therefore , living organisms persistently explore their environment for extending their boundaries into favorable regions .
one fundamental difference we are taught early on between motile animal and nonmotile plant cells is the way in which they carry out their boundary extension .
motile cells achieve whole - body displacement or locomotion , which allows them to leave resource - depleted domains behind while moving on to new areas .
plant cells , however , become rooted to a spot and are only able to extend outwards from it in response to different stimuli .
two major conditions have to be met for achieving locomotion ; a flexible bounding layer of the cell , which allows for rapid change in its shape , and the ability to retract one end of the cell while extending the other ( small et al . , 2002a ) .
plant cells do not fulfill these requirements because they are encased in a relatively rigid cellulosic cell wall and , clearly , they do not have rear - end retractability . however , the cell wall of actively expanding plant cells is actually quite labile in contrast to its later , rigid nature in a mature , fully expanded cell ( mathur , 2004 ) .
further , though lacking rear - end retractability , the forward extension of a plant cell , like that of an animal cell , also involves membrane protrusion ( vidali et al . , 2001 )
recent cell - biological and molecular evidence from higher plants suggests that , despite their sessile and wall - encased nature , plant cells possess a core machinery similar to the one required for forward motility of animate cells .
although usually used in the context of crawling amoeboid locomotion , here i use the term  leading edge  for both animal and plant cells as the part of a cell that extends or  leads . 
though descriptions vary from cell to cell , the generalized leading edge of an amoeboid cell comprises a 25-m - wide veil - like , organelle free , cytoplasmic extension called the lamellipodium .
the leading edge has an actin - rich zone comprising of a fine f - actin mesh followed by a microtubule - rich region ( etienne - manneville , 2004 ) ( fig .
1 a ) . a few pioneering microtubules do extend into the f - actin mesh ( small et al .
schematic depiction of similar intracellular zonation at the leading edge of animal and plant cells .
( a ) a generalized motile animal cell with a rear end and leading edge .
enlarged view of the lamellipodium displays an organelle - free area near the absolute edge , followed by an actin - rich zone with dynamic , rapidly polymerizing actin .
the microtubular zone usually lies behind the actin - packed zone , though pioneering microtubules make their way into the actin zone .
( b ) a seedling depicting two of the regions where  diffuse-growing cells , such as hypocotyl cells and  tip-growing cells such as root hairs are found .
( c ) plant cells exhibiting diffuse growth have large expanding vacuoles that press the cytoplasm into a thin layer against the plasma membrane .
a fine f - actin meshwork lying below the plasma membrane is followed by cytoplasmic microtubules on its inner side .
( d ) plant cells that extend by tip - focused growth are characterized by an apical accumulation of vesicles in an organelle - free zone . a fine actin meshwork with its distal region interspersed by dynamic microtubules follows .
a vacuole occupies the rest of the nongrowing , mature part of the tubular cell .
note that the conserved region ( blue speckled ) of rho - gtpase / actin interaction leading to a fine f - actin mesh in each cell type has been based on the localization of rac in live animal cells ( kraynov et al . , 2000 ) and atrop localizations for different plant cells ( fu et al . , 2001 , 2002 ;
a very similar intracellular zonation is seen in plants ( fig . 1 , b  d ; mathur , 2004 ) .
for plant cells , two kinds of expansion modes are recognized ; one called tip - growth , where the growth processes is limited to a small region that extends to form a tubular structure , and the second , designated diffuse growth , where the process of growth is dispersed over a large area of the cell ( mathur , 2004 ) . tip - growing cells ( fig .
1 , b and d ) are best typified by elongating root - hair and pollen tubes and , like lamellipodia , exhibit an apical region with a stretched plasma membrane , followed by a organelle - depleted clear zone ( vidali and hepler , 2001 ) .
a fine , labile f - actin zone is defined next and leads into a dense f - actin region where the filaments become progressively bundled ( fu et al . , 2001 ; ketelaar et al . ,
extend only to the edge of the fine f - actin mesh ( sieberer et al .
a large vacuole fills the rest of the cell ( carol and dolan , 2002 ) . by comparison
, the volume occupied by vacuoles is quite large in diffuse - growing cells so that the cytoplasm is compressed into a thin layer against the plasma membrane ( fig . 1 , b and c ; mathur et al .
although this thin cytoplasmic layer has not allowed intracellular zonation in these cells to be appreciated as clearly as in tip - growing cells , a fine cortical mesh , underlying cytoplasmic f - actin bundles ( fu et al . , 2002 ; mathur et al . , 2003b ) , and microtubules ( saedler et al . , 2004a )
1 c ) . the apparent similarity in zonal relationship between the plasma membrane and cytoskeletal elements in the region of active protrusive growth between animal and plant cells ( fig .
