Genetic Structure and Molecular Mechanisms Underlying the Formation of Tassel, Anther, and Pollen in the Male Inflorescence of Maize (Zea mays L.)

OsLAP3/OsSTRL2, encoding a rice strictosidine synthase, is required for anther cuticle formation and pollen exine patterning in rice

The formation of the anther wall and the development of pollen processes, central to rice fertility and yield, are highly dependent on the synthesis and accumulation of lipid polymers. Although several regulatory factors related to lipid biosynthesis during pollen wall development have been identified, the molecular mechanisms controlling these processes remain poorly understood. In this study, a male-sterile rice mutant, lap3, was identified, characterized by normal vegetative growth but complete male sterility due to delayed programmed cell death (PCD) in tapetal cells and defects in anther cuticle and pollen exine formation. Map-based cloning revealed that OsLAP3 is a new allele of the strictosidine synthase-like gene, OsSTRL2. Functional analysis, including complementation and CRISPR/Cas9-based gene editing, confirmed that the 2-nucleotide deletion in the OsLAP3 is responsible for the male sterility phenotype. OsLAP3 is homologous to the maize ZmMS45, the core recessive nuclear sterile gene of maize Seed Production Technology (SPT), and localizes to the endoplasmic reticulum and plays a conserved role in anther development and pollenformation. Gene expression analysis revealed a significant downregulation of key genes involved in anther development and sporopollenin biosynthesis in lap3 anthers. Furthermore, lipid profiling demonstrated a marked reduction in both wax and cutin content. These findings establish OsLAP3 as a critical regulator of fatty acid synthesis and highlight its role in anther cuticle formation and pollen exine development. The findings of this study provide valuable insights into the molecular regulation of lipid biosynthesis during rice male reproductive development and offer potential applications for hybrid rice breeding.

Ac/Ds-like Transposon Elements Inserted in ZmABCG2a Cause Male Sterility in Maize

Using male sterile (MS) lines instead of normal inbred maternal lines in hybrid seed production can increase the yield and quality with lower production costs. Therefore, developing a new MS germplasm is essential for maize hybrid seed production in the future. Here, we reported a male sterility gene ms*-N125, cloned from a newly found MS mutant ms*-N125. This mutant has an underdeveloped tassel that showed impaired glumes and shriveled anthers without pollen grains. The MS locus of ms*-N125 was mapped precisely to a 112-kb-interval on the chromosome 5. This interval contains only three candidate genes, Zm958, Zm959, and Zm960. Sequencing results showed that only candidate Zm960 harbored a 548-bp transposable element (TE) in its 9th exon, and the two other candidate genes were found to have no genetic variations between the mutant and wild type (WT). Thus, Zm960 is the only candidate gene for male sterility of the mutant ms*-N125. In addition, we screened another recessive MS mutant, ms*-P884, which exhibited similar male sterility phenotypes to ms*-N125. Sequencing Zm960 in ms*-P884 showed a 600-bp TE located in its 2nd exon. Zm960 encodes an ATP-binding cassette in the G subfamily of ABC (ABCG) transporters, ZmABCG2a, with both mutants which harbored an Ac/Ds-like transposon in each. To verify the function of ZmABCG2a for male sterility further, we found an ethyl methanesulfonate (EMS) mutant, zmabcg2a*, which displayed male sterility and tassel phenotypes highly similar to ms*-N125 and ms*-P884, confirming that ZmABCG2a must be the gene for male sterility in maize. In addition, the results of lipid metabolome analysis of ms*-N125 young tassels showed that the total lipid content of the mutant was significantly lower than that of the WT, with 15 subclasses of lipids, including PE (phosphatidylethanolamine), PC (phosphatidylcholine), DG (digalactosyldiacylglycerols), and MGDG (monogalactosyldiacylglycerol) which were significantly down-regulated in the ms*-N125 mutant versus its wild type. In summary, we identified alternate mutations of the ZmABCG2a gene, which may be a potential germplasm for hybrid seed production in maize.

