Evidence for the role of transfer cells in the evolutionary increase in seed and fiber biomass yield in cotton

Cell wall invertase 3 plays critical roles in providing sugars during pollination and fertilization in cucumber

In plants, pollen-pistil interactions during pollination and fertilization mediate pollen hydration and germination, pollen tube growth, and seed set and development. Cell wall invertases (CWINs) help provide the carbohydrates for pollen development; however, their roles in pollination and fertilization have not been well established. In cucumber (Cucumis sativus), CsCWIN3 showed the highest expression in flowers, and we further examined CsCWIN3 for functions during pollination to seed set. Both CsCWIN3 transcript and CsCWIN3 protein exhibited similar expression patterns in the sepals, petals, stamen filaments, anther tapetum, and pollen of male flowers, as well as in the stigma, style, transmitting tract, and ovule funiculus of female flowers. Notably, repression of CsCWIN3 in cucumber did not affect the formation of parthenocarpic fruit but resulted in an arrested growth of stigma integuments in female flowers and a partially delayed dehiscence of anthers with decreased pollen viability in male flowers. Consequently, the pollen tube grew poorly in the gynoecia after pollination. In addition, CsCWIN3-RNA interference plants also showed affected seed development. Considering that sugar transporters could function in cucumber fecundity, we highlight the role of CsCWIN3 and a potential close collaboration between CWIN and sugar transporters in these processes. Overall, we used molecular and physiological analyses to determine the CsCWIN3-mediated metabolism during pollen formation, pollen tube growth, and plant fecundity. CsCWIN3 has essential roles from pollination and fertilization to seed set but not parthenocarpic fruit development in cucumber.

Quantitative trait loci and candidate genes for yield-related traits of upland cotton revealed by genome-wide association analysis under drought conditions

BACKGROUND

Due to the influence of extreme weather, the environment in China's main cotton-producing areas is prone to drought stress conditions, which affect the growth and development of cotton and lead to a decrease in cotton yield.

RESULTS

In this study, 188 upland cotton germplasm resources were phenotyped for data of 8 traits (including 3 major yield traits) under drought conditions in three environments for two consecutive years. Correlation analysis revealed significant positive correlations between the three yield traits. Genetic analysis showed that the estimated heritability of the seed cotton index (SC) under drought conditions was the highest (80.81%), followed by that of boll weight (BW) (80.64%) and the lint cotton index (LC) (70.49%) With genome-wide association study (GWAS) analysis, a total of 75 quantitative trait loci (QTLs) were identified, including two highly credible new QTL hotspots. Three candidate genes (Gh_D09G064400, Gh_D10G261000 and Gh_D10G254000) located in the two new QTL hotspots, QTL51 and QTL55, were highly expressed in the early stage of fiber development and showed significant correlations with SC, LC and BW. The expression of three candidate genes in two extreme materials after drought stress was analyzed by qRT-PCR, and the expression of these two materials in fibers at 15, 20 and 25 DPA. The expression of these three candidate genes was significantly upregulated after drought stress and was significantly higher in drought-tolerant materials than in drought-sensitive materials. In addition, the expression levels of the three candidate genes were higher in the early stage of fiber development (15 DPA), and the expression levels in drought-tolerant germplasm were higher than those in drought-sensitive germplasm. These three candidate genes may play an important role in determining cotton yield under drought conditions.

CONCLUSIONS

This study is helpful for understanding the regulatory genes affecting cotton yield under drought conditions and provides germplasm and candidate gene resources for breeding high-yield cotton varieties under these conditions.

Identification and Characterization of Phytocyanin Family Genes in Cotton Genomes

Phytocyanins (PCs) are a class of plant-specific blue copper proteins that have been demonstrated to play a role in electron transport and plant development. Through analysis of the copper ligand residues, spectroscopic properties, and domain architecture of the protein, PCs have been grouped into four subfamilies: uclacyanins (UCs), stellacyanins (SCs), plantacyanins (PLCs), and early nodulin-like proteins (ENODLs). The present study aimed to identify and characterise the PCs present in three distinct cotton species (Gossypium hirsutum, Gossyium arboreum, and Gossypium raimondii) through the identification of 98, 63, and 69 genes respectively. We grouped PCs into four clades by using bioinformatics analysis and sequence alignment, which exhibit variations in gene structure and motif distribution. PCs are distributed across all chromosomes in each of the three species, with varying numbers of exons per gene and multiple conserved motifs, and with a minimum of 1 and maximum of 11 exons found on one gene. Transcriptomic data and qRT-PCR analysis revealed that two highly differentiated PC genes were expressed at the fibre initiation stage, while three highly differentiated PCs were expressed at the fibre elongation stage. These findings serve as a foundation for further investigations aimed at understanding the contribution of this gene family in cotton fibre production.

Comparative transcriptome and metabolome analyses identified the mode of sucrose degradation as a metabolic marker for early vegetative propagation in bulbs of Lycoris

Vegetative propagation (VP) is an important practice for production in many horticultural plants. Sugar supply constitutes the basis of VP in bulb flowers, but the underlying molecular basis remains elusive. By performing a combined sequencing technologies coupled with ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry approach for metabolic analyses, we compared two Lycoris species with contrasting regeneration rates: high-regeneration Lycoris sprengeri and low-regeneration Lycoris aurea. A comprehensive multi-omics analyses identified both expected processes involving carbohydrate metabolism and transcription factor networks, as well as the metabolic characteristics for each developmental stage. A higher abundance of the differentially expressed genes including those encoding ethylene responsive factors was detected at bulblet initiation stage compared to the late stage of bulblet development. High hexose-to-sucrose ratio correlated to bulblet formation across all the species examined, indicating its role in the VP process in Lycoris bulb. Importantly, a clear difference between cell wall invertase (CWIN)-catalyzed sucrose unloading in high-regeneration species and the sucrose synthase-catalyzed pathway in low-regeneration species was observed at the bulblet initiation stage, which was supported by findings from carboxyfluorescein tracing and quantitative real-time PCR analyses. Collectively, the findings indicate a sugar-mediated model of the regulation of VP in which high CWIN expression or activity may promote bulblet initiation via enhancing apoplasmic unloading of sucrose or sugar signals, whereas the subsequent high ratio of hexose-to-sucrose likely supports cell division characterized in the next phase of bulblet formation.

