What makes pollen tubes grow

Even with such diversity, all PTs share a common purpose, that is, to safely deliver sperm cells to the female gametophyte for double fertilization. Sperm cells remain as a passive cargo during its whole journey Zhang et al. Moreover, mechanics of its elongation is poorly understood. In this review, we discuss several persisting and unfolding issues regarding the nature of PT and mechanics of its response at its growth environment based on the findings in relevant fields.

In vascular plants among all eukaryotes , pollen and PT are the only such structures that harbor cells generative cell and sperm cells, respectively within a cell vegetative cell when the mitochondria and plastids are taken as cell components as they are.

Such structures include hyphae in fungi, rhizoids in algae and several spore—producing plants moss and fern , protonemata in spore producing plants, root hairs in vascular plants; and neuronal axons in animals. All tip—growing structures exhibit tip—focused active metabolism and oscillatory polar growth Albus et al. One major difference between PTs and protonemata and other tip-growing structures is that unlike others wherein the cytoplasm retracts and is salvaged by the host tissue unless the cytoplasm is too damaged during programmed cell death Logi et al.

An elongating angiosperm PT is a one—way flow, which appears more like a moving cell than a growing tissue as the back—flow of its content is blocked by the callose deposition at regular interval Figure 1. Figure 1. Arabidopsis root hairs and pollen tubes PTs. Root hairs and PTs are morphologically very similar A,B , respectively. However, cytoplasm of a growing, single celled root hair always remains connected to its mother cell A while elongating PTs have callose deposited at regular interval thereby disconnecting their front region from the spent growth B—D.

A aniline blue treated PT shows its inner callose-layer of its cell wall except in its extreme tip C. The red-demarcated region in C has been magnified in D. Arrowheads point to the apex of root hairs or PTs and arrows point to the callose plugs in PTs.

While discussing the function of actin earlier, Steer indicated that the PT tip retains many features of the primitive amoeboid motion. Later observation of the PT tip elongation, even in the absence of its spent growth and host pollen in lily, further strengthened such postulation Jauh and Lord, Structurally, the PT can be divided into four distinct zones, namely, apex, shoulder, sub—apex, and shank Figure 2A.

Multiple direct and indirect observational studies have shown that a typical elongating—PT comprises of actin bundles at its cortex barbed—end facing PT—apex and center pointed—end facing PT—apex in its shank region up to the subapical zone Figures 2B,D. Its apex is populated with vesicles in a conical shape, which is encircled with the organellar population including vesicles, which itself is encircled by an actin fringe Figures 2B,D,E Bove et al.

The organelles move forward at the cortex of the PT—shank and rearward at its center, forming a reverse fountain—like appearance Figure 2C , although larger organelles reverse their path near sub—apical zone, mitochondria and dictyosomes and rough endoplasmic reticulum reach up to the base of PT—tip dome.

Additionally PT comprises a large vacuole at its distal end, in front of which reside two sperm cells linked to the vegetative nucleus Figure 2E Derksen et al. Figure 2.

Schematic representation of the pollen tube PT front region. A PT with its tentative four zones. C PT with its cell wall cut opened to show the cytoplasmic stream inside which flows toward apex at the cortex and reverses its direction at the subapical zone and flows back at its core giving a reverse fountain-like appearance.

It comprises callose plug distally formation of which depend on microtubule-assisted incorporation of CalS at the site close and distal to the large vacuole. Two sperm cells linked to the VN move in front of the large vacuole. Cortex comprises of actin bundles with its barbed end oriented toward PT-apex while that of the actin at the core is oriented toward large vacuole. Actomyosin-assisted organellar flow occurs at this region in flow motion illustrated at C. The sub-apical region harbors shorter F-actin fringe and organellar population of mitochondria, dictyosomes, rough endoplasmic reticulum RER , vesicles etc.

