Phenylpropanoid Derivatives Are Essential Components of Sporopollenin in Vascular Plants

Comparative proteomic analysis of papaya bud flowers reveals metabolic signatures and pathways driving hermaphrodite development

Papaya (Carica papaya) is a trioecious species with female, male, and hermaphrodite plants. Given the sex segregation, selecting hermaphroditic plants is vital for orchard establishment due to their greater commercial value. However, selecting hermaphrodite plants through sexing is laborious and costly. Moreover, environmental stressors can exacerbate the issue by potentially inducing abnormal flower development, thus affecting fruit quality. Despite these challenges, the molecular mechanisms governing sex development in papaya remain poorly understood. Thus, this study aimed to identify proteins associated with sex development in female and hermaphrodite flowers of papaya through comparative proteomic analysis. Proteins from flower buds at the early and late developmental stages of three papaya genotypes (UENF-CALIMAN 01, JS12, and Sunrise Solo 72/12) were studied via proteomic analysis via the combination of the shotgun method and nanoESI-HDMSE technology. In buds at an early stage of development, 496 (35.9%) proteins exhibited significantly different abundances between sexes for the SS72/12 genotype, 139 (10%) for the JS12 genotype, and 165 (11.9%) for the UC-01 genotype. At the final stage of development, there were 181 (13.5%) for SS72/12, 113 (8.4%) for JS12, and 125 (9.1%) for UC-01. The large group of differentially accumulated proteins (DAPs) between the sexes was related to metabolism, as shown by the observation of only the proteins that exhibited the same pattern of accumulation in the three genotypes. Specifically, carbohydrate metabolism proteins were up-regulated in hermaphrodite flower buds early in development, while those linked to monosaccharide and amino acid metabolism increased during late development. Enrichment of sporopollenin and phenylpropanoid biosynthesis pathways characterizes hermaphrodite samples across developmental stages, with predicted protein interactions highlighting the crucial role of phenylpropanoids in sporopollenin biosynthesis for pollen wall formation. Most of the DAPs played key roles in pectin, cellulose, and lignin synthesis and were essential for cell wall formation and male flower structure development, notably in the pollen coat. These findings suggest that hermaphrodite flowers require more energy for development, likely due to complex pollen wall formation. Overall, these insights illuminate the molecular mechanisms of papaya floral development, revealing complex regulatory networks and energetic demands in the formation of male reproductive structures.

The Effects of Heavy Metal Pollution on Soil Nitrogen Transformation and Rice Volatile Organic Compounds under Different Water Management Practices

One of the most concerning global environmental issues is the pollution of agricultural soils by heavy metals (HMs), especially cadmium, which not only affects human health through Cd-containing foods but also impacts the quality of rice. The soil's nitrification and denitrification processes, coupled with the release of volatile organic compounds by plants, raise substantial concerns. In this review, we summarize the recent literature related to the deleterious effects of Cd on both soil processes related to the N cycle and rice quality, particularly aroma, in different water management practices. Under both continuous flooding (CF) and alternate wetting and drying (AWD) conditions, cadmium has been observed to reduce both the nitrification and denitrification processes. The adverse effects are more pronounced in alternate wetting and drying (AWD) as compared to continuous flooding (CF). Similarly, the alteration in rice aroma is more significant in AWD than in CF. The precise modulation of volatile organic compounds (VOCs) by Cd remains unclear based on the available literature. Nevertheless, HM accumulation is higher in AWD conditions compared to CF, leading to a detrimental impact on volatile organic compounds (VOCs). The literature concludes that AWD practices should be avoided in Cd-contaminated fields to decrease accumulation and maintain the quality of the rice. In the future, rhizospheric engineering and plant biotechnology can be used to decrease the transport of HMs from the soil to the plant's edible parts.

Gh4CL20/20A involved in flavonoid biosynthesis is essential for male fertility in cotton (Gossypium hirsutum L.)

Flavonoids have been shown to play an essential role in plant growth and fertility. 4-Coumarate CoA ligase (4CL) is one of the indispensable enzymes involved in the biosynthesis of flavonoids. However, the role of 4CL and flavonoids in impact on cotton fertility is still unknown. In this study, on the basis of identification of an additional Gh4CL gene, Gh4CL20A, by using an updated G. hirsutum genome, we found that Gh4CL20A and its homologous Gh4CL20 were preferentially expressed in petals and stamens. The petals of the loss-of-function Gh4CL20/Gh4CL20A mutant generated by CRISPR/Cas9 gene editing remained white until wilting. Notably, the mutant showed indehiscent anthers, reduced number of pollen grains and pollen viability, leading to male sterility. Histological analysis revealed that abnormal degradation of anther tapetum at the tetrad stage and abnormal pollen grain development at the mature stage caused male sterility of the gene editing mutant. Analysis of the anther transcriptome identified a total of 10574 and 11962 genes up- and down-regulated in the mutant, respectively, compared to the wild-type. GO, KEGG, and WGCNA analyses linked the abnormality of the mutant anthers to the defective flavonoid biosynthetic pathway, leading to decreased activity of 4CL and chalcone isomerase (CHI) and reduced accumulation of flavonoids in the mutant. These results imply a role of Gh4CL20/Gh4CL20A in assuring proper development of cotton anthers by regulating flavonoid metabolism. This study elucidates a molecular mechanism underlying cotton anther development and provides candidate genes for creating cotton male sterile germplasm that has the potential to be used in production of hybrid seeds.

Temperature and light reverse the fertility of rice P/TGMS line ostms19 via reactive oxygen species homeostasis

P/TGMS (Photo/thermo-sensitive genic male sterile) lines are crucial resources for two-line hybrid rice breeding. Previous studies revealed that slow development is a general mechanism for sterility-fertility conversion of P/TGMS in Arabidopsis. However, the difference in P/TGMS genes between rice and Arabidopsis suggests the presence of a distinct P/TGMS mechanism in rice. In this study, we isolated a novel P/TGMS line, ostms19, which shows sterility under high-temperature conditions and fertility under low-temperature conditions. OsTMS19 encodes a novel pentatricopeptide repeat (PPR) protein essential for pollen formation, in which a point mutation GTA(Val) to GCA(Ala) leads to ostms19 P/TGMS phenotype. It is highly expressed in the tapetum and localized to mitochondria. Under high temperature or long-day photoperiod conditions, excessive ROS accumulation in ostms19 anthers during pollen mitosis disrupts gene expression and intine formation, causing male sterility. Conversely, under low temperature or short-day photoperiod conditions, ROS can be effectively scavenged in anthers, resulting in fertility restoration. This indicates that ROS homeostasis is critical for fertility conversion. This relationship between ROS homeostasis and fertility conversion has also been observed in other tested rice P/TGMS lines. Therefore, we propose that ROS homeostasis is a general mechanism for the sterility-fertility conversion of rice P/TGMS lines.

Simultaneous suppression of lignin, tricin and wall-bound phenolic biosynthesis via the expression of monolignol 4-O-methyltransferases in rice

Grass lignocelluloses feature complex compositions and structures. In addition to the presence of conventional lignin units from monolignols, acylated monolignols and flavonoid tricin also incorporate into lignin polymer; moreover, hydroxycinnamates, particularly ferulate, cross-link arabinoxylan chains with each other and/or with lignin polymers. These structural complexities make grass lignocellulosics difficult to optimize for effective agro-industrial applications. In the present study, we assess the applications of two engineered monolignol 4-O-methyltransferases (MOMTs) in modifying rice lignocellulosic properties. Two MOMTs confer regiospecific para-methylation of monolignols but with different catalytic preferences. The expression of MOMTs in rice resulted in differential but drastic suppression of lignin deposition, showing more than 50% decrease in guaiacyl lignin and up to an 90% reduction in syringyl lignin in transgenic lines. Moreover, the levels of arabinoxylan-bound ferulate were reduced by up to 50%, and the levels of tricin in lignin fraction were also substantially reduced. Concomitantly, up to 11 μmol/g of the methanol-extractable 4-O-methylated ferulic acid and 5-7 μmol/g 4-O-methylated sinapic acid were accumulated in MOMT transgenic lines. Both MOMTs in vitro displayed discernible substrate promiscuity towards a range of phenolics in addition to the dominant substrate monolignols, which partially explains their broad effects on grass phenolic biosynthesis. The cell wall structural and compositional changes resulted in up to 30% increase in saccharification yield of the de-starched rice straw biomass after diluted acid-pretreatment. These results demonstrate an effective strategy to tailor complex grass cell walls to generate improved cellulosic feedstocks for the fermentable sugar-based production of biofuel and bio-chemicals.

