Regulation of pollen lipid body biogenesis by MAP kinases and downstream WRKY transcription factors in Arabidopsis

Tetrad stage transient cold stress skews auxin-mediated energy metabolism balance in Chinese cabbage pollen

Changing ambient temperature often impairs plant development and sexual reproduction, particularly pollen ontogenesis. However, mechanisms underlying cold stress-induced male sterility are not well understood. Here, we exposed Chinese cabbage (Brassica campestris) to different cold conditions during flowering and demonstrated that the tetrad stage was the most sensitive. After completion of pollen development at optimal conditions, transient cold stress at the tetrad stage still impacted auxin levels, starch and lipid accumulation, and pollen germination, ultimately resulting in partial male sterility. Transcriptome and metabolome analyses and histochemical staining indicated that the reduced pollen germination rate was due to the imbalance of energy metabolism during pollen maturation. The investigation of β-glucuronidase (GUS)-overexpressing transgenic plants driven by the promoter of DR5 (DR5::GUS report system) combined with cell tissue staining and metabolome analysis further validated that cold stress during the tetrad stage reduced auxin levels in mature pollen grains. Low-concentration auxin treatment on floral buds at the tetrad stage before cold exposure improved the cold tolerance of mature pollen grains. Artificially changing the content of endogenous auxin during pollen maturation by spraying chemical reagents and loss-of-function investigation of the auxin biosynthesis gene YUCCA6 by artificial microRNA technology showed that starch overaccumulation severely reduced the pollen germination rate. In summary, we revealed that transient cold stress at the tetrad stage of pollen development in Chinese cabbage causes auxin-mediated starch-related energy metabolism imbalance that contributes to the decline in pollen germination rate and ultimately seed set.

Evidence for dual targeting control of Arabidopsis 6-phosphogluconate dehydrogenase isoforms by N-terminal phosphorylation

The oxidative pentose-phosphate pathway (OPPP) retrieves NADPH from glucose-6-phosphate, which is important in chloroplasts at night and in plastids of heterotrophic tissues. We previously studied how OPPP enzymes may transiently locate to peroxisomes, but how this is achieved for the third enzyme remained unclear. By extending our genetic approach, we demonstrated that Arabidopsis isoform 6-phosphogluconate dehydrogenase 2 (PGD2) is indispensable in peroxisomes during fertilization, and investigated why all PGD-reporter fusions show a mostly cytosolic pattern. A previously published interaction of a plant PGD with thioredoxin m was confirmed using Trxm2 for yeast two-hybrid (Y2H) and bimolecular fluorescent complementation (BiFC) assays, and medial reporter fusions (with both ends accessible) proved to be beneficial for studying peroxisomal targeting of PGD2. Of special importance were phosphomimetic changes at Thr6, resulting in a clear targeting switch to peroxisomes, while a similar change at position Ser7 in PGD1 conferred plastid import. Apparently, efficient subcellular localization can be achieved by activating an unknown kinase, either early after or during translation. N-terminal phosphorylation of PGD2 interfered with dimerization in the cytosol, thus allowing accessibility of the C-terminal peroxisomal targeting signal (PTS1). Notably, we identified amino acid positions that are conserved among plant PGD homologues, with PTS1 motifs first appearing in ferns, suggesting a functional link to fertilization during the evolution of seed plants.

MAPK20-mediated ATG6 phosphorylation is critical for pollen development in Solanum lycopersicum L

In flowering plants, male gametogenesis is tightly regulated by numerous genes. Mitogen-activated protein kinase (MAPK) plays a critical role in plant development and stress response, while its role in plant reproductive development is largely unclear. The present study demonstrated MAPK20 phosphorylation of ATG6 to mediate pollen development and germination in tomato (Solanum lycopersicum L.). MAPK20 was preferentially expressed in the stamen of tomato, and mutation of MAPK20 resulted in abnormal pollen grains and inhibited pollen viability and germination. MAPK20 interaction with ATG6 mediated the formation of autophagosomes. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis showed that ATG6 was phosphorylated by MAPK20 at Ser-265. Mutation of ATG6 in wild-type (WT) or in MAPK20 overexpression plants resulted in malformed and inviable pollens. Meanwhile, the number of autophagosomes in mapk20 and atg6 mutants was significantly lower than that of WT plants. Our results suggest that MAPK20-mediated ATG6 phosphorylation and autophagosome formation are critical for pollen development and germination.

WRKY transcription factors modulate flowering time and response to environmental changes

WRKY transcription factors (TFs), originating in green algae, regulate flowering time and responses to environmental changes in plants. However, the molecular mechanisms underlying the role of WRKY TFs in the correlation between flowering time and environmental changes remain unclear. Therefore, this review summarizes the association of WRKY TFs with flowering pathways to accelerate or delay flowering. WRKY TFs are implicated in phytohormone pathways, such as ethylene, auxin, and abscisic acid pathways, to modulate flowering time. WRKY TFs can modulate salt tolerance by regulating flowering time. WRKY TFs exhibit functional divergence in modulating environmental changes and flowering time. In summary, WRKY TFs are involved in complex pathways and modulate response to environmental changes, thus regulating flowering time.