The positions of cytosolic components are not to the true scale. Of the two cell wall layers, the outer pectin layer remains esterified at the PT—tip which gets de—esterified distally starting from the sub—apical zone Figure 2E. Callose is crucial for cushioning the elongating PT against tensile and compression stresses Parre and Geitmann, Unlike in PT, the primary cell wall of root hairs comprises randomly oriented cellulose microfibrils soft at its tip, and the tough secondary cell wall comprising cellulose fibrils in a parallel orientation to the growth axis at the shank Hirano et al.

Studies have shown that the cellulose synthase complex harboring multiple cellulose synthase catalytic subunits, callose synthase complex, and sucrose synthase are carried by the dictyosomes along the actin bundles up to the sub—apical zone, which moves along the actin filaments of the actin fringe, and is later incorporated into the plasma membrane near the tip Cai et al. According to the widely accepted model proposed by Verma and Hong , UGT1 acts as a subunit of callose synthase complex, and interacts with the membrane bound phragmoplastin Phr , a cell plate—associated protein.

After the membrane bound sucrose synthase synthesizes UDP-glucose, the ROP1-activated callose synthase complex converts it to callose and releases across the plasma membrane Figure 3. A review by Dehors et al. Figure 3. Putative callose synthase complex model with known PT-specific and other components. UGT1 interacts with CalS5 and act as its subunit. Membrane bound Phr interacts with UGT1 as well. A unique property of PT is its callose plug at regular intervals, which blocks the cytoplasmic connection to the distal PT region, with the newest plug close to the large vacuole Figures 1C,D , 2E.

This is the reason why PT remains one of the fastest growing structure in plants 0. The plug prevents the backflow of the PT contents thereby maintaining its turgor pressure and integrity Li et al.

Callose plugs are evolutionarily developed only in angiosperms as the PTs of gymnosperms lack such depositions Williams, The interval and the topmost callose plugging sites may vary among different species and even among different ecotypes of the same species Laitiainen et al. As observed in tobacco, the continuous movement of the vegetative nucleus and generative cells toward the tip in the elongating PT is affected when treated with oryzalin, a microtubule polymerization—inhibiting dinitroaniline herbicide with high affinity to plant tubulin monomers Morejohn et al.

The intracellular location of microtubules is reported to be a determinant factor for the site of plug formation in PTs. They are known to incorporate the callose synthase complex in the plasma membrane around the site of plug formation distal to the large vacuole, as observed in tobacco PT Cai et al.

To date, what triggers the machinery to initiate plug formation at a certain interval is unclear. Observing the nature of PT elongation, one possible factor would be the internal turgor pressure, provided that it oscillates with the PT elongation. However, studies show that turgor pressure is not correlated with the elongation rate of the PT, although the pressure may slightly vary within it Benkert et al. Furthermore, changes in PT length per oscillatory cycle of its elongation is much shorter than the distance between two callose plugs Benkert et al.

A related study has further reported constant turgor pressure in growing lily PTs Hill et al. As previously mentioned, PTs have a relatively low cellulose content, which is recycled within the short front region Mogami et al. The recycling involves new membrane incorporation to and membrane retrieval from the plasma membrane, mainly via exocytosis and endocytosis respectively.

A strongly held belief for PT membrane recycling being debated lately assumes that endocytosis is largely restricted at the sub—apical region, whereas exocytosis is restricted at the apex. However, for a fast—elongating structure, concentrating most of its exocytotic activities at the apex that would give drag to its elongation by spewing vesicular contents out and endocytotic activities at the sub—apical region seemed like an ill—designed natural structure.

In a tobacco PT study, Zonia and Munnik used total internal reflection fluorescence microscopy TIRFM which selectively illuminates and excites the fluorophores of the specimen via evanescent waves hence it is also called evanescent wave microscopy Axelrod, They tracked the newly endocytosed vesicles after treating the elongating tobacco PTs with a non-toxic lipophilic FM dye FM 1—43, green for 2 h followed by the treatment with another FM dye with a different emission wavelength FM 4—64, red and observation at 21 s intervals for up to 20 min.

