Silencing of a BYPASS1 homolog results in root-independent pleiotrophic developmental defects in Nicotiana benthamiana

The Arabidopsis bypass1 mutant (bps1) exhibits defective shoot and root growth that is associated with constitutive production of a root-derived carotenoid-related signal (Van Norman et al., Curr Biol 14:1739-1746, 2004). Since the identity of the signal and the function of BPS1 are still unknown, we investigated effects of BPS1 depletion in Nicotiana benthamiana to elucidate BPS1 function in plant growth and development. The predicted protein of NbBPS1, a BPS1 homolog of N. benthamiana, contains a central transmembrane domain, and a NbBPS1:GFP fusion protein was mainly associated with the endoplasmic reticulum. Virus-induced gene silencing (VIGS) of NbBPS1 resulted in pleiotrophic phenotypes, including growth retardation and abnormal leaf development. At the cellular level, the plants exhibited hyperproliferation of the cambial cells and defective xylem differentiation during stem vascular development. Hyperactivity of the cambium was associated with an elevated auxin and cytokinin response. In contrast, the leaves had reduced numbers of cells with increased cell size and elevated endoreduplication. Cell death in NbBPS1 VIGS leaves started with vacuole collapse, followed by degeneration of the organelles. Interestingly, these phenotypes were mainly caused by silencing of NbBPS1 in the aerial parts of the plants, different from the case of the Arabidopsis bps1 mutant. These results suggest that NbBPS1 plays a role in the control of cell division and differentiation in the cambium of N. benthamiana, and BPS homologs may have a diverse function in different tissues and in different species.