Sporopollenin exine capsules modulate the function of microglial cells

Immune cells are the housekeepers of the human body. They protect the body from pathogens, cellular damage, and foreign matter. Proper activation of immune cells is of great significance to diseases such as infection, inflammation, and neurodegeneration. However, excessive activation of cells can be detrimental. An ideal biomaterial could enhance the cellular immune function without proinflammation. In this work, we used sporopollenin exine capsules (SEC) from pollen to promote functions of primary microglia, a typical resident immune cell of the brain. We found that microglia aggregated around SEC and did not undergo any proinflammation. SEC improved the viability, migration, phagocytosis, and anti-inflammatory ability of microglia. By exploring the underlying mechanism of microglial activation without the production of cytotoxic pro-inflammatory cytokines, we found that SEC protects microglia against inflammation induced by lipopolysaccharide (LPS), an immunostimulatory factor, through the toll-like receptor 4 (TLR4) signaling pathway in a myeloid differentiation factor 88-dependent manner. These findings might shed light on the potential application of SEC in microglia transplantation for treatment of microglia-associated degenerative central nervous system diseases.

CRISPR/Cas9-based genome editing of 14 lipid metabolic genes reveals a sporopollenin metabolon ZmPKSB-ZmTKPR1-1/-2 required for pollen exine formation in maize

Lipid biosynthesis and transport are essential for plant male reproduction. Compared with Arabidopsis and rice, relatively fewer maize lipid metabolic genic male-sterility (GMS) genes have been identified, and the sporopollenin metabolon in maize anther remains unknown. Here, we identified two maize GMS genes, ZmTKPR1-1 and ZmTKPR1-2, by CRISPR/Cas9 mutagenesis of 14 lipid metabolic genes with anther stage-specific expression patterns. Among them, tkpr1-1/-2 double mutants displayed complete male sterility with delayed tapetum degradation and abortive pollen. ZmTKPR1-1 and ZmTKPR1-2 encode tetraketide α-pyrone reductases and have catalytic activities in reducing tetraketide α-pyrone produced by ZmPKSB (polyketide synthase B). Several conserved catalytic sites (S128/130, Y164/166 and K168/170 in ZmTKPR1-1/-2) are essential for their enzymatic activities. Both ZmTKPR1-1 and ZmTKPR1-2 are directly activated by ZmMYB84, and their encoded proteins are localized in both the endoplasmic reticulum and nuclei. Based on protein structure prediction, molecular docking, site-directed mutagenesis and biochemical assays, the sporopollenin biosynthetic metabolon ZmPKSB-ZmTKPR1-1/-2 was identified to control pollen exine formation in maize anther. Although ZmTKPR1-1/-2 and ZmPKSB formed a protein complex, their mutants showed different, even opposite, defective phenotypes of anther cuticle and pollen exine. Our findings discover new maize GMS genes that can contribute to male-sterility line-assisted maize breeding and also provide new insights into the metabolon-regulated sporopollenin biosynthesis in maize anther.

Xylan clustering on the pollen surface is required for exine patterning

Xylan is a crosslinking polymer that plays an important role in the assembly of heterogeneous cell wall structures in plants. The pollen wall, a specialized cell wall matrix, exhibits diverse sculpted patterns that serve to protect male gametophytes and facilitate pollination during plant reproduction. However, whether xylan is precisely anchored into clusters and its influence on pollen wall patterning remain unclear. Here, we report xylan clustering on the mature pollen surface in different plant species that is indispensable for the formation of sculpted exine patterns in dicot and monocot plants. Chemical composition analyses revealed that xylan is generally present at low abundance in the mature pollen of flowering plants and shows plentiful variations in terms of substitutions and modifications. Consistent with the expression profiles of their encoding genes, genetic characterization revealed IRREGULAR XYLEM10-LIKE (IRX10L) and its homologous proteins in the GT47 family of glycosyltransferases as key players in the formation of these xylan micro-/nano-compartments on the pollen surface in Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa). A deficiency in xylan biosynthesis abolished exine patterning on pollen and compromised male fertility. Therefore, our study outlines a mechanism of exine patterning and provides a tool for manipulating male fertility in crop breeding.

VAMP726 from maize and Arabidopsis confers pollen resistance to heat and UV radiation by influencing lignin content of sporopollenin

Sporopollenin in the pollen cell wall protects male gametophytes from stresses. Phenylpropanoid derivatives, including guaiacyl (G) lignin units, are known to be structural components of sporopollenin, but the exact composition of sporopollenin remains to be fully resolved. We analyzed the phenylpropanoid derivatives in sporopollenin from maize and Arabidopsis by thioacidolysis coupled with nuclear magnetic resonance (NMR) and gas chromatography-mass spectrometry (GC-MS). The NMR and GC-MS results confirmed the presence of p-hydroxyphenyl (H), G, and syringyl (S) lignin units in sporopollenin from maize and Arabidopsis. Strikingly, H units account for the majority of lignin monomers in sporopollenin from these species. We next performed a genome-wide association study to explore the genetic basis of maize sporopollenin composition and identified a vesicle-associated membrane protein (ZmVAMP726) that is strongly associated with lignin monomer composition of maize sporopollenin. Genetic manipulation of VAMP726 affected not only lignin monomer composition in sporopollenin but also pollen resistance to heat and UV radiation in maize and Arabidopsis, indicating that VAMP726 is functionally conserved in monocot and dicot plants. Our work provides new insight into the lignin monomers that serve as structural components of sporopollenin and characterizes VAMP726, which affects sporopollenin composition and stress resistance in pollen.

Pollen variability in Quercus L. species and relative systematic implications

This study aims to address one of the challenges related to the complexity of the Quercus L. genus, that is the identification of structural elements favouring the systematic identification of the oak pollen. Thus, in this contribution, we explored the variation of morphometric and chemical parameters in pollen samples collected from 47 different Quercus species and hybrids. Several qualitative (e.g., outline in polar view, class, aperture structures) and quantitative (e.g., diameter, exine and sporoderm thickness, autofluorescence, content in proteins and plant metabolites) features were evaluated by optic microscopy and spectrophotometric assays. Statistical analyses were also carried out to assess significant correlations and clustering effects among the studied taxa, based on phenotypical and biochemical data, to identify the parameters which could be useful for taxonomic discrimination at inter- and intra-specific level. Only few morphological traits showed the potentiality to be diagnostic, such as pollen diameter and outline in polar view. The intensity of pollen autofluorescence varied among the samples but it did not seem to correlate with protein, carotenoid, phenolic and flavonoid content. However, differences in protein and carotenoid levels were detected, suggesting them as possible taxonomic discriminants for oak pollen. Thus, our work represents a step forward in understanding morphology and biochemistry of oak pollen, constitutes an experimental set-up applicable in future systematic studies on other genera, and opens new perspectives for further molecular investigations on Quercus species.

Phenylpropanoid Derivatives and Their Role in Plants' Health and as antimicrobials

Phenylpropanoids belong to a wide group of compounds commonly secreted by plants and involved in different roles related with plant growth and development and the defense against plant pathogens. Some key intermediates from shikimate pathway are used to synthesize these compounds. In this way, by the phenylpropanoid pathway several building blocks are achieved to obtain flavonoids, isoflavonoids, coumarins, monolignols, phenylpropenes, phenolic acids, stilbenes and stilbenoids, and lignin, suberin and sporopollenin for plant-microbe interactions, structural support and mechanical strength, organ pigmentation, UV protection and acting against pathogens. Some reviews have revised phenylpropanoid biosynthesis and regulation of the biosynthetic pathways. In this review, the most important chemical structures about phenylpropanoid derivatives are summarized grouping them in different sections according to their structure. We have put special attention on their different roles in plants especially in plant health, growth and development and plant-environment interactions. Their interaction with microorganisms is discussed including their role as antimicrobials. We summarize all new findings about new developed structures and their involvement in plants health.

