Genetic Control Of Organ Internal Asymmetry In Legumes

Floral zygomorphy (flowers with bilateral symmetry) has multiple origins and typically manifests two kinds of asymmetries, dorsoventral (DV) and organ internal (IN) asymmetries in floral and organ planes, respectively, revealing the underlying key regulators in plant genomes that generate and superimpose various mechanisms to build up complexity and different floral forms during plant development. In this study, two types of mutants with defects on IN asmmetry were identified from large scale mutagenesis in Pisum sativum and Lotus japonicus respectively.Previously undescribed regulator, SYMMETRIC PETALS1 (SYP1), has been isolated as specificly controlling IN asymmetry. Two allelic mutants, syp1-1 and syp1-2 bearing petals with altered IN asymmetry were identified. In syp1-1, nearly all petals are bilaterally symmetrical but maintain their DV identities. syp1-2, a weaker allele, has a highly variable effect on IN asymmetry of the lateral but not the ventral petals. Genetic analysis indicated that syp1 was crosponding to a single recessive locus in pea. By comparative approach syp1 was located in linkage group II of pea and was anchored to genome of two legume model plants, Medicago truncatula and L. japonicus. To investigate how DV and IN regulators interact with each other during zygomorphic development, syp1-1 was introduced into k, lst1, and k lst1 genetic backgrounds, respectively. The lst1-1 syp1-1 double mutant displayed an additive phenotype: lobed standards and petals in the lateral and ventral positions without IN asymmetry. However, k-1 syp1-1 flowers possess abnormal standards with an altered shape apart from the expected keels in both lateral and ventral regions, revealing a hidden function of SYP1 during the development of the dorsal petal. In the k-1 lst1-1 syp1-1 triple mutant, the flowers display a radial symmetry, and all petals possess ventralized identity with a keel structure. Thus, a default form without DV and IN asymmetries was identified.Two pea mutants, elephant ear-like leaf 1 (ele1) and ele2, display predominant deficiency on the asymmetry in both lateral and ventral petals, as well as the enlarged sizes of organs. Furthermore, a similar mutant, big organs (bio), was identified from large scale mutagenesis in L. japonicus. Genetic analysis showed that bio was controlled by a single semi-dominant locus in the long arm of chromosomeâ…£of L. japonicus, ele1 was corresponding to a single recessive one in the long arm of linkage group II of pea genome and ele2 was corresponding to a single recessive one in the long arm of linkage groupâ…£of pea genome. Through comparative genomic approach, ELE1 was narrowed down to a region in pea linkage group II, which is closely linked with KEW locus and shares micro-synteny with a region in the genome of L. japonicus, and BIO and ELE2 were found to locate in a syntenic region among papilionoid legume genomes, suggesting that they are the homologous locus. Analysis of the conserved segments of the syntenic region between the L. japonicus and M. truncatula genomes, the candidate for BIO and ELE2 was narrowed down to eleven putative orthologous genes in the two legume model plants. Based on synteny among legumes, ELE1 and BIO/ELE2 were isolated. Genetical interation between ELE1 and ELE2 indicated ELE1 and ELE2 act in the same genetic pathway to establish IN aymmetry. In addition, ELE1 physically interacts with ELE2 in yeast. Based on genetic and physical interactions, we speculated ELE1 and ELE2 form a complex to regulate the establishment of IN asymmetry. To investigate whether there are geneticaly interaction between PsCYCs and ELE, ele2 was introduced into k-1, lst1-1, and k-1 lst1-1 genetic backgrounds, respectively. The k-1 ele2 double mutant seemed to display an additive phenotype: the flower bore symmetric lateral and ventral petals with a keel structure; however, the enlarged size of dorsal petal was subtle. The lst1-1 ele2 double mutant could be also considered as an additive phenotype: the lateral and ventral petals lost IN asymmetry, but the enlargement of lobed standard not less evident either. In the k-1 lst1-1 ele2 triple mutant, the flower was radial symmetric and all petals possess a keel structure, which looked the same as the ones in k-1 lst1-1 syp1-1 triple mutant. Particularly, the size of dorsal petal in the triple mutant was little different from that in k-1 lst1-1 syp1-1 triple mutant, indicating that the enlargement effect of ele2 on the dorsal petal size is suppressed when the PsCYC genes were mutated. Genetic interaction of K, LST1 and ELE2 showed that enlargement of dorsal petal in ele2 requires PsCYCs activities. We also showed that ELE2 gentically interacted with SYP1 and PsCYCs to regulate IN asymmetry in legumes.Genetic analysis demonstrates that DV and IN asymmetries could be controlled independently by the two kinds of regulators in pea, and their interactions help to specify the type of zygomorphy. Based on the genetic analysis in pea, we suggest that variation in both the functions and interactions of these regulators could give rise to the wide spectrum of floral symmetries among legume species and other flowering plants. Cloning ELE1, BIO/ELE2, SYP1 and other key components in the IN asymmetry pathway and analyzing their functions in the coming future should shed light on the molecular basis about the interaction and superimposition of DV and IN asymmetries, and further provide the molecular basis for crop improvement in the coming future.