Starch biosynthesis during pollen maturation is associated with altered patterns of gene expression in maize

Transcriptome profile analysis of young floral buds of fertile and sterile plants from the self-pollinated offspring of the hybrid between novel restorer line NR1 and Nsa CMS line in Brassica napus

BACKGROUND

The fertile and sterile plants were derived from the self-pollinated offspring of the F1 hybrid between the novel restorer line NR1 and the Nsa CMS line in Brassica napus. To elucidate gene expression and regulation caused by the A and C subgenomes of B. napus, as well as the alien chromosome and cytoplasm from Sinapis arvensis during the development of young floral buds, we performed a genome-wide high-throughput transcriptomic sequencing for young floral buds of sterile and fertile plants.

RESULTS

In this study, equal amounts of total RNAs taken from young floral buds of sterile and fertile plants were sequenced using the Illumina/Solexa platform. After filtered out low quality data, a total of 2,760,574 and 2,714,441 clean tags were remained in the two libraries, from which 242,163 (Ste) and 253,507 (Fer) distinct tags were obtained. All distinct sequencing tags were annotated using all possible CATG+17-nt sequences of the genome and transcriptome of Brassica rapa and those of Brassica oleracea as the reference sequences, respectively. In total, 3231 genes of B. rapa and 3371 genes of B. oleracea were detected with significant differential expression levels. GO and pathway-based analyses were performed to determine and further to understand the biological functions of those differentially expressed genes (DEGs). In addition, there were 1089 specially expressed unknown tags in Fer, which were neither mapped to B. oleracea nor to B. rapa, and these unique tags were presumed to arise basically from the added alien chromosome of S. arvensis. Fifteen genes were randomly selected and their expression levels were confirmed by quantitative RT-PCR, and fourteen of them showed consistent expression patterns with the digital gene expression (DGE) data.

CONCLUSIONS

A number of genes were differentially expressed between the young floral buds of sterile and fertile plants. Some of these genes may be candidates for future research on CMS in Nsa line, fertility restoration and improved agronomic traits in NR1 line. Further study of the unknown tags which were specifically expressed in Fer will help to explore desirable agronomic traits from wild species.

Perspectives on deciphering mechanisms underlying plant heat stress response and thermotolerance

Global warming is a major threat for agriculture and food safety and in many cases the negative effects are already apparent. The current challenge of basic and applied plant science is to decipher the molecular mechanisms of heat stress response (HSR) and thermotolerance in detail and use this information to identify genotypes that will withstand unfavorable environmental conditions. Nowadays X-omics approaches complement the findings of previous targeted studies and highlight the complexity of HSR mechanisms giving information for so far unrecognized genes, proteins and metabolites as potential key players of thermotolerance. Even more, roles of epigenetic mechanisms and the involvement of small RNAs in thermotolerance are currently emerging and thus open new directions of yet unexplored areas of plant HSR. In parallel it is emerging that although the whole plant is vulnerable to heat, specific organs are particularly sensitive to elevated temperatures. This has redirected research from the vegetative to generative tissues. The sexual reproduction phase is considered as the most sensitive to heat and specifically pollen exhibits the highest sensitivity and frequently an elevation of the temperature just a few degrees above the optimum during pollen development can have detrimental effects for crop production. Compared to our knowledge on HSR of vegetative tissues, the information on pollen is still scarce. Nowadays, several techniques for high-throughput X-omics approaches provide major tools to explore the principles of pollen HSR and thermotolerance mechanisms in specific genotypes. The collection of such information will provide an excellent support for improvement of breeding programs to facilitate the development of tolerant cultivars. The review aims at describing the current knowledge of thermotolerance mechanisms and the technical advances which will foster new insights into this process.

Starch metabolism in Arabidopsis

Starch is the major non-structural carbohydrate in plants. It serves as an important store of carbon that fuels plant metabolism and growth when they are unable to photosynthesise. This storage can be in leaves and other green tissues, where it is degraded during the night, or in heterotrophic tissues such as roots, seeds and tubers, where it is stored over longer time periods. Arabidopsis accumulates starch in many of its tissues, but mostly in its leaves during the day. It has proven to be a powerful genetic system for discovering how starch is synthesised and degraded, and new proteins and processes have been discovered. Such work has major significance for our starch crops, whose yield and quality could be improved by the application of this knowledge. Research into Arabidopsis starch metabolism has begun to reveal how its daily turnover is integrated into the rest of metabolism and adapted to the environmental conditions. Furthermore, Arabidopsis mutant lines deficient in starch metabolism have been employed as tools to study other biological processes ranging from sugar sensing to gravitropism and flowering time control. This review gives a detailed account of the use of Arabidopsis to study starch metabolism. It describes the major discoveries made and presents an overview of our understanding today, together with some as-yet unresolved questions.

