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In flowering plants, sexual reproduction is a very important process and the pollen tubes growth plays an important role in plant sexual reproduction. Pollen tubes are important and they formed when the pollen grain lands on stigma and germinates during fertilization process where they penetrate the style of the plant to transfer the sperm cells to ovule during fertilization process (Ren & Xiang, 2007; Fu, 2010). Depending on the species, the stigma surface can be wet or dry and dry pollen germination is normally triggered by the pollen hydration while for tobacco plant, where the stigma is wet and sticky, pollen hydration is not a problem for them to form pollen (Howell, 1998). During this process, it is depends on the actin cytoskeleton, directional and highly polarized (Vidali et al, 2001; Zhang et al, 2010). In order to maintain the polarity of growth, the actin cytoskeleton needs to be maintained carefully by the pollen tubes (Ren & Xiang, 2007). A successful pollen tube growth required four stages. Firstly is pollen tube growth on the cell wall of papillary cell by piercing the cuticle layer and secondly is grows basipetally down through the transmitting tract of the style (Howell, 1998). Next, is the emergence of pollen tube from septum near funiculus of an ovule, and lastly this pollen tubes will growth into the micropyle of the ovule (Howell, 1998). For a pollen tube to growth, actin plays an important role as it act as a transporter to transfer signals that triggered the germination.
Pollen tubes cell wall is consisted by three main portions which are pectins, cellulose and callose and they growth like a lateral root hair cells. A growing pollen tube can be divided into three regions, which is shank, subapex, and apex (Ren & Xiang, 2007). Basically in shank region, actin filaments will arrange into bundles with evenly distributed polarity, structural support and acts as pathway for the cytoplasmic streaming (Ren & Xiang, 2007; Zhang et al, 2010). According to Vidali et al, (2001), they found that secretory vesicle which is important for pollen tubes growth is delivered through the cytoplasmic streaming system which is controlled by the actin bundles. Hence, when actin system was inhibited, the pollen tubes will stop growing due to interdiction of the vesicle transport (Vidali et al, 2001). At subapex, filamentous actin or known as F-actin will organize into more collar and fringe like structure which allows the reverse-fountain cytoplasmic streaming pattern instead of organize into straight line like shank region and the dynamics of F-actin is important for the polarization of pollen tube growth (Ren & Xiang, 2007; Zhang et al, 2010). Lastly in the apex or clear zone at the tip of growing tube, which is a region of expansion and also have the vesicle that containing cell wall precursor (Massawe, 2010; Ren & Xiang, 2007).
During pollen tubes formation, actin cytoskeleton is very important and plays a crucial role as they transfer the Golgi vesicles and materials that needed to build up cell wall. Actin cytoskeleton must exist in equal ratio between G-actin and F-actin where disruption of the ratio stops the pollen tube from growing (Krichevsky et al, 2007). Besides there are also many organelles and vesicles that move to and fro along the axis of central actin tracks and keep recycling. This track is important because they deliver the membraneous and Golgi vesicles to the growing site of pollen tube (Cai & Cresti, 2008). In subapex, membrane trafficking will also occurs together with the position of specific actin array and there is also an accumulation of Golgi-derived secretory vesicle at the tip and moving along the peripheral actin cables. Studies also shown that actin fringe plays a role in delivering this vesicle to the tip and inhibition of actin affects the Golgi integrity thus causes the structural disruption (Cai & Cresti, 2008; Samaj et al, 2006). Besides, there are two types of growing pollen tube, which is fast and slow. In fast growing pollen tubes, where most of the endocytosis will occurs, there was a localization of apical F-actin cortical fringe were found at the position and this suggests that actin has a role in rapid endocytosis too (Samaj et al, 2006). While for slow growing pollen tube such as Arabidopsis, endocytosis will occurs at the extreme tip where actin filaments were found. All the studies shown that actin has a role in endocytosis process.
