Written by: Octavia Rowe ‘24
Edited by: Angelina Cho ‘24
For most people, the life cycle of a plant is something that they learned about in junior high school and haven’t considered since. Plants are pollinated, make seeds and sometimes fruit, the seed grows into a plant, flowers, and the cycle begins again. Many of my friends were surprised at the fact that pollination itself is not the same as fertilization when I spoke with them about this article. Though this may not be the most scientific survey or the most representative sample group, this article will begin by going over what is currently known about how reproduction works in angiosperms, as derived from studies on the model species Arabidopsis thaliana and others.
When flowers are pollinated, grains of dehydrated pollen rest on the top of the stigma. On the stigmatic surface, pollen grains are hydrated and a pollen tube holding within it two sperm cells begins polarized growth down the style[1]. Only one end of the pollen grain will grow, creating a polarized cell growth pattern. As the pollen tube grows down into the ovary, it is drawn toward ovules by special LURE proteins, where it passes over two synergid cells, a pair of cells named for their function as conductors. Both synergids coordinate pollen tube burst, and the receptive synergid bursts along with the pollen tube. If double fertilization is successful–one sperm fusing with the egg cell and the other with the central cell–then the remaining persistent synergid eventually degrades and fuses with the central cell. If not, then the cycle begins again and the persistent synergid becomes the new receptive synergid[2].
However, before bursting, as it grows towards the ovule, the pollen tube releases Rapid Alkalinization Factors (RALFs) that act as ligands for the FERONIA (FER) protein receptor complex within the membrane of the synergid cells. When the pollen tube reaches the synergids, the FER complex perceives the RALFs and a signaling cascade begins that eventually causes the pollen tube to burst[2].
Two recent papers, by Sheng Zhong et al and Yan Ju et al respectively, describe processes that occur between this reception and pollen tube burst centering on failures caused by polytubey or pollen tubes not bursting. Polytubey is when more than one pollen tube grows into an ovule, and polyspermy is when more than one sperm fuse with the egg and central cell respectively.
In control groups, Arabidopsis thaliana plants are fertilized successfully nearly 100% of the time because of a series of tightly regulated processes that prevent failure events. It was previously known that during pollen tube reception NORTIA (NTA), a transmembrane protein normally localized to the Golgi apparatus, is re-localized to the filiform apparatus, a thickened structure at the end of the synergid known to secrete LURE proteins, at the receptive end of the synergids[3], though the purpose or role of it was not clear. Ju’s paper now tells us that this NTA is localized after pollen tube reception because it promotes pollen tube bursting.
The most important experiment performed in Ju’s paper is as follows: It is known that feronia (fer) loss of function mutants increase infertility to 80%, and that NTA is only re-localized after FER is initiated. If NTA promotes pollen tube burst, then NTA already localized to the filiform apparatus in FER mutants will increase fertility. To test this hypothesis, a FER and filiform apparatus localized NTA mutant, faNTA, was created and pollinated with wild type Arabidopsis thaliana pollen. Loss of function fer mutants and faNTA were hybridized to create fer;faNTA plants. Indeed, it was observed that infertility was halved from 80% to 40% in fer versus fer;faNTA mutants, a significant decrease against controls. The results indicated that by localizing NTA to the filiform apparatus, pollen tube burst is promoted and fertility is increased.
However, the FER complex does not only cause this one NTA trafficking event. Pollen tubes are drawn to ovules indiscriminately by LURE proteins, so how does the plant know to only draw in one pollen tube per ovule, and why is this behavior preferred? The answer to the latter question is that polytubey and subsequently polyspermy eventually cause cell death because of the excess DNA[4]. The answer to the first is a mechanism known as a block that prevents more than one tube from entering the ovule. Zhong’s paper tells us how the polytuby block is established and released.
When a pollen tube grows into an ovule, RALFs expressed on the surface of the pollen tube act as ligands for the FER complex and contemporaneously trigger the trafficking of NTA and a block to polytubey. The block remains for as long as the pollen tube remains intact, but after bursting, the RALFs quickly degrade and the block is released. If fertilization is successful, the persistent synergid degrades and stops producing LURE proteins, otherwise, if fertilization is unsuccessful, it remains and attracts a second pollen tube.
Similar to Ju’s methods, Zhong created and used mutants to test both sides of the hypothesis that RALFs and the FER complex are necessary to establish the block to polyspermy. Using ralf mutants to pollinate wild type flowers showed significant polytubey compared to control crosses. The same was true for fer mutants pollinated with wild type pollen–polytubey increased. In order to study how the block was released in the case of unsuccessful fertilization, Zhong also created plants with RALFs expressing fluorescent proteins to record its concentration before and after the pollen tubes’ bursting.
Taken and understood together, these studies show that during pollen tube reception, two processes are initiated by the triggering of the FERONIA complex to raise the chance of successful double fertilization: a block to polytubey, and the release of a secondary pollen tube burst signal. Previously, it was only known that a polytubey block, or many blocks, existed and was necessary to maintain the high success rate of wild type fertilization, and FERONIA mutants in contrast have a much higher rate of failure. Both articles have important insights and further the field of plant science as a whole, leaving the door open for future research.
What happens after pollination is not so simple, and many processes still elude scientists today. But it is certainly more complicated than just pollination followed by seeds and fruit. In the intervening time between pollination and fertilization, many intricate systems must be coordinated to maintain a high rate of successful fertilization events and the continued persistence of flowering plants.
References
[2] Johnson MA, Harper JF, Palanivelu R. A Fruitful Journey: Pollen Tube Navigation from Germination to Fertilization. Annu Rev Plant Biol. 2019 Apr 29;70:809-837. doi: 10.1146/annurev-arplant-050718-100133.
[3] Ju Y, Yuan J, Jones DS, Zhang W, Staiger CJ, Kessler SA. Polarized NORTIA accumulation in response to pollen tube arrival at synergids promotes fertilization. Dev Cell. 2021 Nov 8;56(21):2938-2951.e6.
[4] Brian Dale, Polyspermy,Encyclopedia of Reproduction (Second Edition), Academic Press, 2018, Pages 309-313, ISBN 9780128151457,https://doi.org/10.1016/B978-0-12-801238-3.64472-0.