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Arabinogalactan proteins mediate intercellular crosstalk in the ovule of apple flowers

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AGP-rich glycoproteins mediate pollenovule interactions and cell patterning in the embryo sac of apple before and after fertilization.


Glycoproteins are significant players in the dialog that takes place between growing pollen tubes and the stigma and style in the angiosperms. Yet, information is scarce on their possible involvement in the ovule, a sporophytic organ that hosts the female gametophyte. Apple flowers have a prolonged lapse of time between pollination and fertilization, offering a great system to study the developmental basis of glycoprotein secretion and their putative role during the last stages of the progamic phase and early seed initiation. For this purpose, the sequential pollen tube elongation within the ovary was examined in relation to changes in arabinogalactan proteins (AGPs) in the tissues of the ovule before and after fertilization. To evaluate what of these changes are developmentally regulated, unpollinated and pollinated flowers were compared. AGPs paved the pollen tube pathway in the ovules along the micropylar canal, and the nucellus entrance toward the synergids, which also developmentally accumulated AGPs at the filiform apparatus. Glycoproteins vanished from all these tissues following pollen tube passage, strongly suggesting a role in pollen–ovule interaction. In addition, AGPs marked the primary cell walls of the haploid cells of the female gametophyte, and they further built up in the cell walls of the embryo sac and developing embryo, layering the interactive walls of the three generations hosted in the ovule, the maternal sporophytic tissues, the female gametophyte, and the developing embryo.

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Authors are grateful to Reyes Lopez for technical assistance, and Nuria Blanco-Moure for helping with graphs and data analysis. We thank William E. Friedman for his support and for providing laboratory equipment during part of this work. This work was supported by Ministerio de Ciencia e Innovación (MICINN)-FEDER [AGL2006-13529-C02-01, AGL 12621-C02-01, AGL 2012–40239] and Gobierno de Aragón [group A43]. JIM antibodies distribution was partly supported by NSF grants [DBI-0421683, RCN 009281]. JML was supported by a FPI fellowship [BES-2007-16059] from MICINN.

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Correspondence to Juan M. Losada.

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Communicated by Tetsuya Higashiyama.

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Supplementary Fig. 1

AGPs labeled with JIM13 mAb in the nucellus of apple ovules before and after pollen tube penetration. A. Glycoprotein epitopes in the surface of a row of cells leading to the egg apparatus (white arrows), at anthesis (0 daa). B. At the time of pollen tube penetration and fertilization, these glycoproteins vanish from the areas of pollen tube growth. Two-micrometer sections of ovules were immunolocalized with JIM13 mAb and detected with an Alexa 488 anti-rat secondary antibody conjugated with FITC that shows a green color. Daa, days after anthesis; dap, days after pollination; nu, nucellus; pt, pollen tube. Scale bars: 20 µm. (TIFF 4493 kb)

Supplementary Fig. 2

Changes in the filiform apparatus of the synergids during the post-pollination stages in apple. A. While the embryo sac accumulates starch, the filiform apparatus of the synergids (black arrows) shows accumulation of polysaccharides three days after pollination (3 dap). B. Concomitant with pollen tube arrival, one synergid degenerates (black arrow). C. Following fertilization, upon zygote formation, the filiform apparatus (black arrows) degenerates, losing polysaccharide staining. Two-micrometer sections of ovules stained with periodic acid–Shiff’s reagent (PAS) for insoluble polysaccharides (pink to purple color). Dap, days after pollination; es, embryo sac; fa, filiform apparatus; pt, pollen tube; syn, synergid; zyg, zygote. Scale bars: 20 µm. (JPEG 2931 kb)

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Losada, J.M., Herrero, M. Arabinogalactan proteins mediate intercellular crosstalk in the ovule of apple flowers. Plant Reprod 32, 291–305 (2019).

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