Normal Pollen Grains Research Articles

Tapetum-specific expression of cysteine protease induces male sterility in tomato

Male sterile plants play a significant role in developing hybrid varieties to exploit the benefits of hybrid vigour in crops. Cysteine proteases play critical functions, including proteolysis and programmed cell death in plants. In this study, we have generated male-sterile transgenic tomato plants using AdCP (Arachis diogoi cysteine protease) gene under the control of a tapetum-specific promoter (TA-29). The transgenic tomato plants produced non-functional pollen grains. The aborted pollen grains of the male sterile plant did not germinate even after 24 h of incubation compared to normal pollen grains. PCR analysis confirmed the stable integration of transgenes in transgenic plants. Semi-quantitave RT-PCR analysis showed the tissue-specific AdCP gene expression in the anthers of transgenic tomato plants. A back-cross was conducted between the transgenic male-sterile plants (female parent) and control (untransformed) plants (male parent). The T1 progeny indicated the segregation into female fertile and male-sterile plants, showing normal fruit development and seed set. High levels of AdCP transcripts were detected in anther tissues, confirming tapetum-specific expression of the TA29 promoter. The male-sterile tomato plants with targeted expression of the AdCP gene in tapetum could potentially be used to develop novel varieties through hybrid seed production.

Comment on “The ‘seed-fern’ Lepidopteris mass-produced the abnormal pollen Ricciisporites during the end-Triassic biotic crisis” by V. Vajda, S. McLoughlin, S. M. Slater, O. Gustafsson, and A. G. Rasmusson [Palaeogeography, Palaeoclimatology, Palaeoecology, 627 (2023), 111,723

Recently, Ricciisporites Lundblad and Cycadopites Wodehouse (=Monosulcites Cookson) pollen types have been found cooccurring in Antevsia zeilleri (Nathorst) Harris pollen sacs and it was assumed that the tetrads of Ricciisporites resulted from atypical pollen developmental pathway in which the pollen grains failed to separate. The pollen grains of Cycadopites type were interpreted as normal pollen grains of a peltasperm with Antevsia zeilleri pollen sacs and Lepidopteris ottonis (Göppert) Schimper foliage, and Ricciisporites, as abnormal pollen tetrads of this parent plant. However, if it were true, the exines in the pollen grains of the two types should have something in common in the ultrastructure. Examples of aberrant pollen in living seed plants show that changes occurring in the predetermined succession of events of exine development are minor and even if they finally lead to a significantly altered mature exine, the original structural plan remains recognizable. Contrariwise, the two pollen types are too dissimilar by their exine ultrastructure as well as by the general morphology and exine sculpture. Comparison with ontogenetic data on suitable modern analogues also allows one to suppose that exines of Ricciisporites and Cycadopites developed differently, in unrelated parent plants. Another explanation should be found for the presence of Ricciisporites tetrads in these pollen sacs.

Integrated analysis of transcriptome and proteome changes related to the male-sterile mutant MS7-2 in wucai (Brassica Campestris L.)

