Liu et al. Horticulture Research (2019) 6:88 Horticulture Research https://doi.org/10.1038/s41438-019-0170-2 www.nature.com/hortres ARTICLE Open Access A putative bHLH transcription factor is a candidategeneformale sterile 32,alocus affecting pollen and tapetum development in tomato Xiaoyan Liu1,2,MengxiaYang2, Xiaolin Liu2,KaiWei2,XueCao2, Xiaotian Wang2,XiaoxuanWang2,YanmeiGuo2, Yongchen Du2,JunmingLi2,LeiLiu2,JinshuaiShu2,YongQin1 and Zejun Huang 2 Abstract The tomato (Solanum lycopersicum) male sterile 32 (ms32) mutant has been used in hybrid seed breeding programs largely because it produces no pollen and has exserted stigmas. In this study, histological examination of anthers revealed dysfunctional pollen and tapetum development in the ms32 mutant. The ms32 locus was fine mapped to a 28.5 kb interval that encoded four putative genes. Solyc01g081100, a homolog of Arabidopsis bHLH10/89/90 and rice EAT1, was proposed to be the candidate gene of MS32 because it contained a single nucleotide polymorphism (SNP) that led to the formation of a premature stop codon. A codominant derived cleaved amplified polymorphic sequence (dCAPS) marker, MS32D, was developed based on the SNP. Real-time quantitative reverse-transcription PCR showed that most of the genes, which were proposed to be involved in pollen and tapetum development in tomato, were downregulated in the ms32 mutant. These findings may aid in marker-assisted selection of ms32 in hybrid breeding 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; programs and facilitate studies on the regulatory mechanisms of pollen and tapetum development in tomato. Introduction can produce normal pollen, but the pollen cannot reach Tomato (Solanum lycopersicum) is one of the most the stigma because of indehiscent anthers or dehiscent important vegetable crops in the world. Hybrid vigor has anthers with morphological abnormalities, such as posi- been extensively utilized in tomato production because tional sterile (ps)7, ps-28, and exserted stigma (ex)9. F1-hybrid cultivars usually exhibit a greater yield and Sporogenous male-sterile mutants exhibit relatively nor- higher disease resistance than open-pollinated varieties1,2. mal floral morphology but produce little or no fertile Male sterility has proven to be a useful trait for hybrid pollen. Most tomato male-sterile mutants belong to this seed production3. Male-sterile mutants are classified as type10. Thus, fine mapping and identifying the genes for structural, functional, and sporogenous types4. Structural sporogenous male-sterile mutants of tomato will not only male-sterile mutants of tomato usually bear extremely help us understand the regulation of pollen development deformed stamens and do not produce pollen, such as but also facilitate the use of these mutants in hybrid seed stamenless (sl)5 and sl-26. Functional male-sterile mutants breeding programs. Pollen development has been widely studied in Arabi- dopsis and rice (Oryza sativa)11. Several transcription Correspondence: Yong Qin ([email protected]) or Zejun Huang ([email protected]) factor genes have been identified as regulators involved in 1College of Forestry and Horticulture, Xinjiang Agricultural University, 830052 pollen development in Arabidopsis, such as DYSFUNC- Urumqi, China TIONAL TAPETUM1 (DYT1, referred to as AtDYT1 2Key Laboratory of Biology and Genetic Improvement of Horticultural Crops of the Ministry of Agriculture, Institute of Vegetables and Flowers, Chinese hereafter), DEFECTIVE TAPETUM DEVELOPMENT Academy of Agricultural Sciences, 100086 Beijing, China © The Author(s) 2019 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a linktotheCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Liu et al. Horticulture Research (2019) 6:88 Page 2 of 11 AND FUNCTION1 (TDF1, AtTDF1, syn. AtMYB35), breeding programs and facilitate studies on the regulatory ABORTED MICROSPORES (AMS, AtAMS), basic helix- mechanisms of pollen and tapetum development in loop-helix 10 (bHLH10, AtbHLH10), bHLH89 tomato. (AtbHLH89), bHLH91 (AtbHLH91), and MYB80 – (AtMYB80, syn. AtMYB103)12 17. AtDYT1 encodes a Results bHLH transcription factor that directly regulates the Phenotypic investigation of the ms32 mutant expression of AtTDF118. AtTDF1 affects the expression of The morphologies of the ms32 mutant and WT plants AtAMS19. AtAMS protein can interact with AtbHLH89/ were the same until the flowering stage. At the flowering 91 and regulate the expression of AtMYB8020,21. The stage, the ms32 mutant had longer sepals and shorter interaction between AtDYT1 and AtbHLH10/89/91 is petals than the WT (Fig. 1). In addition, the stamens in also important for the regulation of pollen development22. ms32 flowers were greatly reduced and shrunken