Carbon assimilation and distribution in cotton photosynthetic organs is a limiting factor affecting boll weight formation under drought

Previous studies have documented cotton boll weight reductions under drought, but the relative importance of the subtending leaf, bracts and capsule wall in driving drought-induced reductions in boll mass has received limited attention. To investigate the role of carbon metabolism in driving organ-specific differences in contribution to boll weight formation, under drought conditions. Controlled experiments were carried out under soil relative water content (SRWC) (75 ± 5)% (well-watered conditions, control), (60 ± 5)% (moderate drought) and (45 ± 5)% (severe drought) in 2018 and 2019 with two cultivars Yuzaomian 9110 and Dexiamian 1. Under severe drought, the decreases of photosynthetic rate (Pn) and carbon isotope composition (δ¹³C) were observed in the subtending leaf, bract and capsule wall, suggesting that carbon assimilation of three organs was restricted and the limitation was most pronounced in the subtending leaf. Changes in the activities of sucrose phosphate synthase (SPS), sucrose synthase (SuSy), invertases as well as the reduction in expression of sucrose transporter (GhSUT1) led to variabilities in the sucrose content of three organs. Moreover, photosynthate distribution from subtending leaf to seeds plus fibers (the components of boll weight) was significantly restricted and the photosynthetic contribution rate of subtending leaf to boll weight was decreased, while contributions of bracts and capsule wall were increased by drought. This, in conjunction with the observed decreases in boll weight, indicated that the subtending leaf was the most sensitive photosynthetic organ to drought and was a dominant driver of boll weight loss under drought. Therefore, the subtending leaf governs boll weight loss under drought due to limitations in carbon assimilation, perturbations in sucrose metabolism and inhibition of sucrose transport.

Suppression of GhGLU19 encoding β-1,3-glucanase promotes seed germination in cotton

BACKGROUND

In eudicots, germination begins with water uptake by the quiescent dry seed and is greatly related to the permeability of micropyle enriched callose layers. Once imbibition starts, seeds undergo a cascade of physiological, biochemical, and molecular events to initiate cellular activities. However, the effects of callose on water uptake and following seed metabolic events during germination are largely unknown. Cotton (Gossypium hirsutum) is a eudicot plant with natural fiber and edible oil production for humans. Here, we addressed this question by examining the role of GhGLU19, a gene encoding β-1,3-glucanase, in cotton seed germination.

RESULTS

GhGLU19 belongs to subfamily B and was expressed predominately in imbibed seeds and early seedlings. Compared to wild type, GhGLU19-suppressing and GhGLU19-overexpressing transgenic cotton lines showed the higher and lower seed germination percentage, respectively. Callose was enriched more at inner integument (ii) than that in embryo and seed coat in cotton seeds. In GhGLU19-suppressing lines, callose at ii of cotton seeds was greatly increased and brought about a prolonged water uptake process during imbibition. Both proteomic and transcriptomic analysis revealed that contrary to GhGLU19-overexpressing lines, the glycolysis and pyruvate metabolism was decreased, and abscisic acid (ABA) biosynthesis related genes were downregulated in imbibed seeds of GhGLU19-suppressing lines. Also, endogenous ABA was significantly decreased in GhGLU19-suppressing line while increased in GhGLU19-overexpressing line.

CONCLUSIONS

Our results demonstrate that suppression of GhGLU19 improves cotton seed germination via accumulating callose of inner integument, modulating glycolysis and pyruvate metabolism, and decreasing ABA biosynthesis. This study provides a potential way for improving germination percentage in cotton seed production, and other eudicot crops.

Genome-wide association analysis reveals quantitative trait loci and candidate genes involved in yield components under multiple field environments in cotton (Gossypium hirsutum)

BACKGROUND

Numerous quantitative trait loci (QTLs) and candidate genes associated with yield-related traits have been identified in cotton by genome-wide association study (GWAS) analysis. However, most of the phenotypic data were from a single or few environments, and the stable loci remained to be validated under multiple field environments.

RESULTS

Here, 242 upland cotton accessions collected from different origins were continuously investigated for phenotypic data of four main yield components, including boll weight (BW) and lint percentage (LP) under 13 field environments, and boll number per plant (BN) and seed index (SI) under 11 environments. Correlation analysis revealed a positive correlation between BN and LP, BW and SI, while SI had a negative correlation with LP and BN. Genetic analysis indicated that LP had the highest heritability estimates of 94.97%, followed by 92.08% for SI, 86.09% for BW, and 72.92% for BN, indicating LP and SI were more suitable traits for genetic improvement. Based on 56,010 high-quality single nucleotide polymorphisms (SNPs) and GWAS analysis, a total of 95 non-redundant QTLs were identified, including 12 of BN, 23 of BW, 45 of LP, and 33 of SI, respectively. Of them, 10 pairs of homologous QTLs were detected between A and D sub-genomes. We also found that 15 co-located QTLs with more than two traits and 12 high-confidence QTLs were detected under more than six environments, respectively. Further, two NET genes (GH_A08G0716 and GH_A08G0783), located in a novel QTL hotspot (qtl24, qtl25 and qlt26) were predominately expressed in early fiber development stages, exhibited significant correlation with LP and SI. The GH_A07G1389 in the stable qtl19 region encoded a tetratricopeptide repeat (TPR)-like superfamily protein and was a homologous gene involved in short fiber mutant ligon lintless-y (Li_y), implying important roles in cotton yield.

