Actin and Pollen Tube Growth Coursework Essay

7/29/2019 Actin and Pollen Tube Growth Coursework Essay

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This paper represent my own work in accordance with University regulation .

Nottingham University

School of Bioscience

[D235P2: Plant Cell Signalling (Actin and the growth of pollen tube)]

[Daniel Bin Ahmad Zamri]

Student ID : [004821]

M. Coordinator : [Dr Chin Chiew Foan]


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Flowering plant reproduction system involves delivery of pollen grain onto stigmatic

tissues. A compatible reaction between pollen and stigma will induce germination of pollen

through hydration of pollen grain. After that, pollen tube will penetrates through cuticle

before growing through cell wall of style. The elongation process continues until the pollen

tube reaches micropyle. The process of elongation of pollen tube is known to be driven by

several biochemical mechanisms. These mechanisms are the concentration gradient of

calcium within pollen tube tip and involvement of actin microfilament. However, the

mechanism of actin involvement in pollen tube growth is still poorly understood although

there were extensive researches. Therefore, this essay will discuss on one of the major

biochemical mechanism involves in pollen tube growth; actin microfilament, in terms of its

properties, roles and regulation based on recent discovery.

Actin is globular protein that consists of a component known as microfilament.

Microfilament can be identified within cells as a thin, single-stranded filament. Actin

filament existed in most organisms because it is an important component of cytoskeleton;

basic cells structural support. Other than that, actin filament also drives cells motility. Most

unicellular and simple multicellular organisms depend on actin for their motility. However,

plant cells feature; cell wall may disrupt this function. It is agreed that most plant cells grow

by using internal turgor pressure; however, recent discovery found that growth of apices

such as root and pollen tube undergo different process. In fact, growth of apices is related

to actin microfilament.

During pollen tube growth, it is observed that clear zone exist within extreme apex

of pollen tube. This is called clear zone because of that in this region, small organelles and


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vesicles have low refractivity under light microscope (Vidali and Hepler, 2001). Examples of

organelles in this region are secretory vesicles and smooth endoplasmic reticulum. Further

behind this clear zone, organelles such as Golgi stack and mitochondria are abundant. This

clear zone is important in understanding a phenomenon called reverse fountain

streaming. Reverse fountain streaming is described as movement of particle acropetally

along the edge of pollen tube and basipetally through central of the core (Iwanami, 1956).

This is important in order to transport materials needed for pollen tube growth at the apex

in the process of exocytosis; process of transfer of exocytic vesicles along actin cytoskeleton

near the cell periphery and endocytosis; process of transfer of endocytic vesicles at clear

zone towards the central core of pollen tube (Zonia and Munnik, 2009). The movement of

particles and existence of clear zone might have been influenced by localization of actin in

pollen tube. Several studies have been done to recognize the location of filamentous actin

(F-actin) in pollen tube. Kost and colleagues (1998) discovered that there is little amount of

F-actin at the extreme apex of pollen tube through the use of green fluorescent protein on

tobacco pollen tube. However, behind the clear zone, F-actin is organized as longitudinally

oriented cable and there are differences of arrangement between F-actin at the core and

the periphery of cell. At the periphery of cell, F-actin is organized into barbed end toward

the apex. Meanwhile at the core, F-actin is organized into pointed end toward the apex. The

arrangement of F-actin is discovered to have relation to the phenomenon of reverse

fountain streaming (Lenartowska and Michalska, 2008).

The pollen tube growth does not only involve localization of F-actin. It also involved

the regulation and stability of actin itself. This is discovered when microfilament

organization of actin is disrupted by Latrunculin B (LATB); a type of inhibitor, that show an


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effect of reduce number of protrusion of tube out of pollen and length of pollen tube along

an increase of concentration of LATB(Gibbon et.al, 1999). In order to maintain the growth of

pollen tube at the apex, actin is needed to be regulated through process of polymerization.

The regulation of actin polymerization or actin dynamics involved several components.

These components are regulatory enzymes and actin-binding proteins (ABPs). Examples of

regulatory enzymes for maintenance of actin polymerization are Rho-type GTPases (Rops)

and Phosphatidylinositol 4, 5-bisphosphate (PIP2). Rops is a plant specific subfamily of the

Rho family of monomeric GTPases and considered to be important signaling molecules for

actin dynamic. In Arabidopsis, eleven ROPs genome has been encoded. It is found that

pollen tube germination and growth is inhibited through study of anti-body microinjection

of anti-Rop1 (Kost et.al, 1999). This might suggest that ROP1; one of encoded genomes,

regulates pollen tube germination and growth. It is also suggested that ROP1 is important

for polarization of pollen tube. This is because over expression of ROP1 causes

delocalization of signaling complex thus, promoting depolarization growth of tube (Ying Gu

et.al, 2002). The evidence of relation between activation of ROP1 and F-actin formation is

investigated and it is found that inactivation of ROP1 through over expression of RoPGAP1

depleted F-actin at the tip of pollen tube (Fu et.al, 2001). There are several suggestions that

stated that ROPs signaling might have been related through the steep gradient of Ca2+,

however extensive research need to be done to elucidate the whole involvement of Ca2+

in

ROP signaling mechanism. Another regulatory enzyme PIP2 is also responsible for actin

dynamics. Apart from that, PIP2 is also responsible for vesicle trafficking and calcium ion

homeostasis (Cremona et.al, 1999, Stevenson et al. 2000). For actin dynamics, it seems that