Integrated transcriptomic and lipidomic analysis provides key insights into lipid content changes during pecan (Carya illinoensis) fruit development

Pecans [Carya illinoinensis (Wangenh.) K. Koch] are highly valued for their abundance of quality healthy lipids, positively impacting human health and making themselves a preferred choice for nutritionally rich foods. However, a comprehensive understanding of the high-resolution characteristics of pecan fruit lipid composition and its dynamic changes, as well as the transfer between embryo and pericarp during development, remains incomplete. In this study, through integrated multi-omics analysis, we observed significant spatiotemporal heterogeneity in lipid changes between the pericarp and embryo. It showed smaller fluctuations and more stable lipid levels in the pericarp while exhibiting a dynamic pattern of initially increasing and then decreasing lipid content in the embryo. In this study, a total of 52 differentially expressed genes were identified, related to fatty acid synthesis and metabolism pathways in the two tissues, with changes in oleic acid and linoleic acid composition being the primary features of the embryo. This research lays the foundation for further understanding the differential regulation mechanisms of lipid metabolism between embryo and pericarp. Overall, this study filled the knowledge gap regarding dynamic changes in pericarp lipid metabolites, provided crucial insights into the lipid metabolism network during pecan fruit development, and established a scientific basis for the genetic improvement of pecan crops.

Transcriptome disclosure of hormones inducing stigma exsertion in Nicotiana tabacum by corolla shortening

BACKGROUND

Stigma exsertion is an essential agricultural trait that can promote cross-pollination to improve hybrid seed production efficiency. However, the molecular mechanism controlling stigma exsertion remains unknown.

RESULTS

In this study, the Nicotiana tabacum cv. K326 and its two homonuclear-heteroplasmic lines, MSK326 (male-sterile) and MSK326SE (male-sterile and stigma exserted), were used to investigate the mechanism of tobacco stigma exsertion. A comparison of the flowers between the three lines showed that the stigma exsertion of MSK326SE was mainly due to corolla shortening. Therefore, the corollas of the three lines were sampled and presented for RNA-seq analysis, which found 338 candidate genes that may cause corolla shortening. These genes were equally expressed in K326 and MSK326, but differentially expressed in MSK326SE. Among these 338 genes, 15 were involved in hormone synthesis or signal transduction pathways. Consistently, the content of auxin, dihydrozeatin, gibberellin, and jasmonic acid was significantly decreased in the MSK326SE corolla, whereas abscisic acid levels were significantly increased. Additionally, seven genes involved in cell division, cell cycle, or cell expansion were identified. Protein-protein interaction network analysis identified 45 nodes and 79 protein interactions, and the largest module contained 20 nodes and 52 protein interactions, mainly involved in the hormone signal transduction and pathogen defensive pathways. Furthermore, a putative hub gene coding a serine/threonine-protein kinase was identified for the network.

CONCLUSIONS

Our results suggest that hormones may play a key role in regulating tobacco stigma exsertion induced by corolla shortening.

Transcription Factors and Their Regulatory Roles in the Male Gametophyte Development of Flowering Plants

Male gametophyte development in plants relies on the functions of numerous genes, whose expression is regulated by transcription factors (TFs), non-coding RNAs, hormones, and diverse environmental stresses. Several excellent reviews are available that address the genes and enzymes associated with male gametophyte development, especially pollen wall formation. Growing evidence from genetic studies, transcriptome analysis, and gene-by-gene studies suggests that TFs coordinate with epigenetic machinery to regulate the expression of these genes and enzymes for the sequential male gametophyte development. However, very little summarization has been performed to comprehensively review their intricate regulatory roles and discuss their downstream targets and upstream regulators in this unique process. In the present review, we highlight the research progress on the regulatory roles of TF families in the male gametophyte development of flowering plants. The transcriptional regulation, epigenetic control, and other regulators of TFs involved in male gametophyte development are also addressed.

Movement of Lipid Droplets in the Arabidopsis Pollen Tube Is Dependent on the Actomyosin System

The growth of pollen tubes, which depends on actin filaments, is pivotal for plant reproduction. Pharmacological experiments showed that while oryzalin and brefeldin A treatments had no significant effect on the lipid droplets (LDs) trafficking, while 2,3-butanedione monoxime (BDM), latrunculin B, SMIFH2, and cytochalasin D treatments slowed down LDs trafficking, in such a manner that only residual wobbling was observed, suggesting that trafficking of LDs in pollen tube is related to F-actin. While the trafficking of LDs in the wild-type pollen tubes and in myo11-2, myo11b1-1, myo11c1-1, and myo11c2-1 single mutants and myo11a1-1/myo11a2-1 double mutant were normal, their trafficking slowed down in a myosin-XI double knockout (myo11c1-1/myo11c2-1) mutant. These observations suggest that Myo11C1 and Myo11C2 motors are involved in LDs movement in pollen tubes, and they share functional redundancy. Hence, LDs movement in Arabidopsis pollen tubes relies on the actomyosin system.