Starch metabolism in Arabidopsis

Starch is the major non-structural carbohydrate in plants. It serves as an important store of carbon that fuels plant metabolism and growth when they are unable to photosynthesise. This storage can be in leaves and other green tissues, where it is degraded during the night, or in heterotrophic tissues such as roots, seeds and tubers, where it is stored over longer time periods. Arabidopsis accumulates starch in many of its tissues, but mostly in its leaves during the day. It has proven to be a powerful genetic system for discovering how starch is synthesised and degraded, and new proteins and processes have been discovered. Such work has major significance for our starch crops, whose yield and quality could be improved by the application of this knowledge. Research into Arabidopsis starch metabolism has begun to reveal how its daily turnover is integrated into the rest of metabolism and adapted to the environmental conditions. Furthermore, Arabidopsis mutant lines deficient in starch metabolism have been employed as tools to study other biological processes ranging from sugar sensing to gravitropism and flowering time control. This review gives a detailed account of the use of Arabidopsis to study starch metabolism. It describes the major discoveries made and presents an overview of our understanding today, together with some as-yet unresolved questions.

Proteome analysis of the wild and YX-1 male sterile mutant anthers of wolfberry (Lycium barbarum L.)

Pollen development is disturbed in the early tetrad stage of the YX-1 male sterile mutant of wolfberry (Lycium barbarum L.). The present study aimed to identify differentially expressed anther proteins and to reveal their possible roles in pollen development and male sterility. To address this question, the proteomes of the wild-type (WT) and YX-1 mutant were compared. Approximately 1760 protein spots on two-dimensional differential gel electrophoresis (2D-DIGE) gels were detected. A number of proteins whose accumulation levels were altered in YX-1 compared with WT were identified by mass spectrometry and the NCBInr and Viridiplantae EST databases. Proteins down-regulated in YX-1 anthers include ascorbate peroxidase (APX), putative glutamine synthetase (GS), ATP synthase subunits, chalcone synthase (CHS), CHS-like, putative callose synthase catalytic subunit, cysteine protease, 5B protein, enoyl-ACP reductase, 14-3-3 protein and basic transcription factor 3 (BTF3). Meanwhile, activities of APX and GS, RNA expression levels of apx and atp synthase beta subunit were low in YX-1 anthers which correlated with the expression of male sterility. In addition, several carbohydrate metabolism-related and photosynthesis-related enzymes were also present at lower levels in the mutant anthers. In contrast, 26S proteasome regulatory subunits, cysteine protease inhibitor, putative S-phase Kinase association Protein 1(SKP1), and aspartic protease, were expressed at higher levels in YX-1 anthers relative to WT anthers. Regulation of wolfberry pollen development involves a complex network of differentially expressed genes. The present study lays the foundation for future investigations of gene function linked with wolfberry pollen development and male sterility.

Calcium dependent protein kinase (CDPK) expression during fruit development in cultivated peanut (Arachis hypogaea) under Ca²⁺-sufficient and -deficient growth regimens

Adequate soil calcium (Ca²⁺) levels are crucial for sustained reproductive development of peanut (Arachis hypogaea). A role for calcium dependent protein kinase was evaluated during peanut fruit development under sufficient and deficient soil Ca²⁺ conditions. Quantitative RT-PCR and protein gel blot analyses confirmed transcriptional upregulation of CDPK in seeds developing under inadequate soil Ca²⁺ regimen, as well as spatiotemporal regulation of CDPK expression during early mitotic growth and later during the storage phase of seed development. However, a consistent basal level of CDPK was present during similar developmental stages of pod tissue, irrespective of the soil Ca²⁺ status. Immunolocalization data showed CDPK decoration primarily in the outer most cell layers of the pericarp and around vascular bundles linked by lateral connections in developing pods, as well as the single vascular trace supplying nutrients to the developing seed. Finally, carbohydrate analyses and qRT-PCR data are provided for peanut genes encoding enzymes involved in sucrose cleavage (orthologs of Vicia faba, VfCWI1 and VfCWI2) and utilization (AhSuSy and AhSpS), and oleosin gene transcripts (AhOleo17.8 and AhOleo18.5) validating a role for CDPK in the establishment and maintenance of sink strength, and subsequent onset of storage product biosynthetic phase during seed maturation.

14-3-3 proteins in plant physiology

Plant 14-3-3 isoforms, like their highly conserved homologues in mammals, function by binding to phosphorylated client proteins to modulate their function. Through the regulation of a diverse range of proteins including kinases, transcription factors, structural proteins, ion channels and pathogen defense-related proteins, they are being implicated in an expanding catalogue of physiological functions in plants. 14-3-3s themselves are affected, both transcriptionally and functionally, by the extracellular and intracellular environment of the plant. They can modulate signaling pathways that transduce inputs from the environment and also the downstream proteins that elicit the physiological response. This review covers some of the key emerging roles for plant 14-3-3s including their role in the response to the plant extracellular environment, particularly environmental stress, pathogens and light conditions. We also address potential key roles in primary metabolism, hormone signaling, growth and cell division.