Mining Candidate Genes for Maize Tassel Spindle Length Based on a Genome-Wide Association Analysis

Maize tassel spindle length is closely related to the number of pollen grains and the duration of the flowering stage, ultimately affecting maize yield and adaptations to stress conditions. In this study, 182 maize inbred lines were included in an association population. A genome-wide association study was conducted on maize tassel spindle length using the Q + K model. With p ≤ 1.0 × 10-4 applied as the significance threshold, 240 SNPs significantly associated with tassel spindle length were detected, which were associated with 99 quantitative trait loci (QTLs), with 21 QTLs detected in two or more environments. Moreover, 51 candidate genes were detected in 21 co-localized QTLs. A KEGG enrichment analysis and candidate gene expression analysis indicated that Zm00001d042312 affects plant hormone signal transduction and is highly expressed in maize tassels. A haplotype analysis of Zm00001d042312 revealed three main haplotypes, with significant differences between Hap1 and Hap2. In conclusion, we propose that Zm00001d042312 is a gene that regulates maize tassel spindle length. This study has further elucidated the genetic basis of maize tassel spindle length, while also providing excellent genetic targets and germplasm resources for the genetic improvement of maize tassel spindle length and yield.

Striking a growth-defense balance: Stress regulators that function in maize development

Maize (Zea mays) cultivation is strongly affected by both abiotic and biotic stress, leading to reduced growth and productivity. It has recently become clear that regulators of plant stress responses, including the phytohormones abscisic acid (ABA), ethylene (ET), and jasmonic acid (JA), together with reactive oxygen species (ROS), shape plant growth and development. Beyond their well established functions in stress responses, these molecules play crucial roles in balancing growth and defense, which must be finely tuned to achieve high yields in crops while maintaining some level of defense. In this review, we provide an in-depth analysis of recent research on the developmental functions of stress regulators, focusing specifically on maize. By unraveling the contributions of these regulators to maize development, we present new avenues for enhancing maize cultivation and growth while highlighting the potential risks associated with manipulating stress regulators to enhance grain yields in the face of environmental challenges.

Deploying QTL-seq rapid identification and separation of the major QTLs of tassel branch number for fine-mapping in advanced maize populations

The tassel competes with the ear for nutrients and shields the upper leaves, thereby reducing the yield of grain. The tassel branch number (TBN) is a pivotal determinant of tassel size, wherein the reduced TBN has the potential to enhance the transmission of light and reduce the consumption of nutrients, which should ultimately result in increased yield. Consequently, the TBN has emerged as a vital target trait in contemporary breeding programs that focus on compact maize varieties. In this study, QTL-seq technology and advanced population mapping were used to rapidly identify and dissect the major effects of the TBN on QTL. Advanced mapping populations (BC4F2 and BC4F3) were derived from the inbred lines 18-599 (8-11 TBN) and 3237 (0-1 TBN) through phenotypic recurrent selection. First, 13 genomic regions associated with the TBN were detected using quantitative trait locus (QTL)-seq and were located on chromosomes 2 and 5. Subsequently, validated loci within these regions were identified by QTL-seq. Three QTLs for TBN were identified in the BC4F2 populations by traditional QTL mapping, with each QTL explaining the phenotypic variation of 6.13-18.17%. In addition, for the major QTL (qTBN2-2 and qTBN5-1), residual heterozygous lines (RHLs) were developed from the BC4F2 population. These two major QTLs were verified in the RHLs by QTL mapping, with the phenotypic variation explained (PVE) of 21.57% and 30.75%, respectively. Near-isogenic lines (NILs) of qTBN2-2 and qTBN5-1 were constructed. There were significant differences between the NILs in TBN. These results will enhance our understanding of the genetic basis of TBN and provide a solid foundation for the fine-mapping of TBN.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01431-y.

Evolution of cereal floral architecture and threshability

Hulled grains, while providing natural protection for seeds, pose a challenge to manual threshing due to the pair of glumes tightly encasing them. Based on natural evolution and artificial domestication, gramineous crops evolved various hull-like floral organs. Recently, progress has been made in uncovering novel domesticated genes associated with cereal threshability and deciphering common regulatory modules pertinent to the specification of hull-like floral organs. Here we review morphological similarities, principal regulators, and common mechanisms implicated in the easy-threshing traits of crops. Understanding the shared and unique features in the developmental process of cereal threshability may not only shed light on the convergent evolution of cereals but also facilitate the de novo domestication of wild cereal germplasm resources through genome-editing technologies.

The Genetic Structures and Molecular Mechanisms Underlying Ear Traits in Maize (Zea mays L.)

