Pathogen Trojan Horse Delivers Bioactive Host Protein to Alter Maize Anther Cell Behavior in Situ

Receptor-like kinases and their signaling cascades for plant male fertility: loyal messengers

Receptor-like kinases (RLKs) are evolved for plant cell-cell communications. The typical RLK protein contains an extracellular and hypervariable N-terminus to perceive various signals, a transmembrane domain to anchor into plasma membrane, and a cytoplasmic, highly conserved kinase domain to phosphorylate target proteins. To date, RLKs have manifested their significance in a myriad of biological processes during plant reproductive growth, especially in male fertility. This review first summarizes a recent update on RLKs and their interacting protein partners controlling anther and pollen development, pollen release from dehisced anther, and pollen function during pollination and fertilization. Then, regulatory networks of RLK signaling pathways are proposed. In addition, we predict RLKs in maize and rice genome, obtain homologs of well-studied RLKs from phylogeny of three subfamilies and then analyze their expression patterns in developing anthers of maize and rice to excavate potential RLKs regulating male fertility in crops. Finally, current challenges and future prospects regarding RLKs are discussed. This review will contribute to a better understanding of plant male fertility control by RLKs, creating potential male sterile lines, and inspiring innovative crop breeding methods.

A Trojan Horse Approach Using Ustilago maydis to Study Apoplastic Maize (Zea mays) Peptides In Situ

Plant peptides are important signaling components in many parts of the plant lifecycle, e.g., development, reproduction, environmental stress response, and plant pathogen defenses. Yet, in maize, one of the most grown crops worldwide, only a few peptides have been identified and studied. In general, molecular research is severely impacted by time-consuming and costly maize transformation, and external application of purified peptides does not allow functional analysis in deeper cell layers due to the thickness of the tissue. In an attempt to bypass these problems while studying the function of small secreted proteins in maize, we established the Trojan Horse approach. Here, tagged peptides are delivered into the maize apoplast in a highly localized fashion by using a genetically modified version of the biotrophic pathogen Ustilago maydis. This technique offers the possibility of rapid testing of predicted maize peptides for in situ functions.

Endocytosis-mediated entry of a caterpillar effector into plants is countered by Jasmonate

Insects and pathogens release effectors into plant cells to weaken the host defense or immune response. While the imports of some bacterial and fungal effectors into plants have been previously characterized, the mechanisms of how caterpillar effectors enter plant cells remain a mystery. Using live cell imaging and real-time protein tracking, we show that HARP1, an effector from the oral secretions of cotton bollworm (Helicoverpa armigera), enters plant cells via protein-mediated endocytosis. The entry of HARP1 into a plant cell depends on its interaction with vesicle trafficking components including CTL1, PATL2, and TET8. The plant defense hormone jasmonate (JA) restricts HARP1 import by inhibiting endocytosis and HARP1 loading into endosomes. Combined with the previous report that HARP1 inhibits JA signaling output in host plants, it unveils that the effector and JA establish a defense and counter-defense loop reflecting the robust arms race between plants and insects.

ZmRPN1 confers quantitative variation in pollen number and boosts hybrid seed production in maize

The number of pollen grains is a critical determinant of reproductive success in seed plants and varies among species and individuals. However, in contrast with many mutant-screening studies relevant to anther and pollen development, the natural genetic basis for variations in pollen number remains largely unexplored. To address this issue, we carried out a genome-wide association study in maize, ultimately revealing that a large presence/absence variation in the promoter region of ZmRPN1 alters its expression level and thereby contributes to pollen number variation. Molecular analyses showed that ZmRPN1 interacts with ZmMSP1, which is known as a germline cell number regulator, and facilitates ZmMSP1 localization to the plasma membrane. Importantly, ZmRPN1 dysfunction resulted in a substantial increase in pollen number, consequently boosting seed production by increasing female-male planting ratio. Together, our findings uncover a key gene controlling pollen number, and therefore, modulation of ZmRPN1 expression could be efficiently used to develop elite pollinators for modern hybrid maize breeding.

Peptide signaling in anther development and pollen-stigma interactions

Polypeptides play irreplaceable roles in cell-cell communication by binding to receptor-like kinases. Various types of peptide-receptor-like kinase-mediated signaling have been identified in anther development and male-female interactions in flowering plants. Here, we provide a comprehensive summary of the biological functions and signaling pathways of peptides and receptors involved in anther development, self-incompatibility, pollen tube growth and pollen tube guidance.

