PECTIN METHYLESTERASE48 is involved in Arabidopsis pollen grain germination.

Christelle Leroux,Sophie Bouton,Arnaud Lehner,Stéphanie Guénin,Françoise Fournet,Marie-Christine Kiefer-Meyer,Patrice Lerouge,Azeddine Driouich,Alain Mareck,Jean-Claude Mollet,Jérôme Pelloux,Tohnyui Ndinyanka Fabrice,Christoph Ringli

doi:10.1104/pp.114.250928

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 pme48-/- pollen grains. In contrast, the PME activity was lower in pme48-/-, partly due to a reduction of PME48 activity revealed by zymogram. Interestingly, the wild-type phenotype was restored in pme48-/- 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.

Highlights

Germination of pollen grains is a crucial step in plant reproduction
The expression of the 14 pollen-specific pectin methylesterases (PMEs) genes was analyzed by quantitative reverse transcription-PCR on total RNA extracted from dry and imbibed pollen grains and 6-h-old pollen tubes
Data have shown an important role for the pectin demethylesterification process in pollen tetrad separation (Francis et al, 2006) and pollen tube growth (Jiang et al, 2005; Tian et al, 2006), the involvement of PMEs during pollen germination is still poorly understood

Summary

Introduction

Germination of pollen grains is a crucial step in plant reproduction. the molecular mechanisms involved remain unclear. AtPPME1 (At1g69940), coding for a pollen-specific group 1 PME, was identified to play an important role in pollen tube growth (Tian et al, 2006). We have investigated in vitro and in vivo pollen germination, pollen tube morphology and growth, as well as the level of PME activity and the DM of the HG by immunolabeling and Fourier transform infrared (FT-IR) spectroscopy in mutant and wild-type plants. Our results show that the group 1 PME48, the second most expressed PME gene in pollen, plays a major role in remodeling the HG of the intine cell wall during Arabidopsis pollen grain maturation, resulting, after rehydration, in normal pollen grain germination

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Conclusion