Response of salt stress resistance in highland barley (Hordeum vulgare L. var. nudum) through phenylpropane metabolic pathway

Highland barley (Hordeum vulgare L. var. nudum) is a grain crop that grows on the plateau under poor and high salt conditions. Therefore, to cultivate high-quality highland barley varieties, it is necessary to study the molecular mechanism of strong resistance in highland barley, which has not been clearly explained. In this study, a high concentration of NaCl (240 mmol/L), simulating the unfavorable environment, was used to spray the treated highland barley seeds. Transcriptomic analysis revealed that the expression of more than 8,000 genes in highland barley seed cells was significantly altered, suggesting that the metabolic landscape of the cells was deeply changed under salt stress. Through the KEGG analysis, the phenylpropane metabolic pathway was significantly up-regulated under salt stress, resulting in the accumulation of polyphenols, flavonoids, and lignin, the metabolites for improving the stress resistance of highland barley seed cells, being increased 2.71, 1.22, and 1.17 times, respectively. This study discovered that the phenylpropane metabolic pathway was a significant step forward in understanding the stress resistance of highland barley, and provided new insights into the roles of molecular mechanisms in plant defense.

OsTKPR1 proteins with a single amino acid substitution fail the synthesis of a specific sporopollenin precursor and cause abnormal exine and pollen development in rice

Fatty acid derivatives are key components of rice pollen exine. The synthesis of aliphatic sporopollenin precursors are initiated in the plastids of the tapetal cells, followed by multiple-step reactions conducted in the endoplasmic reticulum (ER). However, the relative contribution of different precursors to the precise structure of sporopollenin remains largely elusive, let alone the underlying mechanism. Here, we report that two complete male sterile mutants ostkpr1-3 (Tetraketide α-pyrone reductase 1-3, with OsTKPR1P124S substitution) and ostkpr1-4 (with truncated OsTKPR1stop) are defective in pollen exine, Ubisch body and anther cuticle development where ostkpr1-4 display severer phenotypes. Remarkably, OsTKPR1 could produce reduced hydroxylated tetraketide α-pyrone and reduced tetraketide α-pyrone, whereas OsTKPR1P124S fails to produce the latter. Pairwise interaction assays show that mutated OsTKPR1P124S is able to integrate into a recently characterized metabolon, thus its altered catalytic activity is not due to dis-integrity of the metabolon. In short, we find that reduced tetraketide α-pyrone is a key sporopollenin precursor required for normal exine formation, and the conserved 124th proline of OsTKPR1 is essential for the reduction activity. Therefore, this study provided new insights into the sporopollenin precursor constitution critical for exine formation.

Stepwise changes in flavonoids in spores/pollen contributed to terrestrial adaptation of plants

Protecting haploid pollen and spores against UV-B light and high temperature, 2 major stresses inherent to the terrestrial environment, is critical for plant reproduction and dispersal. Here, we show flavonoids play an indispensable role in this process. First, we identified the flavanone naringenin, which serves to defend against UV-B damage, in the sporopollenin wall of all vascular plants tested. Second, we found that flavonols are present in the spore/pollen protoplasm of all euphyllophyte plants tested and that these flavonols scavenge reactive oxygen species to protect against environmental stresses, particularly heat. Genetic and biochemical analyses showed that these flavonoids are sequentially synthesized in both the tapetum and microspores during pollen ontogeny in Arabidopsis (Arabidopsis thaliana). We show that stepwise increases in the complexity of flavonoids in spores/pollen during plant evolution mirror their progressive adaptation to terrestrial environments. The close relationship between flavonoid complexity and phylogeny and its strong association with pollen survival phenotypes suggest that flavonoids played a central role in the progression of plants from aquatic environments into progressively dry land habitats.

Multifaceted roles of pollen in the management of cancer

Oral drug delivery of microparticles demonstrates shortcomings like aggregation, decreased loading capacity and batch-to-batch variation, which limits its scale-up. Later, porous structures gained attention because of their large surface-to-volume ratio, high loading capacity and ability to carry biomacromolecules, which undergo degradation in GIT. But there are pitfalls like non-uniform particle size distribution, the impact of porogen properties, and harsh chemicals. To circumvent these drawbacks, natural carriers like pollen are explored in drug delivery, which withstands harsh environments. This property helps to subdue the acid-sensitive drug in GIT. It shows uniform particle size distribution within the species. On the other side, they contain phytoconstituents like flavonoids and polysaccharides, which possess various pharmacological applications. Therefore, pollen has the capability as a carrier system and therapeutic agent. This review focuses on pollen's microstructure, composition and utility in cancer management. The extraction strategies, characterisation techniques and chemical structure of sporopollenin exine capsule, its use in the oral delivery of antineoplastic drugs, and emerging cancer treatments like photothermal therapy, immunotherapy and microrobots have been highlighted. We have mentioned a note on the anticancer activity of pollen extract. Further, we have summarised the regulatory perspective, bottlenecks and way forward associated with pollen.

Microspore-expressed SCULP1 is required for p-coumaroylation of sporopollenin, exine integrity, and pollen development in wheat

Sporopollenin is one of the most structurally sophisticated and chemically recalcitrant biopolymers. In higher plants, sporopollenin is the dominant component of exine, the outer wall of pollen grains, and contains covalently linked phenolics that protect the male gametes from harsh environments. Although much has been learned about the biosynthesis of sporopollenin precursors in the tapetum, the nutritive cell layer surrounding developing microspores, little is known about how the biopolymer is assembled on the microspore surface. We identified SCULP1 (SKS clade universal in pollen) as a seed plant conserved clade of the multicopper oxidase family. We showed that SCULP1 in common wheat (Triticum aestivum) is specifically expressed in the microspore when sporopollenin assembly takes place, localized to the developing exine, and binds p-coumaric acid in vitro. Through genetic, biochemical, and 3D reconstruction analyses, we demonstrated that SCULP1 is required for p-coumaroylation of sporopollenin, exine integrity, and pollen viability. Moreover, we found that SCULP1 accumulation is compromised in thermosensitive genic male sterile wheat lines and its expression partially restored exine integrity and male fertility. These findings identified a key microspore protein in autonomous sporopollenin polymer assembly, thereby laying the foundation for elucidating and engineering sporopollenin biosynthesis.

Functional and developmental convergence in the reproductive "nurse cells" of flowering plants

The successful sexual reproduction of flowering plants depends upon double fertilisation, during which pollen grains, produced within the male floral organ (the anther) deliver two sperm cells to the ovule, buried deep within the ovary, triggering the development of the embryo and the surrounding tissues of the seed. Although much attention has been given to pollen and embryo development, less has been focused on the supporting tissues surrounding these organisms as they develop, the tapetum and the endosperm. Intriguingly, despite their very different origins, these tissues appear to have converged functionally and developmentally. Here we will discuss this apparent convergence and its molecular and physiological basis.

Interactive effects of changes in UV radiation and climate on terrestrial ecosystems, biogeochemical cycles, and feedbacks to the climate system

Terrestrial organisms and ecosystems are being exposed to new and rapidly changing combinations of solar UV radiation and other environmental factors because of ongoing changes in stratospheric ozone and climate. In this Quadrennial Assessment, we examine the interactive effects of changes in stratospheric ozone, UV radiation and climate on terrestrial ecosystems and biogeochemical cycles in the context of the Montreal Protocol. We specifically assess effects on terrestrial organisms, agriculture and food supply, biodiversity, ecosystem services and feedbacks to the climate system. Emphasis is placed on the role of extreme climate events in altering the exposure to UV radiation of organisms and ecosystems and the potential effects on biodiversity. We also address the responses of plants to increased temporal variability in solar UV radiation, the interactive effects of UV radiation and other climate change factors (e.g. drought, temperature) on crops, and the role of UV radiation in driving the breakdown of organic matter from dead plant material (i.e. litter) and biocides (pesticides and herbicides). Our assessment indicates that UV radiation and climate interact in various ways to affect the structure and function of terrestrial ecosystems, and that by protecting the ozone layer, the Montreal Protocol continues to play a vital role in maintaining healthy, diverse ecosystems on land that sustain life on Earth. Furthermore, the Montreal Protocol and its Kigali Amendment are mitigating some of the negative environmental consequences of climate change by limiting the emissions of greenhouse gases and protecting the carbon sequestration potential of vegetation and the terrestrial carbon pool.