Lipid droplets are versatile organelles involved in plant development and plant response to environmental changes

Since decades plant lipid droplets (LDs) are described as storage organelles accumulated in seeds to provide energy for seedling growth after germination. Indeed, LDs are the site of accumulation for neutral lipids, predominantly triacylglycerols (TAGs), one of the most energy-dense molecules, and sterol esters. Such organelles are present in the whole plant kingdom, from microalgae to perennial trees, and can probably be found in all plant tissues. Several studies over the past decade have revealed that LDs are not merely simple energy storage compartments, but also dynamic structures involved in diverse cellular processes like membrane remodeling, regulation of energy homeostasis and stress responses. In this review, we aim to highlight the functions of LDs in plant development and response to environmental changes. In particular, we tackle the fate and roles of LDs during the plant post-stress recovery phase.

Functions of WRKYs in plant growth and development

As sessile organisms, plants must overcome various stresses. Accordingly, they have evolved several plant-specific growth and developmental processes. These plant processes may be related to the evolution of plant-specific protein families. The WRKY transcription factors originated in eukaryotes and expanded in plants, but are not present in animals. Over the past two decades, there have been many studies on WRKYs in plants, with much of the research concentrated on their roles in stress responses. Nevertheless, recent findings have revealed that WRKYs are also required for seed dormancy and germination, postembryonic morphogenesis, flowering, gametophyte development, and seed production. Thus, WRKYs may be important for plant adaptations to a sessile lifestyle because they simultaneously regulate stress resistance and plant-specific growth and development.

Comprehensive Analysis of Metabolome and Transcriptome Reveals the Regulatory Network of Coconut Nutrients

Coconut flesh is widely consumed in the market for its good flavor. However, a comprehensive and dynamic assessment of the nutrients in coconut flesh and their molecular regulatory mechanisms is lacking. In this study, the metabolite accumulation and gene expression of three representative coconut cultivars belonging to two subspecies were investigated using ultra performance liquid chromatography/tandem mass spectrometry. A total of 6101 features were detected, of which 52, 8, and 158 were identified as amino acids and derivatives, polyamines, and lipids, respectively. The analysis of the metabolite pathway showed that glutathione and α-linolenate were the main differential metabolites. Transcriptome data revealed significant differences in the expression of five glutathione structural genes and thirteen polyamine-regulated genes, consistent with trends in metabolite accumulation. Weighted correlation network and co-expression analyses showed that a novel gene WRKY28 was implicated in the regulation of lipid synthesis. These results broaden our understanding of coconut nutrition metabolism and provide new insights into the molecular basis of coconut nutrition metabolism.

High-temperature stress in crops: male sterility, yield loss and potential remedy approaches

Global food security is one of the utmost essential challenges in the 21st century in providing enough food for the growing population while coping with the already stressed environment. High temperature (HT) is one of the main factors affecting plant growth, development and reproduction and causes male sterility in plants. In male reproductive tissues, metabolic changes induced by HT involve carbohydrates, lipids, hormones, epigenetics and reactive oxygen species, leading to male sterility and ultimately reducing yield. Understanding the mechanism and genes involved in these pathways during the HT stress response will provide a new path to improve crops by using molecular breeding and biotechnological approaches. Moreover, this review provides insight into male sterility and integrates this with suggested strategies to enhance crop tolerance under HT stress conditions at the reproductive stage.

Exogenous application of salicylic acid improves freezing stress tolerance in alfalfa

Freezing stress is one of the most detrimental environmental factors that can seriously impact the growth, development, and distribution of alfalfa (Medicago sativa L.). Exogenous salicylic acid (SA) has been revealed as a cost-effective method of improving defense against freezing stress due to its predominant role in biotic and abiotic stress resistance. However, how the molecular mechanisms of SA improve freezing stress resistance in alfalfa is still unclear. Therefore, in this study, we used leaf samples of alfalfa seedlings pretreatment with 200 μM and 0 μM SA, which were exposed to freezing stress (-10°C) for 0, 0.5, 1, and 2h and allowed to recover at normal temperature in a growth chamber for 2 days, after which we detect the changes in the phenotypical, physiological, hormone content, and performed a transcriptome analysis to explain SA influence alfalfa in freezing stress. The results demonstrated that exogenous SA could improve the accumulation of free SA in alfalfa leaves primarily through the phenylalanine ammonia-lyase pathway. Moreover, the results of transcriptome analysis revealed that the mitogen-activated protein kinase (MAPK) signaling pathway-plant play a critical role in SA alleviating freezing stress. In addition, the weighted gene co-expression network analysis (WGCNA) found that MPK3, MPK9, WRKY22 (downstream target gene of MPK3), and TGACG-binding factor 1 (TGA1) are candidate hub genes involved in freezing stress defense, all of which are involved in the SA signaling pathway. Therefore, we conclude that SA could possibly induce MPK3 to regulate WRKY22 to participate in freezing stress to induced gene expression related to SA signaling pathway (NPR1-dependent pathway and NPR1-independent pathway), including the genes of non-expresser of pathogenesis-related gene 1 (NPR1), TGA1, pathogenesis-related 1 (PR1), superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), glutathione-S-transferase (GST), and heat shock protein (HSP). This enhanced the production of antioxidant enzymes such as SOD, POD, and APX, which increases the freezing stress tolerance of alfalfa plants.

