filamentous cells represent an important advance in cellular growth and are also important devices for the colonization of land by fungi and plants . for example , filamentous growth in the form of aseptate hyphae evolved as an excellent tool for exploring dry soils . this innovation also enabled the fungi to establish close relationships with land plants , for example , through symbiotic relationships that resemble arbuscular mycorrhizal associations today . in arbuscular mycorrhiza , tip - growing fungal hyphae rapidly enter plant root tissue to establish a mutually beneficial relationship . the first tip - growing cells in land - plants arose about 450 million years ago in bryophytes , which used filamentous rhizoids for anchoring themselves to the soil . in plants today , two types of tip - growing cells dominate ; pollen tubes and root hairs . while pollen tubes provided for water - independent propagation of species , root hairs bestowed an increase in root surface area that could be used to increase nutrient uptake and better anchor plants in the soil . root hairs and pollen tubes are filamentous cells that grow uni - directionally by depositing flexible cell wall material at one site of the cell and exploiting turgor pressure as the driving force for elongation ; a process also referred to as tip growth ( figure 1 ) . root hair and pollen tube tip growth include a variety of cellular components and compounds that work in concert . the intracellular calcium concentration plays a central role in plant tip growth , and regulates actin dynamics via actin interacting proteins , such as villin and adf . these proteins are likely involved in the organization of the actin cytoskeleton at different sites in the root hair . microtubules are located net axially , i.e. along the shank , and reach into the subapical region of the root hair . their dynamic behaviour is regulated by the microtubule plus end - tracking protein ark1 and is important for maintenance of polarity and growth direction . these produce ros ( h2o2 and oh ) which mediate cell wall relaxation and peroxidase - mediated wall stiffening . ros may also partake in a positive feedback loop by activating calcium influx from the apoplast into the cytosol . the channels responsible for calcium influx are not known in root hairs but , in pollen tubes , cngc and glr channels have been identified . furthermore , calcium seems to influence extracellular ph ( possibly via h - atpases ) , which contributes to changes in cell wall structure during growth . cellulose microfibrils are short and randomly oritientated in the cell wall of the growing tip , whereas a well ordered second layer of cellulose microfibrils occurs in the shank . this organization promotes dynamic cell wall extension exclusively at the tip . in pollen tubes pmes mediate the crosslinking of pectins in the shank and remain inactive at the tip after inactivation through pmeis . pectin , pmes and pmeis are delivered from the golgi / trans golgi network to the plasma membrane of the pollen tube tip via actin - dependent vesicle trafficking . vesicles are spacially directed by the distribution of pis in the plasma membrane and are transported along the actin cytoskeleton via motor proteins . tip growth is regulated by several plasma membrane receptor - like kinases , for example , feronia and anxur . feronia is known to regulate rop signaling pathways via interaction with rop guanine nucleutide exchange factors . important rop effectors in pollen tubes are rics that directly influence tip growth by interaction with bundled and fine f - actin and modulation of intracellular calcium concentrations . abbreviations : adf , actin depolymerizing factor ; ark1 , armadillo - repeat kinesin 1 ; cngc , cyclic nucleotide - gated channel ; glr , glutamate receptor - like ; pi , inositol phosphate ; pme , pectin - methyl - esterase ; pmei , pme inhibitor ; ric , rop - interactive crib motif - containing ; rop , rho of plant gtpase ; ros , reactive oxygen species . although these tip - growing systems are very similar , they also display differences in their growth - related molecular machineries . in view of the fact that soil nutrient availability , water accessibility and soil condition are strongly affected by climate change , the study of root hair tip growth is a timely topic to help us to understand and improve plant growth and will therefore be the main focus of this review . while delicate , root hairs are excellent experimental model systems for polarized cell growth . they reach growth rates of typically 1 m min or more   without losing cell wall integrity . this implies an extremely efficient system for delivering and modifying membrane and cell wall material at the tip apex . this system will therefore need to be highly coordinated , and involves a tip - derived calcium gradient , cytoskeleton dynamics , delivery of new cell wall material via vesicle trafficking , regulation of extracellular ph , and production of reactive oxygen