PECTATE LYASE-LIKE10 is associated with pollen wall development in Brassica campestris

Exogenous Methyl Jasmonate (MeJA) Improves 'Ruixue' Apple Fruit Quality by Regulating Cell Wall Metabolism

'Ruixue' apples were used as the test material to study the effect of 10 μM methyl jasmonate (MeJA) on the quality and cell wall metabolism of apples after 18 d of storage. The results showed that MeJA significantly decreased the respiratory rate, reduced the titratable acid content and maintained a high soluble solids content. MeJA has been shown to suppress the activities and gene expressions of WSP, CSP, ISP, and cellulose in contrast to the control group, thereby maintaining a lower cell permeability and higher exocarp firmness. MeJA significantly decreased the expression of MdACS, MdACO, MdPL, Mdgal, and MdPG genes in the apple exocarp when compared to the control group. In addition, the overexpression of MdPL18 increased the content of cell wall polysaccharides such as WSP and CSP, enhanced cell wall-degrading enzyme activities, and accelerated fruit ripening and softening, whereas silencing MdPL18 did the opposite. Together, these results demonstrate that exogenous MeJA maintains the Ruixue apple fruit quality by regulating the metabolism of cell wall substances.

Biosynthesis and transport of pollen coat precursors in angiosperms

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

The hydroxyproline O-arabinosyltransferase FIN4 is required for tomato pollen intine development

The pollen grain cell wall is a highly specialized structure composed of distinct layers formed through complex developmental pathways. The production of the innermost intine layer, composed of cellulose, pectin and other polymers, is particularly poorly understood. Here we demonstrate an important and specific role for the hydroxyproline O-arabinosyltransferase (HPAT) FIN4 in tomato intine development. HPATs are plant-specific enzymes which initiate glycosylation of certain cell wall structural proteins and signaling peptides. FIN4 was expressed throughout pollen development in both the developing pollen and surrounding tapetal cells. A fin4 mutant with a partial deletion of the catalytic domain displayed significantly reduced male fertility in vivo and compromised pollen hydration and germination in vitro. However, fin4 pollen that successfully germinated formed morphologically normal pollen tubes with the same growth rate as the wild-type pollen. When we examined mature fin4 pollen, we found they were cytologically normal, and formed morphologically normal exine, but produced significantly thinner intine. During intine deposition at the late stages of pollen development we found fin4 pollen had altered polymer deposition, including reduced cellulose and increased detection of pectin, specifically homogalacturonan with both low and high degrees of methylesterification. Therefore, FIN4 plays an important role in intine formation and, in turn pollen hydration and germination and the process of intine formation involves dynamic changes in the developing pollen cell wall.

The triticale mature pollen and stigma proteomes - assembling the proteins for a productive encounter

Triticeae crops are major contributors to global food production and ensuring their capacity to reproduce and generate seeds is critical. However, despite their importance our knowledge of the proteins underlying Triticeae reproduction is severely lacking and this is not only true of pollen and stigma development, but also of their pivotal interaction. When the pollen grain and stigma are brought together they have each accumulated the proteins required for their intended meeting and accordingly studying their mature proteomes is bound to reveal proteins involved in their diverse and complex interactions. Using triticale as a Triticeae representative, gel-free shotgun proteomics was used to identify 11,533 and 2977 mature stigma and pollen proteins respectively. These datasets, by far the largest to date, provide unprecedented insights into the proteins participating in Triticeae pollen and stigma development and interactions. The study of the Triticeae stigma has been particularly neglected. To begin filling this knowledge gap, a developmental iTRAQ analysis was performed revealing 647 proteins displaying differential abundance as the stigma matures in preparation for pollination. An in-depth comparison to an equivalent Brassicaceae analysis divulged both conservation and diversification in the makeup and function of proteins involved in the pollen and stigma encounter. SIGNIFICANCE: Successful pollination brings together the mature pollen and stigma thus initiating an intricate series of molecular processes vital to crop reproduction. In the Triticeae crops (e.g. wheat, barley, rye, triticale) there persists a vast deficit in our knowledge of the proteins involved which needs to be addressed if we are to face the many upcoming challenges to crop production such as those associated with climate change. At maturity, both the pollen and stigma have acquired the protein complement necessary for their forthcoming encounter and investigating their proteomes will inevitably provide unprecedented insights into the proteins enabling their interactions. By combining the analysis of the most comprehensive Triticeae pollen and stigma global proteome datasets to date with developmental iTRAQ investigations, proteins implicated in the different phases of pollen-stigma interaction enabling pollen adhesion, recognition, hydration, germination and tube growth, as well as those underlying stigma development were revealed. Extensive comparisons between equivalent Triticeae and Brassiceae datasets highlighted both the conservation of biological processes in line with the shared goal of activating the pollen grain and promoting pollen tube invasion of the pistil to effect fertilization, as well as the significant distinctions in their proteomes consistent with the considerable differences in their biochemistry, physiology and morphology.