Soybean 14-3-3 gene family: identification and molecular characterization

The 14-3-3s are a group of proteins that are ubiquitously found in eukaryotes. Plant 14-3-3 proteins are encoded by a large multigene family and are involved in signaling pathways to regulate plant development and protection from stress. Recent studies in Arabidopsis and rice have demonstrated the isoform specificity in 14-3-3s and their client protein interactions. However, detailed characterization of 14-3-3 gene family in legumes has not been reported. In this study, soybean 14-3-3 proteins were identified and their molecular characterization performed. Data mining of soybean genome and expressed sequence tag databases identified 18 14-3-3 genes, of them 16 are transcribed. All 16 SGF14s have higher expression in embryo tissues suggesting their potential role in seed development. Subcellular localization of all transcribed SGF14s demonstrated that 14-3-3 proteins in soybean have isoform specificity, however, some overlaps were also observed between closely related isoforms. A comparative analysis of SGF14s with Arabidopsis and rice 14-3-3s indicated that SGF14s also group into epsilon and non-epsilon classes. However, unlike Arabidopsis and rice 14-3-3s, SGF14s contained only one kind of gene structure belonging to each class. Overall, soybean consists of the largest family of 14-3-3 proteins characterized to date. Our results provide a solid framework for further investigations into the role of SGF14s and their involvement in legume-specific functions.

Study of the changes in insoluble polysaccharides during pollen development in rice ( Oryza sativa L.) by microscopic multispectral imaging

Multispectral analysis combined with the Periodic Acid-Schiff method was used to investigate cytological features of insoluble polysaccharides and changes in total insoluble polysaccharide content (TPC) during pollen development in rice, including four cytoplasmic male sterility lines (MSLs) and their corresponding fertility-maintaining lines (FMLs). The multispectral curves of the relative transmittance value (RTV) and the images of developing pollen cells were obtained across a range of successive wavelengths (400-720 nm). A minimum RTV was found near 550 nm indicating an absorption peak of the TPC. Thus, the TPC was measured using the RTV of 550 nm. In the four FMLs, the minimum TPC of developing pollen cells occurred at the late microspore stage, while the maximum TPC occurred at the mature pollen grain stage. The TPC levels of pollen cells were significantly higher in the four FMLs than in their corresponding MSLs during and after pollen abortion. Notably, a steep decrease of multispectral curves at 420 nm appeared before the occurrence of abortion, implying a marker associated with pollen abortion in rice. Our results will be helpful for exploring the changes in TPC during pollen ontogenesis in rice and provide a novel method for the study of bio-macromolecules.

Short-term high temperature growth conditions during vegetative-to-reproductive phase transition irreversibly compromise cell wall invertase-mediated sucrose catalysis and microspore meiosis in grain sorghum (Sorghum bicolor)

Grain sorghum (Sorghum bicolor) crop yield is significantly compromised by high temperature stress-induced male sterility, and is attributed to reduced cell wall invertase (CWI)-mediated sucrose hydrolysis in microspores and anthers leading to altered carbohydrate metabolism and starch deficiency in pollen (Jain et al., 2007). Sorghum plants were grown under season-long ambient (30/20 degrees C day-time maximum/night-time minimum) or high temperature stress (HS, 36/26 degrees C) environments, or reciprocally transferred for 5-10 days between either temperature regimens through panicle and microspore developmental stages. Quantitative RT-PCR analyses for CWI gene SbIncw1, plasma membrane H(+)-ATPase (Mha1) and sugar transporter proteins (OsSUT3 and OsMST7 homologs in sorghum), starch deficiency and pollen sterility data are presented to confirm HS-sensitivity of pre- and post-meiotic stages of sorghum microsporogenesis. Heat stress-induced reduction in Incw transcriptional activity during microspore meiosis was irreversible despite return of optimal growth temperature conditions through further reproductive development.