They observed that the mixed fluorescence was consistently detected in the apical vesicle—population, whereas full membrane distention was observed only on the sub-apical zone. From such observations, the authors argued that PT—apex is the exclusive site for endocytosis and membrane retrieval, whereas exocytosis is restricted in the zone adjacent to the apical dome.

Furthermore, authors suggested that the exocytosed membrane pushes toward both directions apically as well as distally thereby providing enough membrane for PT—shank elongation and apical membrane internalizing Zonia and Munnik, , Figure 4. Proposed model of membrane recycling in pollen tube PT. A A schematic representation of a pollen tube with its vesicle dynamics.

Arrows show the path of the vesicle movement. The scale at the left represents color code for relative membrane tension at PT plasma membrane and vesicles for A,D only. The vesicles arriving from the cytoplasmic stream at PT-cortex move toward apex. The dictyosomes and RER may bud-off smaller vesicles, which then move toward the apex along the flow.

The apical and sub-apical regions may serve for both exocytosis and endocytosis with their different modes at certain segment depending on the PT membrane tension. Conventional clathrin-mediated endocytosis CME B takes place at distal region in PT shank in which plasma membrane is actively retrieved Bove et al. Fully distended exocytosis C occurs at the subapical to lower shoulder region which increases PT membrane area and excess membrane is pushed toward the apex.

During the process, the vesicle never reaches to the plasma membrane but gets connected with a lipid nanotube, through which, lipid flows toward the vesicle thereby equilibrating the vesicle membrane tension with the PT plasma membrane Mellander et al. The budding vesicles from dictyosomes may enter the process all over again. At the extreme apex, smooth endocytosis E may occur through which, membrane is directly retrieved without CME while engulfing ECM during the process Bove et al.

ER, endoplasmic reticulum; di, dictyosomes; v, vesicle; PM, plasma membrane; nt, nanotube. An almost simultaneous study by Bove et al.

They observed the accumulation of vesicles arriving from cortical cytoplasmic stream at the shoulder of the apex, but found that the extreme apex never fluoresced with full intensity. Additionally, they proposed smooth endocytosis at the extreme apex Figures 4A,E Bove et al. The observations and postulations made by Zonia and Munnik , as well as Bove et al. In their earlier tobacco PT electron micrographs observational study, Derksen et al.

Interestingly, they observed that PTs encompass smaller vesicles in large population at their apex. However, the coated vesicles close to plasma membrane was relatively larger than those close to the dictyosomes Derksen et al.

Relatively recent study by Prado et al. Secretion of such membrane bound vesicle may account for some proportion of the disparity in membrane turnover observed earlier. However, for a fast elongating structure, like PT, removal of about one—third of its newly added membrane is still questionable.

Disparity in membrane turnover had also been reported for growing root hairs, which led early researchers to contemplate that the excess membrane materials may be actively destroyed by the cellular machineries Steer, However, the study by Ketelaar et al. Their observation indicated that the elongating PT and possibly the growing root hair as well is less likely to destroy or secrete its plasma membrane in significant proportion. Furthermore, the authors also suggested that the vesicles may undergo partial kiss-and-run or full distention with the plasma membrane, depending on the membrane requirement and internal vesicle population.

Although multicellular, the disparity in coleoptile cell wall expansion and vesicle quantity had led to the discovery of the kiss-and-run mode of exocytosis in maize earlier Weise et al. In their study, the authors measured membrane capacitance C m using patch—clamp techniques. Additional studies have shown that kiss-and-run mode can equally contribute to the endocytosis as well Kavalali, ; Wen et al. One recent study reported that pollen germination requires autophagy—mediated compartmental cytoplasmic deletion in tobacco Zhao et al.

Whether it acts as a part of membrane recycling machinery in elongating PT is yet unknown. Post—germination knockdown or knock-out studies of the responsible genes ATG2 , 5 , and 7 may clarify it in the future. The reviews by Grebnev et al. Discussions on the mechanisms of fully distended exocytosis and endocytosis is relatively more prevalent Onelli and Moscatelli, ; Grebnev et al.