CONCLUSIONS

The present study provides a foundation for understanding the regulatory mechanisms of yield components and may enhance yield improvement through molecular breeding in cotton.

Transfer cells: what regulates the development of their intricate wall labyrinths?

Transfer cells (TCs) support high nutrient rates into, or at symplasmic discontinuities within, the plant body. Their transport capacity is conferred by an amplified plasma membrane surface area, enriched in nutrient transporters, supported on an intricately invaginated wall labyrinth (WL). Thus, development of the WL is at the heart of TC function. Enquiry has shifted from describing WL architecture and formation to discovering mechanisms regulating WL assembly. Experimental systems used to examine these phenomena are critiqued. Considerable progress has been made in identifying master regulators that commit stem cells to a TC fate (e.g. the maize Myeloblastosis (MYB)-related R1-type transcription factor) and signals that induce differentiated cells to undergo trans-differentiation to a TC phenotype (e.g. sugar, auxin and ethylene). In addition, signals that provide positional information for assembly of the WL include apoplasmic hydrogen peroxide and cytosolic Ca²⁺ plumes. The former switches on, and specifies the intracellular site for WL construction, while the latter creates subdomains to direct assembly of WL invaginations. Less is known about macromolecule species and their spatial organization essential for WL assembly. Emerging evidence points to a dependency on methyl-esterified homogalacturonan accumulation, unique patterns of cellulose and callose deposition and spatial positioning of arabinogalactan proteins.

Understanding the role of phytohormones in cotton fiber development through omic approaches; recent advances and future directions

Cotton is among the most important fiber crops for the textile-based industry, thanks to its cellulose-rich mature fibers. The fiber initiation and elongation are one of the best models for deciphering mechanisms of single-cell differentiation and growth, that also target of fiber development programs. During the last couple of decades, high yielding omics approaches (genomics, transcriptomics, and proteomics), have helped in the identification of several genes and gene products involved in fiber development along with functional relationship to phytohormones. For example, MYB transcription factor family and Sus gene family have been evidenced by controlling cotton fiber initiation. Most importantly, the biosynthesis, responses, and transporting of phytohormones is documented to participate in the initiation of cotton fibers. Herein, in this review, the reliable genetic evidence by manipulating the above genes in cotton have been summarized to describe the relationships among key phytohormones, transcription factors, proteins, and downstream fiber growth-related genes such as Sus. The effect of other important factors such as ROS, fatty acid metabolism, and actin (globular multi-functional proteins) over fiber development has also been discussed. The challenges and deficiencies in the research of cotton fiber development have been mentioned along with a future perspective to discover new crucial genes using multiple omics analysis.

A panel of criteria for comprehensive assessment of severity of ultraviolet B radiation-induced non-melanoma skin cancers in SKH-1 mice

The study of causes and cures for ultraviolet B radiation (UVB)-induced non-melanoma skin cancers (NMSC) has been greatly facilitated by use of the albino SKH-1 hairless mice. These mice develop multiple tumors of different sizes and the severity of cancer is often measured by one or more of the four criteria, namely the prevalence, multiplicity, area and volume of tumors. However, there are inherent limitations of each criterion: the prevalence and number do not account for size differences among tumors, area measurement ignores the tumor height, and volume measurement overcompensates for the height at the cost of planar dimensions. Here, using our dataset from an ongoing NMSC study, we discuss the limitations of these four criteria, and suggest refinements in measuring prevalence. We recommend the use of three more criteria, namely the Knud Thomsen tridimensional surface that apportions optimal weightage to three tumor dimensions, weekly occurrence of new tumors and tumor growth-rate to reveal initiation and growth of tumors in early and late phase of NMSC development, respectively. Together, use of this comprehensive panel of seven criteria can provide an accurate assessment of severity of NMSC and lead to a testable hypothesis whether the experimental manipulation of mice has affected the early initiation or growth phase of NMSC tumors.

A Gossypium hirsutum GDSL lipase/hydrolase gene (GhGLIP) appears to be involved in promoting seed growth in Arabidopsis

GDSL lipase (GLIP) plays a pivotal role in plant cell growth as a multifunctional hydrolytic enzyme. Herein, a cotton (Gossypium hirsutum L. cv Xuzhou 142) GDSL lipase gene (GhGLIP) was obtained from developing ovules and fibers. The GhGLIP cDNA contained an open reading frame (ORF) of 1,143 base pairs (bp) and encodes a putative polypeptide of 380 amino acid residues. Sequence alignment indicated that GhGLIP includes four enzyme catalytic amino acid residue sites of Ser (S), Gly (G), Asn (N) and His (H), located in four conserved blocks. Phylogenetic tree analysis showed that GhGLIP belongs to the typical class IV lipase family with potential functions in plant secondary metabolism. Subcellular distribution analysis demonstrated that GhGLIP localized to the nucleus, cytoplasm and plasma membrane. GhGLIP was expressed predominantly at 5-15 day post anthesis (dpa) in developing ovules and elongating fibers, measured as mRNA levels and enzyme activity. Ectopic overexpression of GhGLIP in Arabidopsis plants resulted in enhanced seed development, including length and fresh weight. Meanwhile, there was increased soluble sugar and protein storage in transgenic Arabidopsis plants, coupled with the promotion of lipase activity. Moreover, the expression of cotton GhGLIP is induced by ethylene (ETH) treatment in vitro. A 1,954-bp GhGLIP promoter was isolated and expressed high activity in driving green fluorescence protein (GFP) expression in tobacco leaves. Cis-acting element analysis of the GhGLIP promoter (pGhGLIP) indicated the presence of an ethylene-responsive element (ERE), and transgenic tobacco leaves with ectopic expression of pGhGLIP::GFP-GUS showed increased GUS activity after ETH treatment. In summary, these results suggest that GhGLIP is a functional enzyme involved in ovule and fiber development and performs significant roles in seed development.