Extraordinary microcarriers derived from spores and pollens

Spores and pollens refer to the reproductive cells of seed plants and asexually reproducing sporophytes, exhibiting a natural core-shell structure and exquisite surface morphology. They possess extraordinary dimensional homogeneity, porosity, amphiphilicity and adhesion. Their sporopollenin exine layer endows them with chemically stable, UV resistant, and biocompatible properties, which can also be facilely functionalized due to sufficient groups on the surface. The unique characteristics of spores and pollens have facilitated a wide range of applications in drug carriers, biological imaging, food science, microrobotics, environmental purification, flexible electronics, cell scaffolds, 3D printing materials and biological detection. This review showcases the common structural composition and physicochemical properties of spores and pollens, describes the extraction and processing methods, and summarizes the recent research on their applications in various fields. Following these sections, this review analyzes the existing challenges in spores and pollen research and provides a future outlook.

OsCCRL1 is Essential for Phenylpropanoid Metabolism in Rice Anthers

Phenylpropanoid metabolism and timely tapetal degradation are essential for anther and pollen development, but the underlying mechanisms are unclear. In the current study, to investigate this, we identified and analyzed the male-sterile mutant, osccrl1 (cinnamoyl coA reductase-like 1), which exhibited delayed tapetal programmed cell death (PCD) and defective mature pollen. Map-based cloning, genetic complementation, and gene knockout revealed that OsCCRL1 corresponds to the gene LOC_Os09g32020.2, a member of SDR (short-chain dehydrogenase/reductase) family enzyme. OsCCRL1 was preferentially expressed in the tapetal cells and microspores, and localized to the nucleus and cytoplasm in both rice protoplasts and Nicotiana benthamiana leaves. The osccrl1 mutant exhibited reduced CCRs enzyme activity, less lignin accumulation, delayed tapetum degradation, and disrupted phenylpropanoid metabolism. Furthermore, an R2R3 MYB transcription factor OsMYB103/OsMYB80/OsMS188/BM1, involved in tapetum and pollen development, regulates the expression of OsCCRL1. Finally, the osmyb103 osccrl1 double mutants, exhibited the same phenotype as the osmyb103 single mutant, further indicating that OsMYB103/OsMYB80/OsMS188/BM1 functions upstream of OsCCRL1. These findings help to clarify the role of phenylpropanoid metabolism in male sterility and the regulatory network underlying the tapetum degradation.

BrACOS5 mutations induced male sterility via impeding pollen exine formation in Chinese cabbage (Brassica rapa L. ssp. pekinensis)

BrACOS5 mutations led to male sterility of Chinese cabbage verified in three allelic male-sterile mutants. Chinese cabbage (Brassica rapa L. ssp. pekinensis) is one of the major vegetable crops in East Asia, and the utilization of male-sterile line is an important measure for its hybrid seed production. Herein, we isolated three allelic male-sterile mutants, msm1-1, msm1-2 and msm1-3, from an ethyl methane sulfonate (EMS) mutagenized population of Chinese cabbage double-haploid (DH) line 'FT', whose microspores were completely aborted with severely absent exine, and tapetums were abnormally developed. Genetic analyses indicated that the three male-sterile mutants belonged to allelic mutation and were triggered by the same recessive nuclear gene. MutMap-based gene mapping and kompetitive allele-specific PCR (KASP) analysis demonstrated that three different single-nucleotide polymorphisms (SNPs) of BraA09g012710.3C were responsible for the male sterility of msm1-1/2/3, respectively. BraA09g012710.3C is orthologous of Arabidopsis thaliana ACOS5 (AT1G62940), encoding an acyl-CoA synthetase in sporopollenin biosynthesis, and specifically expressed in anther, so we named BraA09g012710.3C as BrACOS5. BrACOS5 localizes to the endoplasmic reticulum (ER). Mutations of BrACOS5 resulted in decreased enzyme activities and altered fatty acid contents in msm1 anthers. As well as the transcript accumulations of putative orthologs involved in sporopollenin biosynthesis were significantly down-regulated excluding BrPKSA. These results provide strong evidence for the integral role of BrACOS5 in conserved sporopollenin biosynthesis pathway and also contribute to uncovering exine development pattern and underlying male sterility mechanism in Chinese cabbage.

Anther development in Arabidopsis thaliana involves symplastic isolation and apoplastic gating of the tapetum-middle layer interface

During flowering plant reproduction, anthers produce pollen grains, the development of which is supported by the tapetum, a nourishing maternal tissue that also contributes non-cell-autonomously to the pollen wall, the resistant external layer on the pollen surface. How the anther restricts movement of the tapetum-derived pollen wall components, while allowing metabolites such as sugars and amino acids to reach the developing pollen, remains unknown. Here, we show experimentally that in arabidopsis thaliana the tapetum and developing pollen are symplastically isolated from each other, and from other sporophytic tissues, from meiosis onwards. We show that the peritapetal strip, an apoplastic structure, separates the tapetum and the pollen grains from other anther cell layers and can prevent the apoplastic diffusion of fluorescent proteins, again from meiosis onwards. The formation and selective barrier functions of the peritapetal strip require two NADPH oxidases, RBOHE and RBOHC, which play a key role in pollen formation. Our results suggest that, together with symplastic isolation, gating of the apoplast around the tapetum may help generate metabolically distinct anther compartments.

The transcription factor TaGAMYB modulates tapetum and pollen development of TGMS wheat YanZhan 4110S via the gibberellin signaling

Male reproductive development in higher plants experienced a series of complex biological processes, which can be regulated by Gibberellins (GA). The transcriptional factor GAMYB is a crucial component of GA signaling in anther development. However, the mechanism of GAMYB in wheat male reproduction is less understood. Here, we found that the thermo-sensitive genic male sterilitywheat line YanZhan 4110S displayed delayed tapetum programmed cell death and pollen abortive under the hot temperature stress. Combined with RNA-Sequencing data analysis, TaGAMYB associated with fertility conversion was isolated, which was located in the nucleus and highly expressed in fertility anthers. The silencing of TaGAMYB in wheat displayed fertility decline, defects in tapetum, pollen and exine formation, where the abortion characteristics were the same as YanZhan 4110S. In addition, either hot temperature or GA3 treatment in YanZhan 4110S caused the downregulation of TaGAMYB at binucleate stage and trinucleate stage, as well as fertility decrease. Further, the transcription factor TaWRKY2 significantly changed under GA3-treatment and directly interacted with the TaGAMYB promoter by W-box cis-element. Therefore, we suggested that TaGAMYB may be essential for anther development and male fertility, and GA3 activates TaGAMYB by TaWRKY2 to regulate fertility in wheat.

bHLH010/089 Transcription Factors Control Pollen Wall Development via Specific Transcriptional and Metabolic Networks in Arabidopsis thaliana

The pollen wall is a specialized extracellular cell wall that protects male gametophytes from various environmental stresses and facilitates pollination. Here, we reported that bHLH010 and bHLH089 together are required for the development of the pollen wall by regulating their specific downstream transcriptional and metabolic networks. Both the exine and intine structures of bhlh010 bhlh089 pollen grains were severely defective. Further untargeted metabolomic and transcriptomic analyses revealed that the accumulation of pollen wall morphogenesis-related metabolites, including polysaccharides, glyceryl derivatives, and flavonols, were significantly changed, and the expression of such metabolic enzyme-encoding genes and transporter-encoding genes related to pollen wall morphogenesis was downregulated in bhlh010 bhlh089 mutants. Among these downstream target genes, CSLB03 is a novel target with no biological function being reported yet. We found that bHLH010 interacted with the two E-box sequences at the promoter of CSLB03 and directly activated the expression of CSLB03. The cslb03 mutant alleles showed bhlh010 bhlh089–like pollen developmental defects, with most of the pollen grains exhibiting defective pollen wall structures.

Mutation of glucose-methanol-choline oxidoreductase leads to thermosensitive genic male sterility in rice and Arabidopsis

Thermosensitive genic male sterility (TGMS) lines serve as the major genetic resource for two-line hybrid breeding in rice. However, their unstable sterility under occasional low temperatures in summer highly limits their application. In this study, we identified a novel rice TGMS line, ostms18, of cultivar ZH11 (Oryza sativa ssp. japonica). ostms18 sterility is more stable in summer than the TGMS line carrying the widely used locus tms5 in the ZH11 genetic background, suggesting its potential application for rice breeding. The ostms18 TGMS trait is caused by the point mutation from Gly to Ser in a glucose-methanol-choline (GMC) oxidoreductase; knockout of the oxidoreductase was previously reported to cause complete male sterility. Cellular analysis revealed the pollen wall of ostms18 to be defective, leading to aborted pollen under high temperature. Further analysis showed that the tapetal transcription factor OsMS188 directly regulates OsTMS18 for pollen wall formation. Under low temperature, the flawed pollen wall in ostms18 is sufficient to protect its microspore, allowing for development of functional pollen and restoring fertility. We identified the orthologous gene in Arabidopsis. Although mutants for the gene were fertile under normal conditions (24°C), fertility was significantly reduced under high temperature (28°C), exhibiting a TGMS trait. A cellular mechanism integrated with genetic mutations and different plant species for fertility restoration of TGMS lines is proposed.

