Mucilage secretion by seeds doubles the chance to escape removal by ants

Post-dispersal seed predation is a risk for plants in semiarid environments, leading to strategies to protect their propagules from seed collection by animals. In this study, we evaluated the importance of mucilage secretion in seeds as a mechanism to reduce seed collection by ants. We selected three Mediterranean species with strong mucilage secretion on their seeds which become sticky upon wetting. Seeds of Rosmarinus officinalis, Fumana ericoides, and Fumana thymifolia were exposed to ants and survival was compared between dry loose seeds and seeds glued to the soil with previously secreted mucilage. The study site was in the Sierra Calderona, 25?km north of Valencia (Spain). The ant?Cplant interaction was analyzed by scrutinizing seed collection by ants and by analyzing the waste piles of ant nests. To test survival, groups of 10 seeds were placed on the ground. Each group consisted of five control (dry) and five?mucilaginous seeds (previously mucilage secreted) and was covered by the cover of a Petri dish modified to permit only the entry of ants. Seeds were inspected weekly for seed disappearance and the survival function (Kaplan?CMeier estimator) was calculated. Seeds of the target species were important food items for ants and were actively collected, and more than 50?% of the experimental seeds that were glued to the ground with their own mucilage survived at the end of the study period but only 0?C20?% of the control seeds survived after the same time of exposure. The implications for plant establishment of these findings are discussed.     

朱唇种子吸水特性及其在干旱胁迫下萌发特性

种子粘液质是植物在长期适应环境过程中形成的,该物质对于种子的扩散、定居、生存力的改善、萌发、幼苗生存乃至抵御有毒化学物质毒害等都具有重要的生态学意义。朱唇为唇形科鼠尾草属多年生草本植物,原产美洲热带地区,现已广泛栽植于世界各地。为了理解朱唇种子表面的粘液物质吸水特性和种子在干旱胁迫下的萌发特性,该研究以朱唇种子为材料,运用光学显微镜和扫描电子显微镜观察以及种子萌发试验的方法,对种子和粘液层的形态结构、粘液质对种子萌发的影响进行了研究。结果表明:朱唇种子为卵形,表面为负网状结构,千粒重为(1.611±0.0084)g,无粘液种子吸水倍数为3,粘液种子吸水倍数为25,粘液层吸水倍数为122。粘液和无粘液种子及粘液层的重量都随吸水时间的延长而增长,但脱水过程要远长于吸水过程。朱唇种子吸水2 h达到饱和,经过36 h可干燥失水恢复原重。不同浓度PEG对朱唇种子的萌发均有影响,发芽势随PEG浓度升高而显著降低。朱唇种子在5%PEG胁迫下发芽率最高达(90.00±8.66)%,20%PEG胁迫下发芽率最低为(76.67±10.41)%,低浓度PEG对朱唇种子萌发有一定促进作用。这说明朱唇种子为速萌型种子,其粘液质在种子吸水过程中起到举足轻重的作用,能保证短时间内有充足的水分供其萌发。     

Hydrated mucilage reduces post-dispersal seed removal of a sand desert shrub by ants in a semiarid ecosystem

Post-dispersal seed removal by animals can lead to extensive seed loss and thus is an important factor in structuring plant communities. However, we know much less about post-dispersal seed predation than about other forms of herbivory. Mucilage plays many ecological roles in adaptation of plants to diverse environments; nevertheless, until now the role of mucilage in ant-mediated seed movement remains largely hypothetical. We studied the role of mucilage in seed removal of Artemisia sphaerocephala by ants in Mu Us Sandland in Inner Mongolia, China. Messor aciculatus was the most active seed predator of Artemisia sphaerocephala. Time to first ant collecting (T _1st) of wet intact seeds was longest and significantly different from that for dry intact seeds, wet demucilaged seeds, and dry demucilaged seeds; number of seeds removed to ant nests was lowest for wet intact seeds. After they were collected by ants, 5 % of wet intact seeds were dropped during transport. Our results indicate that seed mucilage of Artemisia sphaerocephala may play a significant role in post-dispersal seed removal by (1) making seeds less attractive to ants, thus resulting in a delay of collection time; (2) forming a strong bond to soil particles, making it difficult for ants to remove seeds; and (3) making seeds more likely to be dropped during transport, thereby allowing them to escape from predation even after collection by ants. This study demonstrates the importance of mucilage in reducing seed removal by ants and thus in anchoring seeds of desert plants in the vicinity of mother plants.     