Furthermore, actin microfilaments are also a supportive structure which can withstand the turgor pressure where this actin microfilaments network provides a filter to form apical vesicular zone in the clear zone (Vidali & Helper, 2000). Moreover, during pollen tube growth, the dynamic organization and constant elongation at the tip by actin is very important contribution and without this constant elongation, the supplement of vesicles, cell wall precursor will become depolarized and even swell at the pollen tip and no pollen tube formation will be found. During pollen tube growth, activation and localization of ROP1 signaling complex was found to control the actin microfilaments dynamics and also promotes tip growth domain (Krichevsky et al, 2007). When polymerization of actin microfilaments occurred that promoted by jasplakinolide leads to reverse cytoplasmic streaming in apex region (Krichevsky et al, 2007). Actin polymerization is also important during the pollen tubes formation in order to continue the elongation at apex region as disruption of actin polymerization affect pollen tube formation (Vidali & Helper, 2000). Therefore, polymerization of new actin microfilaments is required for the pollen tube growth (Vidali & Helper, 2000).
There are several signaling pathways that been found to regulates cell polarity and growth by forming a complex network where they interacting with each other during the pollen tubes growth (Fu, 2010). In addition, during the pollen tube tip growth, calcium (Ca2+), small guanosine-triphosphatase (GTPase) and lipid-mediated signaling pathways were found to be involved in rapid F-actin turnover, controlling actin network structure and also transmitting signals to the actin binding proteins (Fu, 2010). When F-actin organization and dynamics has been disrupted, it causes the inhibition of the pollen tubes growth (Fu, 2010; Zhang et al, 2010). This show that actin cytoskeleton is essential for the pollen tubes growth in order to develop the new membrane and deliver cell wall materials to the controlled growth site (Fu, 2010). During the elongation process where actin will take part, they act as a force generator and organizer at the apical cytoplasm (Vidali et al, 2001). For GTPase, they play a role where they regulate the actin microfilament dynamics and the vesicular transport (Krichevsky et al, 2007).
In order to maintain the F-actin dynamics, there are various actin-binding proteins (ABPs) that controlling it and is crucial for pollen tube growth which included actin- depolymerizing factor (ADF), Profilin, Formin, vilin and Cofilin (Fu, 2010). Besides, Ca2+ was also found to be involved in pollen tube growth where the F-actin dynamics and organization can be manipulated by Ca2+ as a result of regulating ABPs (Eckardt, 2005; Fu, 2010). Fu, (2010) also stated that the inhibition of Ca2+ pathway will inhibited the pollen tube growth while manually applied Ca2+ gradient at new site will redirect the pollen tube growth. This proved that Ca2+ is controlling and regulating the ABPs and is essential to promote pollen tube growth. In addition, since there are many ABPs that physically bind to F-actin, these ABPs actually directly controlling the F-actin organization and the dynamics through capping, severing, crosslinking and bundling (Fu, 2010). For example, profilin which is actin-sequestering activity that will inhibit elongation of filaments is highly depends on Ca2+ while vilin (P-135-ABP) that has actin bundling, fragmenting, capping, and G-actin binding activity which is crucial for actin bundles formation will also suppress by the Ca2+ gradient (Fu, 2010).
In conclusion, actin filament is important as they are the main pathway for the organelles and vesicles to move around and actin also provides the polarity for the tip to growth. Somehow, Ca2+ gradient will regulates the movement of these organelles and Ca2+ has been found to be important during the pollen tubes growth. Moreover, actin cannot function itself and they must function together with others ABPs and also depends on the gradient of Ca2+ since Ca2+ is controlling and regulating ABPs. ABP also stabilize the actin filaments and leads to formation of higher actin structure for pollen tube formation. All the ABPs are linked to each otherââ‚¬â„¢s where suppression of one ABP will cause another ABP not to function well hence disrupted the pollen tube formation. Although many studies on actin during pollen tubes formation have been carried out, the precise function of actin are still not known. Therefore, continuous effort in this area must be done to acquire further understanding on this actin role during the pollen tubes formation.