Identification and characterization of the MADS-box genes highly expressed in the laticifer cells of Hevea brasiliensis

MADS-box transcription factors possess many functions in plant reproduction and development. However, few MADS-box genes related to secondary metabolites regulation have been identified. In Hevea brasiliensis, natural rubber is a representative cis-polyisoprenoids in secondary metabolism which occurs in the rubber laticifer cells, the molecular regulation basis of natural rubber biosynthesis is not clear. Here, a total of 24 MADS-box genes including 4 type I MADS-box genes and 20 type II MADS-box genes were identified in the transcriptome of rubber tree latex. The phylogenetic analysis was performed to clarify the evolutionary relationships of all the 24 rubber tree MADS-box proteins with MADS-box transcription factors from Arabidopsis thaliana and Oryza sativa. Four type I MADS-box genes were subdivided into Mα (3 genes) and Mβ (1 gene). Twenty type II MADS-box genes were subclassified into MIKC* (8 genes) and MIKC(c) (12 genes). Eight MADS-box genes (HblMADS3, 5, 6, 7, 9, 13, 23, 24) were predominant expression in laticifers. ABA up-regulated the expression of HblMADS9, and the expression of HblMADS3, HblMADS5, HblMADS24 were up-regulated by MeJA. The function of HblMADS24 was elucidated. HblMADS24 bound HbFPS1 promoter in yeast and HblMADS24 activated HbFPS1 promoter in tobacco plants. Moreover, we proposed that HblMADS24 is a transcription activator of HbFPS1 which taking part in natural rubber biosynthesis.