Primary Study On Molecular Mechanism Controlling Compound Leaf Development In Lotus Japonicus

Recent research on leaf development demonstrates two key gene families, the KNOX Iand ARP gene family, involved in this process. KNOX I genes express in undetermined cells,maintaining the meristem identity whereas expression of ARP genes results in recruitment ofleaf founder cells in peripheral zone and exclude KNOX genes expression in the leafprimordia. KNOX I genes and ARP genes function antagonistically to each other in simpleleaf development. Compound leaf development presents a very different case from simpleleaf development. KNOX I genes and ARP genes co-express in the tomato compound leafprimordia and complete their functions. Expression of KNOX I genes maintains the marginalmeristem and ensures the initiation of leaflets primordia. The ARP expression sets up theadaxial domain, providing the right position information for leaflets initiation andparticipating in the adaxial-abaxial pattern formation.Absence of KNOX I expression from Legume compound leaf primordia encounters apuzzle,that is what molecular mechanism maintains the marginal meristem in Legume.Research on pea mutant uni suggests UNI, a ortholog of FLO/LFY floral identity gene, be apossible candidate for meristem maintainer and responsible for complexity of compound leaf.The question is unresolved. How much of compound leaf developmental mechanism iscommon to most lineages and how much is peculiar to Legume?To address these questions, we investigate the compound leaf development in Legumemodel system Lotus japonicus. This study is based on utilization of techniques in reversegenetics and forward genetics.Firstly, 3 KNOX I genes, 2 ARP genes and 3 LOB-like genes have been isolated fromLotus japonicus. Expression patterns of LjKN1 and LjKN3 reveal that KNOX genes areexcluded from Lotus leaf development. Dynamic expression patterns of LjAS1a, LjAS1b andLjFLO suggest these genes may function in key events in Lotus compound leaf development.At early stage LjAS1 and LjAS1b may establish compound leaf primordia identity andadaxial domain for the correct position of leaflets initiation. At late stage LjAS1b is likely tofunction in leaflets identity determination and adaxial-abaxial pattern formation of leaflets.LjFLO has a possible role in maintaining the marginal meristem of compound leaf primordia.LjLOB1 and LjLOB3 co-express on the base of leaflets and probably set up the boundarybetween leaflets and rachis. Secondly, morphology and genetic analysis on the nl1 mutanthave been conducted. The mutant displays defects in both compound leaf architecture andleaflets polarity pattern. Backcross test indicates the nl1 phenotype is caused by mutation ona single recessive locus. Modification in LjAS1a expression pattern and reduced expressionlevel probably causes the premature initiation of leaflets and the decrease in adaxial-abaxialpolarity of leaflets in nl1 mutant. We propose that NL1 locus function in maintaining theexpression pattern of LjAS1a in normal Lotus compound leaf development.