Transcriptomic analysis reveals candidate genes associated with anther development in Lilium Oriental Hybrid 'Siberia'

Lily (Lilium spp. and hybrids) is an important cut flower crop worldwide. Lily flowers have large anthers, which release a large amount of pollen that stains the tepals or clothing and thus can affect the commercial value of cut flowers. In this study, lily Oriental 'Siberia' was used to investigate the regulatory mechanism of lily anther development, which may provide information to prevent pollen pollution in the future. Based on the flower bud length, anther length and color, and anatomical observations, lily anther development was categorized into five stages: green (G), green-to-yellow 1 (GY1), green-to-yellow 2 (GY2), yellow (Y), and purple (P). Total RNA was extracted from the anthers at each stage for transcriptomic analysis. A total of 268.92-Gb clean reads were generated, and 81,287 unigenes were assembled and annotated. The number of differentially expressed genes (DEGs) and unique genes were largest for the pairwise comparison between the G and GY1 stages. The G and P samples were clustered separately, whereas the GY1, GY2, and Y samples were clustered together in scatter plots from a principal component analysis. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses of DEGs detected in the GY1, GY2, and Y stages revealed that the pectin catabolic process, hormone levels, and phenylpropanoid biosynthesis were enriched. The DEGs associated with jasmonic acid biosynthesis and signaling were highly expressed at the early stages (G and GY1), whereas the DEGs associated with phenylpropanoid biosynthesis were mainly expressed in the intermediate stages (GY1, GY2, and Y). The DEGs involved in the pectin catabolic process were expressed at advanced stages (Y and P). Cucumber mosaic virus-induced gene silencing of LoMYB21 and LoAMS caused a strongly inhibited anther dehiscence phenotype, but without affecting the development of other floral organs. These results provide novel insights for understanding the regulatory mechanism of anther development in lily and other plants.

Virus-induced gene silencing for in planta validation of gene function in cucurbits

Virus-induced gene silencing (VIGS) is a powerful tool for high-throughput analysis of gene function. Here, we developed the VIGS vector pCF93, from which expression of the cucumber fruit mottle mosaic virus genome is driven by the cauliflower mosaic virus 35S promoter to produce viral transcripts in inoculated plants. To test the utility of the pCF93 vector, we identified candidate genes related to male sterility (MS) in watermelon (Citrullus lanatus), which is recalcitrant to genetic transformation. Specifically, we exploited previously reported reference-based and de novo transcriptome data to define 38 differentially expressed genes between a male-sterile line and its fertile near-isogenic line in the watermelon cultivar DAH. We amplified 200- to 300-bp fragments of these genes, cloned them into pCF93, and inoculated DAH with the resulting VIGS clones. The small watermelon cultivar DAH enabled high-throughput screening using a small cultivation area. We simultaneously characterized the phenotypes associated with each of the 38 candidate genes in plants grown in a greenhouse. Silencing of 8 of the 38 candidate genes produced male-sterile flowers with abnormal stamens and no pollen. We confirmed the extent of gene silencing in inoculated flowers using reverse transcription-qPCR. Histological analysis of stamens from male-fertile and male-sterile floral buds and mature flowers revealed developmental defects and shrunken pollen sacs. Based on these findings, we propose that the pCF93 vector and our VIGS system will facilitate high-throughput analysis for the study of gene function in watermelons.