Sucrose phloem unloading follows an apoplastic pathway with high sucrose synthase in Actinidia fruit

Phloem unloading plays a pivotal role in photoassimilate partitioning and the accumulation of sugars in sink organs, e.g. fruit. Here, we investigated the pathway of sucrose unloading in kiwifruit (Actinidia deliciasa cv. Qinmei) using a combination of electron microscopy, transport of the phloem-mobile symplastic tracer carboxyfluorescein and enzyme activity and gene expression assays. Our structural investigation revealed that the sieve element-companion cell complex of bundles feeding the fruit flesh was symplastically isolated from its surrounding parenchyma cells throughout fruit development, whereas numerous plasmodesmata were present between the phloem parenchyma cells. Consistent with this, carboxyfluorescein unloading showed that the dye remained confined in the phloem strands during fruit development. The activities and expression of cell wall acid invertase in fruit flesh were lower than those of other enzymes that catalyze sucrose dissociation. However, sucrose synthase showed higher enzyme activities and mRNA expression in fruit flesh compared with other detected enzymes. These results imply that, in kiwifruit flesh, phloem unloading of sucrose is predominantly an apoplastic pathway during fruit development, and that sucrose synthase is a key enzyme for sucrose post-unloading pathways.

EcoTILLING revealed SNPs in GhSus genes that are associated with fiber- and seed-related traits in upland cotton

Cotton is the most important textile crop in the world due to its cellulose-enriched fibers. Sucrose synthase genes (Sus) play pivotal roles in cotton fiber and seed development. To mine and pyramid more favorable alleles for cotton molecular breeding, single nucleotide polymorphisms (SNPs) of GhSus family genes were investigated across 277 upland cotton accessions by EcoTILLING. As a result, a total of 24 SNPs in the amplified regions of eight GhSus genes were identified. These SNPs were significantly associated with at least one fiber- or seed-related trait measured in Nanjing, Anyang and Kuche in 2007-2009. Four main-effect quantitative trait nucleotides (QTNs) and five epistatic QTNs, with 0.76-3.56% of phenotypic variances explained by each QTN (PVE), were found to be associated with yield-related traits; six epistatic QTNs, with the 0.43-3.48% PVE, were found to be associated with fiber quality-related traits; and one main-effect QTN and one epistatic QTN, with the PVE of 1.96% and 2.53%, were found to be associated with seed oil content and protein content, respectively. Therefore, this study provides new information for molecular breeding in cotton.

Genome-scale analysis of the cotton KCS gene family revealed a binary mode of action for gibberellin A regulated fiber growth

Production of β-ketoacyl-CoA, which is catalyzed by 3-ketoacyl-CoA synthase (KCS), is the first step in very long chain fatty acid (VLCFA) biosynthesis. Here we identified 58 KCS genes from Gossypium hirsutum, 31 from G. arboreum and 33 from G. raimondii by searching the assembled cotton genomes. The gene family was divided into the plant-specific FAE1-type and the more general ELO-type. KCS transcripts were widely expressed and 32 of them showed distinct subgenome-specific expressions in one or more cotton tissues/organs studied. Six GhKCS genes rescued the lethality of elo2Δelo3Δ yeast double mutant, indicating that this gene family possesses diversified functions. Most KCS genes with GA-responsive elements (GAREs) in the promoters were significantly upregulated by gibberellin A3 (GA). Exogenous GA3 not only promoted fiber length, but also increased the thickness of cell walls significantly. GAREs present also in the promoters of several cellulose synthase (CesA) genes required for cell wall biosynthesis and they were all induced significantly by GA3 . Because GA treatment resulted in longer cotton fibers with thicker cell walls and higher dry weight per unit cell length, we suggest that it may regulate fiber elongation upstream of the VLCFA-ethylene pathway and also in the downstream steps towards cell wall synthesis.

Down-regulation of the cotton endo-1,4-β-glucanase gene KOR1 disrupts endosperm cellularization, delays embryo development, and reduces early seedling vigour

Towards the aim of examining the potential function of KORRIGAN (KOR), a highly conserved membrane-bound endoglucanase, in reproductive development, here transgenic evidence is provided that a cotton (Gossypium hirsutum) endoglucanase, GhKOR1, plays significant roles in endosperm and embryo development. RNA interference (RNAi)- and co-suppression-mediated down-regulation of GhKOR1 resulted in smaller filial tissue and reduced seed weight, which were characterized by disrupted endosperm cellularization and delayed embryo development, leading to a delayed germination and a weak growth of seedlings early in development. The transgenic seeds exhibited fewer and smaller endosperm cells with irregular and brittle cell walls, and their embryos developed only to the globular stage at 10 days post-anthesis (DPA) when the wild-type endosperm has become highly cellularized and the embryo has progressed to the heart stage. The transgenic seed also displayed a significant reduction of callose in the seed coat transfer cells and reduced cellulose content both in the seed coat and in mature fibres. These findings demonstrate that GhKOR1 is required for the developmental of both seed filial and maternal tissues and the establishment of seedling vigour.

Tomato ovary-to-fruit transition is characterized by a spatial shift of mRNAs for cell wall invertase and its inhibitor with the encoded proteins localized to sieve elements

Central to understanding fruit development is to elucidate the processes mediating a successful transition from pre-pollination ovaries to newly set fruit, a key step in establishing fruit yield potential. In tomato, cell wall invertase (CWIN) LIN5 and its inhibitor INH1 are essential for fruit growth. However, the molecular and cellular basis by which they exert their roles in ovary-to-fruit transition remains unknown. To address this issue, we conducted a study focusing on ovaries and fruitlets at 2 days before and 2 days after anthesis, respectively. In situ hybridization analyses revealed that LIN5 and INH1 exhibited a dispersed expression in ovaries compared with their phloem-specific expression in fruitlets. Remarkably, LIN5 and INH1 proteins were immunologically co-localized to cell walls of sieve elements (SEs) in ovaries immediately prior to anthesis and in young fruitlets, but were undetectable in provascular bundles of younger ovaries. A burst in CWIN activity occurred during ovary-to-fruit transition. Interestingly, the ovaries, but not the fruitlets, exhibited high expression of a defective invertase, SldeCWIN1, an ortholog of which is known to enhance inhibition of INH on CWIN activity in tobacco. Imaging of a fluorescent symplasmic tracer indicated an apoplasmic phloem unloading pathway operated in ovaries, contrary to the previously observed symplasmic unloading pathway in fruit pericarp. These new data indicate that (1) a phloem-specific patterning of the CWIN and INH mRNAs is induced during ovary-to-fruit transition, and (2) LIN5 protein functions specifically in walls of SEs and increases its activity during ovary-to-fruit transition, probably to facilitate phloem unloading and to generate a glucose signal positively regulating cell division, hence fruit set.

