ABCG26-Mediated Polyketide Trafficking and Hydroxycinnamoyl Spermidines Contribute to Pollen Wall Exine Formation in Arabidopsis  

Abstract

Pollen grains are encased by a multilayered, multifunctional wall. The sporopollenin and pollen coat constituents of the outer pollen wall (exine) are contributed by surrounding sporophytic tapetal cells. Because the biosynthesis and development of the exine occurs in the innermost cell layers of the anther, direct observations of this process are difficult. The objective of this study was to investigate the transport and assembly of exine components from tapetal cells to microspores in the intact anthers of Arabidopsis thaliana. Intrinsically fluorescent components of developing tapetum and microspores were imaged in intact, live anthers using two-photon microscopy. Mutants of ABCG26, which encodes an ATP binding cassette transporter required for exine formation, accumulated large fluorescent vacuoles in tapetal cells, with corresponding loss of fluorescence on microspores. These vacuolar inclusions were not observed in tapetal cells of double mutants of abcg26 and genes encoding the proposed sporopollenin polyketide biosynthetic metabolon (ACYL COENZYME A SYNTHETASE5, POLYKETIDE SYNTHASE A [PKSA], PKSB, and TETRAKETIDE a-PYRONE REDUCTASE1), providing a genetic link between transport by ABCG26 and polyketide biosynthesis. Genetic analysis also showed that hydroxycinnamoyl spermidines, known components of the pollen coat, were exported from tapeta prior to programmed cell death in the absence of polyketides, raising the possibility that they are incorporated into the exine prior to pollen coat deposition. We propose a model where ABCG26-exported polyketides traffic from tapetal cells to form the sporopollenin backbone, in coordination with the trafficking of additional constituents, prior to tapetum programmed cell death.

Two ATP Binding Cassette G (ABCG) Transporters, OsABCG26 and OsABCG15, Collaboratively Regulate Rice Male Reproduction

“…9, M-P), suggesting that OsABCG15 is likely involved mainly in the transport of lipid molecules synthesized in tapetum to microspores for the formation of pollen wall. This assumption can be supported indirectly by the finding that AtABCG26, the OsABCG15 ortholog in Arabidopsis, is involved in exine formation and pollen development (Quilichini et al, 2010(Quilichini et al, , 2014. Furthermore, because of the defective anther cuticle phenotype of osabcg15, OsABCG15 may also function in transporting lipid precursors from the tapetum to anther surface.…”

“…It is generally accepted that sporopollenin, the pollen exine constituent, is produced exclusively in tapetum, and many genes involved in lipid synthesis and exine formation were identified in tapetum (Li et al, 2010;Quilichini et al, 2010Quilichini et al, , 2014Ariizumi and Toriyama, 2011;Chen et al, 2011b;Shi et al, 2011;Zhu et al, 2013;Lou et al, 2014;Yang et al, 2014). However, there is no Figure 11.…”

“…Some plant ABCG proteins have been reported to contribute to the synthesis of extracellular barriers. In Arabidopsis, ABCG11 (Cuticular Defect and Organ Fusion1/DESPERADO/White-Brown Complex11; Bird et al, 2007;Panikashvili et al, 2007Panikashvili et al, , 2010, ABCG12 (Eceriferum5/WBC12; Pighin et al, 2004), ABCG13 (Panikashvili et al, 2010), ABCG29 (Alejandro et al, 2012), ABCG32 (Bessire et al, 2011), and ABCG26 (WBC27; Xu et al, 2010;Quilichini et al, 2010Quilichini et al, , 2014Choi et al, 2011;Dou et al, 2011) have been shown to be involved in transport of lipidic compounds. Notably, it has been reported that these ABCG proteins have a broad substrates spectrum (for example, ABCG11 transports both cutin and wax monomers, and ABCG13 and ABCG32 mainly transport cutin monomers, whereas ABCG12 mainly transports wax precursors, and ABCG29 mainly transports monolignol).…”

Role of Glycosyltransferases in Pollen Wall Primexine Formation and Exine Patterning

“…1) indicated that the irx9l microspores had similar morphology to wild type at this stage. We also imaged irx9l pollen using two-photon (2P) microscopy, which allows live cell imaging of microspores in intact anthers using the intrinsic autofluorescence of exine under UV excitation (Quilichini et al, 2014b(Quilichini et al, , 2015b. As seen in Figure 1, irx9l microspores appeared to have a thinner pollen wall compared to wild type with less intense emission, but the exine showed similar autofluorescence.…”

“…These polyketide sporopollenin precursors are secreted from tapetal cells by the action of the ATP binding cassette transporter ABCG26 (Quilichini et al, 2010(Quilichini et al, , 2014b. Based on the coincidence of probaculae formation and primexine deposition, it is hypothesized that the primexine matrix plays a key role in providing a substrate for anchoring sporopollenin deposition on the microspore surface, and may play a role in exine patterning (Blackmore et al, 2007;Ariizumi and Toriyama, 2011;Quilichini et al, 2014a).…”