Functional characterization of E3 ubiquity ligase Bra015092 in pollen development of Brassica campestris ssp. Chinensis

Pollen development plays an important role in the sexual reproduction of seed-type plants. Ubiquitination of proteins is an essential link in the post-translational modification of proteins. E3 ubiquity ligase is a key protein that recognizes substrates in the protein ubiquitination pathway. The hybrid line "Bcajh97-01A/B" of Chinese cabbage (Brassica campestris L. ssp. Chinensis) was used as test material. The gene Bra015092, with a size of 642 bp, was amplified. Semi-quantitative (RT-PCR) and quantitative real-time PCR (qRT-PCR) techniques were utilized to analyze the expression of Bra015092 in the dual-purpose line of Chinese cabbage. It was found that Bra015092 had a higher expression level in inflorescence. Subcellular localization analysis showed that Bra015092 and GFP fusion expression protein widely exist in tobacco epidermal cells. Bra015092 was transformed into "Youqing49" cabbage to obtain Bra015092^OE overexpressing transgenic lines. The morphological observation of Bra015092^OE plants showed that the pollen of BcMF29^OE plants became deformed and inactive, and the vegetative and reproductive nuclei were abnormally developed. The in vitro germination experiments showed that about 24.5% of the pollen in Bra015092^OE plants could not germinate. The results of the semi-thin section showed that the pollen development of Bra015092^OE plants was abnormal at the stage of binuclear pollen grains. Transmission electron microscopy revealed that the pollen grains of Bra015092^OE plants gradually degraded from the binuclear to the trinucleate pollen grain stage, and the pollen inner wall was abnormally developed, indicating that Bra015092 plays a major role in the process of pollen development.

The SlHB8 acts as a negative regulator in tapetum development and pollen wall formation in Tomato

Pollen development is crucial for the fruit setting process of tomatoes, but the underlying regulatory mechanism remains to be elucidated. Here, we report the isolation of one HD-Zip III family transcription factor, SlHB8, whose expression levels decreased as pollen development progressed. SlHB8 knockout using CRISPR/Cas9 increased pollen activity, subsequently inducing fruit setting, whereas overexpression displayed opposite phenotypes. Overexpression lines under control of the 35 s and p2A11 promoters revealed that SlHB8 reduced pollen activity by affecting early pollen development. Transmission electron microscopy and TUNEL analyses showed that SlHB8 accelerated tapetum degradation, leading to collapsed and infertile pollen without an intine and an abnormal exine. RNA-seq analysis of tomato anthers at the tetrad stage showed that SlHB8 positively regulates SPL/NZZ expression and the tapetum programmed cell death conserved genetic pathway DYT1-TDF1-AMS-MYB80 as well as other genes related to tapetum and pollen wall development. In addition, DNA affinity purification sequencing, electrophoretic mobility shift assay, yeast one-hybrid assay and dual-luciferase assay revealed SlHB8 directly activated the expression of genes related to pollen wall development. The study findings demonstrate that SlHB8 is involved in tapetum development and degradation and plays an important role in anther development.

Arabidopsis Novel Microgametophyte Defective Mutant 1 Is Required for Pollen Viability via Influencing Intine Development in Arabidopsis

The pollen intine layer is necessary for male fertility in flowering plants. However, the mechanisms behind the developmental regulation of intine formation still remain largely unknown. Here, we identified a positive regulator, Arabidopsis novel microgametophyte defective mutant 1 (AtNMDM1), which influences male fertility by regulating intine formation. The AtNMDM1, encoding a pollen nuclei-localized protein, was highly expressed in the pollens at the late anther stages, 10-12. Both the mutations and the knock-down of AtNMDM1 resulted in pollen defects and significantly lowered the seed-setting rates. Genetic transmission analysis indicated that AtNMDM1 is a microgametophyte lethal gene. Calcofluor white staining revealed that abnormal cellulose distribution was present in the aborted pollen. Ultrastructural analyses showed that the abnormal intine rather than the exine led to pollen abortion. We further found, using transcriptome analysis, that cell wall modification was the most highly enriched gene ontology (GO) term used in the category of biological processes. Notably, two categories of genes, Arabinogalactan proteins (AGPs) and pectin methylesterases (PMEs) were greatly reduced, which were associated with pollen intine formation. In addition, we also identified another regulator, AtNMDM2, which interacted with AtNMDM1 in the pollen nuclei. Taken together, we identified a novel regulator, AtNMDM1 that affected cellulose distribution in the intine by regulating intine-related gene expression; furthermore, these results provide insights into the molecular mechanisms of pollen intine development.

