Phosphorylation of Serine 186 of bHLH Transcription Factor SPEECHLESS Promotes Stomatal Development in Arabidopsis

The initiation of stomatal lineage and subsequent asymmetric divisions in Arabidopsis require the activity of the basic helix-loop-helix transcription factor SPEECHLESS （SPCH）. It has been shown that SPCH controls entry into the stomatal lineage as a substrate either of the MITOGEN-ACTIVATED PROTEIN KINASE （MAPK） cascade or GSK3-1ike kinase BRASSlNOSTEROID INSENSITIVE 2 （BIN2）. Here we show that three serine residues of SPCH appear to be the primary phosphorylation targets of Cyclin-Dependent Kinases A;1 （CDKA;1） in vitro, and among them Serine 186 plays a crucial role in stomatal formation. Expression of an SPCH construct harboring a mutation that results in phosphorylation deficiencies on Serine 186 residue failed to rescue stomatal defects in spch null mutants. Expression of a phosphorylation-mimic mutant SPCHS186D complemented stomatal production defects in the transgenic lines harboring the targeted expression of dominant-negative CDKA;1.N146. Therefore, in addition to MAPK- and BIN2-mediated phos- phorylation on SPCH, phosphorylation at Serine 186 is positively required for SPCH function in regulating stomatal development.

AtTMEM18 plays important roles in pollen tube and vegetative growth in Arabidopsis

In flowering plants, pollen tube growth is essential for delivery of male gametes into the female gametophyte or embryo sac for double fertilization. Although many genes have been identified as being involved in the process, the molecular mechanisms of pollen tube growth remains poorly understood. in this study, we identified that the Arabidopsis Transmembrane Protein 18 （AtTMEM18） gene played important roles in pollen tube growth. The AtTMEM18 shares a high similarity with the Transmembrane 18 proteins （TMEM18s） that are conserved in most eukaryotes and may play important roles in obesity in humans. Mutation in the AtTMEM18 by a Ds insertion caused abnormal callose deposition in the pollen grains and had a significant impact on pollen germination and pollen tube growth. AtTMEM18 is expressed in pollen grains, pollen tubes, root tips and other vegetative tissues. The pollen-rescued assays showed that the mutation in AtTMEM18 also caused defects in roots, stems, leaves and transmitting tracts. AtTMEM18-GFP was located around the nuclei. Genetic assays demonstrated that the localization of AtTMEM18 around the nuclei in the generative cells of pollen grains was essential for the male fertility.Furthermore, expression of the rice TM EM18-homologous protein （OsTMEM18） driven by LAT52 promoter could recover the fertility of the Arabidopsis attmem18 mutant. These results suggested that the TMEM18 is important for plant growth in Arabidopsis.

WBC27,an Adenosine Tri-phosphate-binding Cassette Protein,Controls Pollen Wall Formation and Patterning in Arabidopsis

In flowering plants, the exine components are derived from tapetum. Despite its importance to sexual plant reproduction, little is known about the translocation of exine materials from tapetum to developing microspores. Here we report functional characterization of the arabidopsis WBC27 gene. WBC27 encodes an adenosine tri-phosphate binding cassette （ABC） transporter and is expressed preferentially in tapetum. Mutation of WBC27 disrupted the exine formation. The wbc27 mutant microspores began to degenerate once released from tetrads and most of the microspores collapsed at the uninucleate stage. Only a small number of wbc27-1 microspores could develop into tricellular pollen grains. These survival pollen grains lacked exine and germinated in the anther before anthesis. All of these results suggest that the ABC transporter, WBC27 plays important roles in the formation of arabidopsis exine, possibly by translocation of lipidic precursors of sporopollenin from tapetum to developing microspores.