Male sterility is a complex developmental process involving the differentiation of different tissues and the coordinated expression of genes. It is not only used to produce hybrids with effective utilization of heterosis but also an ideal tool to study plant development networks. Here, we carried out morphological and cytological observations on the flower buds of the previously studied male-sterile mutant MS7-2 and its wild-type fertile plant MF7-2 at different developmental stages and performed transcriptomic and proteomic analyses. Compared with the anthers of MF7-2, the anthers of MS7-2 were obviously atrophied, the anther surface was concave, the tapetum was abnormally expanded at the mononuclear microspore stage and normal pollen grains could not be produced. Proteomic analysis showed that the differentially expressed proteins (DEPs) were mainly involved in lipid transport, lipid binding and lipase activity and were mainly concentrated in carbohydrate metabolism, biosynthesis of other secondary metabolites and lipid metabolism. Transcriptomic results showed that the differentially expressed genes (DEGs) were mainly related to cell membrane components and were significantly enriched in carbohydrate metabolism, lipid metabolism, membrane transport and other pathways. In addition, transcription factors involved in anther and pollen development were also found to be significantly down-regulated in MS7-2, such as AMS, bHLH66, bHLH69 and WRKY 34. Weighted gene coexpression network analysis (WGCNA) screened two key modules of anther abortion in wucai. The DEGs in the two modules were mainly enriched in the “terpenoid backbone biosynthesis,” “pentose and glucuronate interconversions” and “cutin, suberine and wax biosynthesis” pathways, and three hub genes were screened as BraA02g028550.3C (LACS5), BraA01g016670.3C (VHA-G3) and BraA03g043400. 3C (AMS). Through transcriptomic and proteomic analyses, a regulatory network of MS7-2 male sterility of was constructed. It is speculated that the down-regulation of these genes and proteins in MS7-2 anthers inhibits the synthesis of some lipids and endogenous metabolites and accumulates a large amount of reactive oxygen species (ROS) in the tapetum, leading to the failure of normal PCD and the abortion of MS7-2. This study provides new insights into the molecular mechanism of male sterility in wucai.

Biological features of Crocus speciosus M. Bieb. (Iridaceae family)

The paper presents the results of studying the decorative species Crocus speciosus. Some features of the formation and development of male and female generative spheres are presented. It is shown that the formed anther wall is generated by the epidermis, endothecium, middle layer and tapetum. The wall of the mature anther is formed by the epidermis, fibrous endothecium with fibrous thickening, and a tapetal spathella with sporopollenin orbicles. Analysis of mature pollen grains showed a rather high proportion of normal pollen grains (80%), which under favorable conditions indicates a high ability to effectively pollinate. The characteristics of the female generative sphere are also shown: three-celled ovary; anatropic, bitegmal, funicular, crassinucellate ovule; linear tetrad of megaspores; Polygonum-type of embryonic sac development. The processes of flowering, pollination and seed formation are described.

Arabidopsis antiporter CHX23 and auxin transporter PIN8 coordinately regulate pollen growth

Both the antiporter CHX23 (Cation/Proton Exchangers 23) and auxin transporter PIN8 (PIN-FORMED 8) are localized in the ER and regulate pollen growth in Arabidopsis. But how these two proteins regulate pollen growth remains to be studied. Here, we report that CHX23 and PIN8 act coordinately in regulating pollen growth. The chx23 mutant was reduced in pollen growth and normally shaped pollen grains, and complementation with CHX23 restored both pollen growth and normal pollen morphology. NAA treatments showed that CHX23 was crucial for pollen auxin homeostasis. The pin8 chx23 double mutant was decreased in pollen growth and normal pollen grains, indicating the joint effort of CHX23 and PIN8 in pollen growth. In vivo germination assay showed that CHX23 and PIN8 were involved in the early stage of pollen growth. CHX23 and PIN8 also function collaboratively in maintaining pollen auxin homeostasis. PIN8 depends on CHX23 in regulating pollen morphology and response to NAA treatments. CHX23 co-localized with PIN8, but there was no physical interaction. KCl and NaCl treatments showed that pollen growth of chx23 was reduced less than Col-0; pin8 chx23 was reduced less than chx23 and pin8. Together, CHX23 may regulate PIN8 function and hence pollen growth through controlling K+ and Na+ homeostasis mediated by its transport activity.

Characterization and functional analysis of LoUDT1, a bHLH transcription factor related to anther development in the lily oriental hybrid Siberia (Lilium spp.)

