Expression Analysis Of Genes Controlling Flower And Floral Organ Development By Cytokinins In Arabidopsis

Plant hormone cytokinin participates in regulation of diverse plant growth and development processes including cell division, flowering, leaf senescence and maintenance of apical dominance. The functions of Arabidopsis thaliana floral meristem identity genes and floral organ identity genes have been well studied, and some environmental factors such as light and temperature were implicated in floral development modulation. In Arabidopsis, cytokinin level in pAP1:IPT4 transgenic plants is elevated compared with wild type plants, resulted in abnormal phenotypes including compacted flower arrangement, increased floral primordium and floral organs,reduced apical dominance and so on (Li et al., unpublished data). To investigate the roles of cytokinin regulation in flower development, we examined the expression patterns of genes controlling flower and floral organ development in pAP1:IPT4 transgenic plants of Arabidopsis.In situ hybridization and GUS staining analyses indicated that floral meristem identity gene LFY was up-regulated in pAP1:IPT4 transgenic plants, whereas its expression domain was not affected compared with that in wild type plants. Analyses of in situ hybridization revealed that the up-regulation of floral organ identity genes AP1, PI and AG occurs in pAP1:IPT4 transgenic plants. AP1 mRNA was first detectable in flower primordia. As flower development proceeds, AP1 trancripts accumulated in sepal primordium, as well the central meristem interior to sepal primordial in pAP1:IPT4 transgenic plants, suggested that the AP1 expression pattern was affected by transgenic expresssion of IPT4. PI was expressed in the floral meristem and stage 2 flower of pAP1:IPT4 transgenic plants, and the PI mRNA level increased along with subsequently flower development. AG signal was detectable in the floral primordium of pAP1:IPT4 transgenic plants and stronger in floral meristem than that of wild type in stage 3 and 4 flowers. Additionally, In pAP1:IPT4 plants, AG signal was discovered in sepal primordium, which differed from that of wild type plants. Thus, it suggested that changed expression patterns of these genes partially contribute to the phenotype of pAP1:IPT4 plants.Genome-wide expression analyses by gene chip revealed that the expression of LBD3, a member of the LATERAL ORGAN BOUNDARIES DOMAIN (LBD) gene family in Arabidopsis, was up-regulated in pAP1:IPT4 plants. To further study the function of LBD3, 35S:LBD3 and 35S:anti-LBD3 construct was introgressed into wild type plants of Arabidopsis, respectively. Overexpression of LBD3 resulted in reduced apical dominance, while 35S:anti-LBD3 transgenic plants had no detectable phenotypic altreration. Previous studies have uncovered that MAX pathway represses lateral bud growth by suppression of polar auxin transport, and BRANCHED1(BRC1) functions downstream of MAX pathway. The results of RT-PCR analyses indicated that the expression of MAX4 was evidently up-regulated and MAX2 down-regulated in LBD3-overexpressing plants. In addition, the abundance of BRC1 mRNA was elevated in 35S:LBD3 plants. These data raised the possible relationships between LBD3 and polar auxin transport as well as genes controlling lateral bud growth.