species ( ros ) . how these players are geared to each other is only partly known , but recent studies have made great progress leading to a better understanding of the coordination . two of the major signal transducers for root hair growth and function are ca and ros ( figure 1 ) . calcium is a crucial and general regulator of plant cell growth and development , and is perhaps the most important factor for maintenance of tip growth where it is distributed as a continuous gradient in the cytosol , with the highest concentration close to the tip apex . furthermore , the cytosolic ca concentration displays periodic cycling that follows the rate of growth of the tip . this cycling appears to be coordinated with a rise and decline of both apoplastic ph and ros production . these findings suggest that changes in ca concentrations might regulate growth by the activation of ros production , and by increasing the ph , at the cell surface . the production of cell surface ros in turn activates ca influx into the cell and thereby completes a positive feedback loop . the increase in cytosolic ca is typically facilitated by membrane located ca channels . while the identity of these channels are not known in root hairs , cyclic nucleotide - gated ( cngc ) , and glutamate receptor - like ( glr ) , channels were identified as ca influx channels [ 24 ] in pollen tubes , and perhaps a similar scenario could be envisioned during root hair growth . the function of the fluctuations in intracellular ca may modulate components important for tip growth , such as cytoskeleton - organizing factors , and ros and ph homeostasis as alluded to above . however , it may also alter the apoplastic ca levels that can influence changes in the cell wall . in tip growing cells , the actin cytoskeleton is typically organized as thick actin bundles that run in parallel with the root hair shank supporting an inverse fountain flow of cytoplasmic streaming , and as fine filamentous structures at the apex of the tip that keep this region free of larger cell organelles [ 58 ] . the organization of the actin cytoskeleton is controlled by a variety of actin binding proteins ( abps ) , which aid in nucleation , depolymerisation and cross - linking of the actin cytoskeleton . for example , ca may influence actin depolymerizing factor ( adf ) in maize that can increase actin turnover by severing actin filaments , and villins that can influence the bundling of actin . the role of the actin cytoskeleton as a scaffold for polarized membrane trafficking is well established in root hairs . in particular , new membrane and cell wall material must be delivered to the tip apex to support the rapid growth . the cell wall material is engulfed in vesicles that move along the actin cytoskeleton via motor proteins . two recently characterized motor proteins of the myosin xi family that function in actin - dependent transport of cell organelles , vesicles and protein complexes , are required for normal actin dynamics and fast root hair growth . vesicle secretion can also be controlled by other factors , including rab gtpases   and inositol phosphates ( pis ) . for example , pi-4p and pi-4,5p2 accumulate in the plasma membrane at the root hair apex where they might orchestrate growth by recruiting other important proteins containing pi-4p or pi-4,5p2 binding domains . one such example is the adp ribosylation factor gtpase - activating protein agd1 , which is necessary for maintenance of polarity during root hair growth . membrane secretion in root hair growth is also influenced by the exocyst complex , a rho and rop gtpase effector , that tethers vesicles to their target membranes . cortical microtubules are oriented net axially along the fringe of the root hair tube , while endoplasmic microtubules reach from the perinuclear cytoplasm towards the tip . microtubule stabilizing or destabilizing treatments using taxol or oryzalin , respectively , lead to wavy and occasionally branched root hair phenotypes . similar phenotypes were also found in ark1 mutants lacking the functional microtubule plus - end tracking protein armadillo - repeat kinesin 1 ( ark1 ) . ark1 plays a key role in root hair tip growth by promoting microtubule catastrophe events . in summary , while the actin cytoskeleton plays an important role as a dynamic scaffold for proper vesicle delivery in polarized root hair elongation , the microtubules are required for maintenance of cell polarity and directed growth . the cell wall is a dynamic structure that determines the shape of the root hair ( figure 1 ) . here , one can illustrate the cell wall as a corset that readily allows for expansion at the tip , but provides resistance against the turgor pressure along the shank . cellulose is a major load - bearing structure of the cell wall and is therefore the main contributor to the corset function . two partially redundant members of the cellulose synthase ( cesa)-like ( csl ) superfamily , csld2 and csld3 , are located at the root hair tip and