Mutation in SUMO E3 ligase, SIZ1, Disrupts the Mature Female Gametophyte in Arabidopsis

Female gametophyte is the multicellular haploid structure that can produce embryo and endosperm after fertilization, which has become an attractive model system for investigating molecular mechanisms in nuclei migration, cell specification, cell-to-cell communication and many other processes. Previous reports found that the small ubiquitin-like modifier (SUMO) E3 ligase, SIZ1, participated in many processes depending on particular target substrates and suppression of salicylic acid (SA) accumulation. Here, we report that SIZ1 mediates the reproductive process. SIZ1 showed enhanced expression in female organs, but was not detected in the anther or pollen. A defect in the siz1-2 maternal source resulted in reduced seed-set regardless of high SA concentration within the plant. Moreover, aniline blue staining and scanning electron microscopy revealed that funicular and micropylar pollen tube guidance was arrested in siz1-2 plants. Some of the embryo sacs of ovules in siz1-2 were also disrupted quickly after stage FG7. There was no significant affects of the siz1-2 mutation on expression of genes involved in female gametophyte development- or pollen tube guidance in ovaries. Together, our results suggest that SIZ1 sustains the stability and normal function of the mature female gametophyte which is necessary for pollen tube guidance.雌配子体（female gametophyte）是一类多细胞单倍体结构，可在受精后产生胚胎与胚乳，现已成为研究核迁移、细胞特化、细胞间通讯等诸多过程分子机制的极具吸引力的模型系统。既往研究显示，类泛素小修饰蛋白（small ubiquitin-like modifier, SUMO）E3连接酶SIZ1可通过靶向特定底物及抑制水杨酸（salicylic acid, SA）积累参与多种生理过程。本研究证实SIZ1介导植物生殖进程：SIZ1在雌性生殖器官中表达上调，但在花药与花粉中未检测到其表达。siz1-2突变体的母本缺陷会导致结籽率降低，即便植株体内水杨酸浓度升高亦无法改善该表型。此外，苯胺蓝染色与扫描电子显微镜分析显示，siz1-2植株的珠柄及珠孔端花粉管导向过程受阻。siz1-2突变体的部分胚珠内胚囊在发育至FG7时期后会快速出现结构紊乱。siz1-2突变并未对子房内参与雌配子体发育或花粉管导向的基因表达产生显著影响。综上，本研究结果表明，SIZ1可维持成熟雌配子体的稳定性与正常功能，而这一过程是花粉管导向所必需的。