Primary Cell Wall Modifying Proteins Regulate Wall Mechanics to Steer Plant Morphogenesis

Plant morphogenesis involves multiple biochemical and physical processes inside the cell wall. With the continuous progress in biomechanics field, extensive studies have elucidated that mechanical forces may be the most direct physical signals that control the morphology of cells and organs. The extensibility of the cell wall is the main restrictive parameter of cell expansion. The control of cell wall mechanical properties largely determines plant cell morphogenesis. Here, we summarize how cell wall modifying proteins modulate the mechanical properties of cell walls and consequently influence plant morphogenesis.

Integrative analysis of transcriptomic and proteomic changes related to male sterility in Tagetes erecta

Male sterile and male fertile two-type lines are important in heterosis utilization and breeding in Tagetes erecta, but the genes and pathways involved in male sterility are poorly understood. To explore these topics, transcriptome data (by RNA-seq) and proteome data (by iTRAQ) were gathered from flower buds of the male sterile line 'MS2-2' and male fertile line 'MF2-2' and integrated for a better understanding of the underlying molecular mechanisms of male sterility in T. erecta. The RNA-seq procedure generated 285,139,740 clean reads and 63359 unigenes and 6640 differentially expressed genes (DEGs) were identified, of which 4136 were downregulated and 2504 were upregulated in 'MS2-2'. DEGs related to flower development, pollen development, pollen wall assembly, endogenous hormones and transcription factors were identified. The iTRAQ analysis identified 3950 proteins in total; 789 were differentially expressed proteins (381 upregulated, 408 downregulated), which were mainly annotated to the Ribosome, Carbon metabolism and Biosynthesis of amino acids pathways. An association analysis revealed strong correlation (r Pearson = 0.6019) between the transcriptomic and proteomic data, and 256 and 34 proteins showed the same and opposite expression patterns with regard to their transcripts, respectively. Pathways such as photosynthesis, fatty acid biosynthesis and phenylpropanoid biosynthesis which influence tapetum and pollen development in male sterile plants, were significantly enriched at the transcript and protein levels. Most genes involved in these pathways were downregulated in 'MS2-2'. The low expression of these genes or functional loss of proteins could be associated with flower development, pollen development and related to changes in fertility in T. erecta. This study provided transcriptomic and proteomic information for T. erecta that could illuminate the mechanism of male sterility.

Comparative Transcriptomics Analysis and Functional Study Reveal Important Role of High-Temperature Stress Response Gene GmHSFA2 During Flower Bud Development of CMS-Based F1 in Soybean

High-temperature (HT) is one of the most important environmental factors that negatively impact the yield of some soybean cytoplasmic male sterility (CMS)-based hybrid (F₁) combinations. The response of soybean to HT, especially at the male organ development stage, is poorly understood. To investigate the molecular mechanisms of the response from soybean CMS-based F₁ male organ to HT, a detailed transcriptomics analysis was performed during flower bud development of soybean HT-tolerant and HT-sensitive CMS-based F₁ combinations (NF₁ and YF₁) under normal-temperature and HT conditions. Obvious HT damage was observed by subjecting YF₁ with HT, such as indehiscent anthers and decreased pollen fertility, whereas the male fertility of NF₁ was normal. In total, 8,784 differentially expressed genes (DEGs) were found to respond to HT stress, which were mainly associated with anther/pollen wall development, carbohydrate metabolism and sugar transport, and auxin signaling. The quantitative real-time PCR (qRT-PCR) analysis and substance content detection also revealed that HT caused male fertility defects in YF₁ by altering pectin metabolism, auxin, and sugar signaling pathways. Most importantly, the sugar signaling-PIF-auxin signaling pathway may underlie the instability of male fertility in YF₁ under HT. Furthermore, HT induced the expression of heat shock factor (HSF) and heat shock protein (HSP) gene families. Overexpression of GmHSFA2 in Arabidopsis can promote the expression of HT protective genes (such as HSP20) by binding to the HSE motifs in their promoters, so as to improve the HT tolerance during flowering. Our results indicated that GmHSFA2 acted as a positive regulator, conferring HT tolerance improvement in soybean CMS-based F₁. GmHSFA2 may be directly involved in the activation of male fertility protection mechanism in the soybean CMS-based F₁ under HT stress.