are required for its growth . notably , at the growing tip of the root hair the cellulose microfibrils are short and randomly orientated , as revealed by ultrastructural studies [ 2426 ] . in contrast , the shank behind the actively growing tip contains a second layer of highly ordered cellulose microfibrils oriented transversely to the axis of expansion [ 2628 ] . lesions in enzymes that synthesize some of these matrix polymers , for example , xyloglucans , led to root hair growth defects , perhaps due to mis - alignment of cellulose fibrils . whereas the xyloglucans thus influence root hair growth , the role of pectins is not well explored . however , pectins do have a prominent role in pollen tube growth . here , pectin - methyl - esterases ( pmes ) , which de - methylesterify carboxyl residues on the pectin backbone , can affect the cell wall viscosity . through the pme action , carboxyl residues are set free to bind ca , which results in pectin cross - linking and presumably cell wall tightening at the pollen tube tip [ 3134 ] . pme and its inhibitors , pmeis , are , furthermore , spatio - temporally distributed along the pollen tube with a preferential activity of pmes at the shank and pmeis at the tip , which results in a distinct pectic composition of the pollen tube cell wall . it is possible that similar distributions also may play a role in root hair growth . in addition , extensins , cell wall structural proteins , are of crucial importance for root hair development , possibly for maintaining cell wall stability . these proteins may be post - translationally modified by o - linked glycosylation , and this modification alters cell wall properties during root hair growth . recently , pft1/med25 , a transcriptional mediator complex subunit , was revealed to orchestrate the expression of cell wall remodelling genes including a set of peroxidases and cell wall cross - linking extensins and being required for redox homeostasis in roots and normal root hair formation . these results indicate that the cell wall proteome could be a useful source to find cell wall and , therefore , tip growth modulators . however , results on the proteomic composition of root hair cell walls are scarce , not only because separating cell wall proteins from the cell wall matrix is generally challenging , but also because isolating a reasonable amount of protein from root hairs for analysis is difficult . not only calcium / pectin interactions but also surface ros and ph are thought to directly influence cell wall viscosity of the tip surface . on the other hand , h2o2 is a substrate for peroxidases that help to stiffen the wall via crosslinking of cell wall polysaccharides . this may seem like a paradox and a tight coordination between these processes is likely to occur , but how this is achieved remains unclear . surface ph may also influence the cell wall viscosity by activation of the expansins , non - hydrolytic cell wall - loosening proteins , and other cell wall modifying enzymes , which promote cell wall relaxation [ 4244 ] . two of the major signal transducers for root hair growth and function are ca and ros ( figure 1 ) . calcium is a crucial and general regulator of plant cell growth and development , and is perhaps the most important factor for maintenance of tip growth where it is distributed as a continuous gradient in the cytosol , with the highest concentration close to the tip apex . furthermore , the cytosolic ca concentration displays periodic cycling that follows the rate of growth of the tip . this cycling appears to be coordinated with a rise and decline of both apoplastic ph and ros production . these findings suggest that changes in ca concentrations might regulate growth by the activation of ros production , and by increasing the ph , at the cell surface . the production of cell surface ros in turn activates ca influx into the cell and thereby completes a positive feedback loop . the increase in cytosolic ca is typically facilitated by membrane located ca channels . while the identity of these channels are not known in root hairs , cyclic nucleotide - gated ( cngc ) , and glutamate receptor - like ( glr ) , channels were identified as ca influx channels [ 24 ] in pollen tubes , and perhaps a similar scenario could be envisioned during root hair growth . the function of the fluctuations in intracellular ca may modulate components important for tip growth , such as cytoskeleton - organizing factors , and ros and ph homeostasis as alluded to above . however , it may also alter the apoplastic ca levels that can influence changes in the cell wall . in tip growing cells , the actin cytoskeleton is typically organized as thick actin bundles that run in parallel with the root hair shank supporting an inverse fountain flow of cytoplasmic streaming , and as fine filamentous structures at the apex of the tip that keep this region free of larger cell organelles [ 58 ] . the organization of the actin cytoskeleton is controlled by a variety of actin