Two polyamines -responsive WRKY transcription factors from Anoectochilus roxburghii play opposite functions on flower development

Anoectochilus roxburghii is a rare and precious plant with medicinal and healthcare functions. Embryo abortion caused the lack of resources. Polyamine promoted its flowering and stress resistance in our previous study. But the mechanism remains unclear. The WRKY transcription factor family has been linked to a variety of biological processes in plants. In this study, two WRKY TFs (ArWRKY5 and ArWRKY20) of A. roxburghii, which showed significant response to Spd treatment, were identified and functionally analyzed. Tissue specific expression analyzation showed both of them mostly present in the flower. And ArWRKY5 expressed highest in the flower bud stage (-1 Flowering), while ArWRKY20 showed the highest expression in earlier flower bud stage (-2 Flowering) and the expression gradually decreased with flowering. The transcriptional activation activity assay and subcellular localization revealed that ArWRKY5 and ArWRKY20 were located in the nucleus and ArWRKY20 showed transcriptional activity. The heterologous expression of ArWRKY5 in Arabidopsis thaliana showed earlier flowering, while overexpression of ArWRKY20 delayed flowering. But the OE-ArWRKY20 lines had a robust body shape and a very significant increase in the number of rosette leaves. Furthermore, stamens and seed development were positively regulated by these two ArWRKYs. These results indicated that ArWRKY5 and ArWRKY20 not only play opposite roles in the floral development, but also regulate the plant growth and seed development in A. thaliana. But their specific biological functions and mechanism in A. roxburghii need to be investigated further.

Bioenergetics of pollen tube growth in Arabidopsis thaliana revealed by ratiometric genetically encoded biosensors

Pollen tube is the fastest-growing plant cell. Its polarized growth process consumes a tremendous amount of energy, which involves coordinated energy fluxes between plastids, the cytosol, and mitochondria. However, how the pollen tube obtains energy and what the biological roles of pollen plastids are in this process remain obscure. To investigate this energy-demanding process, we developed second-generation ratiometric biosensors for pyridine nucleotides which are pH insensitive between pH 7.0 to pH 8.5. By monitoring dynamic changes in ATP and NADPH concentrations and the NADH/NAD⁺ ratio at the subcellular level in Arabidopsis (Arabidopsis thaliana) pollen tubes, we delineate the energy metabolism that underpins pollen tube growth and illustrate how pollen plastids obtain ATP, NADPH, NADH, and acetyl-CoA for fatty acid biosynthesis. We also show that fermentation and pyruvate dehydrogenase bypass are not essential for pollen tube growth in Arabidopsis, in contrast to other plant species like tobacco and lily.

Sucrose rather than GA transported by AtSWEET13 and AtSWEET14 supports pollen fitness at late anther development stages

Both sugar and the hormone gibberellin (GA) are essential for anther-enclosed pollen development and thus for plant productivity in flowering plants. Arabidopsis (Arabidopsis thaliana) AtSWEET13 and AtSWEET14, which are expressed in anthers and associated with seed yield, transport both sucrose and GA. However, it is still unclear which substrate transported by them directly affects anther development and seed yield. Histochemical staining, cross-sectioning and microscopy imaging techniques were used to investigate and interpret the phenotypes of the atsweet13;14 double mutant during anther development. Genetic complementation of atsweet13;14 using AtSWEET9, which transports sucrose but not GA, and the GA transporter AtNPF3.1, respectively, was conducted to test the substrate preference relevant to the biological process. The loss of both AtSWEET13 and AtSWEET14 resulted in reduced pollen viability and therefore decreased pollen germination. AtSWEET9 fully rescued the defects in pollen viability and germination of atsweet13;14, whereas AtNPF3.1 failed to do so, indicating that AtSWEET13/14-mediated sucrose rather than GA is essential for pollen fertility. AtSWEET13 and AtSWEET14 function mainly at the anther wall during late anther development stages, and they probably are responsible for sucrose efflux into locules to support pollen development to maturation, which is vital for subsequent pollen viability and germination.

Identification of the WRKY Gene Family and Characterization of Stress-Responsive Genes in Taraxacum kok-saghyz Rodin

WRKY transcription factors present unusual research value because of their critical roles in plant physiological processes and stress responses. Taraxacum kok-saghyz Rodin (TKS) is a perennial herb of dandelion in the Asteraceae family. However, the research on TKS WRKY TFs is limited. In this study, 72 TKS WRKY TFs were identified and named. Further comparison of the core motifs and the structure of the WRKY motif was analyzed. These TFs were divided into three groups through phylogenetic analysis. Genes in the same group of TkWRKY usually exhibit a similar exon-intron structure and motif composition. In addition, virtually all the TKS WRKY genes contained several cis-elements related to stress response. Expression profiling of the TkWRKY genes was assessed using transcriptome data sets and Real-Time RT-PCR data in tissues during physiological development, under abiotic stress and hormonal treatments. For instance, the TkWRKY18, TkWRKY23, and TkWRKY38 genes were significantly upregulated during cold stress, whereas the TkWRKY21 gene was upregulated under heat-stress conditions. These results could provide a basis for further studies on the function of the TKS WRKY gene family and genetic amelioration of TKS germplasm.

