Abstract
Proper growth of the pollen tube depends on an elaborate mechanism that integrates several molecular and cytological sub-processes and ensures a cell shape adapted to the transport of gametes. This growth mechanism is controlled by several molecules among which cytoplasmic and apoplastic polyamines. Spermine (Spm) has been correlated with various physiological processes in pollen, including structuring of the cell wall and modulation of protein (mainly cytoskeletal) assembly. In this work, the effects of Spm on the growth of pear pollen tubes were analyzed. When exogenous Spm (100 μM) was supplied to germinating pollen, it temporarily blocked tube growth, followed by the induction of apical swelling. This reshaping of the pollen tube was maintained also after growth recovery, leading to a 30–40% increase of tube diameter. Apical swelling was also accompanied by a transient increase in cytosolic calcium concentration and alteration of pH values, which were the likely cause for major reorganization of actin filaments and cytoplasmic organelle movement. Morphological alterations of the apical and subapical region also involved changes in the deposition of pectin, cellulose, and callose in the cell wall. Thus, results point to the involvement of Spm in cell wall construction as well as cytoskeleton organization during pear pollen tube growth.
Highlights
Pollen tube growth represents a remarkable example of polarized expansion, which consists in the deposition of new cell wall material only at the extending tip (Hepler et al, 2013)
We have recently demonstrated that the PA spermine (Spm) can enter through the apex of pear pollen tubes and spread in the sub-apical region thereby determining enlargement of the apex
When treated with Spm, pollen tubes were characterized by a lower growth velocity, 0.41 (±0.15) μm min−1; the tube apex isotropically enlarged into the so-called balloon stage within the first 15 min (Figure 2D)
Summary
Pollen tube growth represents a remarkable example of polarized expansion, which consists in the deposition of new cell wall material only at the extending tip (Hepler et al, 2013). Synthetized ductile methyl-esterified pectins are first secreted at the apex of pollen tubes (O’ Neill et al, 1990) After deposition, they are chemically converted into acid pectins at the subapex edge (Rockel et al, 2008) where they bind calcium, thereby contributing to strengthen the cell wall (Palin and Geitmann, 2012; Wolf and Greiner, 2012). They are chemically converted into acid pectins at the subapex edge (Rockel et al, 2008) where they bind calcium, thereby contributing to strengthen the cell wall (Palin and Geitmann, 2012; Wolf and Greiner, 2012) This prevents additional deformation of the cell wall and contributes to maintain the cylindrical shape of pollen tubes.
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