binding proteins ( abps ) , which aid in nucleation , depolymerisation and cross - linking of the actin cytoskeleton . for example , ca may influence actin depolymerizing factor ( adf ) in maize that can increase actin turnover by severing actin filaments , and villins that can influence the bundling of actin . the role of the actin cytoskeleton as a scaffold for polarized membrane trafficking is well established in root hairs . in particular , new membrane and cell wall material must be delivered to the tip apex to support the rapid growth . the cell wall material is engulfed in vesicles that move along the actin cytoskeleton via motor proteins . two recently characterized motor proteins of the myosin xi family that function in actin - dependent transport of cell organelles , vesicles and protein complexes , are required for normal actin dynamics and fast root hair growth . vesicle secretion can also be controlled by other factors , including rab gtpases   and inositol phosphates ( pis ) . for example , pi-4p and pi-4,5p2 accumulate in the plasma membrane at the root hair apex where they might orchestrate growth by recruiting other important proteins containing pi-4p or pi-4,5p2 binding domains . one such example is the adp ribosylation factor gtpase - activating protein agd1 , which is necessary for maintenance of polarity during root hair growth . membrane secretion in root hair growth is also influenced by the exocyst complex , a rho and rop gtpase effector , that tethers vesicles to their target membranes . cortical microtubules are oriented net axially along the fringe of the root hair tube , while endoplasmic microtubules reach from the perinuclear cytoplasm towards the tip . microtubule stabilizing or destabilizing treatments using taxol or oryzalin , respectively , lead to wavy and occasionally branched root hair phenotypes . similar phenotypes were also found in ark1 mutants lacking the functional microtubule plus - end tracking protein armadillo - repeat kinesin 1 ( ark1 ) . ark1 plays a key role in root hair tip growth by promoting microtubule catastrophe events . in summary , while the actin cytoskeleton plays an important role as a dynamic scaffold for proper vesicle delivery in polarized root hair elongation , the microtubules are required for maintenance of cell polarity and directed growth . the cell wall is a dynamic structure that determines the shape of the root hair ( figure 1 ) . here , one can illustrate the cell wall as a corset that readily allows for expansion at the tip , but provides resistance against the turgor pressure along the shank . cellulose is a major load - bearing structure of the cell wall and is therefore the main contributor to the corset function . two partially redundant members of the cellulose synthase ( cesa)-like ( csl ) superfamily , csld2 and csld3 , are located at the root hair tip and are required for its growth . notably , at the growing tip of the root hair the cellulose microfibrils are short and randomly orientated , as revealed by ultrastructural studies [ 2426 ] . in contrast , the shank behind the actively growing tip contains a second layer of highly ordered cellulose microfibrils oriented transversely to the axis of expansion [ 2628 ] . neighboring cellulose fibrils are interlinked with matrix polymers , such as hemicelluloses and pectins . lesions in enzymes that synthesize some of these matrix polymers , for example , xyloglucans , led to root hair growth defects , perhaps due to mis - alignment of cellulose fibrils . whereas the xyloglucans thus influence root hair growth , the role of pectins is not well explored . however , pectins do have a prominent role in pollen tube growth . here , pectin - methyl - esterases ( pmes ) , which de - methylesterify carboxyl residues on the pectin backbone , can affect the cell wall viscosity . through the pme action , carboxyl residues are set free to bind ca , which results in pectin cross - linking and presumably cell wall tightening at the pollen tube tip [ 3134 ] . pme and its inhibitors , pmeis , are , furthermore , spatio - temporally distributed along the pollen tube with a preferential activity of pmes at the shank and pmeis at the tip , which results in a distinct pectic composition of the pollen tube cell wall . it is possible that similar distributions also may play a role in root hair growth . in addition , extensins , cell wall structural proteins , are of crucial importance for root hair development , possibly for maintaining cell wall stability . these proteins may be post - translationally modified by o - linked glycosylation , and this modification alters cell wall properties during root hair growth . recently , pft1/med25 , a transcriptional mediator complex subunit , was revealed to orchestrate the expression of cell wall remodelling genes including a set of peroxidases and cell wall cross - linking extensins and being required for redox homeostasis in roots and normal root hair formation . these results indicate that the