PALD encoding a lipid droplet-associated protein is critical for the accumulation of lipid droplets and pollen longevity in Arabidopsis

Pollen longevity is critical for plant pollination and hybrid seed production, but few studies have focused on pollen longevity. In this study, we identified an Arabidopsis thaliana gene, Protein associated with lipid droplets (PALD), which is strongly expressed in pollen and critical for the regulation of pollen longevity. PALD was expressed specifically in mature pollen grains and the pollen tube, and its expression was upregulated under dry conditions. PALD encoded a lipid droplet (LD)-associated protein and its N terminus was critical for the LD localization of PALD. The number of LDs and diameter were reduced in pollen grains of the loss-of-function PALD mutants. The viability and germination of the mature pollen grains of the pald mutants were comparable with those of the wild-type, but after the pollen grains were stored under dry conditions, pollen germination and male transmission of the mutant were compromised compared with those of the wild-type. Our study suggests that PALD was required for the maintenance of LD quality in mature pollen grains and regulation of pollen longevity.

Study and QTL mapping of reproductive and morphological traits implicated in the autofertility of faba bean

Autofertility describes the ability of faba bean flowers to self-fertilize thereby ensuring the productivity of this crop in the absence of pollinators or mechanical disturbance. In the legume crop faba bean (Vicia faba L.), lack of autofertility in a context of insufficient pollination can lead to a severe decrease in grain yield. Here we performed the first QTL analysis aimed at identifying the genomic regions controlling autofertility in this crop. We combined pod and seed setting scores from a recombinant inbred population (RIL) segregating for autofertility in different environments and years with measurements of morphological floral traits and pollen production and viability. This approach revealed 19 QTLs co-localizing in six genomic regions. Extensive co-localization was evident for various floral features whose QTLs clustered in chrs. I, II and V, while other QTLs in chrs. III, IV and VI revealed co-localization of flower characteristics and pod and seed set data. The percentage of phenotypic variation explained by the QTLs ranged from 8.9 for style length to 25.7 for stigma angle. In the three QTLs explaining the highest phenotypic variation (R ² > 20), the marker alleles derived from the autofertile line Vf27. We further inspected positional candidates identified by these QTLs which represent a valuable resource for further validation. Our results advance the understanding of autofertility in faba bean and will aid the identification of responsible genes for genomic-assisted breeding in this crop.

WRKY2 and WRKY10 regulate the circadian expression of PIF4 during the day through interactions with CCA1/LHY and phyB

WRKY transcription factors are known mostly for their function in plant defense, abiotic stress responses, senescence, seed germination, and development of the pollen, embryo, and seed. Here, we report the regulatory functions of two WRKY proteins in photomorphogenesis and PIF4 expression. PIF4 is a critical signaling hub in light, temperature, and hormonal signaling pathways. Either its expression or its accumulation peaks in the morning and afternoon. WRKY2 and WRKY10 form heterodimers and recognize their target site in the PIF4 promoter near the MYB element that is bound by CCA1 and LHY under red and blue light. WRKY2 and WRKY10 interact directly with CCA1/LHY to enhance their targeting but interact indirectly with SHB1. The two WRKY proteins also interact with phyB, and their interaction enhances the targeting of CCA1 and LHY to the PIF4 promoter. SHB1 associates with the WRKY2 and WRKY10 loci and enhances their expression in parallel with the PIF4 expression peaks. This forward regulatory loop further sustains the accumulation of the two WRKY proteins and the targeting of CCA1/LHY to the PIF4 locus. In summary, interactions of two WRKY proteins with CCA1/LHY and phyB maintain an optimal expression level of PIF4 toward noon and afternoon, which is essential to sketch the circadian pattern of PIF4 expression.

MAP kinase cascades in plant development and immune signaling

Mitogen-activated protein kinase (MAPK) cascades are important signaling modules regulating diverse biological processes. During the past 20 years, much progress has been made on the functions of MAPK cascades in plants. This review summarizes the roles of MAPKs, known MAPK substrates, and our current understanding of MAPK cascades in plant development and innate immunity. In addition, recent findings on the molecular links connecting surface receptors to MAPK cascades and the mechanisms underlying MAPK signaling specificity are also discussed.

Isolation and functional characterization of a glucose-6-phosphate/phosphate translocator (IbG6PPT1) from sweet potato (Ipomoea batatas (L.) Lam.)

Sweet potato (Ipomoea batatas (L.) Lam.) is a good source of carbohydrates, an excellent raw material for starch-based industries, and a strong candidate for biofuel production due to its high starch content. However, the molecular basis of starch biosynthesis and accumulation in sweet potato is still insufficiently understood. Glucose-6-phosphate/phosphate translocators (GPTs) mediate the import of glucose-6-phosphate (Glc6P) into plastids for starch synthesis. Here, we report the isolation of a GPT-encoding gene, IbG6PPT1, from sweet potato and the identification of two additional IbG6PPT1 gene copies in the sweet potato genome. IbG6PPT1 encodes a chloroplast membrane-localized GPT belonging to the GPT1 group and highly expressed in storage root of sweet potato. Heterologous expression of IbG6PPT1 resulted in increased starch content in the leaves, root tips, and seeds and soluble sugar in seeds of Arabidopsis thaliana, but a reduction in soluble sugar in the leaves. These findings suggested that IbG6PPT1 might play a critical role in the distribution of carbon sources in source and sink and the accumulation of carbohydrates in storage tissues and would be a good candidate gene for controlling critical starch properties in sweet potato.

