Pollen grains are important to plant male reproductive organs. The growth and development of pollen grains are closely related to the differentiation of the microspore mother cells, the degradation of the tapetum cells, and the formation of the callose and pollen walls [22]. Mutations in genes related to pollen grain development or environmental changes can also cause abnormal pollen development, resulting in male sterility. Previous studies have found that the DYT1-TDF1-AMS-MYB80 gene pathway strictly controls tapetum development and plays an important role in maintaining normal degradation of tapetum cells and ensuring pollen fertility. This has been confirmed in A. thaliana, rice, tomatoes, and asparagus [23–26].
The TDF1 gene belongs to the DYT1-TDF1-AMS-MYB80 pathway. Mutation or deletion of its base sequence leads to abnormal degradation of tapetum cells, leading to an imbalance in nutrient metabolism and, eventually, to pollen abortion. Zhu et al. created a mutant of A. thaliana attdf1 using EMS mutagenesis technology. Compared to the tapetum cells of wild A. thaliana, the tapetum cells of the stamen anthers were swollen, vacuolated, and excessively divided, preventing them from being unable to transform into the secretory tapetum. Simultaneously, callose around the tetrad was deposited for a long time and the microspores could not be released in time, leading to the failure to form pollen grains, resulting in pollen abortion [27]. The tapetum cell development of ogu-CMS in black cabbage was studied, and the tapetum cell development characteristics were like those of the rice mutant ostdf1. Tapetum vacuoles rapidly form after microspore meiosis, and they cannot secrete enough callose enzyme, so most microspores cannot be released in time. Even if a few microspores are released, the tapetum cells squeeze them until they degenerate and disappear, indicating male sterility. Furthermore, overexpression of the rapeseed BcTDF1 gene in the A. thaliana attdf1 mutant restored the pollen fertility of the A. thaliana attdf1 mutant to some extent and produced some mature and fertile pollen grains [28]. The ability of the tdf1 mutant to induce plant pollen abortion was confirmed in this study. In this study, TaTDF1 was overexpressed in A. thaliana and dominant chimerism was used to suppress the silencing of the TDF1 gene. It was found that the TaTDF1-OE and TDF1-EAR transgenic lines bloomed normally, and the average bolting time of the TaTDF1-OE overexpression lines was 7 d earlier. The results of Alexander staining of the anthers are shown in red. However, the silencing lines of the TDF1-EAR dominant chimera suppression delayed the bolting period by an average of 4 d. Its anthers are shriveled and thin, and Alexander staining of the anthers is blue-green. Virus-induced gene-silencing technology for TaTDF1 silencing in wheat showed that the anthers of the wheat-silent strain tatdf1 were shriveled, thin, and dull in the loose pollen stage; the seed-setting rate decreased significantly. DAPI staining of pollen grains revealed abnormal nuclear development, 82.05% were seedless or unable to form normal fish-like sperm nuclei. However, 17.95% of the pollen grains developed into mature and fertile fish-like sperm nuclei (Fig. 18). After 1% I2-KI staining, most pollen grains were yellow-brown, and a few pollen grains remained blue-black. The results showed that downregulation of TaTDF1 could induce pollen abortion in wheat. Tapetum cells from the ostdf1 gene knockout mutant showed a vacuolated phenotype similar to the Arabidopsis attdf1 mutant, and TUNEL stained nuclei were not observed in anthers. PCD defects were confirmed in the tapetum cells of ostdf1 anthers, leading to pollen abortion [18].
In summary, these findings suggest that MYB family transcription factor TDF1 may play a key role in regulating tapetal development and late function during wheat anther development. Similar to AtTDF1 of Arabidopsis thaliana, TDF1 affects pollen fertility by regulating tapetal production and degradation. Thus, the process of tapetum development in wheat may be similar to that of Arabidopsis thaliana, which is controlled by the conserved genetic pathway DYT1-TDF1-AMS-MYB80-MS1. In this study, the protein TaMAP65 interacting with TaTDF1 was screened by yeast two-hybrid test. Further qRT-PCR was performed on tatdf1 and tamap65 silent strains, and the results preliminarily indicated that TaTDF1 was located in the upstream of TaMP65, which was a supplement to the above genetic pathway (Fig. 19). Unfortunately, this study could not further study the regulatory relationship between DYT1 and MAP65 to further improve this genetic pathway. The process of pollen abortion in tatdf1 silenced strains could not be studied during the process of tapetum cell development. And their relations could not be illustrated. Compared with MF-90-110, the expression level of TaTDF1 in MS-90-110 was lower, suggesting that TaTDF1 may be related to pollen abortion in wheat K-type male sterile lines. This results was confirmed in previous study. Wu et al. showed that TaTDRL ( AtAMS homolog gene) is down-regulated during the development of anther in MS-90-110 [21]. Meanwhile, AtTDF1 is the upstream gene of AtAMS in Arabidopsis thaliana, indicating TaTDRL is possibly located at downstream of TaTDF1. It can be inferred that TaTDF1 and TaTDRL are also key genes involved in pollen abortion during the development of anther in MS-90-110. But this hypothesis needs further study.
