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During anther development, the transformation of the microspore into mature pollen occurs under the protection of first the tetrad wall and later the pollen wall. Mutations in genes involved in this wall transition often lead to microspore rupture and male sterility; some such mutants, such as the reversible male sterile (rvms) mutant, are thermo/photoperiod-sensitive genic male sterile (P/TGMS) lines. Previous studies have shown that slow development is a general mechanism of P/TGMS fertility restoration. In this study, we identified restorer of rvms-2 (res2), which is an allele of QUARTET 3 (QRT3) encoding a polygalacturonase that shows delayed degradation of the tetrad pectin wall. We found that MS188, a tapetum-specific transcription factor essential for pollen wall formation, can activate QRT3 expression for pectin wall degradation, indicating a non-cell-autonomous pathway involved in the regulation of the cell wall transition. Further assays showed that a delay in degradation of the tetrad pectin wall is responsible for the fertility restoration of rvms and other P/TGMS lines, whereas early expression of QRT3 eliminates low temperature restoration of rvms-2 fertility. Taken together, these results suggest a likely cellular mechanism of fertility restoration in P/TGMS lines, that is, slow development during the cell wall transition of P/TGMS microspores may reduce the requirement for their wall protection and thus support their development into functional pollens, leading to restored fertility., (Copyright © 2021 The Author. Published by Elsevier Inc. All rights reserved.)

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., (© The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

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.)