酮类物质合成酶OsPKS1和Os PKS2对水稻花粉外壁形成的作用

The Function of the Polyketide Synthase OsPKS1 and OsPKS2 in Regulating Pollen Wall Formation in Rice

【背景】植物花粉外围包裹的花粉外壁作为植物雄性配子的天然保护屏障对植物的生殖发育起到非常重要的作用。植物花粉外壁的主要成分是孢粉素,主要由脂类物质和酚类物质构成。因此,脂类物质和酚类物质的代谢是植物花药内外壁形成和花粉外壁形成的关键步骤。在其合成过程中,PKS1/PKSA/LAP6和PKS2/PKSB/LAP5在不同物种间发挥保守的生化功能。【目的】通过研究水稻OsPKS1和OsPKS2在花药内外壁和花粉外壁发育过程中的作用,为水稻花药内外壁和花粉外壁合成机理提供新认识。【方法】水稻花药发育基因共表达网络AntherNet预测到一个可能参与孢粉素合成的基因OsPKS1,利用CRISPR/Cas9技术在野生型9522背景和突变体ospks2背景下敲除OsPKS1获得ospks1单突变体和ospks1 ospks2双突变体。在同一生长条件下比较野生型和突变体植株表型,分析突变体植株的营养生长和花器官发育情况。通过I2-KI染色分析ospks1和ospks1 ospks2的花粉活力。通过半薄切片观察野生型和突变体各个时期花药四层细胞发育及小孢子发育,利用扫描电子显微镜观察野生型和突变体花药外壁、花药内壁和花粉外壁表面的精细结构,利用透射电子显微镜观察野生型和突变体花药壁细胞、花粉外壁和乌氏体的精细结构。【结果】获得4个ospks1单突变体和4个ospks1 ospks2双突变体,其中,ospks1-3是纯合的单突变体,ospks1-4 ospks2是纯合的双突变体。ospks1-3和ospks1-4 ospks2均呈现雄性不育的表型。ospks1-3与ospks2的花粉外壁和乌氏体结构均不正常,但两者结构不同。ospks1-3花药表面可形成凸起的外壁结构;绒毡层可正常降解。花粉外壁内部形成大量微小的空洞,柱状体变短,无法有效连接覆盖层和花粉外壁内层;乌氏体的底部结构减小,顶部结构增多,并且较野生型更为尖锐。ospks1-4 ospks2花药外壁角质层减少;绒毡层无法正常降解。小孢子表面无花粉外壁,在11期降解;乌氏体在9期形态异常,数目变少,到11期从绒毡层脱落。【结论】PKS1/PKSA/LAP6和PKS2/PKSB/LAP5的功能在多个物种间均保守,可以影响孢粉素的合成和堆积。但在水稻中两者对于花粉外壁内部结构身碎骨和乌式体形成的功能不同:OsPKS1对柱状层的形成以及乌氏体的底部结构形成更为重要;而OsPKS2对于覆盖层的形成以及乌氏体的顶部结构更为重要。两者互相补充,共同调控花粉外壁、花药外壁的形成和绒毡层的降解。

【Background】 Plant pollen is surrounded by pollen wall which acts as a natural protective barrier for male gametes and plays a pivotal role in plant reproductive development. The main component of pollen wall is sporopollenin, which is mainly composed of lipidic and phenolic substances. Therefore, the metabolism of these two substances is a key step for anther wall and pollen wall formation. PKS1/PKSA/LAP6 and PKS2/PKSB/LAP5 show conserved biochemical functions in sporopollenin biosynthesis pathways among different species.【Objective】 The role of OsPKS1 and OsPKS2 in rice anther wall and pollen wall development was studied to provide a new understanding for the mechanism of this process.【Method】 A gene co-expressed network, AntherNet predicted a gene OsPKS1 that might be involved in sporopollenin biosynthesis, using the CRISPR/Cas9 genome editing system to generate ospks1 and ospks1 ospks2 in Japonica subspecies 9522 background and ospks2 background, respectively. Under the same growth condition, the vegetative growth and floral organ development of the mutant plants were analyzed by comparing the phenotypes of the wild type and the mutants. I2-KI staining was utilized to analyze the pollen viability of ospks1 and ospks1 ospks2. Semi-thin section was performed to observe four cell layers and microspore development in the wild type and the mutants at different stages. Scanning electron microscope (SEM) was used to observe the fine structures of anther wall outer and inner surface as well as pollen wall surface both in the wild type and the mutants. And transmission electron microscopy (TEM) was performed to observe the fine structures of anther wall cell, Ubisch body and pollen wall of the wild type and the mutants.【Result】 Four ospks1 and four ospks1 ospks2 were obtained by CRISPR/Cas9 approach, among which ospks1-3 and ospks1-4 ospks2 were homozygous mutants. Both ospks1-3 and ospks1-4 ospks2 were male sterile. ospks1-3 and ospks2 displayed abnormal pollen wall and Ubisch body, however, the detailed morphology was different between two mutants. ospks1-3 could form convex wall structure on the surface of pollen wall and the tapetal layer could be normally degraded;a large number of tiny cavities were formed in the inner structure of pollen wall and the bacula became shorter, which might cause invalid connection between tectum and nexine;the bottom structure of Ubisch bodies was decreased while the top structure was increased and Ubisch bodies were sharper than those of the wild type. By observing ospks1-4 ospks2, it was found that the cuticle was reduced and the tapetal layer could not be degraded normally. Besides, there was no obvious pollen wall structure on the surface of microspores and the microspores were degraded at stage 11;Ubisch bodies were less formed with abnormal structure at stage 9 and were detached from the anther wall at stage 11.【Conclusion】 The function of PKS1/PKSA/LAP6 and PKS2/PKSB/LAP5 are conserved among various species and could affect the biosynthesis and accumulation of sporopollenin. However, these two genes show different function for the formation of the inner structure of pollen wall and Ubisch bodies in rice: OsPKS1 is more important for the formation of bacula and the bottom structure of Ubisch bodies;OsPKS2 is more important for the formation of tectum and the top structure of Ubisch bodies. They regulate the formation of pollen wall, anther wall and the degradation of tapetum.