Carbon starved anther encodes a MYB domain protein that regulates sugar partitioning required for rice pollen development

In flowering plants, sink tissues rely on transport of carbohydrates from photosynthetic tissues (sources) for nutrition and energy. However, how sugar partitioning in plants is regulated at the molecular level during development remains unknown. We have isolated and characterized a rice (Oryza sativa) mutant, carbon starved anther (csa), that showed increased sugar contents in leaves and stems and reduced levels of sugars and starch in floral organs. In particular, the csa mutant had reduced levels of carbohydrates in later anthers and was male sterile. The csa mutant had reduced accumulation of (14)C-labeled sugars in anther sink tissue. CSA was isolated by map-based cloning and was shown to encode an R2R3 MYB transcription factor that was expressed preferentially in the anther tapetal cells and in the sugar-transporting vascular tissues. In addition, the expression of MST8, encoding a monosaccharide transporter, was greatly reduced in csa anthers. Furthermore, CSA was found to be associated in vivo and in vitro with the promoter of MST8. Our findings suggest that CSA is a key transcriptional regulator for sugar partitioning in rice during male reproductive development. This study also establishes a molecular model system for further elucidation of the genetic control of carbon partitioning in plants.

The legwd mutant uncovers the role of starch phosphorylation in pollen development and germination in tomato

Starches extracted from most plant species are phosphorylated. alpha-Glucan water dikinase (GWD) is a key enzyme that controls the phosphate content of starch. In the absence of its activity starch degradation is impaired, leading to a starch excess phenotype in Arabidopsis and in potato leaves, and to reduced cold sweetening in potato tubers. Here, we characterized a transposon insertion (legwd::Ds) in the tomato GWD (LeGWD) gene that caused male gametophytic lethality. The mutant pollen had a starch excess phenotype that was associated with a reduction in pollen germination. SEM and TEM analyses indicated mild shrinking of the pollen grains and the accumulation of large starch granules inside the plastids. The level of soluble sugars was reduced by 1.8-fold in mutant pollen grains. Overall, the transmission of the mutant allele was only 0.4% in the male, whereas it was normal in the female. Additional mutant alleles, obtained through transposon excision, showed the same phenotypes as legwd::Ds. Moreover, pollen germination could be restored, and the starch excess phenotype could be abolished in lines expressing the potato GWD homolog (StGWD) under a pollen-specific promoter. In these lines, where fertility was restored, homozygous plants for legwd::Ds were isolated, and showed the starch excess phenotype in the leaves. Overall, our results demonstrate the importance of starch phosphorylation and breakdown for pollen germination, and open up the prospect for analyzing the role of starch metabolism in leaves and fruits.

How microspores transform into haploid embryos: changes associated with embryogenesis induction and microspore-derived embryogenesis

Microspore embryogenesis is the most powerful androgenic pathway to produce haploid and doubled haploid plants. To deviate a microspore toward embryogenesis, a number of factors, different for each species, must concur at the same time and place. Once induced, the microspore undergoes numerous changes at different levels, from overall morphology to gene expression. Induction of microspore embryogenesis not only implies the expression of an embryogenic program, but also a stress-related cellular response and a repression of the gametophytic program to revert the microspore to a totipotent status. In this review, we compile the most recent advances in the understanding of the changes undergone by the induced microspore to readapt to the new developmental scenario. We devote special attention to the efforts made to uncover changes in the transcriptome of the induced microspore and microspore-derived embryo (MDE). Finally, we discuss the influence that an in vitro environment exerts over the MDE, as compared with its zygotic counterpart.

Cloning and expression analyses of sucrose non-fermenting-1-related kinase 1 (SnRK1b) gene during development of sorghum and maize endosperm and its implicated role in sugar-to-starch metabolic transition

A full-length cDNA clone, SbSnRK1b (1530 bp, GenBank accession no. EF544393), encoding a putative serine/threonine protein kinase homologue of yeast (Saccharomyces cerevisiae) SNF1, was isolated from developing endosperm of sorghum [Sorghum bicolor (L.) Moench]. Multiple sequence alignment data showed a phylogenetic affiliation of the sorghum clone with the SnRK1b group of protein kinases that are highly expressed in cereal seed endosperm. The DNA gel blot analyses indicated that SbSnRK1b gene is present as a single- or low copy number gene in sorghum. The RNA and protein gel blot analyses confirmed the expression of SbSnRK1b in developing sorghum caryopses, overlapping with the starch biosynthesis phase, 12-24 days after fertilization. In situ hybridization and immunolocalization data resolved the spatial specificity of SbSnRK1b expression in the basal endosperm transfer cell layer, the unique port of assimilate unloading in the growing sorghum seed. Expression of SbSnRK1b was also evident in the developing sorghum microspores, coincident with the onset of starch deposition phase. As in sorghum, similar spatiotemporal specificity of SnRK1b expression was observed during maize (Zea mays L.) seed development. However, discordant in situ hybridization and immunolocalization data indicated that the expression of SbSnRK1b homologue in maize is under posttranscriptional control during endosperm development.

