The mutation of this complex impairs
tethering and fusion of the vesicles that results in accumulation of vesicles
inside cell. (Novick et al. 1980; Heider and Munson, 2012).

During cell division, cell plate
formation is carried out by the accumulation of the vesicles at the site of
cytokinesis (Seguí-Simarro et al. 2004) and their fusion by the
exocyst complex (Žárský et al. 2013). Mutation of exo 70A1 shows some defects
in cell plate formation (Fendrych et al. 2010), inability in root growth, loss
of apical dominance, impaired flower development and smaller organs which
proves that they are important in growth and development process (Synek et al.
2006). There are
different isoforms of the exocyst subunits, which regulate exocytosis related
to biotic (Pe?enková et al. 2011) and abiotic stress (Lin et al. 2013; Žárský
et al. 2013) and other functions such as membrane recycling, autophagy related
vesicular transport (Žárský et al. 2013). Sec3 are the primary subunit to
connect with the target membrane (Finger et al. 1998), their mutants in yeast
resulted accumulation of the secretory vesicle in cytoplasm as they were unable
to dock with the membrane (Finger and Novick, 1997). Sec 3 mutants are reported
with root hair growth defects that leads to various growth defects in plants
(Wen et al. 2005).  Mutation of Sec 8
shows defective pollen germination and tube growth (Cole et al. 2005). The
subunit exo84 plays key role in formation of exocyst complex and targeting
(Zhang et al. 2005). Study of Exo 84 mutation in yeast indicate their role in
post Golgi secretion process (Zhang et al. 2005). Exo 84 homolog exo84b
mutation resulted vesicles accumulation in cytoplasm, cytokinesis defects, irregular
phenotype with retarded growth and sterility (Fendrych et al. 2010).  The accumulated vesicles in exo84 mutants
contain compounds such as pectin and xyloglucan (Fendrych et al. 2010). Other
subunits such as SEC6, SEC8, SEC15b and EXO70A1 are also detected in various
stages of cell plate formation (Fendrych et al. 2010). In plants mutation of
Sec5, Sec6, Sec8, Sec15a resulted less pollen germination with reduced growth
(Hala et al. 2008). During cytokinesis secretory vesicles are directed to the
cell plate formation matrix where exocyst complex assist tethering and fusion (Fredrych
et al. 2010). After fusion vesicles are elongated projecting dumbbell shape,
connect other vesicles and form a perforated layer with network of tubes and vesicles
with callose deposition (Fendrych et al. 2010).

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Till date study of exocyst complex
towards defense is limited however, from the perspective of cellular response
towards various biological activity as they relate repair and growth,
speculation could be made from these experiments that they use these similar
strategies in defense responses (Pe?enková, et al. 2011, 2017). All the exocyst
subunits have their homologs in plants (Elias et al. 2003; Synek et al. 2006). Mutation
of exocyst subunits; Sec5, Sec6, Sec8, Sec15A, Exo70B, and Exo 84B in Nicotiana benthamiana plants resulted
impaired resistance to Phytophthora infestans
(Du et al. 2017). Also, Sec5, Sec6, and Sec10 mutants in N. benthamiana showed more bacterial infection and growth whereas
mutants of other subunits had no effect on resistance (Du et al. 2017). The
exocyst subunit Sec5 is related with the secretion of Pathogenesis-Related (PR)
proteins as their mutants produced less PR proteins with higher susceptibility
to fungal pathogen (Du et al. 2015). Plants pathogens with their virulence gene
attack components of vesicle transport system by impairing secretion of
Golgi-derived vesicles (Driouich et al. 1997), callose deposition and
penetration resistance (Nielsen et al. 2012).

Thus, although limited experiments
have been conducted regarding their defense responses however as described
above, earlier experiments related to either growth, development and repair or
defense process and mechanism, indicate that the octameric protein complex
exocyst is involved in the defense processes providing genetic resistance in
plants thereby playing a vital role in the vesicle membrane fusion.