An observational membrane fusion study on artificial cells protein—free liposome system earlier led to the confirmation of the two modes of exocytosis: first, full distention, wherein, smaller daughter vesicles were completely fused to the membrane of the mother vesicle harboring the former in the inside after the lipid nanotube formed between them extinguished leading the vesicle to grow larger under pressure; and second, the partial distention, wherein, the daughter vesicle never reached to the cell membrane, but formed a lipid nanotube between them which transiently enlarged allowing some contents of the vesicles to be released, followed by narrowing the nanotube again comparable to Figures 4A,D Mellander et al.

The latter form was similar to the kiss-and-run mode of exocytosis, but with extended time and pore opening. Furthermore, they observed that the size of the vesicles that underwent partial distention was relatively smaller than those that underwent full distention, and that the relative membrane tension between the daughter and mother vesicles plays a determining role on which mode of exocytosis would follow.

They proposed that during partial distention, lipid flows from the mother vesicle to the growing daughter vesicle when the latter has higher membrane tension, which stops at the equilibrium.

Studies in PT show that its tip has the lowest plasma membrane tension and stiffness which steeply increases toward the shank Figure 4A Hepler et al. Could the observation made by Derksen et al. It demands further evidences to support such assumption at the moment.

PT elongation is closely linked to the membrane recycling we discussed earlier. Its directional elongation relies on the internalization of external cues, triggering series of molecular chain reaction, that leads to cytoskeletal rearrangement and change in angle of vesicle—population positioning at the apex Bove et al.

In this section, we discuss on how PT translates its interaction with external surface to its elongation rate. A recent PT—elongation study by Reimann et al. However, not all cells show a positive response to the stiffer matrices in terms of their movement speed. Recent mathematical modeling for a single-cell migration on an elastic matrix suggests that durotactic motion is determined by the ratio of the stiffness gradient to the absolute stiffness of the growth matrix which is elastic and deformed under the force exerted by the migrating cell , and lifetime of the focal adhesions membrane site which adheres to the matrix depends on the force exerted by the migrating cell Malik et al.

While no such focal adhesions has been reported in PTs yet, their durotropic nature of elongation suggests for similar, if not same structure or phenomenon in them. Contradictory to the observation made by Reimann et al. Since most of the PT—related microscopic observations and analytic studies have been conducted on the in vitro cultured PTs, they may not exhibit true behavior of their in vivo counterparts.

The nutrient-rich extracellular matrix does not only provide a path for PT elongation but also actively supports and augments the process Lord, , ; Qin et al. The single—cell movement characteristics are not discussed that much in plants, since cells are often encapsulated in a hard cell wall. However, several observations on single cell movement in animal or microbe models are interestingly similar to the PT elongation.

During blebbing, membrane protrusion bleb is initiated either through the localized detachment of plasma membrane from the acto—myosin cortex or the local cortical rupture. Cellular turgor, without any supporting cortex under the membrane at the protrusion, leads the bleb to expand while further detaching away from the cortex at its base. After a new actin cortex is formed under the bleb membrane and myosin is recruited, the bleb retracts.

By repeating these steps, cells exhibit a blebbing movement Charras and Paluch, The model used in the study was very different from PT, and the oscillatory blebbing movement reported in E. In most angiosperms, the stigma—style interface is embedded with densely packed cells exposing a reduced secretory surface for growing PTs. Hence, they need to elongate invasively to pass through the interface Lora et al.

Interestingly, all of the tissues showed deformations while passing through the narrow gaps. PTs may undergo similar constraint at the stigma—style junction and narrow intracellular spaces at TT. The elongating PTs that are adjacent to one another, often share a common cell wall Lennon et al.

How such surroundings affect PT-elongation physically and their potential involvement in its durotactic nature are yet to be elucidated. During lamellipodial movement, the actin layer above the less dynamic actin bundles at the cortex gets polymerized at the front, thereby evoking its contraction until the actin network reaches the adhesion site where myosin II—cluster is formed, which plays a key role in bending the upper dynamic actin layer, leading to edge retraction and initiation of a new adhesion site Giannone et al.

