Hanging On To Your Date, Avoiding Incest And Not Getting Infected – All In A Day’s Work For Flowers Of The Mustard Family

In the mustard family, which includes the important crop Brassica (rapeseed/canola) and the model plant Arabidopsis, the surfaces of the female stigmatic cells are dry. These cells – on which the pollen grains land – are thought to be more advanced than those of the wet stigmas of many other plants because they do not trap fungal spores (or pollen grains of many other species), and can discriminate between self and cross pollen of their own species and thus avoid self-pollination. The evolution of these stigmas has, however, involved the development of a good deal of complex cell biology – primarily to enable their own pollen to adhere to the dry surface, and to hydrate, germinate and penetrate the stigmatic cuticle.

The molecular interactions underpinning this so-called basal compatibility response (BCR) are currently being unraveled in Daphne Goring’s laboratory at the University of Toronto. In presenting the 2015 Annals of Botany Special Lecture at Oxford University’s Department of Plant Sciences on June 15^th, Professor Goring described her most recent work on the signaling pathways regulating the BCR in Brassica and Arabidopsis species, and explained how these systems are linked to the genetically-based system of self-incompatibility that prevents self-pollen development on the stigma surface (See figure below for summary).

Her talk focused on the different signaling components that regulate the stigmatic papillar (hair) cell responses to compatible (out-crossing) pollen and self (incompatible) pollen (reviewed in Indriolo et al., 2014). In response to challenge by compatible pollen, the BCR signalling pathway is activated in the stigmatic cytoplasm below the point of pollen contact, which leads to vesicle secretion in Arabidopsis species, and secretion by multivesicular bodies (MVBs) in Brassica species (Elleman and Dickinson, 1990, 1996; Safavian and Goring, 2013; Indriolo et al., 2014). This polarized secretion under the pollen/stigma contact site is mediated by components of the exocyst complex (Samuel et al., 2009; Safavian et al., 2015) which are known to tether vesicles to specific ‘target’ membranes for fusion and the release of their cargo (reviewed in Zarsky et al., 2013). Professor Goring suggested that this cargo facilitates pollen hydration, germination and pollen tube entry into the stigma. Although the identity of this cargos is unknown, one promising candidate is the ACA13 Ca²⁺-ATPase – proposed to release Cas2+ for the growing pollen tube (Iwano et al., 2014). Once in the stigmatic cell wall, the pollen tube continues to grow down the pistil and eventually reaches the egg-cell containing ovule, where it bursts to release sperm cells for fertilization.

Professor Goring then described how the BCR signalling pathway is also initiated in response to self-incompatible pollen, leading again to vesicle/MVB formation in the cytoplasm. However, in Arabidopsis species she showed new data suggesting that autophagy is then induced, resulting in the inhibition of exocytosis and destruction of cytoplasmic material – including secretory vesicles. In Brassica, exocytosis also fails to occur for MVBs are rerouted to the vacuole for degradation (Safavian and Goring, 2013; Indriolo et al., 2014). It has been known for some time that this self-incompatibility cellular response is initiated by a polymorphic pollen ligand (SP11/SCR) and a polymorphic stigma S Receptor Kinase (SRK; reviewed in Iwano and Takayama, 2012). Professor Goring described recent experiments showing how the ARC1 E3 ubiquitin ligase functions downstream of SRK in this self-incompatibility signalling pathway and may act to inhibit an exocyst subunit, Exo70A1, to prevent exocytosis (Stone et al., 1999; Stone et al., 2003; Samuel et al., 2009; Indriolo et al., 2012; Indriolo et al., 2014). Her experimental data also show that ARC1 is linked to the autophagy response, but precisely how autophagy is initiated in the self-incompatibility pathway is as yet unclear (reviewed in Goring et al., 2014). Of course, by blocking exocytosis, the cargo needed for pollen ‘acceptance’ remains undelivered, and self-incompatible pollen is thus rejected, being unable to hydrate and/or germinate.

Pollination systems of this type are emerging as important paradigms for cell-cell signalling in plants. Furthermore, understanding how pollen is accepted and rejected on dry stigmatic surfaces will also be essential for the development of new plant breeding strategies for brassica crops, and may even help identify mechanisms by which plants recognise and reject fungal pathogens.

