Seeds of Change

01 October 2018 - By: Jessy Silva

Seeds of Change

By Jessy Silva, MSC Student, University of Porto, Portugal

Different love stories that lead to the formation of seeds reunited some experienced researchers, PhD students, early career researchers and academics from all over the world to share their common passion for plant reproduction at the SEB plant satellite meeting “Advances in Plant Reproduction – From Gametes to Seeds” in Florence, Italy this summer.

Prof. Sílvia Coimbra (University of Porto) from the Organising Committee welcomed all the participants highlighting the importance of “understanding the factors regulating plant reproduction, a complex biological process, to improve agricultural productivity”. Here’s an overview of these two days.

UNDERSTANDING FEMALE GERMLINE DEVELOPMENT

In angiosperms, the female gametophyte development occurs within the ovule where a diploid megaspore mother cell (MMC) undergoes meiosis and gives rise to four haploid megaspores1. Only one of these megaspores will become the functional megaspore which will form the embryo sac. The first session of the meeting “Ovule Development – Germline Development and Function” started with Prof. Lucia Colombo (University of Milan, Italy) talking about the development of the ovule, the “extremely complex and extraordinary organ that upon double fertilization develops into seeds”, she said. Lucia detailed the importance of the auxin responsive factor MONOPTEROS and its network during ovule development. Afterwards, Prof. Matthew Tucker (University of Adelaide, Australia) used a quote from Gandalf’s character in the movie Lord of the Rings: The Fellowship of the Ring: “You shall not pass” to explain the mechanism underlying the female germline isolation during the ovule development. Using specific marker lines, he found out that the symplastic protein transport is blocked and therefore the germline is isolated. His results led to the following question: “What is the MMC trying to keep out… or in?”.

THE LOVE STORY

During double fertilization, when a mature pollen grain is released from an anther and reaches the pistil, it adheres to the stigmatic cells and hydration begins, leading to the germination of the pollen into a pollen tube2. The pollen tube grows along the female tissues carrying the two sperm cells into the embryo sac. Then, the pollen tube enters one of the synergids releasing the sperm cells, the receptive synergid degenerates, double fertilization takes place and the persistent synergid is eliminated. We took a step in the theme “Pollen Germination and Pollen Tube Growth - Fertilization and Polyspermy Block” with Prof. José Feijó (University of Maryland, USA) who highlighted the importance of glutamate receptor-like (GLRs) channels in plants. José showed that a mutation of GLR genes in Physcomitrella patens leads to sperm failure in targeting the female reproductive organs3. As GLR genes encode non-selective Ca2+ permeable channels that can regulate cytoplasmic Ca2+ and they are involved in sperm chemotaxis, he suggested that GLRs may have a conserved role in male gamete signal perception.

Later, we heard from Dr. Marta Mendes (University of Milan) about how cytokinins play a role in synergid cell death in Arabidopsis. Marta presented two targets of the SEEDSTICK transcription factor that interact to control the death of the receptive synergid cell: VERDANDI (VDD) and VALKYRIE (VAL), both named after two Norse goddesses who determined the fate of men4. She explained how VDD and VAL form a complex that controls the expression of the cytokines oxidase/ dehydrogenase CKX7 and she discussed how the correct maintenance of cytokinin levels is required for the receptive synergid death.

We also learned about a love story that “can’t get no satisfaction” with Dr. Ana Marta Pereira (University of Milan) who talked about JAGGER, an arabinogalactan protein named after the rock’n’roll god Mick Jagger. JAGGER is an important molecule involved in the mechanism responsible for preventing polyspermy in Arabidopsis5. In jagger mutant, “the persistent synergid does not degenerate after double fertilization and continues to attract more pollen tubes into the embryo sac”, explained Ana Marta.
Later, Prof. Sílvia Coimbra highlighted the importance of the role of arabinogalactan proteins (AGPs) and Ca2+ in pollen tube growth and guidance. Sílvia performed a pollen tube transcriptome of a double mutant of two pollen-specific AGPs, AGP6 and AGP11. She verified that the expression levels of calciumand signalling-related genes were altered, thus supporting the role of AGPs in pollen tube growth regulated by a calcium gradient6. Sílvia also presented us a regulatory triple role of AGPs as: a primary source of cytosolic Ca2+, a plasticizer of wall pectin and Ca2+ signposts to the ovule7.

SEED FORMATION

When double fertilization takes place, one sperm cell fertilizes the egg cell, giving rise to the embryo and the second sperm cell fuses with the diploid central cell, giving rise to the triploid endosperm, which surrounds and provides nutrients to the developing embryo2. In the last session “Embryogenesis and Endosperm Development - Seed and Fruit Development”, we stayed “stick” with PhD student Ana Lopes’ (University of Porto) presentation about the SEEDSTICK (STK) transcription factor control frame. Mature seeds of stk mutant do not detach from the mother plant siliques, they stay “stick”. Furthermore, STK is expressed in the integument tissues and funiculus of developing seeds, indicating that STK plays a role during seed development8. RNA and ChIP sequencing data results (stk vs wt) identified several direct targets of STK, such as arabinogalactan proteins and cytokinin oxidase/dehydrogenase genes. The purpose of Ana’s work is to understand the role of these molecules and their network during seed development.

Prof. Emidio Albertini (University of Perugia, Italy) presented us the “APOSTART: a candidate gene involved in embryo and parthenogenesis”. Apomixis, a naturally occurring mode of asexual reproduction in flowering plants, avoids both meiotic reduction and egg fertilization9 and seed derived progenies are genetically identical to the maternal parent. Emidio isolated APOSTART in Poa pratensis and started characterising the Arabidopsis APOSTART members.