Myosin II is known to play role in the protrusion and contraction of cell and its attachment to the matrix or other cells during its movement in non-plant models Sayyad et al. Additionally, the highly dynamic actin filaments in PTs are localized at the sub—apical region, whereas its shank comprises actin bundles in an opposite orientation pointed to barbed end at the cortex and barbed to pointed end at the center Figure 2D Lenartowska and Michalska, Furthermore, CAP1, a protein known to play a role in cell adhesion Zhang et al.

Whether it plays role in PT—adhesion as reported in animal models Zhang et al. Furthermore, the focal adhesion dynamics in the spreading cells is reportedly regulated by integrin ligands in animal models. A study showed that the cells plated on media nano-patterned with RGD argenine—glycine—aspartate nanoparticles at longer intervals nm spread slower as compared to their counterparts cultured at plate nanopatterned with RGD at shorter interval 58— nm indicating the crucial importance of matrix, and available integrin binding sites for their migration Cavalcanti-Adam et al.

Could the reduced elongation observed in the study be due to the lack of available active domains of integrin-like proteins which would otherwise interact with the stigma and TT-specific ligands to adhere? It is probable that the durotropic response of the hPTs observed by Reimann et al. However, it may require further evidence to confirm such postulations.

Reports show that blebbing requires relatively lesser energy as compared to the lamellipodial movement Charras and Paluch, ; Bovellan et al. Moreover, blebbing led movement is relatively faster than the lamellipodial movement Charras and Paluch, ; Ikenouchi and Aoki, Such phenomenon of cellular movement appear to be a close parallel to the faster rate of sPTs elongation with lesser energy as compared to hPTs observed by Reimann et al.

Moreover, PTs have inner layer of callose in their cell wall which cushions the forward flowing cytoplasmic contents by resisting the compression stress Parre and Geitmann, It is possible that sPTs favor blebbing—like movement while hPTs favor lamellipodial movement. Future experimental evidences may shed more light on the aspect.

Animal-derived single cells often transit from one mode of movement to another during their migration Bergert et al.

Not all cells exhibiting lamellipodial or blebbing movement would necessarily show increased movement at stiffer matrices as the movement patterns largely depend on cellular machinery organization at the focal adhesion and the matrix stiffness Malik et al. Plants have mechanosensing mechanism that is not explored as much as that in their animal counterparts Hamant and Haswell, One crucial plant cytoskeletal component responsive to mechanical stresses is the microtubule.

It is known to direct cellulose microfibril deposition at the site of the plasma membrane under maximal stress by regulating the membrane incorporation of cellulose synthase complexes Williamson, ; Kesten et al. As previously mentioned, the tip of the elongating PT has the lowest membrane tension that sharply increases toward the shank Hepler et al.

Furthermore, the apical methyl—esterified PT—wall starts de-esterification at the sub—apical zone, and microtubules—assisted incorporation of callose synthase complex, sucrose synthase, and cellulose synthase complex occurs at the site near the apex Cai et al. In animal model, RhoA—mDia1 signaling pathway is activated during the durotropic movement, leading to the formation of detyrosinated—microtubule network, thereby positively regulating adhesion site formation Wang et al.

Microtubules are proposed to be associated with the plasma membrane through p and with actin filaments through uncharacterized proteins Cai and Cresti, Whether PT microtubules function in similar way remains to be elucidated. Rho GTPases are known to control protrusion of migrating cell and its adhesion to the matrix by modulating actin organization and reorganization in animal model Sit and Manser, ; Warner et al.

The process is crucial for oscillatory PT elongation Fu et al. This was initially thought to be caused solely by a pistil defect, since SCA was known to be secreted from the stylar TTE and to act as a female factor to guide lily pollen tube growth.