Triphasic regulation of ZmMs13 encoding an ABCG transporter is sequentially required for callose dissolution, pollen exine and anther cuticle formation in maize

INTRODUCTION

ATP Binding Cassette G (ABCG) transporters are associated with plant male reproduction, while their regulatory mechanisms underlying anther and pollen development remain largely unknown.

OBJECTIVES

Identify and characterize a male-sterility gene ZmMs13 encoding an ABCG transporter in modulating anther and pollen development in maize.

METHODS

Phenotypic, cytological observations, and histochemistry staining were performed to characterize the ms13-6060 mutant. Map-based cloning and CRISPR/Cas9 gene editing were used to identify ZmMs13 gene. RNA-seq data and qPCR analyses, phylogenetic and microsynteny analyses, transient dual-luciferase reporter and EMSA assays, subcellular localization, and ATPase activity and lipidomic analyses were carried out to determine the regulatory mechanisms of ZmMs13 gene.

RESULTS

Maize ms13-6060 mutant displays complete male sterility with delayed callose degradation, premature tapetal programmed cell death (PCD), and defective pollen exine and anther cuticle formation. ZmMs13 encodes a plasm membrane (PM)- and endoplasmic reticulum (ER)-localized half-size ABCG transporter (ZmABCG2a). The allele of ZmMs13 in ms13-6060 mutant has one amino acid (I311) deletion due to a 3-bp deletion in its fourth exon. The I311 and other conserved amino acid K99 are essential for the ATPase and lipid binding activities of ZmMS13. ZmMs13 is specifically expressed in anthers with three peaks at stages S5, S8b, and S10, which are successively regulated by transcription factors ZmbHLH122, ZmMYB84, and ZmMYB33-1/-2 at these three stages. The triphasic regulation of ZmMs13 is sequentially required for callose dissolution, tapetal PCD and pollen exine development, and anther cuticle formation, corresponding to transcription alterations of callose-, ROS-, PCD-, sporopollenin-, and anther cuticle-related genes in ms13-6060 anthers.

CONCLUSION

ms13-6060 mutation with one key amino acid (I311) deletion greatly reduces ZmMS13 ATPase and lipid binding activities and displays multiple effects during maize male reproduction. Our findings provide new insights into molecular mechanisms of ABCG transporters controlling anther and pollen development and male fertility in plants.

The methyl-CpG-binding domain family member PEM1 is essential for Ubisch body formation and pollen exine development in rice

Pollen exine is composed of finely-organized nexine, bacula and tectum, and is crucial for pollen viability and function. Pollen exine development involves a complicated molecular network that coordinates the interaction between pollen and tapetal cells, as well as the biosynthesis, transport and assembly of sporopollenin precursors; however, our understanding of this network is very limited. Here, we report the roles of PEM1, a member of methyl-CpG-binding domain family, in rice pollen development. PEM1 expressed constitutively and, in anthers, its expression was detectable in tapetal cells and pollen. This predicted PEM1 protein of 240 kDa had multiple epigenetic-related domains. pem1 mutants exhibited abnormal Ubisch bodies, delayed exine occurrence and, finally, defective exine, including invisible bacula, amorphous and thickened nexine and tectum layer structures, and also had the phenotype of increased anther cuticle. The mutation in PEM1 did not affect the timely degradation of tapetum. Lipidomics revealed much higher wax and cutin contents in mutant anthers than in wild-type. Accordingly, this mutation up-regulated the expression of a set of genes implicated in transcriptional repression, signaling and diverse metabolic pathways. These results indicate that PEM1 mediates Ubisch body formation and pollen exine development mainly by negatively modulating the expression of genes. Thus, the PEM1-mediated molecular network represents a route for insights into mechanisms underlying pollen development. PEM1 may be a master regulator of pollen exine development.

Transcriptome Mining Provides Insights into Cell Wall Metabolism and Fiber Lignification in Agave tequilana Weber

Resilience of growing in arid and semiarid regions and a high capacity of accumulating sugar-rich biomass with low lignin percentages have placed Agave species as an emerging bioenergy crop. Although transcriptome sequencing of fiber-producing agave species has been explored, molecular bases that control wall cell biogenesis and metabolism in agave species are still poorly understood. Here, through RNAseq data mining, we reconstructed the cellulose biosynthesis pathway and the phenylpropanoid route producing lignin monomers in A. tequilana, and evaluated their expression patterns in silico and experimentally. Most of the orthologs retrieved showed differential expression levels when they were analyzed in different tissues with contrasting cellulose and lignin accumulation. Phylogenetic and structural motif analyses of putative CESA and CAD proteins allowed to identify those potentially involved with secondary cell wall formation. RT-qPCR assays revealed enhanced expression levels of AtqCAD5 and AtqCESA7 in parenchyma cells associated with extraxylary fibers, suggesting a mechanism of formation of sclerenchyma fibers in Agave similar to that reported for xylem cells in model eudicots. Overall, our results provide a framework for understanding molecular bases underlying cell wall biogenesis in Agave species studying mechanisms involving in leaf fiber development in monocots.

Documenting the Sporangium Development of the Polypodiales Fern Pteris multifida

Reconstructing the development of sporangia in seed-free vascular plants provides crucial information about key processes enabling the production of spores that are important in the life cycle of these plants. By applying fluorescence imaging in intact tissues using dyes and confocal microscopy, this study aimed to reconstruct the key steps during the development of sporangia. Special emphasis was taken on the cell wall structures of tapetum and spore mother cells that have been challenged by microscopical documentation in the past. After staining the cell wall and cytoplasm using calcofluor white and basic fuchsin, the sporangium development of Pteris multifida was observed using confocal microscopy. The clear cell lineages from the sporangial initial cell to stalk, epidermis, inner tapetum, outer tapetum, and sporogenous cells were revealed by confocal imaging. The sporangium development improved in this work will be useful for a general understanding of fern spore formation.

The True Identity of the “Second Pollen Morphology” of Camellia oleifera—Stomium Cells

Previous studies on Camellia oleifera pollen morphology have indicated dual morphologies, defined as “dimorphism”. However, they were limited to morphological studies at the end of final development and did not elucidate the origin, structure, and function of the second pollen morphology (striate pollen). In this study, the differences between the two “pollen” types were compared by paraffin sections, scanning electron microscopy (SEM), fluorescence microscopy, and in vitro germination. The results clearly showed that the second pollen type was formed by stomium cells of the anther, which is responsible for anther dehiscence. The nucleus and vesicles of the stomium cell were specifically distributed during anther development, which may be related to the formation of the septum, pollen dispersal activity, and the increase in stomium cell count; at the microscopic level, the stomium cell mainly consisted of the cell wall, large vesicles, and nucleus. The large vesicles facilitate the rapid dehydration of stomium cells under suitable conditions for anther dehiscence. Furthermore, studies on other species of the genus Camellia have suggested that the second type of pollen morphology is pseudopollen, which is capable of partaking in deceptive pollination. The present study refuted this theory and suggested that the pseudopollen are stomium cells, whose structure relates to their function. These results provide the basis for further research on C. oleifera pollen physiology toward the improvement of pollination rates with agricultural practices or breeding interventions.

OsSPLs Regulate Male Fertility in Response to Different Temperatures by Flavonoid Biosynthesis and Tapetum PCD in PTGMS Rice

Photoperiod and thermo-sensitive genic male sterile (PTGMS) rice is an important resource for two line hybrid rice production. The SQUAMOSA–promoter binding, such as the (SPL) gene family, encode the plant specific transcription factors that regulate development and defense responses in plants. However, the reports about SPLs participating in male fertility regulation are limited. Here, we identified 19 OsSPL family members and investigated their involvement in the fertility regulation of the PTGMS rice lines, PA2364S and PA2864S, with different fertility transition temperatures. The results demonstrated that OsSPL2, OsSPL4, OsSPL16 and OsSPL17 affect male fertility in response to temperature changes through the MiR156-SPL module. WGCNA (weighted gene co-expression network analysis) revealed that CHI and APX1 were co-expressed with OsSPL17. Targeted metabolite and flavonoid biosynthetic gene expression analysis revealed that OsSPL17 regulates the expression of flavonoid biosynthesis genes CHI, and the up regulation of flavanones (eriodictvol and naringenin) and flavones (apigenin and luteolin) content contributed to plant fertility. Meanwhile, OsSPL17 negatively regulates APX1 to affect APX (ascorbate peroxidase) activity, thereby regulating ROS (reactive oxygen species) content in the tapetum, controlling the PCD (programmed cell death) process and regulating male fertility in rice. Overall, this report highlights the potential role of OsSPL for the regulation of male fertility in rice and provides a new insight for the further understanding of fertility molecular mechanisms in PTGMS rice.