BcPME37c is involved in pollen intine formation in Brassica campestris

Pollen wall development is one of the key processes of pollen development. Several pectin methylesterase (PME) genes participate in pollen germination and pollen tube growth. However, the relationship between PME genes and pollen intine formation remains unclear. In this study, we investigated the expression and subcellular localization of the PME gene BcPME37c in Brassica campestris. Furthermore, morphology and cytology methods were used to examine the phenotype of the CRISPR/Cas9 system-induced BcPME37c mutant. We found that BcPME37c is predominately expressed in mature stamen and located at the cell wall. BcPME37c mutation causes the abnormal thickening of the pollen intine of B. campestris. Our study indicated that BcPME37c is required for pollen intine formation in B. campestris.

Comprehensive analysis of Ogura cytoplasmic male sterility-related genes in turnip (Brassica rapa ssp. rapifera) using RNA sequencing analysis and bioinformatics

Ogura-type cytoplasmic male sterility (Ogura-CMS) has been widely used in the hybrid breeding industry for cruciferous vegetables. Turnip (Brassica rapa ssp. rapifera) is one of the most important local cruciferous vegetables in China, cultivated for its fleshy root as a flat disc. Here, morphological characteristics of an Ogura-CMS line ‘BY10-2A’ and its maintainer fertile (MF) line ‘BY10-2B’ of turnip were investigated. Ogura-CMS turnip showed a reduction in the size of the fleshy root, and had distinct defects in microspore development and tapetum degeneration during the transition from microspore mother cells to tetrads. Defective microspore production and premature tapetum degeneration during microgametogenesis resulted in short filaments and withered white anthers, leading to complete male sterility of the Ogura-CMS line. Additionally, the mechanism regulating Ogura-CMS in turnip was investigated using inflorescence transcriptome analyses of the Ogura-CMS and MF lines. The de novo assembly resulted in a total of 84,132 unigenes. Among them, 5,117 differentially expressed genes (DEGs) were identified, including 1,339 up- and 3,778 down-regulated genes in the Ogura-CMS line compared to the MF line. A number of functionally known members involved in anther development and microspore formation were addressed in our DEG pool, particularly genes regulating tapetum programmed cell death (PCD), and associated with pollen wall formation. Additionally, 185 novel genes were proposed to function in male organ development based on GO analyses, of which 26 DEGs were genotype-specifically expressed. Our research provides a comprehensive foundation for understanding anther development and the CMS mechanism in turnip.

Genome-wide identification of the pectate lyase-like (PLL) gene family and functional analysis of two PLL genes in rice