Fruit and seed heteromorphism in the cold desert annual ephemeral Diptychocarpus strictus (Brassicaceae) and possible adaptive significance

Background and Aims

Diptychocarpus strictus is an annual ephemeral in the cold desert of northwest China that produces heteromorphic fruits and seeds. The primary aims of this study were to characterize the morphology and anatomy of fruits and seeds of this species and compare the role of fruit and seed hetermorphism in dispersal and germination.

Methods

Shape, size, mass and dispersal of siliques and seeds and the thickness of the mucilage layer on seeds were measured, and the anatomy of siliques and seeds, the role of seed mucilage in water absorption/dehydration, germination and adherence of seeds to soil particles, the role of pericarp of lower siliques in seed dormancy and seed after-ripening and germination phenology were studied using standard procedures.

Key Results

Plants produce dehiscent upper siliques with a thin pericarp containing seeds with large wings and a thick mucilage layer and indehiscent lower siliques with a thick pericarp containing nearly wingless seeds with a thin mucilage layer. The dispersal ability of seeds from the upper siliques was much greater than that of intact lower siliques. Mucilage increased the amount of water absorbed by seeds and decreased the rate of dehydration. Seeds with a thick mucilage layer adhered to soil particles much better than those with a thin mucilage layer or those from which mucilage had been removed. Fresh seeds were physiologically dormant and after-ripened during summer. Non-dormant seeds germinated to high percentages in light and in darkness. Germination of seeds from upper siliques is delayed until spring primarily by drought in summer and autumn, whereas the thick, indehiscent pericarp prevents germination for >1 year of seeds retained in lower siliques.

Conclusions

The life cycle of D. strictus is morphologically and physiologically adapted to the cold desert environment in time and space via a combination of characters associated with fruit and seed heteromorphism.     

Mucilage secretion: an adaptive mechanism to reduce seed removal by soil erosion?

Diaspores of many plant species inhabiting open vegetation in semi‐arid environments secrete mucilage after wetting (myxospermy) that glues the diaspores to the ground and prevents movement when the mucilage dries. In the present study, we test whether mucilage secretion can be considered as a selective response to soil erosion in plant species inhabiting semi‐arid environments. We relate the amount and type of mucilage secretion by seeds of Helianthemum violaceum and Fumana ericifolia (Cistaceae) to the number of raindrop impacts needed to remove these seeds after gluing them with their own mucilage to the ground and also the time that these seeds resist water run‐off without detaching. We also compare the amount of seed mucilage production by plants growing in habitats without erosion and plants affected by severe erosion by fitting mixed effect models. Our results show an important phenotypic variation in the amount of mucilage secretion in both species, although it is suggested that the effect of mucilage secretion in the rate of seed removal by erosion is species‐ and mechanism‐dependent. For F. ericifolia, the amount of mucilage secreted by the seeds is directly proportional to their resistance to raindrop impacts and is positively related to the intensity of the erosive processes that the plants experience. Nevertheless, all the seeds resist the force of run‐off during 60 min, irrespective of the amount of mucilage they produce. In H. violaceum, mucilage secretion per se, and not the amount of mucilage produced by the seeds, has an effect on the rate of seed removal by erosive processes. Furthermore, cellulosic fibrils were found only in the mucilage of F. ericifolia but not in H. violaceum. Overall, our results only partially support the hypothesis that a selective response to soil erosion exists. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 241–251.     