OsLTP47 may function in a lipid transfer relay essential for pollen wall development in rice

In plants, lipid transfer proteins (LTPs) transport pollen wall constituents from the tapetum to the exine, a process essential for pollen wall development. However, the functional cooperation of different LTPs in pollen wall development is not well understood. In this study, we have identified and characterized a grass-specific LTP gene, OsLTP47, an important regulator of pollen wall formation in rice (Oryza sativa). OsLTP47 encodes a membrane-localized LTP and in vitro lipid-binding assays confirms that OsLTP47 has lipid-binding activity. Dysfunction of OsLTP47 causes disordered lipid metabolism and defective pollen walls, leading to male sterility. Yeast two-hybrid and pull-down assays reveal that OsLTP47 physically interacts with another LTP, OsC6. These findings suggest that the plasma membrane-localized OsLTP47 may function as a mediator in a lipid transfer relay through association with cytosolic and/or locular OsC6 for pollen wall development and that various LTPs may function in a coordinated manner to transport lipid molecules during pollen wall development.

Delayed callose degradation restores the fertility of multiple P/TGMS lines in Arabidopsis

Photoperiod/temperature-sensitive genic male sterility (P/TGMS) is widely applied for improving crop production. Previous investigations using the reversible male sterile (rvms) mutant showed that slow development is a general mechanism for restoring fertility to P/TGMS lines in Arabidopsis. In this work, we isolated a restorer of rvms-2 (res3), as the male sterility of rvms-2 was rescued by res3. Phenotype analysis and molecular cloning show that a point mutation in UPEX1 l in res3 leads to delayed secretion of callase A6 from the tapetum to the locule and tetrad callose wall degradation. Electrophoretic mobility shift assay and chromatin immunoprecipitation analysis demonstrated that the tapetal transcription factor ABORTED MICROSPORES directly regulates UPEX1 expression, revealing a pathway for tapetum secretory function. Early degradation of the callose wall in the transgenic line eliminated the fertility restoration effect of res3. The fertility of multiple known P/TGMS lines with pollen wall defects was also restored by res3. We propose that the remnant callose wall may broadly compensate for the pollen wall defects of P/TGMS lines by providing protection for pollen formation. A cellular mechanism is proposed to explain how slow development restores the fertility of P/TGMS lines in Arabidopsis.

The GA-DELLA-OsMS188 module controls male reproductive development in rice

Pollen protects male sperm and allows flowering plants to adapt to diverse terrestrial environments, thereby leading to the rapid expansion of plants into new regions. The process of anther/pollen development is coordinately regulated by internal and external factors including hormones. Currently, the molecular mechanisms underlying gibberellin (GA)-mediated male reproductive development in plants remain unknown. We show here that rice DELLA/SLR1, which encodes the central negative regulator of GA signaling, is essential for rice anther development. The slr1-5 mutant exhibits premature programmed cell death of the tapetum, lacks Ubisch bodies, and has no exine and no mature pollen. SLR1 is mainly expressed in tapetal cells and tetrads, and is required for the appropriate expression of genes encoding key factors of pollen development, which are suggested to be OsMS188-targeted genes. OsMS188 is the main component in the essential genetic program of tapetum and pollen development. Further, we demonstrate that SLR1 interacts with OsMS188 to cooperatively activate the expression of the sporopollenin biosynthesis and transport-related genes CYP703A3, DPW, ABCG15 and PKS1 for rapid formation of pollen walls. Overall, the results of this study suggest that the GA hormonal signal is integrated into the anther genetic program and regulates rice anther development through the GA-DELLA-OsMS188 module.

Comparative Cytological and Transcriptome Analyses of Anther Development in Nsa Cytoplasmic Male Sterile (1258A) and Maintainer Lines in Brassica napus Produced by Distant Hybridization

1258A is a new line of B.napus with Nsa cytoplasmic male sterility (CMS) with potential applications in hybrid rapeseed breeding. Sterile cytoplasm was obtained from XinJiang Sinapis arvensis through distant hybridization and then backcrossed with 1258B for many generations. However, the characteristics and molecular mechanisms underlying pollen abortion in this sterile line are poorly understood. In this study, a cytological analysis revealed normal microsporogenesis and uninucleate pollen grain formation. Pollen abortion was due to non-programmed cell death in the tapetum and the inability of microspores to develop into mature pollen grains. Sucrose, soluble sugar, and adenosine triphosphate (ATP) contents during microspore development were lower than those of the maintainer line, along with an insufficient energy supply, reduced antioxidant enzyme activity, and substantial malondialdehyde (MDA) accumulation in the anthers. Transcriptome analysis revealed that genes involved in secondary metabolite biosynthesis, glutathione metabolism, phenylpropane biosynthesis, cyanoamino acid metabolism, starch and sucrose metabolism, and glycerolipid metabolism may contribute to pollen abortion. The down regulation of nine cytochrome P450 monooxygenases genes were closely associated with pollen abortion. These results suggest that pollen abortion in 1258A CMS stems from abnormalities in the chorioallantoic membranes, energy deficiencies, and dysfunctional antioxidant systems in the anthers. Our results provide insight into the molecular mechanism underlying pollen abortion in Nsa CMS and provide a theoretical basis for better heterosis utilization in B.napus.

A biological nanofoam: The wall of coniferous bisaccate pollen

The outer layer of the pollen grain, the exine, plays a key role in the survival of terrestrial plant life. However, the exine structure in different groups of plants remains enigmatic. Here, modern and fossil coniferous bisaccate pollen were examined to investigate the detailed three-dimensional structure and properties of the pollen wall. X-ray nanotomography and volume electron microscopy are used to provide high-resolution imagery, revealing a solid nanofoam structure. Atomic force microscopy measurements were used to compare the pollen wall with other natural and synthetic foams and to demonstrate that the mechanical properties of the wall in this type of pollen are retained for millions of years in fossil specimens. The microscopic structure of this robust biological material has potential applications in materials sciences and also contributes to our understanding of the evolutionary success of conifers and other plants over geological time.

Insights into the biosynthesis pathway of phenolic compounds in microalgae

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Microalgal PCs are important bioactive molecules beneficial for human health.

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Bioinformatic comparative exploration predicts PCs synthesis in microalgae.

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Ten groups of prokaryotic and eukaryotic microalgae reveal a conserved pathway core.

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Featured PCs can be restricted to diverse microalgae due to ecological implications.

Among the most relevant bioactive molecules family, phenolic compounds (PCs) are well known in higher plants, while their knowledge in microalgae is still scarce. Microalgae represent a novel and promising source of human health benefit compounds to be involved, for instance, in nutraceutical composition. This study aims to investigate the PCs biosynthetic pathway in the microalgal realm, exploring its potential variability over the microalgal biodiversity axis. A multistep in silico analysis was carried out using a selection of core enzymes from the pathway described in land plants. This study explores their presence in ten groups of prokaryotic and eukaryotic microalgae.. Analyses were carried out taking into account a wide selection of algal protein homologs, functional annotation of conserved domains and motifs, and maximum-likelihood tree construction. Results showed that a conserved core of the pathway for PCs biosynthesis is shared horizontally in all microalgae. Conversely, the ability to synthesize some subclasses of phenolics may be restricted to only some microalgal groups (i.e., Chlorophyta) depending on featured enzymes, such as the flavanone naringenin and other related chalcone isomerase dependent compounds.