Maize (Zea mays L.) is one of the world's staple food crops. In order to feed the growing world population, improving maize yield is a top priority for breeding programs. Ear traits are important determinants of maize yield, and are mostly quantitatively inherited. To date, many studies relating to the genetic and molecular dissection of ear traits have been performed; therefore, we explored the genetic loci of the ear traits that were previously discovered in the genome-wide association study (GWAS) and quantitative trait locus (QTL) mapping studies, and refined 153 QTL and 85 quantitative trait nucleotide (QTN) clusters. Next, we shortlisted 19 common intervals (CIs) that can be detected simultaneously by both QTL mapping and GWAS, and 40 CIs that have pleiotropic effects on ear traits. Further, we predicted the best possible candidate genes from 71 QTL and 25 QTN clusters that could be valuable for maize yield improvement.

The ZmMYB84-ZmPKSB regulatory module controls male fertility through modulating anther cuticle-pollen exine trade-off in maize anthers

Anther cuticle and pollen exine are two crucial lipid layers that ensure normal pollen development and pollen-stigma interaction for successful fertilization and seed production in plants. Their formation processes share certain common pathways of lipid biosynthesis and transport across four anther wall layers. However, molecular mechanism underlying a trade-off of lipid-metabolic products to promote the proper formation of the two lipid layers remains elusive. Here, we identified and characterized a maize male-sterility mutant pksb, which displayed denser anther cuticle but thinner pollen exine as well as delayed tapetal degeneration compared with its wild type. Based on map-based cloning and CRISPR/Cas9 mutagenesis, we found that the causal gene (ZmPKSB) of pksb mutant encoded an endoplasmic reticulum (ER)-localized polyketide synthase (PKS) with catalytic activities to malonyl-CoA and midchain-fatty acyl-CoA to generate triketide and tetraketide α-pyrone. A conserved catalytic triad (C171, H320 and N353) was essential for its enzymatic activity. ZmPKSB was specifically expressed in maize anthers from stages S8b to S9-10 with its peak at S9 and was directly activated by a transcription factor ZmMYB84. Moreover, loss function of ZmMYB84 resulted in denser anther cuticle but thinner pollen exine similar to the pksb mutant. The ZmMYB84-ZmPKSB regulatory module controlled a trade-off between anther cuticle and pollen exine formation by altering expression of a series of genes related to biosynthesis and transport of sporopollenin, cutin and wax. These findings provide new insights into the fine-tuning regulation of lipid-metabolic balance to precisely promote anther cuticle and pollen exine formation in plants.

Analysis of QTLs and Candidate Genes for Tassel Symptoms in Maize Infected with Sporisorium reilianum

Heat smut is a fungal soil-borne disease caused by Sporisorium reilianum, and affects the development of male and female tassels. Our previous research found that the tassel symptoms in maize infected with Sporisorium reilianum significantly differed in inbred lines with Sipingtou blood, and exhibited stable heredity over time at multiple locations. In this study, cytological analysis demonstrated that the cellular organization structures of three typical inbred lines (Huangzao4, Jing7, and Chang7-2) showed significant discrepancies at the VT stage. QTLs that control the different symptoms of maize tassels infected with Sporisorium reilianum were located in two F₂ populations, which were constructed using three typical inbred lines. The BSA (bulked segregation analysis) method was used to construct mixed gene pools based on typical tassel symptoms. The QTLs of different symptoms of maize tassels infected with Sporisorium reilianum were detected with 869 SSR markers covering the whole maize genome. The mixed gene pools were screened with polymorphic markers between the parents. Additional SSR markers were added near the above marker to detect genotypes in partially single plants in F₂ populations. The QTL controlling tassel symptoms in the Huangzao4 and Jing7 lines was located on the bin 1.06 region, between the markers of umc1590 and bnlg1598, and explained 21.12% of the phenotypic variation with an additive effect of 0.6524. The QTL controlling the tassel symptoms of the Jing7 and Chang7-2 lines was located on the bin 2.07 region, between the markers of umc1042 and bnlg1335, and explained 11.26% phenotypic variation with an additive effect of 0.4355. Two candidate genes (ZmABP2 and Zm00001D006403) were identified by a conjoint analysis of label-free quantification proteome sequencings.