PIP2 is involved with several of ABPs, namely Arp2 and actin depolymerizing factor


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(ADF)/cofilin mainly in process of activation and impairment respectively. Through process

of hydrolysis, PIP2 may produce another important derivative for actin dynamics, namely

inositol 1, 4, 5-triphosphate (IP3). It is suggested that IP3 may be involved in modulating Ca2+

level, another important mechanism in pollen tube growth. However, it can be argued that

hydrolysis of PIP2 to produced IP3 might not have directly modulate Ca2+

because it is found

that by changing the concentration of PIP2 and IP3 artificially through photolysis of caged

probe produce different effect of Ca2+ level and apical secretion (Fu, 2010).

Another component of regulation of actin dynamics is actin-binding proteins.

Examples of ABPs that is related to actin dynamics are profilin, villin, ADF/cofilin, Arp2/3

complex, caldesmon, and myosin (Vidali and Hepler, 2001). Most of these ABPs are

responsible in severing, capping, bundling and cross linking of actin polymer. It is also stated

that several ABPs are correlated with the Ca2+

in maintaining actin dynamics. For example,

the actin-sequestering activity of profilin is dependent on Ca2+

. This is because profilin is

found to accumulate the apex of pollen tube where Ca2+

concentration is usually high, thus

the connection between profilin and Ca2+

. The evidence that support this is by testing

ZmPRO5 and ZmPRO1; new recombinant protein of maize profilin, over a range of Ca2+

concentration, it is found that the activity of actin-sequestering increase with an increase of

Ca2+

concentration (Kovar et.al, 2000). While profilin is involved in actin-sequestering

activity, formin is responsible in actin cables formation. In Arabidopsis, it is estimated that

almost 23 genes encodes formin homologues. It is reported that over expression of AtFH1

(A.thaliana formin homologue) causes pollen tube swelling and cell membrane deformity at

the pollen tube tip (Cheung and Wu, 2004). This suggested that formin mechanism occur

mostly at the apex and it involves in formation of short F-actin where short F-actin presence


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is abundance at the apex. These discoveries have suggested that ABPs are crucial for

maintaining actin dynamic and pollen tube growth.

Pollen tube formation is essential step in reproduction process of most plant species

and is control by several mechanisms mainly Ca2+

and actin dynamics. There are also

numerous amount of signaling pathway involves in order to maintain actin dynamics. These

pathways are regulated by regulatory enzymes and ABPs. Most of these components in

pollen tube formation have been tested using current technology available such as

antisense and mutant analysis, however, much research needed to be done to elucidate the

complete relation of actin and pollen tube formation.


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References

1. Cheung A.Y. and Wu H.M. (2004) Overexpression of anArabidopsis formin stimulatessupernumerary actin cable formation from pollen tube cell membrane. Plant Cell16:

257269

2. Cremona O., Di Paolo G., Wenk M.R., Lthi A., Kim W.T., Takei K., Daniell L., NemotoY., Shears S.B., Flavell R.A., McCormick D.A., De Camilli P. (1999) Essential role of

phophoinositol metabolism in synaptic vesicle recycling. Cell99: 179188

3. Fu, Y. (2010) The Actin Cytoskeleton and Signaling Network during Pollen Tube TipGrowth.Journal of Integrative Plant Biology, 52 (2): 131137

4. Fu Y, Li H, Yang Z. (2002). The ROP2 GTPase controls the formation of cortical fine F-actin and the early phase of directional cell expansion during Arabidopsis

organogenesis. The Plant Cell14: 777-794

5. Gibbon, B.C., Kovar D.R., Staiger, C.J. (1999) Latrunculin B has different effect onpollen germination and tube growth. Plant Cell, 11: 2349-2363

6. Iwanami Y. (1956) Protoplasmic movement in pollen grains and tubes.Phytomorphology, 6: 288-295

7. Kost B., Lemichez E., Spielhofer P., Hong Y., Tolias K., Carpenter C., Chua N.H. (1999)Rac homologues and compartmentalized phosphatidylinositol 4,5-bisphosphate act

in a common pathway to regulate polar pollen tube growth.J Cell Biol145: 3173308. Kost B., Spielhofer P. and Chua N.H (1998) A GFP-mouse talin fusion protein label

plants actin filament in vivo and visualise the actin cytoskeleton in growing pollen

tubes. Plant J, 16: 393-401


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9. Kovar D.R., Drobak K.B., Staiger C.J (2000) Maize Profilin Isoforms Are FunctionallyDistinct. The Plant Cell, 12:583598

10.Lenartowska M. and Michalska A. (2008) Actin filament organization and polarity inpollen tubes revealed by myosin II subfragment 1 decoration. Planta 228:891896

11.Stevenson, J.M., Perera, I.Y., Heilmann, I., Persson, S., Boss, W.F. (2000) Inositolsignalling and plant growth. Trends Plant Sci, 5:252-258

12.Vidali, L. and Hepler, P.K. (2001). Actin and Pollen Tube Growth. Protoplasma215:64-76

13.Zonia L. and Munnik T (2009). Uncovering hidden treasures in pollen tube growthmechanics. Cell Press, 318-327.

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Actin and Pollen Tube Growth Coursework Essay