Arabidopsis Protein Kinase D6PKL3 Is Involved in the Formation of Distinct Plasma Membrane Aperture Domains on the Pollen Surface

Abstract

Certain regions on the surfaces of developing pollen grains exhibit very limited deposition of pollen wall exine. These regions give rise to pollen apertures, which are highly diverse in their patterns and specific for individual species. Arabidopsis thaliana pollen develops three equidistant longitudinal apertures. The precision of aperture formation suggests that, to create them, pollen employs robust mechanisms that generate distinct cellular domains. To identify players involved in this mechanism, we screened natural Arabidopsis accessions and discovered one accession, Martuba, whose apertures form abnormally due to the disruption of the protein kinase D6PKL3. During pollen development, D6PKL3 accumulates at the three plasma membrane domains underlying future aperture sites. Both D6PKL3 localization and aperture formation require kinase activity. Proper D6PKL3 localization is also dependent on a polybasic motif for phosphoinositide interactions, and we identified two phosphoinositides that are specifically enriched at the future aperture sites. The other known aperture factor, INAPERTURATE POLLEN1, fails to aggregate at the aperture sites in d6pkl3 mutants, changes its localization when D6PKL3 is mislocalized, and, in turn, affects D6PKL3 localization. The discovery of aperture factors provides important insights into the mechanisms cells utilize to generate distinct membrane domains, develop cell polarity, and pattern their surfaces.

Functions of Anionic Lipids in Plants

“…In Arabidopsis, SEC3 contains a PH domain that binds to PI(4,5)P2 in vitro (8). The extensive use of biosensors and the localization of lipid metabolic enzymes in plants allowed the cartography of anionic lipids in cells exhibiting polar secretion highlighting specific lipid accumulation patterns, notably for PI(4,5)P2 (Figure 3) (41,42,57,60,61,115,117,146). Interestingly, the localizations of SEC3 and/or EXO70 isoforms often correlate with the PI(4,5)P2 subcellular accumulation pattern, suggesting that PI(4,5)P2 contribute to their localization (8,48,108).…”

Grass-Specific EPAD1 Is Essential for Pollen Exine Patterning in Rice

“…The lipid-binding activity of EPAD1 was dependent on protein structure: when disulfide bonds were disrupted by Tris-2-carboxyethy1-phosphine, binding of EPAD1 to phospholipids was abolished ( Figure 8). These results indicate that EPAD1 can bind several phospholipids that are found in microspores (Lee et al, 2018).…”

“…EPAD1 displays PI-binding activity in vitro, as does its wheat ortholog TaMs1 where four amino acid changes in the TaMs1 LTP domain (C52Y, V55D, D60N and C94Y) totally abolished this function (Wang et al, 2017; Tucker et al, 2017). PIs, enriched in specific PM domains, play crucial roles in directing cell polarity and vesicular trafficking in plants, and act as docking sites or substrates to recruit peripheral membrane proteins (Heilmann and Heilmann, 2013; Stanislas et al, 2015;Barbosa et al, 2016;Lee et al, 2018). PIs also influence PM biophysical properties directly through their polyanionic head groups or in combination with other lipids(Heilmann, 2016; Gerth et al, 2017), indicating that PIs may stabilize PM nanodomains via multiple mechanisms.…”

“…PIs also influence PM biophysical properties directly through their polyanionic head groups or in combination with other lipids(Heilmann, 2016; Gerth et al, 2017), indicating that PIs may stabilize PM nanodomains via multiple mechanisms. Two PI phosphates, PI(4)P and PI(4,5)P2, are enriched in the PM underneath the aperture site(Lee et al, 2018), where minimal sporopollenin deposition occurs. Although PIs normally locate to the inner surface of the PM, some reports indicate that they also present in the outer leaflet of the PM(Kale et al, 2010;Boss and Im, 2012).…”

Natural Artist: How a Protein Kinase Helps Sculpt the Pollen Grain Surface from the Inside Out

“…However, higher levels of INP1 do not lead to the formation of more or wider apertures, suggesting that it helps execute but does not control aperture formation. Lee et al (2018) looked for other players in this process by screening a collection of 316 natural Arabidopsis accessions for abnormally shaped pollen, an indicator of altered pollen aperture. The Libyan accession Martuba (Mt-0) fit the bill, with rounded pollen grains whose apertures are partially covered with exine or sometimes absent.…”