Jasmonic acid (JA) is an important hormone that is widely involved in plant growth and development, metabolic regulation, stress, and defense responses [29]. Among these, MYB transcription factors promote anther dehiscence by interacting with methyl jasmonate to ensure normal anther development and pollen fertility. Qi et al. found that the development of JA-mediated stamen and grain setting were regulated by the bHLH-MYB complex. Among them, the bHLH transcription factor MYC5, the target gene of the JAZ repressor, plays an important regulatory role in stamen development and grain setting, together with the other bHLH factors MYC2, MYC3, and MYC4. Furthermore, the bHLH transcription factor MYC5 interacts with the MYB transcription factors MYB21 and MYB24 to form a bHLH-MYB transcription complex that synergistically regulates stamen development [30]. The R2R3-MYB transcription factors MYB21, MYB24, and MYB108 have been shown to affect stamen development by regulating the JA content. Its abnormal expression can lead to filament elongation, delayed anther dehiscence, and decreased pollen viability, eventually leading to the abortion of male pollen [31–33]. MYB21 and MYB24 are direct targets of JAZs, and their functions are affected by their content. Once the JA signal was detected, COI1 recruited JAZs into the SCF(COI1) complex and used the 26S proteasome for ubiquitination and degradation. MYB21 and MYB24 are released to activate the expression of various genes for JA-regulated anther development and filament elongation [33]. Both LoMYB21 in silent lilies and overexpression of LoMYB21 in Arabidopsis caused abnormal JA content and signal transduction, delayed anther dehiscence, reduced Jasmonic acid accumulation, and pollen abortion [34]. On top of that, Abscisic acid is also an important plant hormone that regulates plant resistance to various biotic and abiotic stresses. These include drought, cold, and salt stress, seed dormancy, and pollen fertility [35]. The pollen activity of plants (L4) overexpressing AtNCED3 decreased. The results of semi-thin sections and electron microscope observations of anthers showed that tapetum cells in L4 degraded too fast, microspores could not absorb nutrients, and their pollen fertility decreased. However, after exogenous spraying of ABA, the surface of the wax content on the anther surface increased and nutrients in the tapetum cells were continuously supplied, thus improving the fertility of the pollen grains [36]. The atu2af65b mutant showed an early flowering phenotype under both long- and short-d conditions, and the transcript of the flowering inhibition gene FLC decreased in the shoot tip of atu2af65b. This was mainly because of abnormal splicing of ABI5 and a decrease in transcript abundance. Furthermore, ABA promotes the expression of AtU2AF65b during ABA-induced flowering. However, the transition and splicing of FLC and ABI5 in the atu2af65b mutant were impaired; therefore, they could not normally induce male germ cell development [37]. Plants overexpressing the ABA-dependent SnRK2 kinase members SAPK8 and ABF1 showed delayed flowering. It is more sensitive to exogenous spraying of ABA-mediated flowering inhibition, whereas simultaneously knocking out ABF1 and its homologous gene bZIP40 can promote flowering [38]. These results indicate that JA and ABA hormones interact with MYB and bZIP transcription factors, respectively, by recognizing the CGTCA, TGACG-motif, and ABRE (ACGTG/CACGTG) motifs. This interferes with gene expression, affecting pollen fertility. This result is discussed preliminarily in this study.
In this study, the regulatory elements of the TaTDF1 promoter region were analyzed using PlantCARE and NewPLACE. Two cis-acting regulators, the CGTCA motif and the TGACG-motif, related to the methyl jasmonate reaction; one cis-acting regulator, ABRE (ACGTG/CACGTG), related to the abscisic acid reaction; four photoperiod response elements, GT1-motif, ATCT motif, G-box, and GA-motif; and three cis-acting regulators, ARE, GC-motif, and MBS, related to adversity stress, were identified (Fig. 20; Table S4). Among these, MeJA and ABRE cis-acting elements may interact with the upstream MYB and bZIP transcription factors to regulate the development of anthers or pollen grains. Therefore, Yeast One-Hybrid (Y1H) and Electrophoretic Mobility Shift Assay (EMSA) experiments can be used to further study the degradation of transcription factors interacting with MeJA and ABRE elements in TaTDF1 promoter region in wheat tapetum cells and the regulatory mechanism during anther or pollen grain development. This study provides new insights into the extraction of key genes that regulate the development of male reproductive organs in wheat.