Silencing the vacuolar invertase gene GhVIN1 blocks cotton fiber initiation from the ovule epidermis, probably by suppressing a cohort of regulatory genes via sugar signaling

Cotton fibers, the most important source of cellulose for the global textile industry, are single-celled trichomes derived from the ovule epidermis at or just prior to anthesis. Despite progress in understanding cotton fiber elongation and cell-wall biosynthesis, knowledge regarding the molecular basis of fiber cell initiation, the first step of fiber development determining the fiber yield potential, remains elusive. Here, we provide evidence that expression of a vacuolar invertase (VIN) is an early event that is essential for cotton fiber initiation. RNAi-mediated suppression of GhVIN1, a major VIN gene that is highly expressed in wild-type fiber initials, resulted in significant reduction of VIN activity and consequently a fiberless seed phenotype in a dosage dependent manner. The absence of a negative effect on seed development in these fiberless seeds indicates that the phenotype is unlikely to be due to lack of carbon nutrient. Gene expression analyses coupled with in vitro ovule culture experiments revealed that GhVIN1-derived hexose signaling may play an indispensable role in cotton fiber initiation, probably by regulating the transcription of several MYB transcription factors and auxin signaling components that were previously identified as required for fiber initiation. Together, the data represent a significant advance in understanding the mechanisms of cotton fiber initiation, and provide the first indication that VIN-mediated hexose signaling may act as an early event modulating the expression of regulatory genes and hence cell differentiation from the ovule epidermis.

Evolution and development of cell walls in cereal grains

The composition of cell walls in cereal grains and other grass species differs markedly from walls in seeds of other plants. In the maternal tissues that surround the embryo and endosperm of the grain, walls contain higher levels of cellulose and in many cases are heavily lignified. This may be contrasted with walls of the endosperm, where the amount of cellulose is relatively low, and the walls are generally not lignified. The low cellulose and lignin contents are possible because the walls of the endosperm perform no load-bearing function in the mature grain and indeed the low levels of these relatively intractable wall components are necessary because they allow rapid degradation of the walls following germination of the grain. The major non-cellulosic components of endosperm walls are usually heteroxylans and (1,3;1,4)-β-glucans, with lower levels of xyloglucans, glucomannans, and pectic polysaccharides. Pectic polysaccharides and xyloglucans are the major non-cellulosic wall constituents in most dicot species, in which (1,3;1,4)-β-glucans are usually absent and heteroxylans are found at relatively low levels. Thus, the "core" non-cellulosic wall polysaccharides in grain of the cereals and other grasses are the heteroxylans and, more specifically, arabinoxylans. The (1,3;1,4)-β-glucans appear in the endosperm of some grass species but are essentially absent from others; they may constitute from zero to more than 45% of the cell walls of the endosperm, depending on the species. It is clear that in some cases these (1,3;1,4)-β-glucans function as a major store of metabolizable glucose in the grain. Cereal grains and their constituent cell wall polysaccharides are centrally important as a source of dietary fiber in human societies and breeders have started to select for high levels of non-cellulosic wall polysaccharides in grain. To meet end-user requirements, it is important that we understand cell wall biology in the grain both during development and following germination.

Sucrose metabolism: gateway to diverse carbon use and sugar signaling

Sucrose metabolism plays pivotal roles in development, stress response, and yield formation, mainly by generating a range of sugars as metabolites to fuel growth and synthesize essential compounds (including protein, cellulose, and starch) and as signals to regulate expression of microRNAs, transcription factors, and other genes and for crosstalk with hormonal, oxidative, and defense signaling. This review aims to capture the most exciting developments in this area by evaluating (a) the roles of key sucrose metabolic enzymes in development, abiotic stress responses, and plant-microbe interactions; (b) the coupling between sucrose metabolism and sugar signaling from extra- to intracellular spaces; (c) the different mechanisms by which sucrose metabolic enzymes could perform their signaling roles; and (d) progress on engineering sugar metabolism and transport for high yield and disease resistance. Finally, the review outlines future directions for research on sugar metabolism and signaling to better understand and improve plant performance.

Associations between the acclimation of phloem-cell wall ingrowths in minor veins and maximal photosynthesis rate

The companion cells (CCs) and/or phloem parenchyma cells (PCs) in foliar minor veins of some species exhibit invaginations that are amplified when plants develop in high light (HL) compared to low light (LL). Leaves of plants that develop under HL also exhibit greater maximal rates of photosynthesis compared to those that develop under LL, suggesting that the increased membrane area of CCs and PCs of HL-acclimated leaves may provide for greater levels of transport proteins facilitating enhanced sugar export. Furthermore, the degree of wall invagination in PCs (Arabidopsis thaliana) or CCs (pea) of fully expanded LL-acclimated leaves increased to the same level as that present in HL-acclimated leaves 7 days following transfer to HL, and maximal photosynthesis rates of transferred leaves of both species likewise increased to the same level as in HL-acclimated leaves. In contrast, transfer of Senecio vulgaris from LL to HL resulted in increased wall invagination in CCs, but not PCs, and such leaves furthermore exhibited only partial upregulation of photosynthetic capacity following LL to HL transfer. Moreover, a significant linear relationship existed between the level of cell wall ingrowths and maximal photosynthesis rates across all three species and growth light regimes. A positive linear relationship between these two parameters was also present for two ecotypes (Sweden, Italy) of the winter annual A. thaliana in response to growth at different temperatures, with significantly greater levels of PC wall ingrowths and higher rates of photosynthesis in leaves that developed at cooler versus warmer temperatures. Treatment of LL-acclimated plants with the stress hormone methyl jasmonate also resulted in increased levels of wall ingrowths in PCs of A. thaliana and S. vulgaris but not in CCs of pea and S. vulgaris. The possible role of PC wall ingrowths in sugar export versus as physical barriers to the movement of pathogens warrants further attention.

