Auxin is required for the assembly of A-type cyclin-dependent kinase complexes in tobacco cell suspension culture

Although activation of A-type cyclin-dependent kinase (CDKA) is required for plant cell division, little is known about how CDKA is activated before commitment to cell division. Here, we show that auxin is required for the formation of active CDKA-associated complexes, promoting assembly of the complex in tobacco suspension culture Bright Yellow-2 (BY-2) cells. Protein get blot analysis revealed that CDKA Levels increased greatly after stationary-phase BY-2 cells were subcultured into fresh medium to re-enter the cell cycle. However, these increasing Levels subsided when cells were subcultured into auxin-deprived medium, and a subtle increase was observed after subculturing into sucrose-deprived medium. Additionally, p13(suc1)-associated kinase activity did not increase significantly after subculturing into either auxin-or sucrose-deprived medium, but increased strongly after subculturing into medium containing both auxin and sucrose. Using get filtration, we found that p13(suc1)-associated kinase activity against tobacco retinoblastoma-related protein was maximal. in fractions corresponding to the molecular mass of the cyctin/CDKA complex. Interestingly, this peak distribution of high motecular-mass fractions of CDKA disappeared after cells were subcultured into auxin-deprived medium. These findings suggest an important rote for auxin in the assembly of active CDKA-associated complexes. (c) 2007 Elsevier GmbH. All rights reserved.