Lily (Lilium spp.), with its beautiful flower, is an important horticultural crop and a popular ornamental plant, but because the abundant pollen pollutes the flowers and surroundings, its use is restricted. To solve this problem, the mechanism of pollen development in lily needs to be analyzed. However, the complex and delicate process of anther development in lily remains largely unknown. In this study, LoUDT1, a bHLH transcription factor (TF), was isolated and identified in lily. LoUDT1 was closely related to OsUDT1 of Oryza sativa and AtDYT1 of Arabidopsis. It was localized in the cytoplasm and nucleus and showed no transcriptional activation in yeast cells. LoUDT1 interacted with another bHLH TF, LoAMS, and the interaction depended on their BIF domains. LoUDT1 and LoAMS were both expressed in the anthers but showed different expression patterns. LoUDT1 was continuously expressed during the entire development of anthers, whereas LoAMS was only highly expressed early in anther development. With overexpression of LoUDT1 in Arabidopsis, normal anther development was affected and defective pollens were produced, which caused partial male sterility of transgenic plants. These defects depended on the level of LoUDT1 accumulation. By contrast, with the appropriate expression of LoUDT1 in a dyt1-3 mutant, normal pollen grains were produced, showing partial fertility. Thus, LoUDT1 might be a key regulator of anther development in lily. By further increasing the understanding of anther development, the results of this study can provide a theoretical basis for the molecular breeding of pollen-free lilies.

Structural anomalies in pollen grains of Dyckia racinae L.B.Sm. (Bromeliaceae)

The development of the pollen grains is a complex and regulated process. However, failures in sporogenesis and gametogenesis are common and include cell division errors and multiple structural modifications, culminating usually in abortions or, less frequently, in formation of anomalous pollen grains. These anomalies include altered morphology and functionality and are reported in several species, but are still unexplored in Bromeliaceae, a diverse neotropical family. Thus, this work structurally describes anomalous pollen grains in anthers of Dyckia racinae L.B. Sm., a Bromeliaceae species endemic to Southern Brazil. Mature anthers were processed and analyzed using standard methods for light microscopy. In addition to structurally normal pollen grains, unicellular, geminate, voluminous and aborted pollen grains were structurally described. These morphological alterations include differences in developmental destination, pollen grains with varied sizes, fusions with cytoplasmic communications, morphological alterations in exine and intine and variations in the cytoplasmic content. These anomalies are normally related to developmental disruptions, such as altered programmed cell death in tapetum, associated with cytoplasmic male sterility, for example, which usually leading to extensive abortion of the pollen grains in the anther locules. The structural anomalies described in this study, however, occur in anthers with apparently normal development and were first described for Bromeliaceae. Our results suggest a partial male sterility for this species, an unexplored embryological aspect of pollen grains in this family. Future investigations of ultrastructure and development of the anomalous pollen grains should contribute to a better understanding of the reported anomalies.

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

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

Downregulation of the mitogen-activated protein kinase SlMAPK7 gene results in pollen abortion in tomato

Mitogen-activated protein kinases (MAPKs) play important roles in various aspects including plant development and stress response. The biological role of MAPKs in tomato development remains unclear. Here, multiple stable transgenetic lines were developed using either gene overexpression or RNA interference (RNAi) technology to investigate the role of SlMAPK7 in tomato reproductive development. Either upregulation or downregulation of SlMAPK7 did not drastically disturb the vegetative growth of tomato. However, in RNAi-based gene silencing plants, approximately 90 % of the pollen grains were identified to be abnormal and lack viability, which were malformed or collapsed. Cytological observations showed that anthers could develop normal microspore mother cells, but the uninucleate microspores failed to form normal mature pollen grains. Transmission electron microscopy examination demonstrated that accelerated degradation of tapetum occurred in the anther and finally the intine layer of pollen appears abnormal. In addition, although inhibition of SlMAPK7 gene did not alter fruit morphology, seed number in RNAi fruits was markedly reduced. Thus, the results demonstrated that SlMAPK7 is involved in tomato pollen development, possibly via regulation the degradation of tapetum.

Induced cytomictic variations and syncyte formation during microsporogenesis in Phaseolus vulgaris L.