Comparative analysis of gene expression between CMS-D8 restored plants and normal non-restoring fertile plants in cotton by differential display

CMS-D8 and its restorer were developed by introducing the cytoplasm and nuclear gene Rf (2) from the wild diploid Gossypium trilobum (D8) into the cultivated tetraploid Upland cotton (Gossypium hirsutum). No information is available on how the Rf (2) gene interacts with CMS-associated genes and how CMS-D8 cytoplasm affects nuclear gene expression. The objective of this study was to identify differentially expressed genes in anther tissues between the non-restoring fertile maintainer ARK8518 (rf(2) rf(2)) and its isogenic heterozygous D8 restorer line, ARK8518R (Rf(2) rf(2)) with D8 cytoplasm, by mRNA differential display (DD). Out of more than 3,000 DDRT-PCR bands amplified by 31 primer combinations from 12 anchor primers and 8 arbitrary decamer primers, approximately 100 bands were identified as being qualitatively differentially displayed. A total of 38 cDNA fragments including 12 preferentially expressed cDNA bands in anther were isolated, cloned and sequenced. Reverse northern blot analysis showed that only 4 genes, including genes encoding a Cys-3-His zinc finger protein and aminopeptidase, were up-regulated, while 22 genes, including genes for phosphoribosylanthranilate transferase (PAT), starch synthase (SS), 4-coumarate-CoA ligase, electron transporter, calnexin, arginine decarboxylase, and polyubiquitin, were down-regulated in the heterozygous restorer ARK8518R. The down-regulation of SS explains the lack of starch accumulation in sterile rf(2) pollen grains in the heterozygous restored plants. The molecular mechanism of CMS and its restoration, specifically the possible roles of SS and PAT genes in relation to restoration of Rf(2) to CMS-D8, are discussed. This investigation represents the first account of such an analysis in cotton.

Effects of season-long high temperature growth conditions on sugar-to-starch metabolism in developing microspores of grain sorghum (Sorghum bicolor L. Moench)

High temperature stress-induced male sterility is a critical problem in grain sorghum (Sorghum bicolor L. Moench) that significantly compromises crop yields. Grain sorghum plants were grown season-long under ambient (30/20 degrees C, day-time maximum/night-time minimum) and high temperature (36/26 degrees C) conditions in sunlit Soil-Plant-Atmospheric-Research (SPAR) growth chambers. We report data on the effects of high temperature on sugar levels and expression profiles of genes related to sugar-to-starch metabolism in microspore populations represented by pre- and post-meiotic "early" stages through post-mitotic "late" stages that show detectable levels of starch deposition. Microspores from high temperature stress conditions showed starch-deficiency and considerably reduced germination, translating into 27% loss in seed-set. Sugar profiles showed significant differences in hexose levels at both "early" and "late" stages at the two temperature regimes; and most notably, undetectable sucrose and approximately 50% lower starch content in "late" microspores from heat-stressed plants. Northern blot, quantitative PCR, and immunolocalization data revealed a significant reduction in the steady-state transcript abundance of SbIncw1 gene and CWI proteins in both sporophytic as well as microgametophytic tissues under high temperature conditions. Northern blot analyses also indicated greatly altered temporal expression profiles of various genes involved in sugar cleavage and utilization (SbIncw1, SbIvr2, Sh1, and Sus1), transport (Mha1 and MST1) and starch biosynthesis (Bt2, SU1, GBSS1, and UGPase) in heat-stressed plants. Collectively, these data suggest that impairment of CWI-mediated sucrose hydrolysis and subsequent lack of sucrose biosynthesis may be the most upstream molecular dysfunctions leading to altered carbohydrate metabolism and starch deficiency under elevated growth temperature conditions.

OeMST2 encodes a monosaccharide transporter expressed throughout olive fruit maturation

In olive fruits, sugars are the main soluble components providing energy and acting as precursors for olive oil biosynthesis. Large quantities of glucose, fructose and galactose are often found in olive pulp. To analyze sugar transport processes in Olea europaea, a cDNA encoding a monosaccharide transporter, designated OeMST2 (Olea europaea monosaccharide transporter 2) was cloned. An open reading frame of 1,569 bp codes for a protein of 523 amino acids and a calculated molecular weight of 57.6 kDa. The protein is homologous to other sugar transporters identified so far in higher plants. Expression of this cDNA in an hxt-null Saccharomyces cerevisiae strain deficient in glucose transport restored its capacity to grow on and to transport glucose. The encoded protein showed high affinity for D-glucose (K(m), 25 microM) and was also able to recognize D-galactose and the analogs 3-O-methyl-D-glucose and 2-deoxy-D-glucose, but not D-fructose, D-arabinose, sucrose or D-mannitol. Maximal transport activity was high at acidic pH (5.0), and the initial D-[(14)C]glucose uptake rates were strongly inhibited by the protonophore carbonyl cyanide m-chlorophenylhydrazone, confirming that OeMST2 is a H(+)/monosaccharide transporter. The expression of OeMST2 was studied during the ripening process. Transcript levels increased during fruit maturation, suggesting that OeMST2 takes part in the massive accumulation of monosaccharides in olive fruits. Monosaccharide:H(+) transport system activity and OeMST2 expression were negatively regulated by glucose in suspension-cultured cells. Glucose-mediated OeMST2 repression was impaired by mannoheptulose, suggesting the involvement of a hexokinase-dependent signaling pathway.