A subtilisin-like serine protease essential for mucilage release from Arabidopsis seed coats

During Arabidopsis seed development large quantities of mucilage, composed of pectins, are deposited into the apoplast underneath the outer wall of the seed coat. Upon imbibition of mature seeds, the stored mucilage expands through hydration and breaks the outer cell wall that encapsulates the whole seed. Mutant seeds carrying loss-of-function alleles of AtSBT1.7 that encodes one of 56 Arabidopsis thaliana subtilisin-like serine proteases (subtilases) do not release mucilage upon hydration. Microscopic analysis of the mutant seed coat revealed no visible structural differences compared with wild-type seeds. Weakening of the outer primary wall using cation chelators triggered mucilage release from the seed coats of mutants. However, in contrast to mature wild-type seeds, the mutant's outer cell walls did not rupture at the radial walls of the seed coat epidermal cells, but instead opened at the chalazal end of the seed, and were released in one piece. In atsbt1.7, the total rhamnose and galacturonic acid contents, representing the backbone of mucilage, remained unchanged compared with wild-type seeds. Thus, extrusion and solubility, but not the initial deposition of mucilage, are affected in atsbt1.7 mutants. AtSBT1.7 is localized in the developing seed coat, indicating a role in testa development or maturation. The altered mode of rupture of the outer seed coat wall and mucilage release indicate that AtSBT1.7 triggers the accumulation, and/or activation, of cell wall modifying enzymes necessary either for the loosening of the outer primary cell wall, or to facilitate swelling of the mucilage, as indicated by elevated pectin methylesterase activity in developing atsbt1.7 mutant seeds.     

Local Evolution of Seed Flotation in Arabidopsis

Arabidopsis seeds rapidly release hydrophilic polysaccharides from the seed coat on imbibition. These form a heavy mucilage layer around the seed that makes it sink in water. Fourteen natural Arabidopsis variants from central Asia and Scandinavia were identified with seeds that have modified mucilage release and float. Four of these have a novel mucilage phenotype with almost none of the released mucilage adhering to the seed and the absence of cellulose microfibrils. Mucilage release was modified in the variants by ten independent causal mutations in four different loci. Seven distinct mutations affected one locus, coding the MUM2 β-D-galactosidase, and represent a striking example of allelic heterogeneity. The modification of mucilage release has thus evolved a number of times independently in two restricted geographical zones. All the natural mutants identified still accumulated mucilage polysaccharides in seed coat epidermal cells. Using nuclear magnetic resonance (NMR) relaxometry their production and retention was shown to reduce water mobility into internal seed tissues during imbibition, which would help to maintain seed buoyancy. Surprisingly, despite released mucilage being an excellent hydrogel it did not increase the rate of water uptake by internal seed tissues and is more likely to play a role in retaining water around the seed.     

Toward understanding the ecological role of mucilage in seed germination of a desert shrub Henophyton deserti: interactive effects of temperature, salinity and osmotic stress

Aims

Seeds of Henophyton deserti (Brassicaceae), an endemic saharan shrub in south Tunisia, produce a pectinaceous mucilage layer that can imbibe a large amount of water when wetted. The aim of this study was to explore the role of mucilage in seed germination of this shrub under heterogeneous stressful environments.

Methods

Germination of both intact and demucilaged seeds was tested over wide ranges of temperature, and in iso-osmotic solutions of NaCl and PEG. Recovery of germination after NaCl and Polyethylene Glycol (PEG)-6000 treatment was also tested. The effect of mucilage on water uptake was measured and the structure of the seed investigated.

Results

A considerable proportion of seed mass (30 %) is made up of mucilage, which is extremely hydrophilic and able to increase seed mass by 550 % over dry seeds. Mucilage water uptake appears to be unaffected by salt concentration, while higher concentrations of PEG inhibit mucilage hydration. Mucilage decreases germination specifically at 10 °C and this effect can be interpreted in relation to oxygen uptake. High concentrations of NaCl and PEG decrease both germination percentage and rate, with some greater tolerance at 15 °C and 20 °C versus 25 °C. Recovery was higher from higher concentrations of NaCl and PEG and lower temperatures, with a clear inhibitory effect of mucilage.