The intercellular translocation of chromatin material along with other cytoplasmic contents among the proximate meiocytes lying in close contact with each other commonly referred as cytomixis was reported during microsporogenesis in Phaseolus vulgaris L., a member of the family Fabaceae. The phenomenon of cytomixis was observed at three administered doses of gamma rays viz. 100, 200, 300 Gy respectively in the diploid plants of Phaseolus vulgaris L. The gamma rays irradiated plants showed the characteristic feature of inter-meiocyte chromatin/chromosomes transmigration through various means.such as channel formation, beak formation or by direct adhesion between the PMC's (Pollen mother cells). The present study also reports the first instance of syncyte formation induced via cytomictic transmigration in Phaseolus vulgaris L. Though the frequency of syncyteformation was rather low yet these could play a significant role in plant evolution. It is speculated that syncyte enhances the ploidy level of plants by forming 2n gametes and may lead to the production ofpolyploid plants. The phenomenon of cytomixis shows a gradual inclination along with the increasing treatment doses of gamma rays. The preponderance of cytomixis was more frequent during meiosis I as compared to meiosis II. An interesting feature noticed during the present study was the channel formation among the microspores and fusion among the tetrads due to cell wall dissolution. The impact of this phenomenon is also visible on the development of post-meiotic products. The formation of heterosized pollen grains; a deviation from the normal pollen grains has also been reported. The production of gametes with unbalanced chromosomes is of utmost importance and should be given more attention in future studies as they possess the capability of inducing variations at the genomic level and can be further utilized in the improvement of germplasm.

Characterization and Functional Analysis of Transcription Factor LoMYB80 Related to Anther Development in Lily (Lilium Oriental Hybrids)

Anther development is very precise and complicated procedure. In this study, the gene LoMYB80 was isolated from lily (Lilium Oriental Hybrids cv. cultivar ‘Siberia’) anther. Multiple alignment and phylogenetic tree analysis showed that LoMYB80 belongs to the R2R3 MYB family. Furthermore, the protein of LoMYB80 shares a 44 amino acid sequence immediately adjacent to the R3 domain with other MYB80s. Subcellular localization showed that LoMYB80 was predominantly localized in the nucleus. The transcriptional activation experiment revealed that LoMYB80 belongs to the transcription activation factors and its transactivation activity depends on the last 70 amino acids located at the end of the C-terminal region. Real-time quantitative PCR analysis showed that LoMYB80 gene expression was significantly higher in anther than in other organs; the highest level was in anthers of 10-cm flower buds and then was in anthers of 4-cm buds. The relative expression in microspores/pollen of 10-cm buds was much higher than the whole anther in the same stage and also much higher than the microspore/pollen of 8- and 9-cm buds. The gene was mainly expressed in young anthers at the development stages from 6 to 10 by analyzing transgenic Arabidopsis lines which were transformed with LoMYB80 promoter::GUS reporter gene constructs. Complementation experiments revealed that transgenic ms188 mutant lines could produce normal pollen grains and display partial fertility. These results indicated that LoMYB80 was a regulator of lily anther development and pollen formation.

Decreased Pollen Viability and Thicken Pollen Intine in Antisense Silenced Brassica campestris Mutant of BcMF19

Brassica campestris male fertility 19 (BcMF19; GenBank accession number GQ902048.1), a gene that is specially expressed in tapetum cells and microspores during anther development in B. campestris ssp. chinensis, which is learned from the previous in situ hybridization study. In the present study, we constructed antisense-silenced plants of BcMF19 using B. campestris ssp. chinensis to validate this prediction. The morphology of the pistils, long anthers, and short anthers was significantly affected in 35sbcmf19 compared with the control samples. 4′-6-Diamidino-2-phenylindole staining revealed that two generative nuclei and one large vegetative nucleus were not affected in the mutant compared with control. Statistical analysis of Alexander's staining results showed that 96% of the control pollen grains had vitality, whereas only 86% of the mutant pollen grains did. Under scanning electron microscopy, the mutant demonstrated numerous abnormal pollen grains and resembled dried persimmon. The frequency of normal pollen grains was approximately 18%. Under transmission electron microscopy, the pollen intine during the binucleate and mature pollen stages in 35sbcmf19 exhibited abnormal thickening, especially at the germinal furrows, compared with control. In vitro pollen germination test showed that the tips of the mutant pollen tubes transformed into globular alveoli and stopped growing compared with control. On the other hand, in vivo pollen germination test suggested that BcMF19 affected the pollen tube extension in the pistil. These findings indicate that BcMF19 is essential to the pollen development and pollen tube extension of B. campestris ssp. chinensis.

