The pollen tube: a soft shell with a hard core
Hannes VoglerChristian DraegerAlain WeberDimitris FelekisChristof EichenbergerAnne‐Lise Routier‐KierzkowskaAurélien Boisson‐DernierChristoph RingliBradley J. NelsonRichard S. SmithUeli Grossniklaus
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Abstract:
Summary Plant cell expansion is controlled by a fine‐tuned balance between intracellular turgor pressure, cell wall loosening and cell wall biosynthesis. To understand these processes, it is important to gain in‐depth knowledge of cell wall mechanics. Pollen tubes are tip‐growing cells that provide an ideal system to study mechanical properties at the single cell level. With the available approaches it was not easy to measure important mechanical parameters of pollen tubes, such as the elasticity of the cell wall. We used a cellular force microscope ( CFM ) to measure the apparent stiffness of lily pollen tubes. In combination with a mechanical model based on the finite element method ( FEM ), this allowed us to calculate turgor pressure and cell wall elasticity, which we found to be around 0.3 MP a and 20–90 MP a, respectively. Furthermore, and in contrast to previous reports, we showed that the difference in stiffness between the pollen tube tip and the shank can be explained solely by the geometry of the pollen tube. CFM , in combination with an FEM ‐based model, provides a powerful method to evaluate important mechanical parameters of single, growing cells. Our findings indicate that the cell wall of growing pollen tubes has mechanical properties similar to rubber. This suggests that a fully turgid pollen tube is a relatively stiff, yet flexible cell that can react very quickly to obstacles or attractants by adjusting the direction of growth on its way through the female transmitting tissue.Keywords:
Turgor pressure
Elasticity
The behavior of pollen and pollen-tube between 1-8 hours after pollination was reported in the previous pa.per (TATEBE 1950). The present paper describes the behavior between 10-30 minutes after pollination. Results obtained may be summarized as follows : 1. Pollen grains on stigmas in all mating combinations germinated well (table . 1.). However, the total number of pollen grains on stigmas were few in some of the long-styled flowers and also in short-styled flowers generally. It may be due to the reason that most of the pollen grains which failed to germinate had been washed off in the treatment of fixing and staining. Consequently, it seems very likely that in some cases the actual percentage of pollen germination was somewhat 10wer than that of the data. The percentage of empty grains which had exhausted their contents into their tubes was lower in short-styled flowers, and especially in the illegitimate unions. Various experiments of pollen germination on artificial media were tried without success.
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Six wheat genotypes having different cross ability genes were crossed with two rye genotypes to study the pollen germination, pollen tube growth and seed set. The trend of pollen fall did not show much differences between selfings and crosses, but it seemed to have a direct correlation with seed set. The pollen germination was more in crosses than the selfings except in a few crosses. The pollen tube length had a direct correlation with seed set and the pollen tubes were quite long in the selfings. Among the crosses with rye, Chinese Spring had longer pollen tubes with better seed set. The rate of tube growth was faster at 4 hours after pollination in most cases. The frequencies of pollen tube abnormalities were more in the crosses than the selfings. Pollen fall and pollen tube growth had a highly significant positive association with seed set, while the pollen tube abnormalities had a highly significant negative relation with it
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We recently revealed that cyclic nucleotide-gated channel 18 (CNGC18) functioned as the main Ca2+ channel in pollen tube tips for pollen tube guidance to ovules by regulating external Ca2+ influx in Arabidopsis. In this study, we found that the reduction of external Ca2+ concentration ([Ca2+]ext) from 10 mM to 5 mM, and further to 2 mM, led to the decreases of pollen germination percentages, but led to the increases of the percentages of ruptured pollen grains and tubes, and branched pollen tubes in vitro in cngc18-17 compared with wild type. The second point mutant allele cngc18-22 showed similar phenotypes, including reduced pollen germination percentages, increased percentages of ruptured pollen tubes, but did not show obvious different percentages of ruptured pollen grains and branched pollen tubes compared with wild type. These data demonstrate that CNGC18 plays essential roles in pollen germination and tube growth as a Ca2+ channel in Arabidopsis.
