PECTIN METHYLESTERASE48 Is Involved in Arabidopsis Pollen Grain Germination  

Abstract

Germination of pollen grains is a crucial step in plant reproduction. However, the molecular mechanisms involved remain unclear. We investigated the role of PECTIN METHYLESTERASE48 (PME48), an enzyme implicated in the remodeling of pectins in Arabidopsis (Arabidopsis thaliana) pollen. A combination of functional genomics, gene expression, in vivo and in vitro pollen germination, immunolabeling, and biochemical analyses was used on wild-type and Atpme48 mutant plants. We showed that AtPME48 is specifically expressed in the male gametophyte and is the second most expressed PME in dry and imbibed pollen grains. Pollen grains from homozygous mutant lines displayed a significant delay in imbibition and germination in vitro and in vivo. Moreover, numerous pollen grains showed two tips emerging instead of one in the wild type. Immunolabeling and Fourier transform infrared analyses showed that the degree of methylesterification of the homogalacturonan was higher in pme482/2 pollen grains. In contrast, the PME activity was lower in pme482/2, partly due to a reduction of PME48 activity revealed by zymogram. Interestingly, the wild-type phenotype was restored in pme482/2 with the optimum germination medium supplemented with 2.5 mM calcium chloride, suggesting that in the wild-type pollen, the weakly methylesterified homogalacturonan is a source of Ca 2+ necessary for pollen germination. Although pollen-specific PMEs are traditionally associated with pollen tube elongation, this study provides strong evidence that PME48 impacts the mechanical properties of the intine wall during maturation of the pollen grain, which, in turn, influences pollen grain germination.

Exocyst SEC3 and phosphoinositides define sites of exocytosis in pollen tube initiation and growth

“…6F). In agreement with published results on tobacco and Arabidopsis pollen tubes (Chebli et al, 2012;Leroux et al, 2015), the LM20-labeled esterified pectins were observed only at the tip of elongating pollen tubes and at the bud of germinating pollen, where they are secreted (Fig. 6G).…”

“…The function of SEC3a in determining the site of the germination pore is in agreement with the earlier manifestation of the sec3a-1 mutant defect in pollen germination, as compared with other exocyst mutants. Similar to sec3a-1/GFP-SEC3a, pme48 mutant pollen developed multiple tips, indicating that pectin deesterification is required for the selection/maintenance of a single germinating tip (Leroux et al, 2015). In pme48, the LM20-labeled esterified pectins were more abundant and less focally localized (Leroux et al, 2015).…”

“…Similar to sec3a-1/GFP-SEC3a, pme48 mutant pollen developed multiple tips, indicating that pectin deesterification is required for the selection/maintenance of a single germinating tip (Leroux et al, 2015). In pme48, the LM20-labeled esterified pectins were more abundant and less focally localized (Leroux et al, 2015). In contrast, in sec3a-1/GFP-SEC3a pollen, the LM20-labeled esterified pectins were highly focused at the tip of the growing pollen tube and absent from the arrested tube.…”

“…Therefore, it is likely that the mechanisms responsible for the formation of multiple tips are different in the two mutants. Leroux et al (2015) suggested that, in pme48, multiple tips emerged due to the softening of the cell wall that contained higher levels of esterified pectins. It is likely that multiple tips in sec3a-1/GFP SEC3a resulted from the ectopic activity of GFP-SEC3a that, in some cases, supported bud site selection/initiation but failed to maintain tube growth.…”

PECTIN METHYLESTERASE34 Contributes to Heat Tolerance through Its Role in Promoting Stomatal Movement

“…Arabidopsis PME41 and PME48 participate in chilling/freezing tolerance through the regulation of brassinosteroid signaling and in pollen grain germination, respectively (Qu et al, 2011;Leroux et al, 2015). Arabidopsis PME3 has been found to interact directly with the cellulose-binding protein of the cyst nematode (Heterodera schachtii), reducing the level of pectin methylesterification in the cell wall to aid cyst nematode parasitism (Hewezi et al, 2008).…”