Expression of kenaf mitochondrial chimeric genes HM184 causes male sterility in transgenic tobacco plants

Chimeric genes resulting from the rearrangement of a mitochondrial genome were generally thought to be a causal factor in the occurrence of cytoplasmic male sterility (CMS). In the study, earlier we reported that identifying a 47 bp deletion at 3′- flanking of atp9 that was linked to male sterile cytoplasm in kenaf. The truncated fragment was fused with atp9, a mitochondrial transit signal (MTS) and/or GFP, comprised two chimeric genes MTS-HM184-GFP and MTS-HM184. The plant expression vector pBI121 containing chimeric genes were then introduced to tobacco plants by Agrobacterium-mediated T-DNA transformation. The result showed that certain transgenic plants were male sterility or semi-sterility, while some were not. The expression analysis further demonstrated that higher level of expression were showed in the sterility plants, while no expression or less expression in fertility plants, the levels of expression of semi-sterility were in between. And the sterile plant (containing MTS-HM184-GFP) had abnormal anther produced malformed/shriveled pollen grains stained negative that failed to germinate (0%), the corresponding fruits was shrunken, the semi-sterile plants having normal anther shape produced about 10–50% normal pollen grains, the corresponding fruits were not full, and the germination rate was 58%. Meanwhile these transgenic plants which altered on fertility were further analyzed in phenotype. As a result, the metamorphosis leaves were observed in the seedling stage, the plant height of transgenic plants was shorter than wild type. The growth duration of transgenic tobacco was delayed 30–45 days compared to the wild type. The copy numbers of target genes of transgenic tobacco were analyzed using the real-time quantitative method. The results showed that these transgenic plants targeting-expression in mitochondrial containing MTS-HM184-GFP had 1 copy and 2 copies, the other two plants containing MTS-HM184 both had 3 copies, but 0 copy in wild type. In summary, the two manual chimeric genes might be related to male sterility in kenaf.

First evidence of putrescine involvement in mitigating the floral malformation in mangoes: A scanning electron microscope study

Floral malformation is the most destructive disease in mangoes. To date, the etiology of this disease has not been resolved. There are indications that stress-stimulated ethylene production might be responsible for the disease. Putrescine mediates various physiological processes for normal functioning and cellular metabolism. Here, the effect of putrescine in concentration ranging from 10(-1) to 10(-3)M was evaluated on disease incidence during mango flowering seasons of 2012 and 2013. In a scanning electron microscopy (SEM) study, putrescine (10(-2)M)-treated malformed floral buds bloomed into opened flowers with separated sepals and/or petals like healthy, whereas the untreated (control) malformed buds remained deformed. Further, malformed flowers recovered upon putrescine treatment, displaying clearly bilobed anthers, enclosing a large number of normal pollen grains and functional ovary with broad stigmatic surface as compared to control. The present findings provide the first report to demonstrate the role of putrescine in reducing various adverse effects of stress ethylene via decelerating the higher pace of its biosynthesis. It stabilizes the normal morphology, development, and functions of malformed reproductive organs to facilitate successful pollination, fertilization, and, thereby, fruit set in mango flowers. However, putrescine-ethylene-mediated cell signaling network, involving various genes to trigger the response, which regulates a wide range of developmental and physiological processes leading to normal cell physiology, needs to be investigated further.