The rice Raf-like MAPKKK OsILA1 confers broad-spectrum resistance to bacterial blight by suppressing the OsMAPKK4-OsMAPK6 cascade

Mitogen-activated protein kinase kinase kinase (MAPKKK) are the first components of MAPK cascades, which play pivotal roles in signaling during plant development and physiological processes. The genome of rice encodes 75 MAPKKKs, of which 43 are Raf-like MAPKKKs. The functions and action modes of most of the Raf-like MAPKKKs, whether they function as bona fide MAPKKKs and which are their downstream MAPKKs, are largely unknown. Here, we identified the osmapkkk43 mutant, which conferred broad-spectrum resistance to Xanthomonas oryzae pv. oryzae (Xoo), the destructive bacterial pathogen of rice. Oryza sativa (Os)MAPKKK43 encoding a Raf-like MAPKKK was previously known as Increased Leaf Angle 1 (OsILA1). Genetic analysis indicated that OsILA1 functioned as a negative regulator and acted upstream of the OsMAPKK4-OsMAPK6 cascade in rice-Xoo interactions. Unlike classical MAPKKKs, OsILA1 mainly phosphorylated the threonine 34 site at the N-terminal domain of OsMAPKK4, which possibly influenced the stability of OsMAPKK4. The N-terminal domain of OsILA1 is required for its homodimer formation and its full phosphorylation capacity. Taken together, our findings reveal that OsILA1 acts as a negative regulator of the OsMAPKK4-OsMAPK6 cascade and is involved in rice-Xoo interactions.

LARP6C orchestrates posttranscriptional reprogramming of gene expression during hydration to promote pollen tube guidance

Increasing evidence suggests that posttranscriptional regulation is a key player in the transition between mature pollen and the progamic phase (from pollination to fertilization). Nonetheless, the actors in this messenger RNA (mRNA)-based gene expression reprogramming are poorly understood. We demonstrate that the evolutionarily conserved RNA-binding protein LARP6C is necessary for the transition from dry pollen to pollen tubes and the guided growth of pollen tubes towards the ovule in Arabidopsis thaliana. In dry pollen, LARP6C binds to transcripts encoding proteins that function in lipid synthesis and homeostasis, vesicular trafficking, and polarized cell growth. LARP6C also forms cytoplasmic granules that contain the poly(A) binding protein and possibly represent storage sites for translationally silent mRNAs. In pollen tubes, the loss of LARP6C negatively affects the quantities and distribution of storage lipids, as well as vesicular trafficking. In Nicotiana benthamiana leaf cells and in planta, analysis of reporter mRNAs designed from the LARP6C target MGD2 provided evidence that LARP6C can shift from a repressor to an activator of translation when the pollen grain enters the progamic phase. We propose that LARP6C orchestrates the timely posttranscriptional regulation of a subset of mRNAs in pollen during the transition from the quiescent to active state and along the progamic phase to promote male fertilization in plants.

LARP6C orchestrates posttranscriptional reprogramming of gene expression during hydration to promote pollen tube guidance

Increasing evidence suggests that posttranscriptional regulation is a key player in the transition between mature pollen and the progamic phase (from pollination to fertilization). Nonetheless, the actors in this messenger RNA (mRNA)-based gene expression reprogramming are poorly understood. We demonstrate that the evolutionarily conserved RNA-binding protein LARP6C is necessary for the transition from dry pollen to pollen tubes and the guided growth of pollen tubes towards the ovule in Arabidopsis thaliana. In dry pollen, LARP6C binds to transcripts encoding proteins that function in lipid synthesis and homeostasis, vesicular trafficking, and polarized cell growth. LARP6C also forms cytoplasmic granules that contain the poly(A) binding protein and possibly represent storage sites for translationally silent mRNAs. In pollen tubes, the loss of LARP6C negatively affects the quantities and distribution of storage lipids, as well as vesicular trafficking. In Nicotiana benthamiana leaf cells and in planta, analysis of reporter mRNAs designed from the LARP6C target MGD2 provided evidence that LARP6C can shift from a repressor to an activator of translation when the pollen grain enters the progamic phase. We propose that LARP6C orchestrates the timely posttranscriptional regulation of a subset of mRNAs in pollen during the transition from the quiescent to active state and along the progamic phase to promote male fertilization in plants.

Sporophytic control of anther development and male fertility by glucose-6-phosphate/phosphate translocator 1 (OsGPT1) in rice

Coordination between the sporophytic tissue and the gametic pollen within anthers is tightly controlled to achieve the optimal pollen fitness. Glucose-6-phosphate/phosphate translocator (GPT) transports glucose-6-phosphate, a key precursor of starch and/or fatty acid biosynthesis, into plastids. Here, we report the functional characterization of OsGPT1 in the rice anther development and pollen fertility. Pollen grains from homozygous osgpt1 mutant plants fail to accumulate starch granules, resulting in pollen sterility. Genetic analyses reveal a sporophytic effect for this mutation. OsGPT1 is highly expressed in the tapetal layer of rice anther. Degeneration of the tapetum, an important process to provide cellular contents to support pollen development, is impeded in osgpt1 plants. In addition, defective intine and exine are observed in the pollen from osgpt1 plants. Expression levels of multiple genes that are important to tapetum degeneration or pollen wall formation are significantly decreased in osgpt1 anthers. Previously, we reported that AtGPT1 plays a gametic function in the accumulation of lipid bodies in Arabidopsis pollen. This report highlights a sporophytic role of OsGPT1 in the tapetum degeneration and pollen development. The divergent functions of OsGPT1 and AtGPT1 in pollen development might be a result of their independent evolution after monocots and dicots diverged.

