) will negatively impact the quality and quantity of vegetation on our planet first and foremost.

A decreasing amount of flora diversity on our planet may have disastrous outcomes on the global ecosystem.

The continuing efforts from Dr. Robert and her fellow researchers might help in uncovering ways to minimize the damages. Although a long and convoluted journey in search of the answers is likely, the present study already exposes a potential gateway.



References

  1. Robert, HS; Park, C; Gutièrrez, CL; Wójcikowska, B; Pěnčík, A; Novák, O; Chen, J; Grunewald, W; Dresselhaus, T; Friml, J; Laux, T, 2018: Maternal auxin supply contributes to early embryo patterning in Arabidopsis. Nature Plants , doi: 10.1038/s41477-018-0204-z https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3963583/
  2. Robert, H. S.; Khaitova, L. C.; Mroue, S.; Benková, E. The importance of localized auxin production for morphogenesis of reproductive organs and embryos in Arabidopsis. J Exp Bot. 2015, 66, 5029–5042.
  3. University of Cambridge. (2015, January 26). Mother's stress hormone levels may affect fetal growth and long term health of child . ScienceDaily. Retrieved January 15, 2019 from www.sciencedaily.com/releases/2015/01/150126095717.htm
  4. Auxins | The International Plant Growth Substances Association. Retrieved from https://pages.wustl.edu/ipgsa/auxins
  5. Möller, B., & Weijers, D. (2009). Auxin control of embryo patterning. Cold Spring Harbor perspectives in biology, 1(5), a001545.
  6. Jenik, P., Gillmor, C., & Lukowitz, W. (2007). Embryonic Patterning in Arabidopsis thaliana. Annual Review Of Cell And Developmental Biology, 23(1), 207-236.doi: 10.1146/annurev.cellbio.22.011105.102609
  7. Friml, J., Vieten, A., Sauer, M., Weijers, D., Schwarz, H., & Hamann, T. et al. (2003). Efflux-dependent auxin gradients establish the apical–basal axis of Arabidopsis. Nature, 426(6963), 147-153. doi: 10.1038/nature02085
  8. Robert, H., Grones, P., Stepanova, A., Robles, L., Lokerse, A., & Alonso, J. et al. (2013). Local Auxin Sources Orient the Apical-Basal Axis in Arabidopsis Embryos. Current Biology, 23(24), 2506-2512.doi: 10.1016/j.cub.2013.09.039



Written by Sophia Man

Edited by Markus Dettenhofer and Somsuvro Basu


Publication date: 29.03.2019