Analysis of the Maize dicer-like1 Mutant, fuzzy tassel, Implicates MicroRNAs in Anther Maturation and Dehiscence

The roles of non-coding RNAs in male reproductive development and abiotic stress responses during this unique process in flowering plants

Successful male reproductive development is the guarantee for sexual reproduction of flowering plants. Male reproductive development is a complicated and multi-stage process that integrates physiological processes and adaptation and tolerance to a myriad of environmental stresses. This well-coordinated process is governed by genetic and epigenetic machineries. Non-coding RNAs (ncRNAs) play pleiotropic roles in the plant growth and development. The identification, characterization and functional analysis of ncRNAs and their target genes have opened a new avenue for comprehensively revealing the regulatory network of male reproductive development and its response to environmental stresses in plants. This review briefly addresses the types, origin, biogenesis and mechanisms of ncRNAs in plants, highlights important updates on the roles of ncRNAs in regulating male reproductive development and emphasizes the contribution of ncRNAs, especially miRNAs and lncRNAs, in responses to abiotic stresses during this unique process in flowering plants.

Biosynthesis and transport of pollen coat precursors in angiosperms

The pollen coat is a hydrophobic mixture on the pollen grain surface, which plays an important role in protecting male gametes from various environmental stresses and microorganism attacks, and in pollen-stigma interactions during pollination in angiosperms. An abnormal pollen coat can result in humidity-sensitive genic male sterility (HGMS), which can be used in two-line hybrid crop breeding. Despite the crucial functions of the pollen coat and the application prospect of its mutants, few studies have focused on pollen coat formation. In this Review, the morphology, composition and function of different types of pollen coat are assessed. On the basis of the ultrastructure and development process of the anther wall and exine found in rice and Arabidopsis, the genes and proteins involved in the biosynthesis of pollen coat precursors and the possible transport and regulation process are sorted. Additionally, current challenges and future perspectives, including potential strategies utilizing HGMS genes in heterosis and plant molecular breeding, are highlighted.

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.

Heat shock protein 101 contributes to the thermotolerance of male meiosis in maize

High temperatures interfere with meiotic recombination and the subsequent progression of meiosis in plants, but few genes involved in meiotic thermotolerance have been characterized. Here, we characterize a maize (Zea mays) classic dominant male-sterile mutant Ms42, which has defects in pairing and synapsis of homologous chromosomes and DNA double-strand break (DSB) repair. Ms42 encodes a member of the heat shock protein family, HSP101, which accumulates in pollen mother cells. Analysis of the dominant Ms42 mutant and hsp101 null mutants reveals that HSP101 functions in RADIATION SENSITIVE 51 loading, DSB repair, and subsequent meiosis. Consistent with these functions, overexpression of Hsp101 in anthers results in robust microspores with enhanced heat tolerance. These results demonstrate that HSP101 mediates thermotolerance during microsporogenesis, shedding light on the genetic basis underlying the adaptation of male meiocytes to high temperatures.

Male sterile 28 encodes an ARGONAUTE family protein essential for male fertility in maize

Male sterility is a common biological phenomenon in plants and is a useful trait for hybrid seed production. Normal tapetum development is essential for viable pollen generation. Although many genes involved in tapetum differentiation and degradation have been isolated in maize, elements that regulate tapetum development during pollen mother cell (PMC) meiosis are less studied. Here, we characterized a classical male-sterile mutant male sterile 28 (ms28) in maize. The ms28 mutant had a regular male meiosis process, while its tapetum cells showed premature vacuolation at the early meiotic prophase stage. Using map-based cloning, we cloned the Ms28 gene and confirmed its role in male fertility in maize together with two allelic mutants. Ms28 encodes the ARGONAUTE (AGO) family protein ZmAGO5c, and its transcripts primarily accumulate in premeiosis anthers, with more intense signals in PMCs. Transcriptomic analysis revealed that genes related to anther development, cell division, and reproductive structure development processes were differentially expressed between the ms28 mutant and its fertile siblings. Moreover, small RNA (sRNA) sequencing revealed that the small interfering RNA (siRNA) and microRNA (miRNA) abundances were obviously changed in ms28 meiotic anthers, which indicated that Ms28 may regulate tapetal cell development through small RNA-mediated epigenetic regulatory pathways. Taken together, our results shed more light on the functional mechanisms of the early development of the tapetum for male fertility in maize.

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.

Perspectives on microRNAs and Phased Small Interfering RNAs in Maize (Zea mays L.): Functions and Big Impact on Agronomic Traits Enhancement

Small RNA (sRNA) population in plants comprises of primarily micro RNAs (miRNAs) and small interfering RNAs (siRNAs). MiRNAs play important roles in plant growth and development. The miRNA-derived secondary siRNAs are usually known as phased siRNAs, including phasiRNAs and tasiRNAs. The miRNA and phased siRNA biogenesis mechanisms are highly conserved in plants. However, their functional conservation and diversification may differ in maize. In the past two decades, lots of miRNAs and phased siRNAs have been functionally identified for curbing important maize agronomic traits, such as those related to developmental timing, plant architecture, sex determination, reproductive development, leaf morphogenesis, root development and nutrition, kernel development and tolerance to abiotic stresses. In contrast to Arabidopsis and rice, studies on maize miRNA and phased siRNA biogenesis and functions are limited, which restricts the small RNA-based fundamental and applied studies in maize. This review updates the current status of maize miRNA and phased siRNA mechanisms and provides a survey of our knowledge on miRNA and phased siRNA functions in controlling agronomic traits. Furthermore, improvement of those traits through manipulating the expression of sRNAs or their targets is discussed.