cell wall proteome could be a useful source to find cell wall and , therefore , tip growth modulators . however , results on the proteomic composition of root hair cell walls are scarce , not only because separating cell wall proteins from the cell wall matrix is generally challenging , but also because isolating a reasonable amount of protein from root hairs for analysis is difficult . not only calcium / pectin interactions but also surface ros and ph are thought to directly influence cell wall viscosity of the tip surface . hydroxyl radicals can cleave polysaccharides and thereby promote wall relaxation [ 3840 ] . on the other hand , h2o2 is a substrate for peroxidases that help to stiffen the wall via crosslinking of cell wall polysaccharides . this may seem like a paradox and a tight coordination between these processes is likely to occur , but how this is achieved remains unclear . surface ph may also influence the cell wall viscosity by activation of the expansins , non - hydrolytic cell wall - loosening proteins , and other cell wall modifying enzymes , which promote cell wall relaxation [ 4244 ] . although the molecular organization of root hair growth is described with increasing detail , how internal and environmental growth - regulating signals are sensed and integrated remains enigmatic . nevertheless , some recent reports have begun to shed some light on this topic . in plants several receptor - like kinase ( rlk ) family members are implicated in relaying information about cell wall integrity ( cwi ) to the inside of the cell ( figure 1 ) . the redundant cataranthus roseus rlk1-like subfamily members anxur1 ( anx1 ) and anx2 are required for pollen tube growth in arabidopsis . the anx1 and 2 are the closest relatives to feronia ( fer ) , which is essential for both pollen tube and root hair growth [ 4652 ] . fer can affect ros production via the regulation of rho of plant gtpases ( rops ) . rops accumulate in the apical plasma membrane of root hairs and other tip - growing cells , and impact actin dynamics and vesicle trafficking . for example , two counteracting rop - interactive crib motif - containing ( ric ) proteins control pollen tube growth by regulating f - actin dynamics . it is tempting to speculate that cell wall derived compounds may be putative ligands for the cwi related rlks . this has , for example , been shown to be the case for the wall - associated kinase family that binds pectins . furthermore , a secreted small peptide hormone , rapid alkalinisation factor ( ralf ) negatively regulates cell elongation by binding to the fer receptor as a ligand . against this backdrop , other small peptides may be considered as ligands for cwi related rlks in root hair growth . while these data are encouraging , the identification of ligands for receptors is not a trivial task . the activated receptors may subsequently cause alterations in intra- and extracellular ion homeostasis , which brings us back to the ca , ros and additional signalling compounds with which we began this review . recent undertakings have greatly improved our understanding of root hair growth ( figure 1 ) . for example , the development of highly sophisticated calcium , ph and ros sensors has revealed a tight interplay between these factors . the recent identification of several new players in the molecular synergy of root hair growth has also stimulated the field . yet , many questions remain . secondly , cell wall biosynthesis in root hair cells differs from most other cell types . thirdly , calcium , extracellular ph and ros are important regulators of tip growth , implicated by their correlated cyclic patterns . however , the mechanisms that directly link these regulators are still largely missing , as are the identities of the ion channels that contribute to the ca uptake . lastly , what are the cwi sensors that the tip growing cells use , how are they activated and what are they regulating ? these are important factors specifying external status of the root hairs but progress in this field is still in its infancy . experiments aiming at answering these questions can provide us not only with new insights into how root hair growth works but might be applicable to other tip growing systems , and to elucidate general developmental patterns during cell growth .
over the last few decades , our understanding of directed cell growth in different organisms has substantially improved . 
 tip - growing cells in plants elongate rapidly via targeted deposition of cell wall and membrane material at the cell apex , and use turgor pressure as a driving force for expansion . 
 this type of polar growth requires a high degree of coordination between a plethora of cellular and extracellular components and compounds , including calcium dynamics , apoplastic reactive oxygen species and ph , the cytoskeleton , and vesicular trafficking . in this review 
 , we attempt to outline and summarize the factors that control root hair growth and how they work together as a team .