Cytomorphological investigations in Oxyria digyna Hill. from the Kashmir Himalaya, India

In the present paper, detailed cytomorphological investigations in Oxyria digyna Hill. from Kashmir Himalaya-India have been reported for the first time. All the of 14 investigated populations of O. digyna are diploid based on x = 7. Out of these in two populations 0-2B chromosomes have been recorded for the first time while 6 populations differed significantly in their meiotic characteristics. Meiotic abnormalities during male meiosis observed include inter PMC chromatin transfer (cytomixis). Non-synchronous disjunction of some bivalents, laggards and bridges at anaphases and telophases. Consequent to these meiotic anomalies, microsporogenesis in meiocytes is abnormal resulting in to dyads, triads and polyads with or without micronuclei. The overall effect is seen in reduced pollen fertility. Unreduced pollen grains were observed in some populations, which differed significantly in their size compared to the normal (reduced) pollen grains. It is observed that a smaller frequency of pollen grains differed morphologically in Aharbal and Yosmarg populations. The remaining eight populations showed regular meiotic course, normal microsporogenesis and high percentage of pollen fertility (95.09-99.09%).

Genomic relationships between hexaploid Helianthus resinosus and diploid Helianthus annuus (Asteraceae)

Genus Helianthus comprises diploid and polyploid species. An autoallopolyploid origin has been proposed for hexaploid species but the genomic relationships remain unclear. Mitotic and meiotic studies in annual Helianthus annuus (2n = 2x = 34) and perennial Helianthus resinosus (2n = 6x = 102) as well as the F1 hybrids between both species were carried out. Chromosome counting confirmed the hybrid origin of the latter plants and their tetraploid condition. Bivalents in hybrids ranged from 12 to 28 (\( \bar{x} \) = 20.8). Univalents, trivalents and quadrivalents were also observed. Meiotic products comprised dyads, triads and normal tetrads and pollen grains were heterogeneous in size. These observations suggest the occurrence of 2n pollen in addition to the expected n. Genomic in situ hybridization (GISH) of total H. annuus DNA on H. resinosus chromosomes rendered weak but uniform signals; similar hybridization pattern was observed using three other annual species. Hybridization with H. annuus probe performed on root tip cells of F1H. annuus × H. resinosus hybrids revealed 17 chromosomes with a strong hybridization signal. GISH in hybrid meiocytes distinguished chromosomes from parental species and revealed autosyndetic pairing of H. resinosus chromosomes, allosyndetic pairing in bivalents, trivalents and quadrivalents, and the presence of univalents derived from parents, H. annuus and H. resinosus. Results obtained from classical and molecular cytogenetics do not support H. annuus as a direct ancestor of H. resinosus. The occurrence of allosyndetic pairing and the relatively high fertility of the F1 hybrids point to the possibility that useful genes could be transferred from H. resinosus to cultivate sunflower, although the effective rate of recombination has not been evaluated. GISH method proved effective to recognize parental chromosomes in H. annuus × H. resinosus progeny.

COLLAPSED ABNORMAL POLLEN1Gene Encoding the Arabinokinase-Like Protein Is Involved in Pollen Development in Rice

We isolated a pollen-defective mutant, collapsed abnormal pollen1 (cap1), from Tos17 insertional mutant lines of rice (Oryza sativa). The cap1 heterozygous plant produced equal numbers of normal and collapsed abnormal grains. The abnormal pollen grains lacked almost all cytoplasmic materials, nuclei, and intine cell walls and did not germinate. Genetic analysis of crosses revealed that the cap1 mutation did not affect female reproduction or vegetative growth. CAP1 encodes a protein consisting of 996 amino acids that showed high similarity to Arabidopsis (Arabidopsis thaliana) l-arabinokinase, which catalyzes the conversion of l-arabinose to l-arabinose 1-phosphate. A wild-type genomic DNA segment containing CAP1 restored mutants to normal pollen grains. During rice pollen development, CAP1 was preferentially expressed in anthers at the bicellular pollen stage, and the effects of the cap1 mutation were mainly detected at this stage. Based on the metabolic pathway of l-arabinose, cap1 pollen phenotype may have been caused by toxic accumulation of l-arabinose or by inhibition of cell wall metabolism due to the lack of UDP-l-arabinose derived from l-arabinose 1-phosphate. The expression pattern of CAP1 was very similar to that of another Arabidopsis homolog that showed 71% amino acid identity with CAP1. Our results suggested that CAP1 and related genes are critical for pollen development in both monocotyledonous and dicotyledonous plants.

