| Summary: | Sporopollenin is one of the toughest biopolymers found in nature. It is considered to be the major component of exine, which protects the male gametophyte from environmental stresses. Previous reverse genetic studies have revealed that sporopollenin precursors are synthesized in the tapetum, but how they are anchored to the microspores and what determines the pattern of their polymerization are still unknown. SPONGY2 (SPG2), synonymous with IRX9L, encodes a glycosyltransferase involved in xylan formation. irx9l pollen possesses a thinner primexine polysaccharide layer at the tetrad stage and as development proceeds, irx9l sporopollenin deposition on the primexine forms exine with a non-reticulate pattern, and with wider or higher number of baculae. Immunolabeling of mature irx9l pollen using the LM10 antibody showed the absence of xylan in its exine compared to wild-type. Another gene IRX14L, whose encoded protein also contributes to xylan formation, has an expression pattern similar to IRX9L. The irx14l mutant produces pollen with a similar spongy exine phenotype as irx9l. Based on these data, we hypothesize that IRX9L/SPG2 is required for primexine formation, and that xylan containing hemicellulose is present in the developing pollen wall for the establishment of proper exine patterning in the early stages of pollen wall formation.
The Arabidopsis UPEX gene encodes a galactosyltransferase (GalT) that is tightly co-expressed with other sporopollenin biosynthesis genes. The upex mutant produces pollen with missing and malformed exine. Through the analysis of upex mutant using 2-photon microscopy and TEM, accumulations similar to sporopollenin were found in the anther locule, unlike the wild type. The onset of the upex phenotype is the tetrad stage, as upex primexine appeared to be less dense than wild-type, and immunolabeling with xylan-directed antibody (LM10) produced a xylan signal that was absent in wild-type samples of the tetrad stage. I hypothesize that UPEX contributes to the synthesis or modification of a polysaccharide that contains galactose, which plays a structural role in primexine, allowing proper anchoring of sporopollenin to the microspore surface, helping to establish exine patterning over the course of pollen wall deposition. === Science, Faculty of === Botany, Department of === Graduate
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