Tapetum Degeneration Retardation is Critical for Aliphatic Metabolism and Gene Regulation during Rice Pollen Development

As a complex wall system in flowering plants, the pollen outer wall mainly contains aliphatic sporopollenin; however, the mechanism for synthesizing these lipidic precursors during pollen development remains less well understood. Here, we report on the function of the rice tapetum-expressing TDR （Tapetum Degeneration Retardation） gene in aliphatic metabolism and its regulatory role during rice pollen development. The observations of transmission electron microscopy （TEM） and scanning electron microscopy （SEM） analyses suggested that pollen wall formation was significantly altered in the tdr mutant. The contents of aliphatic compositions of anther were greatly changed in the tdr mutant revealed by GC-MS （gas chromatography-mass spectrometry） testing, particularly less accumulated in fatty acids, primary alcohols, alkanes and alkenes, and an abnormal increase in secondary alcohols with carbon lengths from C29 to C3S in tdr. Microarray data revealed that a group of genes putatively involved in lipid transport and metabolism were significantly altered in the tdr mutant, indicating the critical role of TDR in the formation of the pollen wall. Also, a wide range of genes （236 in total--154 up-regulated and 82 down-regulated） exhibited statistically significant expressional differences between wild-type and tdr. In addition to its function in promoting tapetum PCD, TDR possibly plays crucial regulatory roles in several basic biological processes during rice pollen development.

Varied Transcriptional Efficiencies of Multiple Arabidopsis U6 Small Nuclear RNA Genes

Arabidopsis U6 small nuclear RNA （snRNA） promoters are those transcribed by RNA polymerase Ⅲ, but all the core elements for transcriptional initiation are located in the 5＇ promoter region. Previously, three Arabidopsis U6 snRNA genes （U6-1, U6-26, and U6-29） were identified. Herein, we have further identified three new U6 loci （U6-4, U6-5, and U6-6） in the Arabidopsis genome. Alignment of these revealed that the upstream sequence element and TATA elements were contained in six U6 promoters. In addition, a unique, highly conserved element named the ＂CAT＂ element was observed in the promoter region. To understand the expression patterns of these U6 genes in Arabidopsis, we fused these promoters to the DNA segment of β-glucuronidase and then transferred these six constructs into Arabidopsis. Real-time reverse transcription-polymerase chain reaction analysis of these fused transcripts indicated that the newly identified U6 genes are active in Arabidopsis and that the U6-26 promoter seems to have higher transcriptional activity in leaf, stem, flower and silique. These results help to understand the function of these U6 snRNAs in Arabidopsis.