Diplospory and obligate apomixis in Miconia albicans (Miconieae, Melastomataceae) and an embryological comparison with its sexual congener M. chamissois

Apomixis, or asexual reproduction through seeds, has been reported for species of the tribe Miconieae, Melastomataceae, but details of the process have yet to be described. We analyzed and compared sporogenesis and gametogenesis in the apomictic Miconia albicans and the sexual M. chamissois. The results point to some differences between species, which were related to the apomictic process. In M. albicans microsporogenesis, problems during meiosis and degeneration of its products led to total pollen sterility, while M. chamissois presented normal bicellular pollen grains in the mature anther. The absence or abnormality of meiosis in M. albicans megasporogenesis led to the formation of an unreduced embryo sac and also to egg cell parthenogenesis, which gave rise to the apomictic embryo. Embryo and endosperm development were autonomous, resulting in seeds and fruits independent of pollination and fertilization. Thus, in this species, apomixis can be classified as diplosporic and obligate. In contrast, meiosis was as expected in the sexual M. chamissois, and led to the development of a reduced embryo sac. Despite the divergent pathways, many embryological characteristics were similar between the studied species and other Melastomataceae and they seem to be conservative character states for the family.

High temperature injury and auxin biosynthesis in microsporogenesis

Plant reproductive development is more sensitive than vegetative growth to many environmental stresses. High temperature (HT) injury is becoming an increasingly serious problem due to recent global warming. In wheat, barley, and other crops, the early phase of anther development is most susceptible to HT. I and my colleagues recently demonstrated that HT causes cell proliferation arrest and represses auxin signaling in a tissue-specific manner in the anther cells of barley and Arabidopsis. HT also caused comprehensive alterations in transcription. The application of auxin at the same time blocked the transcriptional alterations, led to the production of normal pollen grains, and restored the normal seed setting rate under increasing temperatures. Although synthetic auxins have been used widely as potent and selective herbicides, these recent results indicate that auxin is useful for the promotion of fertility and maintenance of crop yields under the threat of global warming.

Early Holocene hybridisation between Betula pubescens and B. nana in relation to birch vegetation in Southwest Iceland

The aims of the present research were to gain better insight into the early Holocene vegetation history of Betula in South Iceland and to investigate whether hybridisation between downy tree-birch (Betula pubescens) and dwarf birch (Betula nana) could be discovered in this region, similar to that previously found in North Iceland. A peat monolith spanning the period from about 10.3 to 7.6calkaBP from Eyvík, Southwest Iceland, provided 44 samples for pollen analysis. The samples were dated by known tephra layers, Saksunarvatn and Seydishólar tephras, and two 14C datings. The macrofossil and spore/pollen results showed several progressive phases of vegetation development in the area until birch woodland became established around 9.5calkaBP. The timing of birch colonisation is in good agreement with results from other sites in Iceland. From these Eyvík samples, all morphologically normal Betula pollen grains were size-measured and abnormal pollen grains recorded. Species proportions within samples were estimated statistically on the basis of pollen size. As birch woodland in this area became established rather late, statistical analysis of Betula pollen size was only possible in the uppermost 18 samples, approximately spanning the period between 7.6 and 9.5calkaBP. The frequency distribution of pollen size indicated populations of B. nana predating those of B. pubescens. The proportion of B. pubescens pollen was oscillating but increased with time. In two samples at the top of the peat monolith, more than half of the measured Betula pollen grains belonged to B. pubescens. Evidence of hybridisation based on high frequencies of non-triporate Betula pollen was found in two samples dated at about 8.8 and 7.7calkaBP. Conditions appeared to be comparable to the corresponding hybridisation events previously reported for North Iceland, i.e. in warm periods when downy birch was expanding near dwarf birch habitats.