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Why do plants have cell walls? This could be a typical exam question. One of the correct answers is that walls allow cells to develop turgor pressure, which keeps the plant from wilting. And if a plant experiences drought and turgor pressure is lost, the cell walls help to maintain structural integrity. Although cell walls are essential for a plant, they also pose challenges. The cell wall consists of two layers: the primary cell wall that surrounds the growing cells and the secondary cell wall that is developed after the cell stops expanding. The secondary cell wall is more rigid, while the primary cell wall is thinner and more stretchable, but still tough. The plant cell wall contains many enzymes that constantly remodel the polysaccharides that make up the wall. Transglycosylases are one class of such cell wall-remodeling enzymes. Transglycosylases participate in ‘cutting and pasting’ of sugar residues: they cleave off part of the backbone of a polysaccharide (donor) and graft it onto another (acceptor) (Franková and Fry, 2013). These molecular rearrangements turn the cell wall in a flexible compartment that allows the cell to grow. The cell wall and its remodeling enzymes have been well studied in land plants. It is thought that the cell wall underwent substantial changes during colonization of the land plants, roughly 470–500 million years ago. This is because a terrestrial lifestyle has different demands than an aquatic one. Underwater, plants are supported by their buoyancy, whereas on land, they have to stand up against gravity and resist the wind. To understand how the cell wall changed during terrestrialization, some researchers study charophyte algae, the closest extant relatives of the algal lineage from which land plants evolved. The cell walls of charophytes exhibit major chemical differences from their land plant counterparts. For example, xyloglucan is present in the cell walls of all land plants, whereas most charophytes contain only low levels or completely lack this polysaccharide (Mikkelsen et al., 2021; Popper and Fry, 2003). Little was known about the transglycosylase enzymes that remodel the cell walls of charophytes. In this issue, Franková and Fry (2021) investigated transglycosylases in charophytes to get insight into the enzymatic cell wall machinery that allowed plants to colonize the land. To do this, they used a collection of algal cultures with members of the Coleochaetales, Klebsormidiales, Zygnematales, Chlorokybales, and Charales. From each algal culture they extracted cell wall enzymes, and then they applied the enzyme extracts to a mixture of polymers that can serve as donor substrates, mixed with radioactively labeled polymers that can serve as acceptor substrates. When donor segments get grafted to the acceptor substrates, new radioactive polysaccharides are formed that often have a different molecular weight than the supplied donor substrate, making the product easily distinguishable (Figure). The authors found that transglycosylase activity in charophyte cell walls is different from that in land plants; most notably charophytes possess a particularly high level of trans-β-1,4-mannanase activity, a transglycosylase that acts on mannan substrates. Charophytes contain abundant β-mannans in their cell walls, and this finding suggests that charophytes might prioritize mannan remodeling. By contrast, land plants mainly remodel the hemicellulose xyloglucan, which is carried out by transglucanases. However, the most striking finding was that the charophyte enzyme extracts also possessed trans-β-glucanase activity. Except for a few species, charophytes lack xyloglucan in their cell walls. Therefore, the authors decided to test if they could also find glucanase activity in situ. They fed fluorescently labeled oligoglucans and found that they were incorporated into the cell walls of most of the tested algal cultures (Figure), indicating the presence of xyloglucan transglycanase activity. Fry and Franková are not sure why charophyte algae possess glucanase activity; possibly charophytes might have an uncharacterized β-glucan-based polysaccharide that serves as a substrate. Alternatively, the observed enzymatic activity might not be specialized for glucan-based substrates. Whatever the answer, the finding of glucanase activity in charophyte algae suggests that enzymes with glucanase activity were present in the ancient charophytes from which land plants evolved. Hence, although the polysaccharide composition of the cell wall changed fundamentally during the transition to land, the cell wall remodeling machinery might have remained conserved. In the future, Fry and his group want to identify mannan and xylan substrates on which transglycosylases act in the cell walls of charophytes. Moreover, they would like to know if these substrates were highly conserved in extinct charophytes. Fry notes that although the main sugar composition of algal hemicelluloses has been studied (O’Rourke et al., 2015), the linkages and sugar residue sequences in the polysaccharide backbones and/or their side chains are still unknown.