Boosting seed development as a new strategy to increase cotton fiber yield and quality

Cotton (Gossypium spp.) is the most important textile crop worldwide due to its cellulosic mature fibers, which are single-celled hairs initiated from the cotton ovule epidermis at anthesis. Research to improve cotton fiber yield and quality in recent years has been largely focused on identifying genes regulating fiber cell initiation, elongation and cellulose synthesis. However, manipulating some of those candidate genes has yielded no effect or only a marginally positive effect on fiber yield or quality. On the other hand, evolutionary comparison and transgenic studies have clearly shown that cotton fiber growth is intimately controlled by seed development. Therefore, I propose that enhancing seed development could be a more effective and achievable strategy to increase fiber yield and quality.

Regulation of cell division and expansion by sugar and auxin signaling

Plant growth and development are modulated by concerted actions of a variety of signaling molecules. In recent years, evidence has emerged on the roles of sugar and auxin signals network in diverse aspects of plant growth and development. Here, based on recent progress of genetic analyses and gene expression profiling studies, we summarize the functional similarities, diversities, and their interactions of sugar and auxin signals in regulating two major processes of plant development: cell division and cell expansion. We focus on roles of sugar and auxin signaling in both vegetative and reproductive tissues including developing seed.

A cotton BURP domain protein interacts with α-expansin and their co-expression promotes plant growth and fruit production

Plant growth requires cell wall extension. The cotton AtRD22-Like 1 gene GhRDL1, predominately expressed in elongating fiber cells, encodes a BURP domain-containing protein. Here, we show that GhRDL1 is localized in cell wall and interacts with GhEXPA1, an α-expansin functioning in wall loosening. Transgenic cotton overexpressing GhRDL1 showed an increase in fiber length and seed mass, and an enlargement of endopleura cells of ovules. Expression of either GhRDL1 or GhEXPA1 alone in Arabidopsis led to a substantial increase in seed size; interestingly, their co-expression resulted in the increased number of siliques, the nearly doubled seed mass, and the enhanced biomass production. Cotton plants overexpressing GhRDL1 and GhEXPA1 proteins produced strikingly more fruits (bolls), leading to up to 40% higher fiber yield per plant without adverse effects on fiber quality and vegetative growth. We demonstrate that engineering cell wall protein partners has a great potential in promoting plant growth and crop yield.

Development of flange and reticulate wall ingrowths in maize (Zea mays L.) endosperm transfer cells

Maize (Zea mays L.) endosperm transfer cells are essential for kernel growth and development so they have a significant impact on grain yield. Although structural and ultrastructural studies have been published, little is known about the development of these cells, and prior to this study, there was a general consensus that they contain only flange ingrowths. We characterized the development of maize endosperm transfer cells by bright field microscopy, transmission electron microscopy, and confocal laser scanning microscopy. The most basal endosperm transfer cells (MBETC) have flange and reticulate ingrowths, whereas inner transfer cells only have flange ingrowths. Reticulate and flange ingrowths are mostly formed in different locations of the MBETC as early as 5 days after pollination, and they are distinguishable from each other at all stages of development. Ingrowth structure and ultrastructure and cellulose microfibril compaction and orientation patterns are discussed during transfer cell development. This study provides important insights into how both types of ingrowths are formed in maize endosperm transfer cells.

Proteomic comparison of basal endosperm in maize miniature1 mutant and its wild-type Mn1

Developing endosperm in maize seed is a major site for biosynthesis and storage of starch and proteins, and of immense economic importance for its role in food, feed and biofuel production. The basal part of endosperm performs a major role in solute, water and nutrition acquisition from mother plant to sustain these functions. The miniature1 (mn1) mutation is a loss-of-function mutation of the Mn1-encoded cell wall invertase that is entirely expressed in the basal endosperm and is essential for many of the metabolic and signaling functions associated with metabolically released hexose sugars in developing endosperm. Here we report a comparative proteomic study between Mn1 and mn1 basal endosperm to better understand basis of pleiotropic effects on many diverse traits in the mutant. Specifically, we used iTRAQ based quantitative proteomics combined with Gene Ontology (GO) and bioinformatics to understand functional basis of the proteomic information. A total of 2518 proteins were identified from soluble and cell wall associated protein (CWAP) fractions; of these 131 proteins were observed to be differentially expressed in the two genotypes. The main functional groups of proteins that were significantly different were those involved in the carbohydrate metabolic and catabolic process, and cell homeostasis. The study constitutes the first proteomic analysis of basal endosperm cell layers in relation to endosperm growth and development in maize.