Conclusions

The study has shown that the mucilage of H. deserti may act as a physical barrier for regulating diffusion of water and oxygen to the inner tissue of the seed and thereby prevent germination under unsuitable conditions.     

A mathematical model of mucilage expansion in myxospermous seeds of Capsella bursa-pastoris (shepherd's purse)

Background and Aims

Myxospermy is a term which describes the ability of a seed to produce mucilage upon hydration. The mucilage is mainly comprised of plant cell-wall polysaccharides which are deposited during development of those cells that comprise the seed coat (testa). Myxospermy is more prevalent among those plant species adapted to surviving on arid sandy soils, though its significance in determining the ecological fitness of plants is unclear. In this study, the first mathematical model of myxospermous seed mucilage expansion is presented based on seeds of the model plant species Capsella bursa-pastoris (shepherd''s purse).

Methods

The structures underpinning the expansion process were described using light, electron and time-lapse confocal micrographs. The data and experimental observations were used to create a mathematical model of myxospermous seed mucilage expansion based on diffusion equations.

Key Results

The mucilage expansion was rapid, taking 5 s, during which the cell mucilage volume increased 75-fold. At the level of the seed, this represented a 6-fold increase in seed volume and a 2·5-fold increase in seed surface area. These increases were shown to be a function of water uptake (16 g water g⁻¹ mucilage dry weight), and relaxation of the polymers which comprised the mucilage. In addition, the osmotic pressure of the seed mucilage, estimated by assessing the mucilage expansion of seeds hydrated in solutions of varying osmotic pressure, was –0·54 MPa (equivalent to 0·11 m or 6·6 g L⁻¹ NaCl).

Conclusions

The results showed that the mucilage may be characterized as hydrogel and seed-mucilage expansion may be modelled using the diffusion equation described. The potential of myxospermous seeds to affect the ecological services provided by soil is discussed briefly.     

Effects of mucilage on seed germination of the desert ephemeral plant Plantago minuta Pall. under osmotic stress and cycles of wet and dry conditions

To test the role of the seed mucilage of Plantago minuta Pall. in regulating germination under osmotic stress and cycles of hydration and dehydration, two experiments were carried out using seeds with intact mucilage and mucilage‐free seeds. In Experiment 1 seeds were immersed in a range of iso‐osmotic polyethylene glycol solutions (?1.15 to 0 MPa) for 14 days; any ungerminated seeds were transferred to deionized water to investigate the recovery germination. In Experiment 2 seeds were immersed in deionized water for 24 h, and were then incubated on filter paper for an additional 13 days to ensure complete desiccation before reimbibition to test the germination recovery percentage. Under mild osmotic stress (?0.73 to 0 MPa), the intact seeds with mucilage were shown to have higher germination rates than the mucilage‐free seeds, indicating that the mucilage led to a “fast sprouting” germination strategy under mild osmotic stress. However, when seeds were exposed to high osmotic stress (?1.15 MPa), the mucilage apparently slowed the germination rate, resulting in a “risk‐balancing” germination strategy. Extreme drought induced by polyethylene glycol solution and the desiccation pretreatment accelerated germination rates compared to non‐pretreated seeds; both germination potential and recovery percentage of the mucilage seeds were significantly higher than that of the mucilage‐free seeds. Our results revealed that the seed mucilage of P. minuta plays a crucial role in regulating seed germination rates and the germination strategies adopted by controlling seed water absorption when the seeds experience different osmotic stresses or alternating wet and dry conditions.     