Sucrose and starch catabolism in the anther of Lilium during its development: a comparative study among the anther wall, locular fluid and microspore/pollen fractions

In order to better understand the various pathways of sucrose and starch catabolism in the anther of lily (Lilium hybrida var. "Enchantment"), invertase (EC 3.2.1.26) and amylase (EC 3.2.1.1, EC 3.2.1.2) activities were measured separately in different fractions (anther wall, locular fluid and microspore/pollen) and correlated with the sugar content during anther development. Our findings showed significant differences among the fractions analyzed, suggesting that the regulation of sucrose and starch catabolism could follow distinct pathways in each fraction. Glucose and fructose amounts progressively decreased from anther wall to fluid and from fluid to microspore/pollen. Thus, the developing pollen could act as a sink for the carbohydrates that reach the anther. In this sense, cell wall-bound invertases seem to play a major role in soluble sugar partitioning in the different fractions of the anther. Sucrose concentration was found to be substantially higher in the locular fluid than in the other fractions, indicating a probable site for storage. On the other hand, the anther wall tissues could have a buffering function, storing nutrient surplus in starch grains and thus regulating the availability of soluble sugars in the whole anther. All these results proved the advantages of the experimental model proposed here, as well as its usefulness to investigate sugar metabolism in Lilium anthers.

Rice Immature Pollen 1 (RIP1) is a regulator of late pollen development

We isolated a pollen-preferential gene, RICE IMMATURE POLLEN 1 (RIP1), from a T-DNA insertional population of japonica rice that was trapped by a promoterless beta-glucuronidase (GUS) gene. Semi-quantitative reverse transcription-PCR (RT-PCR) analyses confirmed that the RIP1 transcript was abundant at the late stages of pollen development. Transgenic plants carrying a T-DNA insertion in the RIP1 gene displayed the phenotype of segregation distortion of the mutated rip1 gene. Moreover, rip1/rip1 homozygous progeny were not present. Reciprocal crosses between Rip1/rip1 heterozygous plants and the wild type showed that the rip1 allele could not be transmitted through the male. Microscopic analysis demonstrated that development in the rip1 pollen was delayed, starting at the early vacuolated stage. Close examination of that pollen by transmission electron microscopy also showed delayed formation of starch granules and the intine layer. In addition, development of the mitochondria, Golgi apparatus, lipid bodies, plastids and endoplasmic reticulum was deferred in the mutant pollen. Under in vitro conditions, germination of this mutant pollen did not occur, whereas the rate for wild-type pollen was >90%. These results indicate that RIP1 is necessary for pollen maturation and germination. This gene encodes a protein that shares significant homology with a group of proteins containing five WD40 repeat sequences. The green fluorescent protein (GFP)-RIP1 fusion protein is localized to the nucleus. Therefore, RIP1 is probably a nuclear protein that may form a functional complex with other proteins and carry out essential cellular and developmental roles during the late stage of pollen formation.

Understanding storage starch biosynthesis in plants: a means to quality improvement

The many varied uses of starch in food and industrial applications often requires an understanding of its physicochemical properties and the detailed variations in granule structure that underpin these properties. The ability to manipulate storage starch structures depends on understanding the biosynthetic pathway, and in particular, how the many components of the pathway are coordinated and regulated. This article presents a current overview of starch structure and the known enzymes involved in the synthesis of the granule, with an emphasis on how current knowledge on the regulation of the pathway in cereals and other crops may be applied to the production of different functional starches.

Characterisation and immunolocalisation of a pollen-specific calmodulin-binding protein from rice

A novel calmodulin (CaM)-binding protein of rice (Oryza sativa pollen CaM-binding protein, OsPCBP) was isolated. It contains six tetratrcopeptide repeats (TPRs) with a molecular mass of ~76.7 kDa. Database searches show that OsPCBP is conserved in monocots and dicots. The results from CaM-agarose pull-down assays show that OsPCBP binds to CaM in a Ca²⁺-dependent manner and its CaM-binding domain (CBD) is located in a segment VSKGWRLLALVLSAQQRY. Northern blot and reverse transcription-polymerase chain reaction (RT-PCR) analysis have demonstrated that OsPCBP is a pollen-specific and late-expressed gene. Immunolocalisation showed that OsPCBP was localised in the amyloplast and intine during the late stage of pollen development. However, upon pollen hydration, there was some leakage of protein and the distribution of OsPCBP in the intine varied with the duration of hydration. After pollen germination, much OsPCBP was observed in the pollen tube wall. These results indicate the relationship of OsPCBP with starch accumulation and its involvement in pollen germination.

