Analysis of Genes Expressed in Rice Anthers during the Stage of Maximal Chilling Sensitivity

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Rice enters its most chilling-sensitive stage at the onset of microspore release. The microsporocytes produce a wall of callose between the primary cell wall and the plasma membrane, and it has been shown that precise regulation of callose synthesis and degradation in the anther is essential for fertile pollen formation. A cDNA for β-1,3-glucanase was isolated from rice anther and named Osg1. RT-PCR analysis revealed that Osg1 transcripts were present in leaves, roots, and anthers. Genes for 10 callose synthases in the rice genome were fully annotated and phylogenetically analyzed. Expression analysis of these genes showed that OsGSL5, an ortholog of microsporogenesis-related Arabidopsis AtGSL2, was specifically expressed in anthers, and was notably downregulated by chilling treatment. Rho-type small GTP-binding proteins are molecular switches that control callose synthase activity. Their gene expression profiles in rice anther were also analyzed. The gene expression profile during the microspore development process under chilling stress was revealed using cDNA microarray. Three novel genes whose expression levels were remarkably changed by chilling in rice anther were identified. A new cis element that includes a DNA transposon Castaway sequence was found in the 5' upstream region of two genes which were conspicuously down-regulated by chilling temperatures in rice anther. Analyses of these chilling-responsive genes are not only fundamental to the understanding of mechanisms that underlie chilling injury in rice pollen, but are also essential to the creation of a chilling-tolerant rice plant.

イネ葯は小胞子解離期に冷温感受性が最も高い。この時期に小胞子を覆う主にβ-1,3-グルカンからなるカロース壁の合成・分解を精密に制御することが稔性花粉の形成に必須である。その分子機構を明らかにするため, イネ葯で発現するβ-1,3-グルカナーゼ遺伝子Osg1 を単離し, その葯および葉, 根での発現を確認した。イネゲノム中の全カロース合成酵素遺伝子の分子系統樹と発現の解析を行い, 小胞子形成に関与するシロイヌナズナAtGSL2 のオルソログ (異種間での相同遺伝子) であるOsGSL5 の発現が葯特異的で冷温により顕著に抑制されることを見出した。また, カロース合成酵素活性の調節因子であるRho 型低分子GTP 結合タンパク遺伝子のイネ葯における発現を解析した。小胞子形成期イネ葯の冷温下での遺伝子発現変動を俯瞰することも重要と考えcDNA マイクロアレイ解析を行い, 3 つの新規イネ葯冷温応答遺伝子を同定するとともに, 冷温下のイネ葯で発現が抑制される遺伝子の5'上流域に, DNA トランスポゾン Castaway を含む新規なシス因子 (転写制御因子タンパク質 (トランス因子) が結合して遺伝子の転写発現の調節を行う) を確認した。これら冷温感受性期に発現する遺伝子の解析は, 花粉発育障害発生機構の解明, ひいては耐冷性イネ作出に向けた重要な足がかりとなるものである。

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