More than just a coating: Ecological importance,taxonomic occurrence and phylogenetic relationships of seed coat mucilage

Studies on the ecological importance of seed coat mucilage have provided valuable information about its roles in critical stages of the plant life cycle. Seed mucilage may, by providing a moist environment and maintaining metabolic activity in the seed, promote seed development. In seed dispersal, seed mucilage influences topochory, epizoochory, endozoochory and hydrochory by anchorage of seeds to soil surface, lubrication or changing the specific weight of the seed. In arid environments, seed mucilage can prevent seeds from drying or initiate DNA repair mechanisms, thereby maintaining the soil seed bank. Seed mucilage reduces oxygen diffusion to the seed and thus has a role in regulating seed dormancy. Due to it being hydrophilous, acting as a physical barrier and containing chemicals, seed mucilage is proposed to promote seed germination in favorable environments. In seedling growth, seed mucilage may lubricate the radicle as it penetrates the soil and be degraded by soil microfloras and thus promote seedling growth. Further investigation of seed mucilage for more species in diverse habitats from the perspectives of evolution, genetics, proteomics, phylogeny and plant–microbe interactions would contribute substantially to our understanding about its ecological importance.     

新疆短命植物抱茎独行菜种子粘液质特性的研究

以新疆荒漠植物抱茎独行菜为材料,运用光镜与扫描电镜观察以及紫外吸收光谱法、化学反应及种子萌发实验等方法,对粘液质的形态和结构,物理化学特性,粘液质对种子萌发及萌发后的影响进行了研究.结果显示:(1)完整干种子表面覆盖着一层膜状物质(完全脱水的粘液质),并呈同一走向的山脊状突出的网状结构,遇水后粘液物质呈射线状向外发射出来,化学反应实验结果表明,粘液质的组成可能是某种多糖,如β-葡聚糖.(2)粘液质约占干种子重量的1/4,有很强的吸水能力,完全浸润10 min后,种子重量增加约30~40倍,种子长度、宽度、厚度的增加分别多于1倍、2倍、4倍;完全润湿的种子能够粘附相当于其干种子重量68倍的沙粒.(3)种皮粘液质对于不同土壤基质中的种子萌发有重要作用,但是对萌发后幼苗的生长没有作用.     

The FEI2-SOS5 pathway and CELLULOSE SYNTHASE 5 are required for cellulose biosynthesis in the Arabidopsis seed coat and affect pectin mucilage structure

A common adaptation in angiosperms is the deposition of hydrophilic mucilage into the apoplast of seed coat epidermal cells during the course of their differentiation. Upon imbibition, seed mucilage, composed mainly of carbohydrates (i.e. pectins, hemicelluloses and glycans) expands rapidly, encapsulating the seed and aiding in seed dispersal and germination. The FEI1/FEI2 receptor-like kinases and the SOS5 extracellular GPI-anchored protein were previously shown to act on a pathway regulating cellulose biosynthesis during Arabidopsis root elongation. In the highlighted study, we demonstrated that FEI2 and SOS5 regulate the production of the cellulosic rays deposited across the inner adherent-layer of seed mucilage. Mutations in either fei2 or sos5 disrupted the formation of rays, which was associated with an increase in the soluble, outer layer of pectin mucilage and accompanied by a reduction in the inner adherent-layer. Mutations in CELLULOSE SYNTHASE 5 also led to reduced rays and mal-partitioning of the pectic component of seed mucilage, further establishing a structural role for cellulose in seed mucilage. Here, we show that FEI2 expressed from a CaMV 35S promoter complemented both root and seed mucilage defects of the fei1 fei2 double mutant. In contrast, expression of FEI1 from a 35S promoter complemented the root, but not the seed phenotype of the fei1 fei2 double mutant, suggesting that unlike in the root, FEI2 plays a unique and non-redundant role in the regulation of cellulose synthesis in seed mucilage. Altogether, these data suggest a novel role for cellulose in anchoring the pectic component of seed mucilage to the seed surface and indicate that the FEI2 protein has a function distinct from that of FEI1, despite the high sequence similarity of these RLKs.     