An artificial miRNA system reveals that relative contribution of translational inhibition to miRNA-mediated regulation depends on environmental and developmental factors in Arabidopsis thaliana

Development and fitness of any organism rely on properly controlled gene expression. This is especially true for plants, as their development is determined by both internal and external cues. MicroRNAs (miRNAs) are embedded in the genetic cascades that integrate and translate those cues into developmental programs. miRNAs negatively regulate their target genes mainly post-transcriptionally through two co-existing mechanisms; mRNA cleavage and translational inhibition. Despite our increasing knowledge about the genetic and biochemical processes involved in those concurrent mechanisms, little is known about their relative contributions to the overall miRNA-mediated regulation. Here we show that co-existence of cleavage and translational inhibition is dependent on growth temperature and developmental stage. We found that efficiency of an artificial miRNA-mediated (amiRNA) gene silencing declines with age during vegetative development in a temperature-dependent manner. That decline is mainly due to a reduction on the contribution from translational inhibition. Both, temperature and developmental stage were also found to affect mature amiRNA accumulation and the expression patterns of the core players involved in miRNA biogenesis and action. Therefore, that suggests that each miRNA family specifically regulates their respective targets, while temperature and growth might influence the performance of miRNA-dependent regulation in a more general way.

Non-coding RNAs and plant male sterility: current knowledge and future prospects

Latest outcomes assign functional role to non-coding (nc) RNA molecules in regulatory networks that confer male sterility to plants. Male sterility in plants offers great opportunity for improving crop performance through application of hybrid technology. In this respect, cytoplasmic male sterility (CMS) and sterility induced by photoperiod (PGMS)/temperature (TGMS) have greatly facilitated development of high-yielding hybrids in crops. Participation of non-coding (nc) RNA molecules in plant reproductive development is increasingly becoming evident. Recent breakthroughs in rice definitively associate ncRNAs with PGMS and TGMS. In case of CMS, the exact mechanism through which the mitochondrial ORFs exert influence on the development of male gametophyte remains obscure in several crops. High-throughput sequencing has enabled genome-wide discovery and validation of these regulatory molecules and their target genes, describing their potential roles performed in relation to CMS. Discovery of ncRNA localized in plant mtDNA with its possible implication in CMS induction is intriguing in this respect. Still, conclusive evidences linking ncRNA with CMS phenotypes are currently unavailable, demanding complementing genetic approaches like transgenics to substantiate the preliminary findings. Here, we review the recent literature on the contribution of ncRNAs in conferring male sterility to plants, with an emphasis on microRNAs. Also, we present a perspective on improved understanding about ncRNA-mediated regulatory pathways that control male sterility in plants. A refined understanding of plant male sterility would strengthen crop hybrid industry to deliver hybrids with improved performance.

Germline Development and Fertilization Mechanisms in Maize

Maize is the most important agricultural crop used for food, feed, and biofuel as well as a raw material for industrial products such as packaging material. To increase yield and to overcome hybridization barriers, studies of maize gamete development, the pollen tube journey, and fertilization mechanisms were initiated more than a century ago. In this review, we summarize and discuss our current understanding of the regulatory components for germline development including sporogenesis and gametogenesis, the progamic phase of pollen germination and pollen tube growth and guidance, as well as fertilization mechanisms consisting of pollen tube arrival and reception, sperm cell release, fusion with the female gametes, and egg cell activation. Mechanisms of asexual seed development are not considered here. While only a few molecular players involved in these processes have been described to date and the underlying mechanisms are far from being understood, maize now represents a spearhead of reproductive research for all grass species. Recent development of essentially improved transformation and gene-editing systems may boost research in this area in the near future.

SMARTER De-Stressed Cereal Breeding

In cereal breeding programs, improved yield potential and stability are ultimate goals when developing new varieties. To facilitate achieving these goals, reproductive success under stressful growing conditions is of the highest priority. In recent times, small RNA (sRNA)-mediated pathways have been associated with the regulation of genes involved in stress adaptation and reproduction in both model plants and several cereals. Reproductive and physiological traits such as flowering time, reproductive branching, and root architecture can be manipulated by sRNA regulatory modules. We review sRNA-mediated pathways that could be exploited to expand crop diversity with adaptive traits and, in particular, the development of high-yielding stress-tolerant cereals: SMARTER cereal breeding through 'Small RNA-Mediated Adaptation of Reproductive Targets in Epigenetic Regulation'.