SIMYB1 and SIMYB2, two new MYB genes from tomato, transcriptionally regulate cellulose biosynthesis in tobacco

Cellulose, a major constituent of plant biomass, is synthesized by a cellulose synthase complex. It has been demonstrated that MYB genes transcriptionally regulate cellulose synthase in Arabidopsis. However, little is known about this process in tomato. Here, two MYB (SIMYB1/2) and three cellulose synthase (CESA) (SICESA4/5/6) genes were isolated. SIMYB1/2 and SICESA4/5/6 accumulation was found to correspond to cellulose accumulation in different tissues of tomato. Dual luciferase assays indicated that these two MYBs were transcriptional activators that interact with promoters of SICESA4/5/6. Moreover, SIMYB2 could also activate promoters of SIMYB1/2, suggesting the possible underlying auto-activation mechanisms for MYB transcription factors. Transient over-expression of SIMYB1/2 in Nicotiana tabacum up-regulated tobacco endogenous NtCESA genes and increased cellulose accumulation. The function of SIMYB1/2 was further investigated using stable transformation and the results indicated that N. tabacum lines heterologous expressing SIMYB1/2 displayed a pleiotropic phenotype, long and narrow leaves, with NtCESA induced and significant increase of cellulose. In conclusion, our data suggest that tomato SIMYB1/2 have transcriptional regulatory roles in cellulose biosynthesis and SIMYB2 was more effective than SIMYB1, which may due to the transcriptional activation by SIMYB2 on SIMYB1 and itself.