References

C. J. Elleman, H. G. Dickinson, 1990, ‘The role of the exine coating in pollen-stigma interactions in Brassica oleracea L’, New Phytologist, vol. 114, no. 3, pp. 511-518 http://dx.doi.org/10.1111/j.1469-8137.1990.tb00419.x

C. J. Elleman, H. G. Dickinson, 1996, ‘Identification of pollen components regulating pollination-specific responses in the stigmatic papillae of Brassica oleracea’, New Phytologist, vol. 133, no. 2, pp. 197-205 http://dx.doi.org/10.1111/j.1469-8137.1996.tb01886.x

D. R. Goring, E. Indriolo, M. A. Samuel, 2014, ‘The ARC1 E3 Ligase Promotes a Strong and Stable Self-Incompatibility Response in Arabidopsis Species: Response to the Nasrallah and Nasrallah Commentary’, The Plant Cell, vol. 26, no. 10, pp. 3842-3846 http://dx.doi.org/10.1105/tpc.114.131243

E. Indriolo, D. Safavian, D. R. Goring, 2014, ‘The ARC1 E3 Ligase Promotes Two Different Self-Pollen Avoidance Traits in Arabidopsis’, The Plant Cell, vol. 26, no. 4, pp. 1525-1543 http://dx.doi.org/10.1105/tpc.114.122879

Emily Indriolo, Darya Safavian, Daphne R. Goring, 2014, ‘Signaling Events in Pollen Acceptance or Rejection in the Arabidopsis Species’, Sexual Reproduction in Animals and Plants, pp. 255-271 http://dx.doi.org/10.1007/978-4-431-54589-7_22

E. Indriolo, P. Tharmapalan, S. I. Wright, D. R. Goring, 2012, ‘The ARC1 E3 Ligase Gene Is Frequently Deleted in Self-Compatible Brassicaceae Species and Has a Conserved Role in Arabidopsis lyrata Self-Pollen Rejection’, The Plant Cell, vol. 24, no. 11, pp. 4607-4620 http://dx.doi.org/10.1105/tpc.112.104943

Megumi Iwano, Seiji Takayama, 2012, ‘Self/non-self discrimination in angiosperm self-incompatibility’, Current Opinion in Plant Biology, vol. 15, no. 1, pp. 78-83 http://dx.doi.org/10.1016/j.pbi.2011.09.003

M. Iwano, M. Igarashi, Y. Tarutani, P. Kaothien-Nakayama, H. Nakayama, H. Moriyama, R. Yakabe, T. Entani, H. Shimosato-Asano, M. Ueki, G. Tamiya, S. Takayama, 2014, ‘A Pollen Coat-Inducible Autoinhibited Ca2+-ATPase Expressed in Stigmatic Papilla Cells Is Required for Compatible Pollination in the Brassicaceae’, The Plant Cell, vol. 26, no. 2, pp. 636-649 http://dx.doi.org/10.1105/tpc.113.121350

Darya Safavian, Daphne R. Goring, 2013, ‘Secretory Activity Is Rapidly Induced in Stigmatic Papillae by Compatible Pollen, but Inhibited for Self-Incompatible Pollen in the Brassicaceae’, PLoS ONE, vol. 8, no. 12, p. e84286 http://dx.doi.org/10.1371/journal.pone.0084286

D. Safavian, Y. Zayed, E. Indriolo, L. Chapman, A. Ahmed, D. R. Goring, 2015, RNA silencing of exocyst genes in the stigma impairs the acceptance of compatible pollen in Arabidopsis. Submitted

M. A. Samuel, Y. T. Chong, K. E. Haasen, M. G. Aldea-Brydges, S. L. Stone, D. R. Goring, 2009, ‘Cellular Pathways Regulating Responses to Compatible and Self-Incompatible Pollen in Brassica and Arabidopsis Stigmas Intersect at Exo70A1, a Putative Component of the Exocyst Complex’, The Plant Cell, vol. 21, no. 9, pp. 2655-2671 http://dx.doi.org/10.1105/tpc.109.069740

S. L. Stone, E. M. Anderson, R. T. Mullen, D. R. Goring, 2003, ‘ARC1 Is an E3 Ubiquitin Ligase and Promotes the Ubiquitination of Proteins during the Rejection of Self-Incompatible Brassica Pollen’, The Plant Cell, vol. 15, no. 4, pp. 885-898 http://dx.doi.org/10.1105/tpc.009845

S. L. Stone, M. Arnoldo, D.R. Goring, 1999, ‘A Breakdown of Brassica Self-Incompatibility in ARC1 Antisense Transgenic Plants’, Science, vol. 286, no. 5445, pp. 1729-1731 http://dx.doi.org/10.1126/science.286.5445.1729

Viktor Žárský, Ivan Kulich, Matyáš Fendrych, Tamara Pečenková, 2013, ‘Exocyst complexes multiple functions in plant cells secretory pathways’, Current Opinion in Plant Biology, vol. 16, no. 6, pp. 726-733 http://dx.doi.org/10.1016/j.pbi.2013.10.013