Pectate lyase catalyses the eliminative cleavage of de-esterified pectin, which is a major component of primary cell walls in many higher plants. Pectate lyase-like (PLL) genes have been identified in various plant species and are involved in a broad range of physiological processes associated with pectin degradation. Previous studies have functionally identified two PLL genes in rice (Oryza sativa. L). However, the knowledge concerning genome-wide analysis of this family remains limited, and functions of the other PLL genes have not been thoroughly elucidated to date. In this study, we identified 12 PLL genes based on a genome-wide investigation in rice. A complete overview of this gene family is presented, including chromosomal locations, exon-intron structure, cis-acting elements and conserved motifs. PLL protein sequences from multiple plant species were compared and divided into five groups based on phylogenetic analysis. Quantitative RT-PCR analysis revealed that only a portion of OsPLL genes (4 of 12) exhibits detectable expression levels. Notably, OsPLL1, OsPLL3, OsPLL4 and OsPLL12 exhibit strong and preferential expression in panicles suggesting that the potential roles of these genes are crucial during rice panicle development. Moreover, knockdown of OsPLL3 and OsPLL4 by artificial microRNA (amiRNA) disrupted normal pollen development and resulted in partial male sterility. These results could provide valuable information for characterising the functions and dissecting the molecular mechanisms of the OsPLL genes.

The distinct functions of two classical arabinogalactan proteins BcMF8 and BcMF18 during pollen wall development in Brassica campestris

Arabinogalactan proteins (AGPs) are extensively glycosylated hydroxyproline-rich glycoproteins ubiquitous in all plant tissues and cells. AtAGP6 and AtAGP11, the only two functionally known pollen-specific classical AGP encoding genes in Arabidopsis, are reported to have redundant functions in microspore development. BcMF18 and BcMF8 isolated from Brassica campestris are the orthologues of AtAGP6 and AtAGP11, respectively. In contrast to the functional redundancy of AtAGP6 and AtAGP11, single-gene disruption of BcMF8 led to deformed pollen grains with abnormal intine development and ectopic aperture formation in B. campestris. Here, we further explored the action of BcMF18 and its relationship with BcMF8. BcMF18 was specifically expressed in pollen during the late stages of microspore development. Antisense RNA transgenic lines with BcMF18 reduction resulted in aberrant pollen grains with abnormal cellulose distribution, lacking intine, cytoplasm and nuclei. Transgenic plants with repressive expression of both BcMF8 and BcMF18 showed a hybrid phenotype, expressing a mixture of the phenotypes of the single gene knockdown plant lines. In addition, we identified functional diversity between BcMF18/BcMF8 and AtAGP6/AtAGP11, mainly reflected by the specific contribution of BcMF18 and BcMF8 to pollen wall formation. These results suggest that, unlike the orthologous genes AtAGP6 and AtAGP11 in Arabidopsis, BcMF18 and BcMF8 are both integral to pollen biogenesis in B. campestris, acting through independent pathways during microspore development.

Characterization and Functional Analysis of the Poplar Pectate Lyase-Like Gene PtPL1-18 Reveal Its Role in the Development of Vascular Tissues

Pectin is a major component of plant cell walls, and the structure of pectin impacts on the properties of wood. Although we know that pectate lyase (PL, EC 4.2.2.2) has a major influence on the structure of pectin, our knowledge of Pectate lyase-like genes (PLL) in tree species remains limited. To better understand the characteristics of PLL genes in trees and to identify novel PLL genes that are potentially involved in the development of wood, we performed comprehensive analyses of gene structures, phylogenetic relationships, chromosomal locations, gene duplication events, conserved protein motifs, and gene expression patterns of 30 PLLs in Populus trichocarpa (PtPL1s). We performed an in silico gene expression profiling and quantitative real-time PCR analysis and found that most of the PtPL1 genes from subgroups Ia and Ib were highly expressed in xylem. PtPL1-18 from subgroup Ia was preferentially expressed in developing primary xylem and in xylem cells that were developing secondary walls. Overexpression of PtPL1-18 in poplar reduced plant growth and xylem development. Reduced secondary cell wall thickening and irregular xylem cells were observed in the transgenic trees, probably due to their lower pectin content. Although pectin is not a major component of plant secondary cell walls, our results are consistent with the PtPL1 genes performing important functions during wood formation.

Transcriptome profiling of differentially expressed genes in floral buds and flowers of male sterile and fertile lines in watermelon

Background

Male sterility is an important mechanism for the production of hybrid seeds in watermelon. Although fruit development has been studied extensively in watermelon, there are no reports on gene expression in floral organs. In this study, RNA-sequencing (RNA-seq) was performed in two near-isogenic watermelon lines (genic male sterile [GMS] line, DAH3615-MS and male fertile line, DAH3615) to identify the differentially expressed genes (DEGs) related to male sterility.

