In flowering plants, penetration of the pollen tube through stigma, style, and transmitting tract is essential for delivery of sperm nuclei to the egg cells embedded deeply within female tissues. Despite its importance in plant reproduction, little is known about the underlying molecular mechanisms that regulate the navigation of the pollen tube through the stigma, style, and transmitting tract. Here, we report the identification and characterization of an Arabidopsis thaliana gene, VANGUARD1 (VGD1) that encodes a pectin methylesterase (PME)-homologous protein of 595 amino acids and is required for enhancing the growth of pollen tubes in the style and transmitting tract tissues. VGD1 was expressed specifically in pollen grain and the pollen tube. The VGD1 protein was distributed throughout the pollen grain and pollen tube, including the plasma membrane and cell wall. Functional interruption of VGD1 reduced PME activity in the pollen to 82% of the wild type and greatly retarded the growth of the pollen tube in the style and transmitting tract, resulting in a significant reduction of male fertility. In addition, the vgd1 pollen tubes were unstable and burst more frequently when germinated and grown on in vitro culture medium, compared with wild-type pollen tubes. Our study suggests that the VGD1 product is required for growth of the pollen tube, possibly via modifying the cell wall and enhancing the interaction of the pollen tube with the female style and transmitting tract tissues

In flowering plants, pollen formation depends on the differentiation and interaction of two cell types in the anther: the reproductive cells, called microsporocytes, and somatic cells that form the tapetum. The microsporocytes generate microspores, whereas the tapetal cells support the development of microspores into mature pollen grains. Despite their importance to plant reproduction, little is known about the underlying genetic mechanisms that regulate the differentiation and interaction of these highly specialized cells in the anther. Here, we report the identification and characterization of a novel TAPETUM DETERMINANT1 ( TPD1 ) gene that is required for the specialization of tapetal cells in the Arabidopsis anther. Analysis of the male-sterile mutant, tpd1 , showed that functional interruption of TPD1 caused the precursors of tapetal cells to differentiate and develop into microsporocytes instead of tapetum. As a results, extra microsporocytes were formed and tapetum was absent in developing tpd1 anthers. Molecular cloning of TPD1 revealed that it encodes a small protein of 176 amino acids. In addition, tpd1 was phenotypically similar to excess microsporocytes1/extra sporogenous cells ( ems1 / exs ) single and tpd1 ems1/exs double mutants. These data suggest that the TPD1 product plays an important role in the differentiation of tapetal cells, possibly in coordination with the EMS1/EXS gene product, a Leu-rich repeat receptor protein kinase.