Department of Genetics, Friedrich Schiller University Jena, Philosophenweg 12, D-07743 Jena, Germany.
Semin Cell Dev Biol. 2010 Feb;21(1):118-28. doi: 10.1016/j.semcdb.2009.11.015. Epub 2009 Nov 26.
20 years after establishment of the ABC model many of the molecular mechanisms underlying development of the angiosperm flower are relatively well understood. Central players in the gene regulatory network controlling flower development are SQUA-like, DEF/GLO-like, AG-like and AGL6/SEP1-like MIKC-type MADS-domain transcription factors. These provide class A, class B, class C and the more recently defined class E floral homeotic functions, respectively. There is evidence that the floral homeotic proteins recognize the DNA of target genes in an organ-specific way as multimeric protein complexes, thus constituting 'floral quartets'. In contrast to the detailed insights into flower development, how the flower originated during evolution has remained enigmatic. However, while orthologues of all classes of floral homeotic genes appear to be absent from all non-seed plants, DEF/GLO-like, AG-like, and AGL6-like genes have been found in diverse extant gymnosperms, the closest relatives of the angiosperms. While SQUA-like and SEP1-like MADS-box genes appear to be absent from extant gymnosperms, reconstruction of MADS-box gene phylogeny surprisingly suggests that the most recent common ancestor of gymnosperms and angiosperms possessed representatives of both genes, but that these have been lost in the lineage that led to extant gymnosperms. Expression studies and genetic complementation experiments indicate that both angiosperm and gymnosperm AG-like and DEF/GLO-like genes have conserved functions in the specification of reproductive organs and in distinguishing male from female organs, respectively. Based on these findings novel models about the molecular basis of flower origin, involving changes in the expression patterns of DEF/GLO-like or AGL6/SEP1/SQUA-like genes in reproductive structures, were developed. While in angiosperms SEP1-like proteins play an important role in floral quartet formation, preliminary evidence suggests that gymnosperm DEF/GLO-like and AG-like proteins alone can already form floral quartet-like complexes, further corroborating the view that the formation of floral quartet-like complexes predated flower origin during evolution.
20 年后，人们对被子植物花发育的许多分子机制有了相对较好的理解。在控制花发育的基因调控网络中，核心参与者是 SQUA 样、DEF/GLO 样、AG 样和 AGL6/SEP1 样 MIKC 型 MADS 结构域转录因子。它们分别提供 A 类、B 类、C 类和最近定义的 E 类花同源异形功能。有证据表明，花同源异形蛋白作为多聚体蛋白复合物，以器官特异性的方式识别靶基因的 DNA，从而构成“花四聚体”。与花发育的详细见解相比，花在进化过程中是如何起源的仍然是个谜。然而，尽管所有花同源异形基因的同源物似乎都不存在于所有非种子植物中，但 DEF/GLO 样、AG 样和 AGL6 样基因已在各种现存的裸子植物中被发现，裸子植物是被子植物的最亲近的亲戚。虽然 SQUA 样和 SEP1 样 MADS 盒基因似乎不存在于现存的裸子植物中，但 MADS 盒基因系统发育的重建令人惊讶地表明，裸子植物和被子植物的最近共同祖先都拥有这两种基因的代表，但这些基因在导致现存裸子植物的谱系中丢失了。表达研究和遗传互补实验表明，被子植物和裸子植物的 AG 样和 DEF/GLO 样基因在生殖器官的特化和分别区分雄性和雌性器官方面都具有保守功能。基于这些发现，关于花起源的分子基础的新模型被开发出来，涉及到在生殖结构中 DEF/GLO 样或 AGL6/SEP1/SQUA 样基因表达模式的变化。虽然在被子植物中，SEP1 样蛋白在花四聚体形成中起着重要作用，但初步证据表明，裸子植物的 DEF/GLO 样和 AG 样蛋白本身就可以形成花四聚体样复合物，这进一步证实了花四聚体样复合物的形成早于进化过程中的花起源的观点。