Combined analysis of transcriptome and metabolome reveals that sugar, lipid, and phenylpropane metabolism are essential for male fertility in temperature-induced male sterile rice

Photoperiod- and thermosensitive genic male sterility (PTGMS) rice is a vital germplasm resource consisting of two-line hybrid rice in which light and temperature strictly control their fertility changes. Variable environmental conditions present huge risks to the two-lines hybrid seed production. Explaining the regulatory mechanism of male fertility in rice PTGMS lines is an essential prerequisite to ensuring food security production. A group of near-isogenic lines (NILs) of a rice PTGMS line unique to this research group was used for this study. These lines have the same genetic background and regulate male fertility by responding to different temperature changes. Transcriptomic analysis revealed that 315 upregulated genes and 391 regulated genes regulated male fertility in response to temperature changes, and differentially expressed genes (DEGs) were mainly characterized in enrichment analysis as having roles in the metabolic pathways of sugar, lipid and phenylpropanoid. Electron microscopy analysis revealed that a lack of starch accumulation in sterile pollen grains induced by high temperature, with an abnormal exine development and a lack of inner pollen grains. Defective processes for sporopollenin synthesis, sporopollenin transport and pollen wall formation in sterile anthers were verified using qPCR. Targeted metabolomics analysis revealed that most lipids (phospholipids, sphingolipids and fatty acids) and flavonoids (flavones and flavanones) were upregulated in fertile anthers and involved in pollen wall development and male fertility formation, while lignin G units and C-type lignin were the major contributors to pollen wall development. The coding genes for trehalose 6-phosphate phosphatase, beta-1,3-glucanase, phospholipase D and 4-coumarate-CoA ligase are considered essential regulators in the process of male fertility formation. In conclusion, our results indicated that the expression of critical genes and accumulation of metabolites in the metabolism of sugar, lipid, and phenylpropanoid are essential for male fertility formation. The results provide new insights for addressing the negative effects of environmental variation on two-line hybrid rice production.

CAND1 is required for pollen viability in Arabidopsis thaliana-a test of the adaptive exchange hypothesis

The dynamic assembly of SKP1•CUL1•F-box protein (SCF) ubiquitin ligases is important for protein ubiquitination and degradation. This process is enabled by CAND1, which exchanges F-box proteins associated with the common CUL1 scaffold, and thereby, recycles the limited CUL1 core and allows diverse F-box proteins to assemble active SCFs. Previous human cell biological and computational studies have led to the adaptive exchange hypothesis, which suggests that the CAND1-mediated exchange confers plasticity on the SCF system, allowing cells to tolerate large variations in F-box protein expression. Here, we tested this hypothesis using Arabidopsis thaliana, a multicellular organism expressing hundreds of F-box protein genes at variable levels in different tissues. The cand1 null mutant in Arabidopsis is viable but produce almost no seeds. Bioinformatic, cell biological, and developmental analyses revealed that the low fertility in the cand1 mutant is associated with cell death in pollen, where the net expression of F-box protein genes is significantly higher than any other Arabidopsis tissue. In addition, we show that the transmission efficiency of the cand1 null allele was reduced through the male but not the female gametophyte. Our results suggest that CAND1 activity is essential in cells or tissues expressing high levels of F-box proteins. This finding is consistent with the proposed adaptive exchange hypothesis, demonstrating the necessity of the evolutionarily conserved CAND1-mediated exchange system in the development of a multicellular organism.

Iron insufficiency in floral buds impairs pollen development by disrupting tapetum function

Reduction of crop yield due to iron (Fe) deficiency has always been a concern in agriculture. How Fe insufficiency in floral buds affects pollen development remains unexplored. Here, plants transferred to Fe-deficient medium at the reproductive stage had reduced floral Fe content and viable pollen and showed a defective pollen outer wall, all restored by supplying floral buds with Fe. A comparison of differentially expressed genes (DEGs) in Fe-deficient leaves, roots, and anthers suggested that changes in several cellular processes were unique to anthers, including increased lipid degradation. Co-expression analysis revealed that ABORTED MICROSPORES (AMS), DEFECTIVE IN TAPETAL DEVELOPMENT AND FUNCTION1, and BASIC HELIX-LOOP-HELIX 089/091/010 encode key upstream transcription factors of Fe deficiency-responsive DEGs involved in tapetum function and development, including tapetal ROS homeostasis, programmed cell death, and pollen outer wall formation-related lipid metabolism. Analysis of RESPIRATORY-BURST OXIDASE HOMOLOG E (RBOHE) gain- and loss-of-function under Fe deficiency indicated that RBOHE- and Fe-dependent regulation cooperatively control anther reactive oxygen species levels and pollen development. Since DEGs in Fe-deficient anthers were not significantly enriched in genes related to mitochondrial function, the changes in mitochondrial status under Fe deficiency, including respiration activity, density, and morphology, were probably because the Fe amount was insufficient to maintain proper mitochondrial protein function in anthers. To sum up, Fe deficiency in anthers may affect Fe-dependent protein function and impact upstream transcription factors and their downstream genes, resulting in extensively impaired tapetum function and pollen development.

Tetraketide α-pyrone reductases in sporopollenin synthesis pathway in Gerbera hybrida: diversification of the minor function

The structurally robust biopolymer sporopollenin is the major constituent of the exine layer of pollen wall and plays a vital role in plant reproductive success. The sporopollenin precursors are synthesized through an ancient polyketide biosynthetic pathway consisting of a series of anther-specific enzymes that are widely present in all land plant lineages. Tetraketide α-pyrone reductase 1 (TKPR1) and TKPR2 are two reductases catalyzing the final reduction of the carbonyl group of the polyketide synthase-synthesized tetraketide intermediates to hydroxylated α-pyrone compounds, important precursors of sporopollenin. In contrast to the functional conservation of many sporopollenin biosynthesis associated genes confirmed in diverse plant species, TKPR2's role has been addressed only in Arabidopsis, where it plays a minor role in sporopollenin biosynthesis. We identified in gerbera two non-anther-specific orthologues of AtTKPR2, Gerbera reductase 1 (GRED1) and GRED2. Their dramatically expanded expression pattern implies involvement in pathways outside of the sporopollenin pathway. In this study, we show that GRED1 and GRED2 are still involved in sporopollenin biosynthesis with a similar secondary role as AtTKPR2 in Arabidopsis. We further show that this secondary role does not relate to the promoter of the gene, AtTKPR2 cannot rescue pollen development in Arabidopsis even when controlled by the AtTKPR1 promoter. We also identified the gerbera orthologue of AtTKPR1, GTKPR1, and characterized its crucial role in gerbera pollen development. GTKPR1 is the predominant TKPR in gerbera pollen wall formation, in contrast to the minor roles GRED1 and GRED2. GTKPR1 is in fact an excellent target for engineering male-sterile gerbera cultivars in horticultural plant breeding.

From plants to antimicrobials: Natural products against bacterial membranes

Bacterial membrane barrier provides a cytoplasmic environment for organelles of bacteria. The membrane is composed of lipid compounds containing phosphatide protein and a minimal amount of sugars, and is responsible for intercellular transfers of chemicals. Several antimicrobials have been found that affect bacterial cytoplasmic membranes. These compounds generally disrupt the organization of the membrane or perforate it. By destroying the membrane, the drugs can permeate and replace the effective macromolecules necessary for cell life. Furthermore, they can disrupt electrical gradients of the cells through impairment of the membrane integrity. In recent years, considering the spread of microbial resistance and the side effects of antibiotics, natural antimicrobial compounds have been studied by researchers extensively. These molecules are the best alternative for controlling bacterial infections and reducing drug resistance due to the lack of severe side effects, low cost of production, and biocompatibility. Better understanding of the natural compounds' mechanisms against bacteria provides improved strategies for antimicrobial therapies. In this review, natural products with antibacterial activities focusing on membrane damaging mechanisms were described. However, further high-quality research studies are needed to confirm the clinical efficacy of these natural products.