Anther development-The long road to making pollen

Anthers express the most genes of any plant organ, and their development involves sequential redifferentiation of many cell types to perform distinctive roles from inception through pollen dispersal. Agricultural yield and plant breeding depend on understanding and consequently manipulating anthers, a compelling motivation for basic plant biology research to contribute. After stamen initiation, two theca form at the tip, and each forms an adaxial and abaxial lobe composed of pluripotent Layer 1-derived and Layer 2-derived cells. After signal perception or self-organization, germinal cells are specified from Layer 2-derived cells, and these secrete a protein ligand that triggers somatic differentiation of their neighbors. Historically, recovery of male-sterile mutants has been the starting point for studying anther biology. Many genes and some genetic pathways have well-defined functions in orchestrating subsequent cell fate and differentiation events. Today, new tools are providing more detailed information; for example, the developmental trajectory of germinal cells illustrates the power of single cell RNA-seq to dissect the complex journey of one cell type. We highlight ambiguities and gaps in available data to encourage attention on important unresolved issues.

Plant biomacromolecule delivery methods in the 21st century

The 21st century witnessed a boom in plant genomics and gene characterization studies through RNA interference and site-directed mutagenesis. Specifically, the last 15 years marked a rapid increase in discovering and implementing different genome editing techniques. Methods to deliver gene editing reagents have also attempted to keep pace with the discovery and implementation of gene editing tools in plants. As a result, various transient/stable, quick/lengthy, expensive (requiring specialized equipment)/inexpensive, and versatile/specific (species, developmental stage, or tissue) methods were developed. A brief account of these methods with emphasis on recent developments is provided in this review article. Additionally, the strengths and limitations of each method are listed to allow the reader to select the most appropriate method for their specific studies. Finally, a perspective for future developments and needs in this research area is presented.

Characterization of germline development and identification of genes associated with germline specification in pineapple

Understanding germline specification in plants could be advantageous for agricultural applications. In recent decades, substantial efforts have been made to understand germline specification in several plant species, including Arabidopsis, rice, and maize. However, our knowledge of germline specification in many agronomically important plant species remains obscure. Here, we characterized the female germline specification and subsequent female gametophyte development in pineapple using callose staining, cytological, and whole-mount immunolocalization analyses. We also determined the male germline specification and gametophyte developmental timeline and observed male meiotic behavior using chromosome spreading assays. Furthermore, we identified 229 genes that are preferentially expressed at the megaspore mother cell (MMC) stage during ovule development and 478 genes that are preferentially expressed at the pollen mother cell (PMC) stage of anther development using comparative transcriptomic analysis. The biological functions, associated regulatory pathways and expression patterns of these genes were also analyzed. Our study provides a convenient cytological reference for exploring pineapple germline development and a molecular basis for the future functional analysis of germline specification in related plant species.

Oxygen, secreted proteins and small RNAs: mobile elements that govern anther development

Correct anther development is essential for male fertility and subsequently agricultural yield. Defects in anther development range from the early stage of stamen formation until the late stage of tapetum degeneration. In particular, the specification of the four distinct somatic layers and the inner sporogenous cells need perfect orchestration relying on precise cell-cell communication. Up to now, several signals, which coordinate the anther´s developmental program, have been identified. Among the known signals are phytohormones, environmental conditions sensed via glutaredoxins, several receptor-like kinases triggered by ligands like MAC1, and small RNAs such as miRNAs and the monocot-prevalent reproductive phasiRNAs. Rather than giving a full review on anther development, here we discuss anther development with an emphasis on mobile elements like ROS/oxygen, secreted proteins and small RNAs (only briefly touching on phytohormones), how they might act and interact, and what the future of this research area might reveal.

Understanding Ustilago maydis Infection of Multiple Maize Organs

Ustilago maydis is a smut fungus that infects all aerial maize organs, namely, seedling leaves, tassels, and ears. In all organs, tumors are formed by inducing hypertrophy and hyperplasia in actively dividing cells; however, the vast differences in cell types and developmental stages for different parts of the plant requires that U. maydis have both general and organ-specific strategies for infecting maize. In this review, we summarize how the maize–U. maydis interaction can be studied using mutant U. maydis strains to better understand how individual effectors contribute to this interaction, either through general or specific expression in a cell type, tissue, or organ. We also examine how male sterile maize mutants that do not support tumor formation can be used to explore key features of the maize anthers that are required for successful infection. Finally, we discuss key unanswered questions about the maize–U. maydis interaction and how new technologies can potentially be used to answer them.

Overexpression of MaTPD1A impairs fruit and pollen development by modulating some regulators in Musa itinerans

BACKGROUND

Pollen formation and development is important for crop fertility and is a key factor for hybrid development. Previous reports have indicated that Arabidopsis thaliana TAPETUM DETERMINANT1 (AtTPD1) and its rice (Oryza sativa) homolog, OsTPD1-like (OsTDL1A), are required for cell specialization and greatly affect pollen formation and development. Little is known about the role of the TPD1 homolog in banana pollen development.

