Search

Pollen wall consists of several complex layers which form elaborate species-specific patterns. In Arabidopsis, the transcription factor ABORTED MICROSPORE (AMS) is a master regulator of exine formation, and another transcription factor, TRANSPOSABLE ELEMENT SILENCING VIA AT-HOOK (TEK), specifies formation of the nexine layer. However, knowledge regarding the temporal regulatory roles of TEK in pollen wall development is limited. Here, TEK-GFP driven by the AMS promoter was prematurely expressed in the tapetal nuclei, leading to complete male sterility in the pAMS:TEK-GFP (pat) transgenic lines with the wild-type background. Cytological observations in the pat anthers showed impaired callose synthesis and aberrant exine patterning. CALLOSE SYNTHASE5 (CalS5) is required for callose synthesis, and expression of CalS5 in pat plants was significantly reduced. We demonstrated that TEK negatively regulates CalS5 expression after the tetrad stage in wild-type anthers and further discovered that premature TEK-GFP in pat directly represses CalS5 expression through histone modification. Our findings show that TEK flexibly mediates its different functions via different temporal regulation, revealing that the temporal regulation of TEK is essential for exine patterning. Moreover, the result that the repression of CalS5 by TEK after the tetrad stage coincides with the timing of callose wall dissolution suggests that tapetum utilizes temporal regulation of genes to stop callose wall synthesis, which, together with the activation of callase activity, achieves microspore release and pollen wall patterning., Competing Interests: The authors have declared that no competing interests exist.

The sexine layer of pollen grain is mainly composed of sporopollenins. The sporophytic secretory tapetum is required for the biosynthesis of sporopollenin. Although several enzymes involved in sporopollenin biosynthesis have been reported, the regulatory mechanism of these enzymes in tapetal layer remains elusive. ABORTED MICROSPORES (AMS) and MALE STERILE 188/MYB103/MYB80 (MS188/MYB103/MYB80) are two tapetal cell-specific transcription factors required for pollen wall formation. AMS functions upstream of MS188. Here we report that AMS and MS188 target the CYP703A2 gene, which is involved in sporopollenin biosynthesis. We found that AMS and MS188 were localized in tapetum while CYP703A2 was localized in both tapetum and locule. Chromatin immunoprecipitation (ChIP) showed that MS188 directly bound to the promoter of CYP703A2 and luciferase-inducible assay showed that MS188 activated the expression of CYP703A2. Yeast two-hybrid and electrophoretic mobility shift assays (EMSAs) further demonstrated that MS188 complexed with AMS. The expression of CYP703A2 could be partially restored by the elevated levels of MS188 in the ams mutant. Therefore, our data reveal that MS188 coordinates with AMS to activate CYP703A2 in sporopollenin biosynthesis of plant tapetum., (© 2016 The Authors The Plant Journal © 2016 John Wiley & Sons Ltd.)

Sporophytic pollen coat proteins (sPCPs) derived from the anther tapetum are deposited into pollen wall cavities and function in pollen–stigma interactions, pollen hydration, and environmental protection. In Arabidopsis, 13 highly abundant proteins have been identified in pollen coat, including seven major glycine-rich proteins GRP14, 16, 17, 18, 19, 20, and GRP–oleosin; two caleosin-related family proteins (AT1G23240 and AT1G23250); three lipase proteins EXL4, EXL5 and EXL6, and ATA27/BGLU20. Here, we show that GRP14, 17, 18, 19, and EXL4 and EXL6 fused with green fluorescent protein (GFP) are translated in the tapetum and then accumulate in the anther locule following tapetum degeneration. The expression of these sPCPs is dependent on two essential tapetum transcription factors, MALE STERILE188 (MS188) and MALE STERILITY 1 (MS1). The majority of sPCP genes are up-regulated within 30 h after MS1 induction and could be restored by MS1 expression driven by the MS188 promoter in ms188 , indicating that MS1 is sufficient to activate their expression; however, additional MS1 downstream factors appear to be required for high-level sPCP expression. Our ChIP, in vivo transactivation assay, and EMSA data indicate that MS188 directly activates MS1. Together, these results reveal a regulatory cascade whereby outer pollen wall formation is regulated by MS188 followed by synthesis of sPCPs controlled by MS1. [ABSTRACT FROM AUTHOR]

During anther development, the tapetum provides materials and nutrients for pollen development. In Arabidopsis, several transcription factors have been identified to form a genetic pathway (DYT1-TDF1-AMS-MS188-MS1) for tapetal development and function. DEFECTIVE in TAPETAL DEVELOPMENT and FUNCTION1 (TDF1) is a member of the R2R3 MYB family and is essential for early tapetum development in Arabidopsis. Here, we characterized an ortholog of Arabidopsis TDF1 in rice, OsTDF1 (LOC_Os03g18480). OsTDF1 shares 69 % amino acid sequence identity with AtTDF1 in the putative MYB domain near the N-terminal region. RT-PCR and in situ hybridization show that OsTDF1 is specifically expressed in tapetal cells of rice anthers. The expression of OsTDF1 in Arabidopsis tdf1 mutant restores its fertility, suggesting that this homolog can fulfill the normal function of TDF1 in Arabidopsis. The ostdf1 knockout mutant exhibits a male-sterile phenotype. Its tapetal cells exhibit a vacuolated and hypertrophic phenotype similar to that of Arabidopsis tdf1 mutants. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assays revealed OsTDF1 acts as an essential regulator for tapetum programmed cell death. The qRT-PCR analysis revealed that OsTDF1 acts downstream of UDT1 and upstream of TDR, EAT1, OsMYB103 and PTC1 in rice, suggesting that the genetic pathway for tapetum development is generally conserved between rice and Arabidopsis. [ABSTRACT FROM AUTHOR]