1 ) extends to the molecular mechanisms responsible for the creation of a leading edge .
a considerable body of information exists about the multiple protein complexes and regulatory molecules that get activated to create the leading edge in an animal cell ( kraynov et al . , 2000 ;
three key interacting components  members of the rho / rac / cdc42 superfamily of gtpases , and actin and microtubule cytoskeletons 
although rho - gtpase loops interact with and regulate both actin and microtubule cytoskeletons and microtubules provide directionality to the leading edge , it is actin dynamics that play a major role in membrane protrusion ( small et al .
cytoskeleton regulation in animal cells involves cdc42/rac - gtpase  triggered activation of a suppressor of camp receptor from dictyostelium ( scar)/wiskott - aldrich syndrome protein family verprolin - homologous protein ( wave ) complex .
this complex in turn activates a 7-subunit actin - related protein ( arp2/3 ) complex to enhance actin polymerization ( millard et al . , 2004 ;
the final outcome is a fine dendritic mesh of filamentous actin seen at the leading edge ( fig .
1 ) . plants , too , possess a unique subfamily of rho - family gtpases , called rho - related gtpase of plants ( rop ; vernoud et al . , 2003 ) , whose members localize to areas of active growth ( fu et al . , 2001 , 2002 ; fig
. 1 , c and d ) and play numerous roles , including that of actin  cytoskeleton regulation ( gu et al . ,
further , different components of a putative scar / wave - like complex ( millard et al . , 2004 ; vartiainen and machesky , 2004 ) , namely scar - related proteins ( frank et al . ,
2004 ) , nap125/gnarled ( brembu et al . , 2004 ; deeks et al . , 2004 ; el - din el - assal et al
, 2004 ; li et al . , 2004 ; zimmermann et al . , 2004 ) , pir121/klunker / pirogi ( basu et al . , 2004 ;
brembu et al . , 2004 ; li et al . , 2004 ; saedler et al .
, 2004b ) , hspc300/brick1 ( frank and smith , 2002 ) , and ab1 - 1like proteins ( deeks et al . , 2004 ) , have been cloned from arabidopsis .
at least one rop , atrop2 , has been shown to interact with pir121 , strongly suggesting that a scar / wave - like pathway mediates the rop 
finally , homologues of the different subunit of the arp2/3 complex have been cloned from plants and in some cases shown to be interchangeable with their animal orthologues ( le et al .
, 2003 ; li et al . , 2003 ; mathur et al . , 2003a , b ; el - assal et al . , 2004 ; saedler et al .
are believed to affect microtubule plus - end growth by removing a stearic hindrance that allows cytoplasmic microtubules localized access to cortical domains ( rodriguez et al . , 2003 ) .
in plants , studies on actin  microtubule interactions are just beginning ( mathur , 2004 ) .
however , observations of an aberrant aggregation and stabilization of cytoplasmic microtubules in actin - compromised cells suggests an actin control over microtubules very similar to that observed in animal cells ( saedler et al . , 2004a ) .
alternatively , microtubule control over the actin cytoskeleton has also been demonstrated in drosophila by providing evidence that eb1 , a microtubule plus end  binding protein , mediates the delivery of drhogef2 , an activator of rho1 to cortical domains for its actomyosin - related functions in cell retraction ( rogers et al . , 2004 ) . though not yet demonstrated , similar microtubule 
rop interactions could be envisaged in plants too , as several microtubule plus end  binding proteins ( bisgrove et al . ,
2004 ) as well as a putative rop - gef , spike1 , have been cloned from arabidopsis .
interestingly , cells of the spike1 mutant display an aberrant microtubule cytoskeleton ( qiu et al . , 2002 ) .
numerous other modulators ( table i ) involved in fine - tuning of the core elements and their interactions appear to be conserved between animals and plants ( goode et al .