However, the reciprocal cross-pollination study showed that the defect in ltp in pollen tube growth was mainly dependent on pollen itself Fig. The ltp pollen tubes grew only up to the middle of the wild-type pistil in 12 h Fig. Therefore, no fertilized ovules were found in the bottom half of the pistil in this cross Fig. When in vitro grown, the ltp pollen tubes displayed abnormally swollen tips and growth cessation in 6 h Fig.

The LTP5 gene expression was found in pollen tubes at a low level Fig. The ltp pistil seed sets were decreased when wild-type pollen was used in a cross Fig. Further study revealed that ltp was a gain-of-function mutant Chae et al. A—D In vivo reciprocal cross-pollination of ltp to wild-type plants.

Flowers at stage 12 Smyth et al. At 12 h after pollination, 6—7 pistils were fixed, and pollen tube growth was examined by aniline blue staining.

The remaining pollinated pistils ripened into mature siliques in 8 d. Siliques were then dissected for examination of fertilized ovules. Arrows indicate the pollen tube front in the pistil. Asterisks designate unfertilized ovules in the silique. E, F Pollen from mature flowers was grown on solid germination medium in vitro for 6 h at room temperature. Arrows indicate pollen tube tips. G A weak level of gene expression arrow was identified in pollen tubes grown on the solid medium in vitro for 6 h.

H A dissected pistil showed a low level of gene expression in the pistil TT arrow. I Superposition of ribbon representations of the structures of LTP5 and ltp The structures were generated using homology modelling and 1 ns molecular dynamics simulations. The additional, predominantly hydrophobic, C-terminal tail of ltp is shown to cap one side of the protein, which is known to be an entrance for a putative ligand to the internal hydrophobic cavity in maize LTP Han et al.

J A focused view of the superposition of I is shown, with residues of interest Arg45, Tyr81, Val91 and Tyr91 depicted in ball and stick representations. The colouring scheme is the same as in I. This research was originally published in The Plant Cell www. Copyright American Society of Plant Biologists. However, ltp was shown to have an additional C-terminal tail Fig. Interestingly, Tyr91 in the ltp tail sequence was predicted to localize in close proximity to Arg45 and Tyr81, which are crucial residues in maize LTP that interact with a lipid molecule Han et al.

Although there is no evidence that SCAs or Arabidopsis LTP5 have any ligand in their hydrophobic cavities, the structural studies suggest that these LTPs may function in pollen tube tip growth by interacting with a putative binding partner. The ballooned pollen tube tip of ltp is highly similar to those of ROP signalling mutants Li et al.

In the A. Although functional redundancy appears to occur in Arabidopsis SCA-like LTPs, a recent study proposes that they may be highly diversified in their roles in plant growth and fertilization Chae et al. LTP1 is present most abundantly in the stigma and the style Fig. LTP3 showed its specific expression in the ovules Fig. LTP6 also showed gene expression in the style and the ovule Fig. These diversified gene expression patterns of SCA -like LTP genes suggest that each gene plays its own role in the pistil for pollen tube growth and guidance.

The values on the branches indicate the number of bootstrap replicates supporting the branch. This research was originally published in Journal of Experimental Botany Chae et al. Copyright the Society of Experimental Biology. Plantacyanins are small ECM proteins that belong to the ancient, plant-specific phytocyanins, which are classified as a subfamily of blue copper proteins Ryden and Hunt, The blue copper proteins have a conserved copper-binding site, formed by two histidines, one cysteine, and one methionine, glutamine or leucine.

Unlike other blue copper proteins, two histidines in the copper-binding site of plantacyanins were shown to be exposed to the surface Einsle et al. Plantacyanins from Arabidopsis , spinach and cucumber harbour one methionine at the fourth copper-binding site, which is thought to provide a high redox potential Nersissian et al. A ragweed plantacyanin, Ra3, does not contain histidines in the binding site and does not display copper-binding activity Hunt et al. Their copper-binding abilities and reactive oxygen species ROS production with respect to structural changes need to be further evaluated to understand their biological roles in plants.

