Tip Growth

Abstract

Pollen tubes and root hairs grow by a highly focused deposition of new wall and membrane materials at their growing apex. Comparison of the machinery that localises such growth between these cell types has revealed common components, providing important insight into how plant cells control cell expansion. Such elements include the small GTPases (e.g. ROPs and RABs), gradients and intricate spatial patterning in the fluxes of ions (e.g. Ca2+ and H+) and partitioning of membrane lipids (such as the phosphoinositides). These regulators are coupled to focused action of the secretory machinery (e.g. the exocyst) and cytoskeletal dynamics, with integral roles emerging for actin, tubulin and their associated motor proteins. These components form an integrated regulatory network that imposes robust spatial localisation of the growth machinery and so supports the production of an elongating tube‐like growth form where cell expansion is limited to the very apex, that is, tip growth.

Key Concepts

  • Tip growth represents the focused expansion of plant cells, such as root hairs and pollen tubes, localised to the apex of an elongating, tube‐like growth form.
  • Expansion is limited to where the tip cell wall yields to internal turgor pressure.
  • Maintained growth requires the focused delivery of cell wall and membrane materials to the growing apex, while strengthening the sides of the tube to resist further expansion.
  • A network of cellular regulators combine to focus the activity of the growth machinery to the growing tip.
  • Localised action of components such as small GTPases, gradients of ions, the cytoskeleton and maintenance of specialised regions of membrane composition interact to provide the spatial information that limits the activity of the secretory machinery to the very tip of the cell.

Keywords: actin; calcium; microtubules; phosphoinositide; root hair; pollen tube; RAB; ROP; small GTPase; tip growth

Figure 1. General features of tip growth. In tip growth, highly localised delivery of new membrane and wall materials to the tip support localised, turgor‐driven expansion. As the tip grows on, the wall that is left behind becomes progressively more cross‐linked and able to withstand the forces of turgor in both the longitudinal and radial directions, restricting cell expansion. This pattern of growth control results in a rigidified tube behind a constantly expanding tip. The tube therefore gets longer by constant addition of new tube material at the elongating apex.
Figure 2. Tip growth in pollen tubes and root hairs. (a) Pollen tube tip growth fits well with the requirements of plant fertilisation. A pollen grain lands on a receptive stigmatic surface, takes up water and germinates, producing a pollen tube that emerges from the operculum, a preformed pore in the pollen wall. The tube then enters tip growth and using chemical cues both within the style and from the ovary is guided to the micropile, a pore allowing entry to the ovary. The fine balance between simultaneously allowing growth but also resisting the expansive forces of turgor within the growing tube tip now shifts towards expansion and the tip of the tube bursts, releasing the sperm inside the pollen tube to effect fertilisation. (b) Tip growth in root hairs starts with the process of initiation, where a bulge forms in a precisely controlled point in the lateral wall of a trichoblast, an epidermal cell genetically preprogrammed to be able to produce root hairs. Cytoplasm accumulates in the bulge which then transitions to tip growth, a genetically distinct process from initiation. Apically growing root hairs have a cytoplasm‐rich tip where the machinery of tip growth is localised. Growth continues until the hair reaches a mature length at which point the tip growth machinery dissipates, the vacuole protrudes to the cell apex and the apical wall becomes strengthened and no longer able to support cell expansion.
Figure 3. Regulatory networks at the growing apex. (a) Exocytosis of new wall polymers and insertion of new plasma membrane material supports expansion focused to the apex of tip growing cells. Simultaneously, endocytosis recycles excess membrane delivered to the tip. The actin cytoskeleton plays a key role in mediating this membrane trafficking system. (b) Examples of regulatory loops controlling tip growth activities in pollen tubes and root hairs. Ca2+ influx has been shown to lead to a tip‐focused Ca2+ gradient promoting multiple processes at the growing apex including exocytosis, regulation of actin dynamics and activation of the ROS producing NADPH oxidases of the RBOH family. RBOH produces ROS to the apoplast promoting wall rigidification. ROS also enter the cytosol via channels in the plasma membrane (possibly aquaporins) where they regulate Ca2+ channel activity. These activities are modulated by parallel, overlapping regulatory cassettes. For example, in pollen tubes a tip‐focused accumulation of the active (GTP‐bound) form of the ROP1 GTPase acts (in part) through RIC3 and RIC4 to modulate both Ca2+ and actin dynamics (RIC3 promoting disassembly and RIC4 assembly). ROP activity itself is modulated by regulators such as Guanine nucleotide Exchange Factors (GEF) that determine whether ROP1 is bound to GTP (active: ROPGTP) or GDP (inactive: ROPGDP). REN1: ROP1 ENHANCER, a Rop GTPase Activating Protein (GAP). REN1 contains a pleckstrin homology (PH) domain implying it can interact with the phosphoinositide regulatory system.
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Further Reading

Cai G, Parrotta L and Cresti M (2015) Organelle trafficking, the cytoskeleton, and pollen tube growth. Journal of Integrative Plant Biology 57: 63–78.

Chebli Y, Kroeger J and Geitmann A (2013) Transport logistics in pollen tubes. Molecular Plant 6: 1037–1052.

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Steinhorst L and Kudla J (2013) Calcium – a central regulator of pollen germination and tube growth. Biochimica et Biophysica Acta 1833: 1573–1581.

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Gilroy, Simon, and Swanson, Sarah J(Apr 2017) Tip Growth. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023746]