and pale Therefore, they form a DYT1-TDF1-AMS-bHLH10/89/ in color, usually with the stigma remaining exserted 91-MYB80 transcriptional cascade to precisely regulate (Fig. 1). Pollen viability analysis detected no pollen in the pollen and tapetum development in Arabidopsis17,23,24.A flowers of the ms32 mutant (Fig. 1). The ms32 mutant similar signaling pathway including the UDT1-TDR1- could not set fruit after self-pollination with an electric TIP2-EAT1 transcriptional cascade is also involved in the regulation of pollen and tapetum development in rice18. UDT1 (UNDEVELOPED TAPETUM1, OsUDT1)isa homolog of AtDYT125, TDR1 (TAPETUM DEGENERA- TION RETARDATION1, OsTDR1) is a homolog of AtAMS26, and TIP2 (TDR INTERACTING PROTEIN2, OsTIP2) and EAT1 (ETERNAL TAPETUM 1, OsEAT1) are homologs of AtbHLH10/89/9018,27. These findings suggest that the signaling pathway regulating pollen and tapetum development is highly conserved in higher plants23. In tomato, until now, only one regulator gene, SlMS10, a homolog of AtDYT1 and OsUDT1, has been found to be involved in pollen and tapetum development. A putative regulatory network for pollen and tapetum development in tomato, which is similar to the networks in Arabidopsis and rice, was presented based on the evolutionary relationships and conserved functions of the genes differentially expressed between the wild type (WT) and male sterile 1035 (ms1035)24. However, except for SlMS10, the regulators in the network need further investigation. The tomato ms32 mutant is completely sterile because it does not produce pollen28. It has been used in hybrid seed breeding programs3. In this study, histological examination revealed dysfunctional microgametogenesis and tapetum degradation in the ms32 mutant. The ms32 locus was fine mapped to a 28.5 kb interval that encoded four putative genes. Solyc01g081100 was proposed to be the candidate gene of MS32 as it contained a single nucleotide polymorphism (SNP) that resulted in the for- mation of a premature stop codon. Solyc01g081100 is a homolog of AtbHLH10/89/90 and OsEAT1. Real-time quantitative reverse-transcription PCR (qPCR) showed Fig. 1 Flower phenotypes and pollen production of the ms32 that most of the regulator genes, which were proposed to mutant. a, b Flower of WT (a) and ms32 plants (b). Scale bars, 1 cm. c, be involved in pollen and tapetum development, were d Analysis of pollen viability by acetocarmine staining of pollen from downregulated in the ms32 mutant. However, the anthers of WT (c) and ms32 plants (d). Scale bars, 50 μm. e Length of expression of SlMS10 did not show a significant difference floral organs in WT and ms32 flowers at anthesis. Values represent the fi means ± SEs. Asterisks indicate a significant difference (p < 0.05) in the ms32 mutant compared to the WT. These ndings between the WT and ms32 mutant may help in marker-assisted selection of ms32 in hybrid Liu et al. Horticulture Research (2019) 6:88 Page 3 of 11 vibrator, but it was able to produce fruit when manually determined to be homozygous for the ms32 mutant and pollinated with pollen from WT plants (Fig. S1). These for just one of these two markers, were selected for fur- findings were consistent with a previous study of ms3228. ther genotyping. Based on the genotype and phenotype of these recombinants, the ms32 locus was narrowed to an Histological examination of ms32 anthers interval between markers LXY1 and LXY5, a 28.5 kb Histological examination of anthers at different devel- interval on chromosome 1 (Fig. 3c). opmental stages was performed to detect the spatial and temporal occurrence of defects in ms32 anthers. At the Candidate gene analysis premeiotic stage, the cell layer differentiation in ms32 Four putative genes were identified in the 28.5 kb region anthers seemed to be similar to that in WT anthers that contained the ms32 locus by searching the tomato (Fig. 2a, d). At the meiotic stage of WT anthers, spor- genome annotation database (ITAG release 3.2) in the ogenous cells developed in pollen mother cells (PMCs), Sol Genomics Network (SGN, https://solgenomics.net/; and meiosis was initiated and completed (Fig. 2b). How- Fig. 3d and Table 1). qPCR analysis revealed that ever, PMCs began to crush and separate from each other Solyc01g081090 and Solyc01g081100 were highly expres- in ms32 anthers (Fig. 2e). At the tetrad, microspore, sed in stamens (Fig. S3). However, the expression of mitotic and dehiscence stages, dramatic changes in mor- Solyc01g081080 was barely detected in any organs ana- phology were detected between WT and ms32 anthers. In lyzed in this study (Fig. S3), and the expression of WT anthers, PMCs successively developed into tetrads Solyc01g081110 in sepals and petals was higher than that (Fig.