No functionality of plantacyanins had been identified until chemocyanin, the lily plantacyanin secreted from the pistil, was shown to act as an external signal to regulate in vitro pollen tube reorientation Kim et al. Localization of external chemocyanin at the tip of in vitro growing pollen tubes Kim et al.

ROS are known to influence intracellular signalling by activating calcium channels in the plasma membrane Pei et al. Activated calcium channels trigger calcium influx through the plasma membrane of the growing pollen tube tip Hepler et al. A reorientation of the tip-focused calcium gradient occurs during pollen tube tip reorientation Hepler et al.

There is a single plantacyanin gene At2g found in the Arabidopsis genome. Unfortunately, the knock-down did not display any phenotype. The gene expression was most abundant in the inflorescence, especially in the stigma and the style Dong et al.

Immunolocalization showed that plantacyanin is present in the surface of the stigmatic papillar cell and in the TT from the style to the ovary, where pollen tubes germinate and are guided to the ovule Dong et al.

This loss of directionality might be due to a disturbed gradient of guidance cues with over-expression of plantacyanin. This failure in pollen tube guidance resulted in smaller seed sets in plantacyanin over-expression lines, compared with the wild type Fig. Plantacyanin over-expression lines were used to examine the effect of increased levels of Arabidopsis plantacyanin in the stigma on pollination with wild-type COL pollen.

Plantacyanin protein levels in the pistils A at flower stage 12—13 of OXPs homozygous T 2 generation are much higher than those of the wild type COL , as revealed by protein blots. The protein loading control used was Ponceau S staining of the Rubisco large subunit.

B, C Over-expression pistils pollinated with wild-type pollen produce siliques with fewer seeds than the wild type COL.

D Scanning electron microscope images of wild-type pollen on wild-type stigma two images. Dotted lines trace the path of the pollen tube after it penetrates the papilla cell wall.

P, papilla cell, Po, pollen grain. E Wild-type pollen on over-expression stigmas showed aberrant tube growth after penetration of the papilla cell wall. Pollen tubes make many turns around the papilla cell in the over-expression stigmas OXP12; left. One pollen tube shown OXP12; right grew away from the style and ended up at the papilla cell tip arrow. In a semi- in vivo analysis F , the over-expression stigma left and the wild-type stigma right were pollinated with wild-type pollen and cultured on an Arabidopsis pollen growth medium.

No significant difference in number was found between the transgenic and control samples. This research was originally published in Plant Physiology www. A gradient of Arabidopsis plantacyanin was found in the embryo sac Dong et al. When travelling through the pistil TT, Arabidopsis pollen tubes emerge through breaks in the septum epidermis and adhere to the surface of this secretory epidermis until precisely targeted to the ovules Lord, Further study is necessary to determine whether plantacyanin acts as a signalling cue in pollen tube guidance to the ovule.

The activity of lily chemocyanin is synergistically enhanced by the presence of SCA in the pollen tube reorientation assay Kim et al. A series of biochemical and genetic studies on plantacyanin Kim et al. By analogy, they may be proposed to function through a putative interacting partner such as a membrane receptor. In Arabidopsis , small, secreted proteins function in diverse receptor-mediated signalling events.

One plant guidance molecule may trigger either attractive or repulsive signalling, depending on the combination of its putative receptors, as in the neuron. Netrin, a secreted chemoattractant for neuronal outgrowth, plays dual-opposite roles via different combinations of its interacting receptors, UNC-5 and UNC Hong et al.

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Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract.

Pollen tube growth and guidance: roles of small, secreted proteins. Keun Chae , Keun Chae. Oxford Academic. But how the tube orients itself when it emerges from the pollen at the papilla surface remains unknown.

Mechanical forces are known to play a major role in plant cell shape by controlling the orientation of cortical microtubules, which in turn mediate the deposition of cellulose microfibrils. For their study, Riglet and her team combined imaging, genetic and chemical approaches to show that the enzyme KATANIN, which cuts microtubules, also acts on cellulose microfibril orientation and confers mechanical properties to the papilla cell wall that allow for correct pollen tube orientation.

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