Irregular xylem 7 (IRX7) is required for anchoring seed coat mucilage in Arabidopsis

Large quantities of mucilage are synthesized in seed coat epidermis cells during seed coat differentiation. This process is an ideal model system for the study of plant cell wall biosynthesis and modifications. In this study, we show that mutation in Irregular Xylem 7 (IRX7) results in a defect in mucilage adherence due to reduced xylan biosynthesis. IRX7 was expressed in the seeds from 4 days post-anthesis (DPA) to 13 DPA, with the peak of expression at 13 DPA. The seed coat epidermis cells of irx7 displayed no aberrant morphology during differentiation, and these cells synthesized and deposited the same amount of mucilage as did wild type (WT) cells. However, the distribution of the water-soluble vs. adherent mucilage layers was significantly altered in irx7 compared to the WT. Both the amount of xylose and the extent of glycosyl linkages of xylan was dramatically decreased in irx7 water-soluble and adherent mucilage compared to the WT. The polymeric structure of water-soluble mucilage was altered in irx7, with a total loss of the higher molecular weight polymer components present in the WT. Correspondingly, whole-seed immunolabeling assays and dot-immunoassays of extracted mucilage indicated dramatic changes in rhamnogalacturonan I (RG I) and xylan epitopes in irx7 mucilage. Furthermore, the crystalline cellulose content was significantly reduced in irx7 mucilage. Taken together, these results indicate that xylan synthesized by IRX7 plays an essential role in maintaining the adhesive property of seed coat mucilage, and its structural role is potentially implemented through its interaction with cellulose.     

Boron-bridged RG-II and calcium are required to maintain the pectin network of the <Emphasis Type="Italic">Arabidopsis</Emphasis> seed mucilage ultrastructure

The structure of a pectin network requires both calcium (Ca²⁺) and boron (B). Ca²⁺ is involved in crosslinking pectic polysaccharides and arbitrarily induces the formation of an “egg-box” structure among pectin molecules, while B crosslinks rhamnogalacturonan II (RG-II) side chain A apiosyl residues in primary cell walls to generate a borate-dimeric-rhamnogalacturonan II (dRG-II-B) complex through a boron-bridge bond, leading to the formation of a pectin network. Based on recent studies of dRG-II-B structures, a hypothesis has been proposed suggesting that Ca²⁺is a common component of the dRG-II-B complex. However, no in vivo evidence has addressed whether B affects the stability of Ca²⁺ crosslinks. Here, we investigated the L-fucose-deficient dwarf mutant mur1, which was previously shown to require exogenous B treatment for phenotypic reversion. Imbibed Arabidopsis thaliana seeds release hydrated polysaccharides to form a halo of seed mucilage covering the seed surface, which consists of a water-soluble outer layer and an adherent inner layer. Our study of mur1 seed mucilage has revealed that the pectin in the outer layer of mucilage was relocated to the inner layer. Nevertheless, the mur1 inner mucilage was more vulnerable to rough shaking or ethylene diamine tetraacetic acid (EDTA) extraction than that of the wild type. Immunolabeling analysis suggested that dRG-II-B was severely decreased in mur1 inner mucilage. Moreover, non-methylesterified homogalacturonan (HG) exhibited obvious reassembly in the mur1 inner layer compared with the wild type, which may imply a possible connection between dRG-II-B deficiency and pectin network transformation in the seed mucilage. As expected, the concentration of B in the mur1 inner mucilage was reduced, whereas the distribution and concentration of Ca²⁺in the inner mucilage increased significantly, which could be the reason why pectin relocates from the outer mucilage to the inner mucilage. Consequently, the disruption of B bridges appears to result in the extreme sensitivity of the mur1 mucilage pectin complex to EDTA extraction, despite the reinforcement of the pectin network by excessive Ca²⁺. Therefore, we propose a hypothesis that B, in the form of dRG-II-B, works together with Ca²⁺to maintain pectin network crosslinks and ultimately the mucilage ultrastructure in seed mucilage. This work may serve to complement our current understanding of mucilage configuration.     