New insights into roles of cell wall invertase in early seed development revealed by comprehensive spatial and temporal expression patterns of GhCWIN1 in cotton

Despite substantial evidence on the essential roles of cell wall invertase (CWIN) in seed filling, it remains largely unknown how CWIN exerts its regulation early in seed development, a critical stage that sets yield potential. To fill this knowledge gap, we systematically examined the spatial and temporal expression patterns of a major CWIN gene, GhCWIN1, in cotton (Gossypium hirsutum) seeds from prefertilization to prestorage phase. GhCWIN1 messenger RNA was abundant at the innermost seed coat cell layer at 5 d after anthesis but became undetectable at 10 d after anthesis, at the onset of its differentiation into transfer cells characterized by wall ingrowths, suggesting that CWIN may negatively regulate transfer cell differentiation. Within the filial tissues, GhCWIN1 transcript was detected in endosperm cells undergoing nuclear division but not in those cells at the cellularization stage, with similar results observed in Arabidopsis (Arabidopsis thaliana) endosperm for CWIN, AtCWIN4. These findings indicate a function of CWIN in nuclear division but not cell wall biosynthesis in endosperm, contrasting to the role proposed for sucrose synthase (Sus). Further analyses revealed a preferential expression pattern of GhCWIN1 and AtCWIN4 in the provascular region of the torpedo embryos in cotton and Arabidopsis seed, respectively, indicating a role of CWIN in vascular initiation. Together, these novel findings provide insights into the roles of CWIN in regulating early seed development spatially and temporally. By comparing with previous studies on Sus expression and in conjunction with the expression of other related genes, we propose models of CWIN- and Sus-mediated regulation of early seed development.

Analysis of grain characters in temperate grasses reveals distinctive patterns of endosperm organization associated with grain shape

Members of the core pooids represent the most important crops in temperate zones including wheat, barley, and oats. Their importance as crops is largely due to the grain, particularly the storage capabilities of the endosperm. In this study, a comprehensive survey of grain morphology and endosperm organization in representatives of wild and cultivated species throughout the core pooids was performed. As sister to the core pooid tribes Poeae, Aveneae, Triticeae, and Bromeae within the Pooideae subfamily, Brachypodium provides a taxonomically relevant reference point. Using macroscopic, histological, and molecular analyses distinct patterns of grain tissue organization in these species, focusing on the peripheral and modified aleurone, are described. The results indicate that aleurone organization is correlated with conventional grain quality characters such as grain shape and starch content. In addition to morphological and organizational variation, expression patterns of candidate gene markers underpinning this variation were examined. Features commonly associated with grains are largely defined by analyses on lineages within the Triticeae and knowledge of grain structure may be skewed as a result of the focus on wheat and barley. Specifically, the data suggest that the modified aleurone is largely restricted to species in the Triticeae tribe.

Genotypic differences in pod wall and seed growth relate to invertase activities and assimilate transport pathways in asparagus bean

BACKGROUND AND AIMS

Coordination of sugar transport and metabolism between developing seeds and their enclosing fruit tissues is little understood. In this study the physiological mechanism is examined using two genotypes of asparagus bean (Vigna unguiculata ssp. sesquipedialis) differing in pod wall and seed growth rates. Pod growth dominates over seed growth in genotype 'Zhijiang 121' but not in 'Zhijiang 282' in which a 'bulging pod' phenotype is apparent from 8 d post-anthesis (dpa) onward.

METHODS

Seed and pod wall growth rates and degree of pod-bulging were measured in the two genotypes together with assays of activities of sucrose-degrading enzymes and sugar content in pod wall and seed and evaluation of cellular pathways of phloem unloading in seed coat using a symplasmic fluorescent dye, 5(6)-carboxyfluorescein (CF).

KEY RESULTS

Activities of cell wall, cytoplasmic and vacuolar invertases (CWIN, CIN and VIN) were significantly smaller in pod walls of '282' than in '121' at 10 dpa onwards. Low INV activities were associated with weak pod wall growth of '282'. In seed coats, CF was confined within the vasculature in '282' but moved beyond the vasculature in '121', indicating apoplasmic and symplasmic phloem unloading, respectively. Higher CWIN activity in '282' seed coats at 6-8 dpa correlated with high hexose concentration in embryos and enhanced early seed growth. However, CWIN activity in '282' decreased significantly compared with '121' from 10 dpa onwards, coinciding with earlier commencement of nuclei endoreduplication in their embryos.

CONCLUSIONS

The study shows genotypic differences between 'bulging pod' and 'non-bulging' phenotypes of asparagus bean in sucrose metabolism in relation to the pathway of phloem unloading in developing seed coats, and to pod and seed growth. Low INV activity in pod wall corresponds to its shortened and weak growth period; by contrast, the apoplasmic path in the seed coat is associated with high CWIN activity and strong early seed growth.

Overexpression of GhSusA1 increases plant biomass and improves cotton fiber yield and quality

Cotton (Gossypium spp.) is an important economic crop and the largest source of textile fiber in the world. However, to date, only a few genes have been identified that exhibit critical roles in fiber development, and few has shown positive effects on fiber yield and quality in transgenic cotton. Here, we report the characterization of a novel sucrose synthase (SusA1) gene from a superior quality fiber germplasm line 7235 in Gossypium hirsutum. By association analysis, GhSusA1 was highly correlated with fiber qualities in (7235× TM-1) recombinant inbred lines based on polymorphism of GhSusA1 between 7235 and TM-1. Subsequently, based on an interspecific population of 141 BC₁ individuals generated from the cross between TM-1 and Gossypium barbadense line, Hai7124, we further mapped GhSusA1 genes on homeologous chromosomes A8 (chro.8) and D8 (chro.24). Suppression of GhSusA1 in transgenic cotton reduced fiber quality and decreased the boll size and seed weight. Importantly, overexpression of this gene increased fiber length and strength, with the latter indicated by the enhanced thickening of cell wall during secondary wall formation stage. Moreover, increasing GhSusA1 transcript abundance in vegetative tissues led to elevated seedling biomass. Together, these findings identified GhSusA1 as a key regulator of sink strength in cotton, which is tightly associated with productivity, and hence a promising candidate gene that can be developed to increase cotton fiber yield and quality.