Turgor pressure
Secondary cell wall
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It is proposed that long distance transport of solutes is controlled by regulation of turgor. Turgor constancy is maintained by import or export of solutes and water from or to the apoplast. This will cause changes in the apoplast pressure which will unbalance waterpotential equilibrium between the apoplast and other cells. Turgor changes are thus caused in these cells which will consequently import or export solutes and water to readjust their turgor.
Turgor pressure
Symplast
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SummaryPressure-volume methodology was used to evaluate the components of plant water potential in expanding leaves, mature leaves and roots of well watered and water stressed plants of Prunus avium x pseudocerasus 'Colt'. Under well watered conditions, expanding leaves, mature leaves and roots lost turgor at water potentials of -1.37, -1.84, and -1.12 MPa, respectively. Comparable tissue from water stressed plants maintained turgor to^water potentials of —2.08, —2.09, and —1.85 MPa. This improved capacity for maintaining turgor resulted primarily from changes in tissue osmotic potential and was not due to changes in tissue elasticity. Regardless of the imposed water regime, roots lost less turgor for a given change in tissue water potential than did leaves. As compared with leaves, the more elastic root tissue compensated for higher tissue osmotic potentials at full turgor by allowing for a greater reduction in relative water content and concentration of tissue solutes as water potentials decreased, thereby reducing the rate at which turgor was lost.
Turgor pressure
Water Stress
Osmotic pressure
Plant tissue
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This study was carried out to determine the effects of heavy metals (Ni, Fe, Pb, Co, Cd, Hg, Al, Zn and Cu) on pollen germination and pollen tube length in the tobacco plant (Nicotiana tabacum L.) cv. Karabalar. The results showed that enhanced concentrations of heavy metals, except Fe, decreased the pollen germination rates and the pollen tube lengths. With Fe concentrations, on the other hand, first a positive, and then a negative relation was determined between the pollen characteristics examined. The most toxic effect on pollen germination was seen with the applications of Cu, Ni and Hg; on pollen tube length, on the other hand, a similar tendency was determined with the applications of Hg, Cd and Ni. The toxic effects of Co, Al and Fe were found to be low on both of the pollen characteristics. As a result, all the heavy metals examined prevented pollen germination and tube growth in the tobacco plant, but their toxicity levels varied.
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After selfpollination of <em>Sinapis alba</em> L. pollen tubes growth is inhibited on the stigma. The pollen grains germinate 3-4 hours after pollination. The pollen give rise to one or more pollen tubes. They grow along the papillae. In the place of contact between the papilla and pollen tube the pellicula is digested. Then the direction of pollen tube growth changes completely. Pollen tubes grow back on the exine of their own pollen grain, or turn into the air. The pollen tubes growth was inhibited in 6-8 hours after selfpollination. After crosspollination usually there is no incompatibility reaction.
Stigma
Sinapis
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Objective:Different concentration growth regulating substances of 6-BA,NAA,2,4-D and PP 333 on pollen germination and pollen tube growth were studied with pyrus pyrifoliaWhangkeumbae.Method:Pollen cultivating liquid medium were confected,with different mass concentrations of 6-BA,NAA,2,4-D and PP 333.To research the effects of different concentrations plant growth regulating substances on pollen germination and pollen tube growth.Result:Results showed that the low concentration of 6-BA and 2,4-D stimulated the pollen germination and pollen tube growth,but when the concentration up to certain value,pollen growth and pollen tube growth were inhibited.NAA and PP333 had inhibition on pollen germination and pollen tube growth.Conclusion:10 mg /L 6-BA and 5 mg /L 2,4-D stimulated the pollen germination and pollen tube growth.
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