Abstract
SummaryHow actin-bundling factors cooperatively regulate shank-localized actin bundles remains largely unexplored. Here we demonstrate that FIM5 and PLIM2a/PLIM2b decorate shank-localized actin bundles and that loss of function of PLIM2a and/or PLIM2b suppresses phenotypes associated with fim5 mutants. Specifically, knockout of PLIM2a and/or PLIM2b partially suppresses the disorganized actin bundle and intracellular trafficking phenotype in fim5 pollen tubes. PLIM2a/PLIM2b generates thick but loosely packed actin bundles, whereas FIM5 generates thin but tight actin bundles that tend to be cross-linked into networks in vitro. Furthermore, PLIM2a/PLIM2b and FIM5 compete for binding to actin filaments in vitro, and PLIM2a/PLIM2b decorate disorganized actin bundles in fim5 pollen tubes. These data together suggest that the disorganized actin bundles in fim5 mutants are at least partially due to gain of function of PLIM2a/PLIM2b. Our data suggest that the balance between FIM5 and PLIM2a/PLIM2b is crucial for the normal bundling and organization of shank-localized actin bundles in pollen tubes.
Highlights
The actin cytoskeleton has been implicated in numerous fundamental physiological cellular processes, such as cell motility, cell division, cytokinesis, cell expansion, and intracellular trafficking (Pollard, 2016; Pollard and Cooper, 2009)
The Expression of PLIMs Is Upregulated in fim5 Mutants and Loss of Function of PLIM2a and/ or PLIM2b Suppresses the Phenotype Associated with fim5 We initially examined the transcript levels of several actin-bundling factors in fim5 pollen and compared them with the levels in WT pollen
It was reported that PLIM2c is biochemically distinct from PLIM2a and PLIM2b as it is the only Arabidopsis LIM to clearly respond to Ca2+ (Papuga et al, 2010)
Summary
The actin cytoskeleton has been implicated in numerous fundamental physiological cellular processes, such as cell motility, cell division, cytokinesis, cell expansion, and intracellular trafficking (Pollard, 2016; Pollard and Cooper, 2009). Within the shank region of angiosperm pollen tubes, actin filaments form parallel actin bundles with their barbed ends facing tipward at the cortex and backward within the middle region (Lenartowska and Michalska, 2008) This unique organization pattern of longitudinal actin bundles together with the barbed-end-directed myosin XIs (Madison et al, 2015) generates the reverse fountain pattern of cytoplasmic streaming and drives various intracellular trafficking events in pollen tubes (Chebli et al, 2013; Chen et al, 2009; Cheung and Wu, 2008; Fu, 2015; Guan et al, 2013; Qin and Yang, 2011; Qu et al, 2015; Ren and Xiang, 2007; Staiger et al, 2010; Vidali and Hepler, 2001). The molecular mechanism by which the shank-oriented actin bundles are generated and maintained in pollen tubes remains incompletely understood
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