Effects of storage,mucilage presence,photoperiod, thermoperiod and salinity on germination of Farsetia aegyptia Turra (Brassicaceae) seeds: implications for restoration and seed banks in Arabian Desert

Seed viability and germination are key factors in the success of restoration efforts, especially when stored seeds are used. However, the effect of seed storage on germination of most of the native Arabian species is not well documented. We investigated the effect of storage time and role of the seed mucilage in regulating germination, dormancy, salinity tolerance and consequential survival strategy of F. aegyptia in an unpredictable arid desert setting. Effect of light and temperature during germination was studied under two photoperiods and two thermoperiods using intact and de-mucilaged seeds. Presence of mucilage and thermoperiod did not affect the germination. However, seed collection year and photoperiod had a highly significant effect on the germination. Increasing salinity levels decreased the germination of F. aegyptia but ungerminated seeds were able to germinate when salinity stress was alleviated. Seed storage at room temperature enhances the germination percentage, indicating that F. aegyptia seeds have physiological dormancy and it can be alleviated by after-ripening at dry storage. In addition, F. aegyptia seeds show ability to germinate at lower salinity concentration and remain viable even at higher saline conditions, indicating their adaptability to cope with such harsh environmental conditions.     

Isolation and Characterization of Mutants Defective in Seed Coat Mucilage Secretory Cell Development in Arabidopsis

In Arabidopsis, fertilization induces the epidermal cells of the outer ovule integument to differentiate into a specialized seed coat cell type producing extracellular pectinaceous mucilage and a volcano-shaped secondary cell wall. Differentiation involves a regulated series of cytological events including growth, cytoplasmic rearrangement, mucilage synthesis, and secondary cell wall production. We have tested the potential of Arabidopsis seed coat epidermal cells as a model system for the genetic analysis of these processes. A screen for mutants defective in seed mucilage identified five novel genes (MUCILAGE-MODIFIED [MUM]1–5). The seed coat development of these mutants, and that of three previously identified ones (TRANSPARENT TESTA GLABRA1, GLABRA2, and APETALA2) were characterized. Our results show that the genes identified define several events in seed coat differentiation. Although APETALA2 is needed for differentiation of both outer layers of the seed coat, TRANSPARENT TESTA GLABRA1, GLABRA2, and MUM4 are required for complete mucilage synthesis and cytoplasmic rearrangement. MUM3 and MUM5 may be involved in the regulation of mucilage composition, whereas MUM1 and MUM2 appear to play novel roles in post-synthesis cell wall modifications necessary for mucilage extrusion.     

Polyploidy and polyembryony in Anemopaegma (Bignonieae, Bignoniaceae)

Polyploidy is a key process in plant evolution, with the asexual formation of embryos representing a way through which polyploids can escape sterility. The association between polyploidy and polyembryony is known to occur in Bignoniaceae. In this study, we investigate polyembryony in four polyploid species of Anemopaegma: A. acutifolium, A. arvense, A. glaucum and A. scabriusculum as well as in one diploid species, A. album. Polyembryony was observed only in polyploid species. We used seed dissection and germination tests to compare the number of polyembryonic seeds. We tested how the pollen source influences the number of polyembryonic seeds and the number of embryos per seed and tested the correlation between the number of viable seeds per fruit and mean number of embryos per seed. The number of polyembryonic seeds observed by seed dissection was higher than the number of polyembryonic seeds determined by the germination test, with the number of embryos produced per seed being higher than the number of seedlings. The dissection of seeds of A. glaucum indicated that a higher number of polyembryonic seeds and a higher number of embryos were present in seeds from cross-pollination than in seeds from self-pollination. On the other hand, germination tests indicated that a higher number of polyembryonic seeds were present in fruits from self-pollination than from cross-pollination. The mean number of embryos per seed was not influenced by the number of viable seeds per fruit in fruits from open pollination. These results indicate a positive relationship between polyembryony and polyploidy in Anemopaegma.