The Toughest Material in the Plant Kingdom: An Update on Sporopollenin

The extreme chemical and physical recalcitrance of sporopollenin deems this biopolymer among the most resilient organic materials on Earth. As the primary material fortifying spore and pollen cell walls, sporopollenin is touted as a critical innovation in the progression of plant life to a terrestrial setting. Although crucial for its protective role in plant reproduction, the inert nature of sporopollenin has challenged efforts to determine its composition for decades. Revised structural, chemical, and genetic experimentation efforts have produced dramatic advances in elucidating the molecular structure of this biopolymer and the mechanisms of its synthesis. Bypassing many of the challenges with material fragmentation and solubilization, insights from functional characterizations of sporopollenin biogenesis in planta, and in vitro, through a gene-targeted approach suggest a backbone of polyhydroxylated polyketide-based subunits and remarkable conservation of biochemical pathways for sporopollenin biosynthesis across the plant kingdom. Recent optimization of solid-state NMR and targeted degradation methods for sporopollenin analysis confirms polyhydroxylated α-pyrone subunits, as well as hydroxylated aliphatic units, and unique cross-linkage heterogeneity. We examine the cross-disciplinary efforts to solve the sporopollenin composition puzzle and illustrate a working model of sporopollenin's molecular structure and biosynthesis. Emerging controversies and remaining knowledge gaps are discussed, including the degree of aromaticity, cross-linkage profiles, and extent of chemical conservation of sporopollenin among land plants. The recent developments in sporopollenin research present diverse opportunities for harnessing the extraordinary properties of this abundant and stable biomaterial for sustainable microcapsule applications and synthetic material designs.

The evolution of the phenylpropanoid pathway entailed pronounced radiations and divergences of enzyme families

Land plants constantly respond to fluctuations in their environment. Part of their response is the production of a diverse repertoire of specialized metabolites. One of the foremost sources for metabolites relevant to environmental responses is the phenylpropanoid pathway, which was long thought to be a land-plant-specific adaptation shaped by selective forces in the terrestrial habitat. Recent data have, however, revealed that streptophyte algae, the algal relatives of land plants, have candidates for the genetic toolkit for phenylpropanoid biosynthesis and produce phenylpropanoid-derived metabolites. Using phylogenetic and sequence analyses, we here show that the enzyme families that orchestrate pivotal steps in phenylpropanoid biosynthesis have independently undergone pronounced radiations and divergence in multiple lineages of major groups of land plants; sister to many of these radiated gene families are streptophyte algal candidates for these enzymes. These radiations suggest a high evolutionary versatility in the enzyme families involved in the phenylpropanoid-derived metabolism across embryophytes. We suggest that this versatility likely translates into functional divergence, and may explain the key to one of the defining traits of embryophytes: a rich specialized metabolism.

A dye combination for the staining of pollen coat and pollen wall

The pollen coat, which forms on the pollen surface, consists of a lipid-protein matrix. It protects pollen from desiccation and is involved in adhesion, pollen-stigma recognition, and pollen hydration during interactions with the stigma. The classical methods used for pollen coat observation are scanning and transmission electron microscopy. In this work, we screened a collection of fluorescence dyes and identified two fluorescent brighteners FB-52 and FB-184. When they were used together with the exine-specific dye, Basic fuchsin, the pollen coat and the exine structures could be clearly visualized in the pollen of Brassica napus. This co-staining method was applied successfully in staining pollen from Fraxinus chinensis, Calystegia hederacea, and Petunia hybrida. Using this method, small pollen coat-containing cavities were detected in the outer pollen wall layer of Oryza sativa and Zea mays. We further showed these dyes are compatible with fluorescent protein markers. In the Arabidopsis thaliana transgenic line of GFP-tagged pollen coat protein GRP19, GRP19-GFP was observed to form particles at the periphery of pollen coat. This simple staining method is expected to be widely used for the studies of the palynology as well as the pollen-stigma interaction.

Cross-Species Metabolic Profiling of Floral Specialized Metabolism Facilitates Understanding of Evolutional Aspects of Metabolism Among Brassicaceae Species

Plants produce a variety of floral specialized (secondary) metabolites with roles in several physiological functions, including light-protection, attraction of pollinators, and protection against herbivores. Pigments and volatiles synthesized in the petal have been focused on and characterized as major chemical factors influencing pollination. Recent advances in plant metabolomics have revealed that the major floral specialized metabolites found in land plant species are hydroxycinnamates, phenolamides, and flavonoids albeit these are present in various quantities and encompass diverse chemical structures in different species. Here, we analyzed numerous floral specialized metabolites in 20 different Brassicaceae genotypes encompassing both different species and in the case of crop species different cultivars including self-compatible (SC) and self-incompatible (SI) species by liquid chromatography-mass spectrometry (LC-MS). Of the 228 metabolites detected in flowers among 20 Brassicaceae species, 15 metabolite peaks including one phenylacyl-flavonoids and five phenolamides were detected and annotated as key metabolites to distinguish SC and SI plant species, respectively. Our results provide a family-wide metabolic framework and delineate signatures for compatible and incompatible genotypes thereby providing insight into evolutionary aspects of floral metabolism in Brassicaceae species.

Fertile Arabidopsis cyp704b1 mutant, defective in sporopollenin biosynthesis, has a normal pollen coat and lipidic organelles in the tapetum

The exine acts as a protectant of the pollen from environmental stresses, and the pollen coat plays an important role in the attachment and recognition of the pollen to the stigma. The pollen coat is made of lipidic organelles in the tapetum. The pollen coat is necessary for fertility, as pollen coat-less mutants, such as those deficient in sterol biosynthesis, show severe male sterility. In contrast, the exine is made of sporopollenin precursors that are biosynthesized in the tapetum. Some mutants involved in sporopollenin biosynthesis lose the exine but show the fertile phenotype. One of these mutants, cyp704b1, was reported to lose not only the exine but also the pollen coat. To identify the cause of the fertile phenotype of the cyp704b1 mutant, the detailed structures of the tapetum tissue and pollen surface in the mutant were analyzed. As a result, the cyp704b1 mutant completely lost the normal exine but had high-electron-density granules localized where the exine should be present. Furthermore, normal lipidic organelles in the tapetum and pollen coat embedded between high-electron-density granules on the pollen surface were observed, unlike in a previous report, and the pollen coat was attached to the stigma. Therefore, the pollen coat is necessary for fertility, and the structure that functions like the exine, such as high-electron-density granules, on the pollen surface may play important roles in retaining the pollen coat in the cyp704b1 mutant.

Integrated Analysis of Small RNA, Transcriptome, and Degradome Sequencing Reveals the MiR156, MiR5488 and MiR399 are Involved in the Regulation of Male Sterility in PTGMS Rice

A photoperiod- and thermo-sensitive genic male sterile (PTGMS) line is the basic material for two-hybrid rice and is an important genetic breeding resource. Peiai64S (PA64S) is an important germplasm resource of PTGMS rice, and it has been applied to two-line hybrid rice systems in China. Pollen fertility in PA64S is regulated by the temperature and photoperiod, but the mechanism of the fertility transition is unclear. In this study, we obtained the male fertile plant PA64S(F) and the male sterile plant PA64S(S) by controlling different temperatures under long light conditions and used the male fertile and sterile plants to investigate the role of microRNAs (miRNAs) in regulating male fertility in rice. We performed the small RNA library sequencing of anthers from PA64S(S) and PA64S(F). A total of 196 miRNAs were identified-166 known miRNAs among 27 miRNA families and 30 novel miRNAs. In the transcriptome analysis, the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of differentially expressed genes revealed significant enrichment in the synthesis and metabolism of fatty acids and some secondary metabolism pathways such as fatty acid metabolism and phenylalanine metabolism. With a comprehensive analysis of miRNA, transcriptome, and degradome sequencing, we identified that 13 pairs of miRNA/target genes regulated male fertility in rice by responding to temperature change, among which the miR156, miR5488, and miR399 affect the male fertility of PA64S by influencing SPLs, the lignin synthesis of anther walls, and the flavonoid metabolism pathway. The results provide a new understanding of PTGMS rice, which will help us better understand the potential regulatory mechanisms of male sterility in the future.

Development of the Middle Layer in the Anther of Arabidopsis

The middle layer is an essential cell layer of the anther wall located between the endothecium and tapetum in Arabidopsis. Based on sectioning, the middle layer was found to be degraded at stage 7, which led to the separation of the tapetum from the anther wall. Here, we established techniques for live imaging of the anther. We created a marker line with fluorescent proteins expressed in all anther layers to study anther development. Several staining methods were used in the intact anthers to study anther cell morphology. We clarified the initiation, development, and degradation of the middle layer in Arabidopsis. This layer is initiated from both the inner and outer secondary parietal cells at stage 4, stopped cell division at stage 6, and finally degraded at stage 11. The neighboring cell layers, the epidermis, and endothecium continued cell division until stage 10, which led to a thin middle layer. The degradation of the tapetum cell wall at stage 7 lead to its isolation from the anther wall. This work presents fundamental information on the development of the middle layer, which facilitates the further investigation of anther development and plant fertility. These live imaging methods could be useful in future studies.