Results

DEG analysis showed that 1259 genes were significantly associated with male sterility at a FDR P-value of < 0.01. Most of these genes were only expressed in the male fertile line. In addition, 11 functional clusters were identified using DAVID functional classification analysis. Of detected genes in RNA-seq analysis, 19 were successfully validated by qRT-PCR.

Conclusions

In this study, we carried out a comprehensive floral transcriptome sequence comparison of a male fertile line and its near-isogenic male sterile line in watermelon. This analysis revealed essential genes responsible for stamen development, including pollen development and pollen tube elongation, and allowed their functional classification. These results provided new information on global mechanisms related to male sterility in watermelon.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2186-9) contains supplementary material, which is available to authorized users.

Genetic and Biochemical Mechanisms of Pollen Wall Development

The pollen wall is a specialized extracellular cell wall matrix that surrounds male gametophytes and plays an essential role in plant reproduction. Uncovering the mechanisms that control the synthesis and polymerization of the precursors of pollen wall components has been a major research focus in plant biology. We review current knowledge on the genetic and biochemical mechanisms underlying pollen wall development in eudicot model Arabidopsis thaliana and monocot model rice (Oryza sativa), focusing on the genes involved in the biosynthesis, transport, and assembly of various precursors of pollen wall components. The conserved and divergent aspects of the genes involved as well as their regulation are addressed. Current challenges and future perspectives are also highlighted.

BcMF26a and BcMF26b Are Duplicated Polygalacturonase Genes with Divergent Expression Patterns and Functions in Pollen Development and Pollen Tube Formation in Brassica campestris

Polygalacturonase (PG) is one of the cell wall hydrolytic enzymes involving in pectin degradation. A comparison of two highly conserved duplicated PG genes, namely, Brassica campestris Male Fertility 26a (BcMF26a) and BcMF26b, revealed the different features of their expression patterns and functions. We found that these two genes were orthologous genes of At4g33440, and they originated from a chromosomal segmental duplication. Although structurally similar, their regulatory and intron sequences largely diverged. QRT-PCR analysis showed that the expression level of BcMF26b was higher than that of BcMF26a in almost all the tested organs and tissues in Brassica campestris. Promoter activity analysis showed that, at reproductive development stages, BcMF26b promoter was active in tapetum, pollen grains, and pistils, whereas BcMF26a promoter was only active in pistils. In the subcellular localization experiment, BcMF26a and BcMF26b proteins could be localized to the cell wall. When the two genes were co-inhibited, pollen intine was formed abnormally and pollen tubes could not grow or stretch. Moreover, the knockout mutants of At4g33440 delayed the growth of pollen tubes. Therefore, BcMF26a/b can participate in the construction of pollen wall by modulating intine information and BcMF26b may play a major role in co-inhibiting transformed plants.

PECTATE LYASE-LIKE 9 from Brassica campestris is associated with intine formation

Brassica campestris pectate lyase-like 9 (BcPLL9) was previously identified as a differentially expressed gene both in buds during late pollen developmental stage and in pistils during fertilization in Chinese cabbage. To characterize the gene's function, antisense-RNA lines of BcPLL9 (bcpll9) were constructed in Chinese cabbage. Self- and cross-fertilization experiments harvested half seed yields when bcpll9 lines were used as pollen donors. In vivo and in vitro pollen germination assays showed that nearly half of the pollen tubes in bcpll9 were irregular with shorter length and uneven surface. Aniline blue staining identified abnormal accumulation of a specific bright blue unknown material in the bcpll9 pollen portion. Scanning electron microscopy observation verified the abnormal outthrust material to be near the pollen germinal furrows. Transmission electron microscopy observation revealed the internal endintine layer was overdeveloped and predominantly occupied the intine. This abnormally formed intine likely induced the wavy structure and growth arrest of the pollen tube in half of the bcpll9 pollen grains, which resulted in less seed yields. Collectively, this study presented a novel PLL gene that has an important function in B. campestris intine formation.