Duarte Figueiredo, junior group leader at the University of Potsdam in Germany, then presented us with some interesting evidence that “auxin regulates endosperm cellularization in Arabidopsis”. In Arabidopsis, the endosperm is initially developed as a syncytium, where nuclear divisions are not followed by cytokinesis and after a defined number of nuclear divisions it cellularizes10. During his talk, Duarte exposed that reduced auxin can restore cellularization in triploid seeds and therefore it can rescue the triploid seed abortion, highlighting a “causal role of increased auxin biosynthesis in preventing endosperm cellularization”. With these results, he proposed that auxin determines the time of endosperm cellularization.

Buzi Raviv (PhD student at Ben Gurion University, Israel) provided a detailed look on the function of seed covering structures. He proposed that dead maternally derived organs enclosing embryos function not only as a physical shield for embryo protection from mechanical stress and microorganism invasion but also as long-term storage for multiple active hydrolases (such as nucleases, proteases and chitinases) that are released upon hydration and that aid in seed persistence and longevity, germination and seedling establishment11. These results explain how seeds persist and retain viability in the soil for so many years. 

Then, Prof. Simona Masiero (University of Milan) enlightened us with a “fruit-talk”. Agricultural production relies on the “yield and quality factors associated with fruits” and therefore it is very important to understand the mechanisms controlling fruit development and maturation. Simona’s group performed a “transcriptome analysis by RNA-deepsequencing covering all the phases of silique development and maturation” to identify genes involved in fruit development and maturation in Arabidopsis thaliana. NAC transcription factors appeared in this analysis and they seem to be involved in the homeostasis of gibberellins and other hormones.

Throughout the two days meeting, the participants were able to appreciate custom Italian food while taking a look at the researcher’s posters showing their ongoing studies. To end this event in great shape there was a Port Wine tasting provided by the Portuguese organising team of the meeting (Sílvia Coimbra, Ana Lúcia Lopes and Ana Marta Pereira). Looking back, the event was a success and all there is left to say is: until the next plant satellite meeting!

References:
1. Reiser, L., & Fischer, R. L. (1993). The ovule and the embryo sac. The Plant Cell, 5(10), 1291.2. Pereira AM, Lopes AL and Coimbra S (2016) Arabinogalactan Proteins as Interactors along the Crosstalk between the Pollen Tube and the Female Tissues. Front. Plant Sci. 7:1895.
3. Ortiz-Ramírez, C., Michard, E., Simon, A. A., Damineli, D. S., Hernández-Coronado, M., Becker, J. D., & Feijó, J. A. (2017). GLUTAMATE RECEPTOR-LIKE channels are essential for chemotaxis and reproduction in mosses. Nature, 549(7670), 91.
4. Mendes, M. A., Guerra, R. F., Castelnovo, B., Velazquez, Y. S., Morandini, P., Manrique, S., ... & Colombo, L. (2016). Live and let die: a REM complex promotes fertilization through synergid cell death in Arabidopsis. Development, dev-134916.
5. Pereira, A. M., Nobre, M. S., Pinto, S. C., Lopes, A. L., Costa, M. L., Masiero, S., & Coimbra, S. (2016). “Love Is Strong, and You’re so Sweet”: JAGGER is essential for persistent synergid degeneration and polytubey block in Arabidopsis thaliana. Molecular plant, 9(4), 601-614.
6. Costa, M., Nobre, M. S., Becker, J. D., Masiero, S., Amorim, M. I., Pereira, L. G., & Coimbra, S. (2013). Expression-based and co-localization detection of arabinogalactan protein 6 and arabinogalactan protein 11 interactors in Arabidopsis pollen and pollen tubes. BMC plant biology, 13(1), 7.
7. Lamport, D. T., Tan, L., Held, M. A., & Kieliszewski, M. J. (2018). Pollen tube growth and guidance: Occam’s razor sharpened on a molecular arabinogalactan glycoprotein Rosetta Stone. New Phytologist, 217(2), 491-500.
8. Mizzotti, C., Ezquer, I., Paolo, D., Rueda-Romero, P., Guerra, R. F., Battaglia, R., ... & Colombo, L. (2014). SEEDSTICK is a master regulator of development and metabolism in the Arabidopsis seed coat. PLoS genetics, 10(12), e1004856.
9. Albertini, E., Marconi, G., Reale, L., Barcaccia, G., Porceddu, A., Ferranti, F., & Falcinelli, M. (2005). SERK and APOSTART. Candidate genes for apomixis in Poa pratensis. Plant Physiology, 138(4), 2185-2199.
10. Figueiredo, D. D., Batista, R. A., & Kohler, C. (2018). Auxin regulates endosperm cellularization in Arabidopsis. bioRxiv, 239301
11. Raviv, B., Aghajanyan, L., Granot, G., Makover, V., Frenkel, O., Gutterman, Y., & Grafi, G. (2017). The dead seed coat functions as a long-term storage for active hydrolytic enzymes. PloS one, 12(7), e0181102.
 
 
Category: Plant Biology
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Jessy Silva

Jessy Silva

I’m a postgraduate student in Functional Biology and Biotechnology at the Faculty of Sciences of the University of Porto in Portugal. My research interests are related to Sexual Plant Reproduction because of its complexity as a biological process and its importance to improve agricultural productivity. I’m currently investigating the role of [methyl] glucuronic acid of arabinogalactan proteins (AGPs) during Arabidopsis sexual reproduction in the Sexual Plant Reproduction and Development laboratory (SPReD) (http://www.fc.up.pt/agplab/index.html).