When microspores decide to become embryos — cellular and molecular changesThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

Cultured microspores can be induced to develop into fully functional haploid embryos instead of mature pollen. The ability of these cells to change their development in response to environmental stimuli is an exceptional example of totipotency in plants. Discovering the triggers of embryo development in microspore cultures could lead to a greater understanding of the early stages of embryogenesis in plants and might be used to increase the range of crop plants to which microspore culture can be applied for the production of double haploid homozygous lines. The information might also help to define the general characteristics of pluripotent cells in any organism. In this review, the changes that occur in cellular organization and gene expression in early-stage microspore cultures of several species are discussed. Responding cells in these cultures enlarge, their nuclei are repositioned to their cell centres, and their cytoplasms become filled with fragmented vacuoles. We used flow cytometry to track cellular changes in canola ( Brassica napus L.) microspore cultures as well as microarray analyses and real-time PCR to compare gene expression in embryogenic and nonembryogenic cells. A model for embryogenic cell activation in plants that involves alkalinization, Ca2+ signaling, and changes in GTPase activity that lead to significant changes in gene expression is discussed.

Cloning and expression analysis of Zmglp1, a new germin-like protein gene in maize

The cDNA and genomic DNA of a green tissue-specific gene were cloned from maize (Zea mays L.) using cDNA-amplified fragment length polymorphism (cDNA-AFLP) and library screening. The deduced protein was highly similar to Hordeum vulgare germin-like protein 1 (HvGLP1), and the maize gene was therefore designated Zmglp1. Northern blot specifically detected the mRNA of Zmglp1 in young whorl leaves at the early-whorl stage. However, at the late-whorl, tassel, and silk stages, Zmglp1 transcripts were highly abundant in young whorl leaves; less abundant in mature leaves, young tassels, and cobs; and not detectable in roots, immature kernels, and stalks. RNA in situ hybridization revealed that Zmglp1 expressed only in mesophyllous, phloem, and guard cells in the young whorl leaves. Deletion analysis of the promoter in transgenic Arabidopsis resulted in the identification of several regions containing important regulatory cis-elements controlling the expression levels and circadian rhythm-oscillated patterns of Zmglp1.

Androgenic switch: an example of plant embryogenesis from the male gametophyte perspective

Embryogenesis in plants is a unique process in the sense that it can be initiated from a wide range of cells other than the zygote. Upon stress, microspores or young pollen grains can be switched from their normal pollen development towards an embryogenic pathway, a process called androgenesis. Androgenesis represents an important tool for research in plant genetics and breeding, since androgenic embryos can germinate into completely homozygous, double haploid plants. From a developmental point of view, androgenesis is a rewarding system for understanding the process of embryo formation from single, haploid microspores. Androgenic development can be divided into three main characteristic phases: acquisition of embryogenic potential, initiation of cell divisions, and pattern formation. The aim of this review is to provide an overview of the main cellular and molecular events that characterize these three commitment phases. Molecular approaches such as differential screening and cDNA array have been successfully employed in the characterization of the spatiotemporal changes in gene expression during androgenesis. These results suggest that the activation of key regulators of embryogenesis, such as the BABY BOOM transcription factor, is preceded by the stress-induced reprogramming of cellular metabolism. Reprogramming of cellular metabolism includes the repression of gene expression related to starch biosynthesis and the induction of proteolytic genes (e.g. components of the 26S proteasome, metalloprotease, cysteine, and aspartic proteases) and stress-related proteins (e.g. GST, HSP, BI-1, ADH). The combination of cell tracking systems with biochemical markers has allowed the key switches in the developmental pathway of microspores to be determined, as well as programmed cell death to be identified as a feature of successful androgenic embryo development. The mechanisms of androgenesis induction and embryo formation are discussed, in relation to other biological systems, in special zygotic and somatic embryogenesis.

The distribution of membrane-bound 14-3-3 proteins in organelle-enriched fractions of germinating lily pollen

Proteins of the 14-3-3 family show a broad range of activities in plants, depending on their localisation in different cellular compartments. Different organelle membranes of pollen grains and pollen tubes of Lilium longiflorum Thunb. were separated simultaneously using optimised discontinuous sucrose density centrifugation. The obtained organelle-enriched fractions were identified as vacuolar, Golgi, endoplasmic reticulum and plasma membranes, according to their marker enzyme activities, and were assayed for membrane-bound 14-3-3 proteins by immunodetection. 14-3-3 proteins were detected in the cytoplasm as well as in all obtained organelle fractions but were also released into the extracellular medium. In pollen grains, much more plasma membrane-bound 14-3-3 proteins were detected than in the PM-enriched fraction of pollen tubes, whereas the level of Golgi- and ER-associated 14-3-3 proteins was similar in pollen grains and tubes. This shift in the localisation of membrane-associated 14-3-3 proteins is probably correlated with a change in the major function of 14-3-3 proteins, e.g., perhaps changing from initiating pollen grain germination by activation of the PM H +-ATPase to recruitment of membrane proteins via the secretory pathway during tube elongation.