Pleiotropy and its dissection through a metabolic gene Miniature1 (Mn1) that encodes a cell wall invertase in developing seeds of maize

The Mn1-encoded endosperm-specific cell wall invertase is a major determinant of sink strength of developing seeds through its control of both sink size, cell number and cell size, and sink activity via sucrose hydrolysis and release of hexoses essential for energy and signaling functions. Consequently, loss-of-function mutations of the gene lead to the mn1 seed phenotype that shows ∼70% reduction in seed mass at maturity and several pleiotropic changes. A comparative analysis of endosperm and embryo mass in the Mn1 and mn1 genotypes showed here significant reductions of both tissues in the mn1 starting with early stages of development. Clearly, embryo development was endosperm-dependent. To gain a mechanistic understanding of the changes, sugar levels were measured in both endosperm and embryo samples. Changes in the levels of all sugars tested, glc, fru, suc, and sorbitol, were mainly observed in the endosperm. Greatly reduced fru levels in the mutant led to RNA level expression analyses by q-PCR of several genes that encode sucrose and fructose metabolizing enzymes. The mn1 endosperm showed reductions in gene expression, ranging from ∼70% to 99% of the Mn1 samples, for both suc-starch and suc--energy pathways, suggesting an in vivo metabolic coordinated regulation due to the hexose-deficiency. Together, these data provide evidence of the Mn1-dependent interconnected network of several pathways as a possible basis for pleiotropic changes in seed development.

Overexpression of a potato sucrose synthase gene in cotton accelerates leaf expansion, reduces seed abortion, and enhances fiber production

Sucrose synthase (Sus) is a key enzyme in the breakdown of sucrose and is considered a biochemical marker for sink strength, especially in crop species, based on mutational and gene suppression studies. It remains elusive, however, whether, or to what extent, increase in Sus activity may enhance sink development. We aimed to address this question by expressing a potato Sus gene in cotton where Sus expression has been previously shown to be critical for normal seed and fiber development. Segregation analyses at T1 generation followed by studies in homozygous progeny lines revealed that increased Sus activity in cotton (1) enhanced leaf expansion with the effect evident from young leaves emerging from shoot apex; (2) improved early seed development, which reduced seed abortion, hence enhanced seed set, and (3) promoted fiber elongation. In young leaves of Sus overexpressing lines, fructose concentrations were significantly increased whereas, in elongating fibers, both fructose and glucose levels were increased. Since hexoses contribute little to osmolality in leaves, in contrast to developing fibers, it is concluded that high Sus activity promotes leaf development independently of osmotic regulation, probably through sugar signaling. The analyses also showed that doubling the Sus activity in 0-d cotton seeds increased their fresh weight by about 30%. However, further increase in Sus activity did not lead to any further increase in seed weight, indicating an upper limit for the Sus overexpression effect. Finally, based on the observed additive effect on fiber yield from increased fiber length and seed number, a new strategy is proposed to increase cotton fiber yield by improving seed development as a whole, rather than solely focusing on manipulating fiber growth.

Maternal control of nutrient allocation in plant seeds by genomic imprinting

Imprinted genes are commonly expressed in mammalian placentas and in plant seed endosperms, where they exhibit preferential uniparental allelic expression. In mammals, imprinted genes directly regulate placental function and nutrient distribution from mother to fetus; however, none of the >60 imprinted genes thus far reported in plants have been demonstrated to play an equivalent role in regulating the flow of resources to the embryo. Here we show that imprinted Maternally expressed gene1 (Meg1) in maize is both necessary and sufficient for the establishment and differentiation of the endosperm nutrient transfer cells located at the mother:seed interface. Consistent with these findings, Meg1 also regulates maternal nutrient uptake, sucrose partitioning, and seed biomass yield. In addition, we generated an imprinted and nonimprinted synthetic Meg1 ((syn)Meg1) dosage series whereby increased dosage and absence of imprinting both resulted in an unequal investment of maternal resources into the endosperm. These findings highlight dosage regulation by genomic imprinting as being critical for maintaining a balanced distribution of maternal nutrients to filial tissues in plants, as in mammals. However, unlike in mammals, Meg1 is a maternally expressed imprinted gene that surprisingly acts to promote rather than restrict nutrient allocation to the offspring.

Diverse functional roles of monosaccharide transporters and their homologs in vascular plants: a physiological perspective

Vascular plants contain two gene families that encode monosaccharide transporter proteins. The classical monosaccharide transporter(-like) gene superfamily is large and functionally diverse, while the recently identified SWEET transporter family is smaller and, thus far, only found to transport glucose. These transporters play essential roles at many levels, ranging from organelles to the whole plant. Many family members are essential for cellular homeostasis and reproductive success. Although most transporters do not directly participate in long-distance transport, their indirect roles greatly impact carbon allocation and transport flux to the heterotrophic tissues of the plant. Functional characterization of some members from both gene families has revealed their diverse roles in carbohydrate partitioning, phloem function, resource allocation, plant defense, and sugar signaling. This review highlights the broad impacts and implications of monosaccharide transport by describing some of the functional roles of the monosaccharide transporter(-like) superfamily and the SWEET transporter family.

Membrane-transport systems for sucrose in relation to whole-plant carbon partitioning

Sucrose is the principal product of photosynthesis used for the distribution of assimilated carbon in plants. Transport mechanisms and efficiency influence photosynthetic productivity by relieving product inhibition and contribute to plant vigor by controlling source/sink relationships and biomass partitioning. Sucrose is synthesized in the cytoplasm and may move cell to cell through plasmodesmata or may cross membranes to be compartmentalized or exported to the apoplasm for uptake into adjacent cells. As a relatively large polar compound, sucrose requires proteins to facilitate efficient membrane transport. Transport across the tonoplast by facilitated diffusion, antiport with protons, and symport with protons have been proposed; for transport across plasma membranes, symport with protons and a mechanism resembling facilitated diffusion are evident. Despite decades of research, only symport with protons is well established at the molecular level. This review aims to integrate recent and older studies on sucrose flux across membranes with principles of whole-plant carbon partitioning.