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.

Expression of a plastid-localized sugar transporter in the suspensor is critical to embryogenesis

Plant growth and development rely on sugar transport between source and sink cells and between different organelles. The plastid-localized sugar transporter GLUCOSE-6-PHOSPHATE TRANSLOCATER1 (GPT1) is an essential gene in Arabidopsis (Arabidopsis thaliana). Using a partially rescued gpt1 mutant and cell-specific RNAi suppression of GPT1, we demonstrated that GPT1 is essential to the function of the embryo suspensor and the development of the embryo. GPT1 showed a dynamic expression/accumulation pattern during embryogenesis. Inhibition of GPT1 accumulation via RNAi using a suspensor-specific promoter resulted in embryos and seedlings with defects similar to auxin mutants. Loss of function of GPT1 in the suspensor also led to abnormal/ectopic cell division in the lower part of the suspensor, which gave rise to an ectopic embryo, resulting in twin embryos in some seeds. Furthermore, loss of function of GPT1 resulted in vacuolar localization of PIN-FORMED1 (PIN1) and altered DR5 auxin activity. Proper localization of PIN1 on the plasma membrane is essential to polar auxin transport and distribution, a key determinant of pattern formation during embryogenesis. Our findings suggest that the function of GPT1 in the embryo suspensor is linked to sugar and/or hormone distribution between the embryo proper and the maternal tissues, and is important for maintenance of suspensor identity and function during embryogenesis.

The YDA-MKK4/MKK5-MPK3/MPK6 Cascade Functions Downstream of the RGF1-RGI Ligand-Receptor Pair in Regulating Mitotic Activity in Root Apical Meristem

The mitotic activity of root apical meristem (RAM) is critical to primary root growth and development. Previous studies have identified the roles of ROOT GROWTH FACTOR 1 (RGF1), a peptide ligand, and its receptors, RGF1 INSENSITIVEs (RGIs), a clade of five leucine-rich-repeat receptor-like kinases, in promoting cell division in the RAM, which determines the primary root length. However, the downstream signaling components remain elusive. In this study, we identify a complete mitogen-activated protein kinase (MAPK or MPK) cascade, composed of YDA, MKK4/MKK5, and MPK3/MPK6, that functions downstream of the RGF1-RGI ligand-receptor pair. Similar to the rgi1/2/3/4/5 quintuple mutant, loss-of-function mutants of MPK3 and MPK6, MKK4 and MKK5, or YDA show a short-root phenotype, which is associated with reduced mitotic activity and lower expression of PLETHORA 1 (PLT1)/PLT2 in the RAM. Furthermore, MPK3/MPK6 activation in response to exogenous RGF1 treatment is impaired in the rgi1/2/3/4/5 quintuple, yda single, and mkk4 mkk5 double mutants. Epistatic analyses demonstrated that the expression of constitutively active MKK4, MKK5, or YDA driven by the RGI2 promoter can rescue the short-root phenotype of the rgi1/2/3/4/5 mutant. Taken together, these results suggest that the YDA-MKK4/MKK5-MPK3/MPK6 cascade functions downstream of the RGF1-RGI ligand-receptor pair and upstream of PLT1/PLT2 to modulate the stem cell population and primary root growth in Arabidopsis.

Genome wide association mapping of epi-cuticular wax genes in Sorghum bicolor

Sorghum accumulates epi-cuticular wax (EW) in leaves, sheaths, and culms. EW reduces the transpirational and nontranspirational (nonstomatal) water loss and protects the plant from severe drought stress in addition to imparting resistance against insect pests. Results presented here are from the analysis of EW content of 387 diverse sorghum accessions and its genome-wide association study (GWAS). EW content in sorghum leaves ranged from 0.1 to 29.7 mg cm^-2 with a mean value of 5.1 mg cm^-2. GWAS using 265,487 single nucleotide polymorphisms identified thirty-seven putative genes associated (P < 9.89E-06) with EW biosynthesis and transport in sorghum. Major EW biosynthetic genes identified included 3-Oxoacyl-[acyl-carrier-protein (ACP)] synthase III, an Ankyrin repeat protein, a bHLH-MYC, and an R2R3-MYB transcription factor. Genes involved in EW regulation or transport included an ABC transporter, a Lipid exporter ABCA1, a Multidrug resistance protein, Inositol 1, 3, 4-trisphosphate 5/6-kinase, and a Cytochrome P450. This GWA study thus demonstrates the potential for genetic manipulation of EW content in sorghum for better adaptation to biotic and abiotic stress.