RESULTS

Here, we report the identification and characterization of TPD1 homologs in diploid banana (Musa itinerans) and examine their role in pollen development by overexpressing the closest homolog, MaTPD1A. MaTPD1A exhibits high expression in stamen and localizes in the plasma membrane. MaTPD1A-overexpressing plants produce no pollen grains and smaller and seedless fruit compared to wild-type plants. Transcriptome analysis showed that in plant hormone, starch and sucrose metabolism, and linolenic acid metabolism-related pathways were affected by overexpression of MaTPD1A, and the expression of several key regulators, such as PTC1 and MYB80, which are known to affect anther development, is affected in MaTPD1A-overexpressing lines.

CONCLUSIONS

Our results indicate that MaTPD1A plays an important role in pollen formation and fruit development in diploid banana, possibly by affecting the expression of some key regulators of pollen development.

Molecular Interactions Between Smut Fungi and Their Host Plants

Smut fungi are a large group of biotrophic plant pathogens that infect mostly monocot species, including economically relevant cereal crops. For years, Ustilago maydis has stood out as the model system to study the genetics and cell biology of smut fungi as well as the pathogenic development of biotrophic plant pathogens. The identification and functional characterization of secreted effectors and their role in virulence have particularly been driven forward using the U. maydis-maize pathosystem. Today, advancing tools for additional smut fungi such as Ustilago hordei and Sporisorium reilianum, as well as an increasing number of available genome sequences, provide excellent opportunities to investigate in parallel the effector function and evolution associated with different lifestyles and host specificities. In addition, genome analyses revealed similarities in the genomic signature between pathogenic smuts and epiphytic Pseudozyma species. This review elaborates on how knowledge about fungal lifestyles, genome biology, and functional effector biology has helped in understanding the biology of this important group of fungal pathogens. We highlight the contribution of the U. maydis model system but also discuss the differences from other smut fungi, which raises the importance of comparative genomic and genetic analyses in future research.

Pre-meiotic anther development

Most genetic and molecular analyses of anther development utilize Arabidopsis thaliana, Oryza sativa (rice), and Zea mays (maize). Especially in maize, early stages of anther development are easy to study because: (1) Maize has unisex flowers. (2) Compared to rice or A. thaliana, maize anthers are relatively large, making dissection for molecular and biochemical analyses easy. (3) Anther developmental stage is strongly correlated with maize anther length. Besides these technical advantages, understanding anther and pollen development in maize is of significant agricultural importance. Today maize is a worldwide cereal crop: approximately 25% of all consumed food contains maize. Yield stability or even increases depend on maintenance of hybrid vigor, and production of hybrid seed requires manual detasseling or genetic control of pollen development. Knowledge of pollen development can also be used to manage transgene containment. In the first section of this chapter, we will describe the current model for sequential cell fate specification in maize anther lobes, with reference to rice and A. thaliana to point out similarities and differences. In the second section of this chapter, we will review what is known about the individual cell types in anther lobes. The diversity of anther organization is addressed to a limited extent by cytological studies of anthers, often directed to clarify taxonomic relationships. In the third section, we will comment on how new lines of investigation could clarify questions remaining in our current appreciation of anther development.

Application of the pathogen Trojan horse approach in maize (Zea mays)

Maize, Zea mays, the second-most-widely-grown crop, yields 20 % of all consumed calories worldwide.1 Despite its agronomic importance, research progress is limited by costly transformation. We recently described the Trojan horse method as a useful tool to study maize proteins in situ that circumvents time- and space-consuming whole plant transformation. The Trojan horse approach uses the protein-folding and secretory properties of the corn smut fungus Ustilago maydis to secrete maize proteins from fungal cells into the maize apoplast. Here, we discuss the timing and location of U. maydis during infection and the protein secretion site in relation to anther anatomy. This spatiotemporal analysis enables the study of apoplastic anther proteins in various premeiotic anther developmental stages, and could be adapted for larger screens.

An apoplastic peptide activates salicylic acid signalling in maize

Localized control of cell death is crucial for the resistance of plants to pathogens. Papain-like cysteine proteases (PLCPs) regulate plant defence to drive cell death and protection against biotrophic pathogens. In maize (Zea mays), PLCPs are crucial in the orchestration of salicylic acid (SA)-dependent defence signalling. Despite this central role in immunity, it remains unknown how PLCPs are activated, and which downstream signals they induce to trigger plant immunity. Here, we discover an immune signalling peptide, Z. mays immune signalling peptide 1 (Zip1), which is produced after salicylic acid (SA) treatment. In vitro studies demonstrate that PLCPs are required to release bioactive Zip1 from its propeptide precursor. Conversely, Zip1 treatment strongly elicits SA accumulation in leaves. Moreover, transcriptome analyses revealed that Zip1 and SA induce highly overlapping transcriptional changes. Consequently, Zip1 promotes the infection of the necrotrophic fungus Botrytis cinerea, while it reduces virulence of the biotrophic fungus Ustilago maydis. Thus, Zip1 represents the previously missing signal that is released by PLCPs to activate SA defence signalling.