Isoform-specific subcellular localization among 14-3-3 proteins in Arabidopsis seems to be driven by client interactions

In most higher eukaryotes, the predominantly phosphoprotein-binding 14-3-3 proteins are the products of a multigene family, with many organisms having 10 or more family members. However, current models for 14-3-3/phosphopeptide interactions suggest that there is little specificity among 14-3-3s for diverse phosphopeptide clients. Therefore, the existence of sequence diversity among 14-3-3s within a single organism begs questions regarding the in vivo specificities of the interactions between the various 14-3-3s and their clients. Chief among those questions is, Do the different 14-3-3 isoforms interact with different clients within the same cell? Although the members of the Arabidopsis 14-3-3 family of proteins typically contain highly conserved regions of sequence, they also display distinctive variability with deep evolutionary roots. In the current study, a survey of several Arabidopsis 14-3-3/GFP fusions revealed that 14-3-3s demonstrate distinct and differential patterns of subcellular distribution, by using trichomes and stomate guard cells as in vivo experimental cellular contexts. The effects of client interaction on 14-3-3 localization were further analyzed by disrupting the partnering with peptide and chemical agents. Results indicate that 14-3-3 localization is both isoform specific and highly dependent upon interaction with cellular clients.

Evolution and function of the sucrose-phosphate synthase gene families in wheat and other grasses

Suc-phosphate synthase (SPS) is a key regulatory enzyme in the pathway of Suc biosynthesis and has been linked to quantitative trait loci controlling plant growth and yield. In dicotyledonous plants there are three SPS gene families: A, B, and C. Here we report the finding of five families of SPS genes in wheat (Triticum aestivum) and other monocotyledonous plants from the family Poaceae (grasses). Three of these form separate subfamilies within the previously described A, B, and C gene families, but the other two form a novel and distinctive D family, which on present evidence is only found in the Poaceae. The D-type SPS proteins lack the phosphorylation sites associated with 14-3-3 protein binding and osmotic stress activation, and the linker region between the N-terminal catalytic glucosyltransferase domain and the C-terminal Suc-phosphatase-like domain is 80 to 90 amino acid residues shorter than in the A, B, or C types. The D family appears to have arisen after the divergence of mono- and dicotyledonous plants, with a later duplication event resulting in the two D-type subfamilies. Each of the SPS gene families in wheat showed different, but overlapping, spatial and temporal expression patterns, and in most organs at least two different SPS genes are expressed. Analysis of expressed sequence tags indicated similar expression patterns to wheat for each SPS gene family in barley (Hordeum vulgare) but not in more distantly related grasses. We identified an expressed sequence tag from rice (Oryza sativa) that appears to be derived from an endogenous antisense SPS gene, and this might account for the apparently low level of expression of the related OsSPS11 sense gene, adding to the already extensive list of mechanisms for regulating the activity of SPS in plants.

The Medicago truncatula sucrose synthase gene MtSucS1 is activated both in the infected region of root nodules and in the cortex of roots colonized by arbuscular mycorrhizal fungi

The MtSucS1 gene encodes a sucrose synthase (EC 2.4.1.13) in the model legume Medicago truncatula. To determine the expression pattern of this gene in different organs and in particular during root endosymbioses, we transformed M. truncatula with specific regions of MtSucS1 fused to the gusAint reporter gene. These fusions directed an induction to the vasculature of leaves, stems, and roots as well as to flowers, developing seeds, young pods, and germinating seedlings. In root nodules, strong promoter activity occurred in the infected cells of the nitrogen-fixing zone but was additionally observed in the meristematic region, the prefixing zone, and the inner cortex, including the vasculature. Concerning endomycorrhizal roots, the MtSucS1 promoter mediated strongest expression in cortical cells harboring arbuscules. Specifically in highly colonized root sections, GUS-staining was furthermore detected in the surrounding cortical cells, irrespective of a direct contact with fungal structures. In accordance with the presence of an orthologous PsSus1 gene, we observed a comparable regulation of MtSucS1 expression in the grain legume Pisum sativum in response to microbial symbionts. Unlike other members of the MtSucS gene family, the presence of rhizobial or Glomus microsymbionts significantly altered and enhanced MtSucS1 gene expression, leading us to propose that MtSucS1 is involved in generating sink-strength, not only in root nodules but also in mycorrhizal roots.