Lipid droplets throughout the evolutionary tree

Intracellular lipid droplets are utilized for lipid storage and metabolism in organisms as evolutionarily diverse as animals, fungi, plants, bacteria, and archaea. These lipid droplets demonstrate great diversity in biological functions and protein and lipid compositions, yet fundamentally share common molecular and ultrastructural characteristics. Lipid droplet research has been largely fragmented across the diversity of lipid droplet classes and sub-classes. However, we suggest that there is great potential benefit to the lipid community in better integrating the lipid droplet research fields. To facilitate such integration, we survey the protein and lipid compositions, functional roles, and mechanisms of biogenesis across the breadth of lipid droplets studied throughout the natural world. We depict the big picture of lipid droplet biology, emphasizing shared characteristics and unique differences seen between different classes. In presenting the known diversity of lipid droplets side-by-side it becomes necessary to offer for the first time a consistent system of categorization and nomenclature. We propose a division into three primary classes that reflect their sub-cellular location: i) cytoplasmic lipid droplets (CYTO-LDs), that are present in the eukaryotic cytoplasm, ii) prokaryotic lipid droplets (PRO-LDs), that exist in the prokaryotic cytoplasm, and iii) plastid lipid droplets (PL-LDs), that are found in plant plastids, organelles of photosynthetic eukaryotes. Within each class there is a remarkable array of sub-classes displaying various sizes, shapes and compositions. A more integrated lipid droplet research field will provide opportunities to better build on discoveries and accelerate the pace of research in ways that have not been possible.

Detection of significant SNP associated with production and oil quality traits in interspecific oil palm hybrids using RARSeq

A RARSeq based Association mapping study was performed in a population of 104 Elaeis oleifera x E. guineensis hybrids of five origins with the aim of finding functional markers associated to six productive and 19 oil quality traits. For this purpose mRNA of each genotype was isolated and double stranded cDNA was synthesized. Following digestion with two restriction enzymes and adapter ligation, a size selected pool of barcoded amplicons was produced and sequenced using Illumina MiSeq. The obtained sequences were processed with a "snakemake" pipeline, filtered and missing values were imputed. For all traits except two significant effects of the origin was observed. Genetic diversity analyses revealed high variability within origins and an excess of heterozygosity in the population. Two GLM models with Q matrix or PCA matrix as covariates and two MLM models incorporating in addition a Kinship matrix were tested for genotype-phenotype associations using GAPIT software. Using unadjusted p values (< 0.01) 78 potential associations were detected involving 25 SNP and 20 traits. When applying FDR multiple testing with p <  0.05, 25 significant associations remained involving eight SNP and six quality traits. Four SNP were located in genes with a potential relevant biological meaning.

Functional Analysis of MaWRKY24 in Transcriptional Activation of Autophagy-Related Gene 8f/g and Plant Disease Susceptibility to Soil-Borne Fusarium oxysporum f. sp. cubense

WRKYs play important roles in plant development and stress responses. Although MaWRKYs have been comprehensively identified in the banana (Musa acuminata), their in vivo roles and direct targets remain elusive. In this study, a transcript profile analysis indicated the common regulation of MaWRKYs transcripts in response to fungal pathogen Fusarium oxysporum f. sp. cubense (Foc). Among these MaWRKYs, MaWRKY24 was chosen for further analysis due to its higher expression in response to Foc. The specific nucleus subcellular location and transcription activated activity on W-box indicated that MaWRKY24 was a transcription factor. The correlation analysis of gene expression indicated that MaWRKYs were closely related to autophagy-associated genes (MaATG8s). Further analysis showed that MaWRKY24 directly regulated the transcriptional level of MaATG8f/g through binding to W-box in their promoters, as evidenced by quantitative real-time Polymerase Chain Reaction (PCR), dual luciferase assay, and electrophoretic mobility shift assay. In addition, overexpression of MaWRKY24 and MaATG8f/g resulted in disease susceptibility to Foc, which might be related to the activation of autophagic activity. This study highlights the positive regulation of MaWRKY24 in transcriptional activation of autophagy-related gene 8f/g in the banana and their common roles in disease susceptibility to soil-borne Foc, indicating the effects of MaWRKY24 on autophagy and disease susceptibility.

Megasporogenesis, microsporogenesis, and female and male gametophyte development in Ziziphus jujuba Mill

Jujube (Ziziphus jujuba Mill.) is an important fruit tree species in China. In this study, we studied the megasporogenesis, microsporogenesis, and female and male gametophyte development of two major jujube cultivars, "Dongzao" and "Mayazao," using the squash technique, improved paraffin section technology, and optical microscopy. Our investigation revealed that both "Dongzao" and "Mayazao" have bilocular ovaries, basal placenta, and anatropous, bitegmic, crassinucellate ovules. The tetrads formed by meiosis of megaspore mother cells are arranged in a straight line or a tetrahedron. Embryo sac development is of the Polygonum type. The flower buds contain five anthers, each having four pollen sacs. The anther wall, which is of the fundamental form, is composed of epidermis, endothecium, one or two middle layers, and glandular tapetum. Mature pollen grains are two-celled and three-colporate. Both "Dongzao" and "Mayazao" can form normal mature pollen grains. Our study, which has revealed the basic phenomena and progression of megasporogenesis, microsporogenesis, and female and male gametophyte development in jujube, has generated important data for further research on jujube cytology and reproductive biology. Finally, our explorations of the cytological mechanism of male sterility in "Dongzao" also have provided a cytological basis for crossbreeding.