, 2000 ; rodriguez et al . , 2003 ; wasteneys and yang , 2004 ) ( table i ) .
adf , actin depolymerizing factor ; eb1 , microtubule plus end  binding protein ; rop , rho - like proteins of plants ; gef , guanine nucleotide exchange factor ; gap , gtpase - activating protein ; gdi , guanine nucleotide dissociation inhibitor .
compromised function of the three key molecular components , as mentioned in the previous section , has major implications for both motility and cell shape .
a brief comparison of cellular phenotypes resulting from defects in the core elements underscores the commonalities .
actin - dependent lamellipodium protrusion and cell motility get greatly attenuated in animal cells with an aberrant actin cytoskeleton ( etienne - manneville , 2004 ) .
likewise , cell expansion is significantly reduced in actin - compromised plant cells ( baluska et al . , 2001 ) .
this is especially apparent in arabidopsis mutants such as nap125/gnarled , pir121/klunker / pirogi ( brembu et al . , 2004 ; deeks et al . , 2004 ; el - assal et al . , 2004 ; li et al . , 2004 ; saedler et al
2004 ) , and hspc300/brick1 ( mutant in maize ; frank and smith , 2002 ) , whose respective gene products possibly feed into the arp2/3 complex regulatory pathway .
mutations in different subunit homologues of the putative plant arp2/3 complex such as arp2/wurm , arp3/distorted1 , arpc2/distorted2 , and arpc5/crooked also exhibit similar phenotypes ( for review see mathur , 2005 ) .
each of these mutants exhibits aberrant f - actin organization and characteristic cellular deformations resulting from abnormal , random expansion .
however , as mentioned earlier in this paper , actin cytoskeleton activity is heavily dependent upon the stimulation provided by rho - gtpases .
animal cells with aberrant rho - gtpase activity display numerous defects in pseudopod extension and chemotaxis ( chung et al . , 2000 ; raftopoulou and hall , 2004 ) .
overexpression of rops in plants results in increased fine f - actin meshworks and a swollen cell morphology , suggesting an overall reduction in growth polarization ( molendijk et al . , 2001 ; fu et al . , 2002 ; jones et al . , 2002
) . because loss of polarity is a characteristic feature of microtubule defective cells too , in both animal ( small et al .
, 2002b ) and plant cells ( mathur , 2004 ) these observations again point to a conserved , intimate relationship between rho - family gtpases , and the actin and microtubule cytoskeletons .
although the similarities described above emphasize conserved mechanisms for membrane protrusion between animal and plant cells , they also reflect the profound impact the mechanism of boundary extension has on the survival ability of an organism . for motile cells ,
loss of motility leads to an inability to move to resource - rich areas and ultimately leads to death . consequently
, many animal mutants for the core elements display lethal phenotypes . for the nonmotile but photosynthetic ( and therefore relatively self - sufficient ) plant cells ,
a change in cell shape does not lead to death directly . as a result , many comparable plant mutants , though misshapen and underweight ( el - assal et al .
, 2004 ; li et al . , 2004 ) , are able to grow and complete their life cycle , giving the erroneous impression that the core genes are not as vital for plants as for animals
. however , when grown in a population of siblings , the inability of mutant plants to extend properly becomes a severe handicap that affects their survival ability .
a comparison of key cell biological and molecular features in two apparently disparate processes , namely , forward motility of animal cells and extension growth of plant cells , reveals a core machinery that has been conserved by both motile and nonmotile life forms for achieving boundary extension ; an essential requirement for survival .
given this realization , future studies on boundary extension mechanisms of different life forms should prove exciting , not only for uncovering the extent of similarities , but also for discovering the range of cell biological and molecular variations , and the adaptations and innovations introduced by different organisms for surviving in their specialized niches .
 locomotion clearly sets plants and animals apart . 
 however , recent studies in higher plants reveal cell - biological and molecular features similar to those observed at the leading edge of animal cells and suggest conservation of boundary extension mechanisms between motile animal cells and nonmotile plant cells . 
Boundary extension: a necessity for living organisms
Boundary extension requires a leading edge
Creating the leading edge
Defects at the leading edge: mutant phenotypes
Conclusions and perspectives