Immunodetection of four mitotic cyclins and the Cdc2a protein kinase in the maize root: their distribution in cell development and dedifferentiation

Cyclin proteins and cyclin-dependent kinases play a key role in the regulat ion of cell division. We have therefore studied the relationship of the lev el of four mitotic cyclin proteins and the Cdc2a kinase protein to cell div ision in maize root tissue with respect to cessation of division as cells l eave the primary meristem region, resumption of division in formation of la teral-root primordia, and induced division following wounding. All four mit otic cyclins and Cdc2a were most abundant in dividing cells. The only exami ned cell cycle protein which was restricted to dividing tissue was cyclin Z mCycB1;2 (previously ZmIb) and may thus be a limiting factor for cell divis ion. All other cyclin proteins, i.e., ZmCycB1;1 (previously ZmIa), ZmCycA1; 1 (previously ZmII), and ZmCycB2;1 (previously ZmIII), and the Cdc2a kinase declined shortly after cells had ceased division. The distance from the ro ot tip at which cells ceased division was tissue-specific and reflected the distance at which decrease of cell cycle proteins was detected. Whereas cy clin ZmCycB1;2 rapidly declined to a hardly detectable level in either nucl eus or cytoplasm, in the nuclei of nondividing cells there was persistence of Cdc2a and of cyclins ZmCycB1;1, ZmCycCA1;1, and ZmCycB2;1, indicating th at there are plant cyclins which are tightly linked to cell division and ot hers that persist, especially in the nuclei, in nondividing cells. The tran sition from division to differentiation may thus partly be triggered and en forced by the decrease of the cell cycle proteins and especially the declin e of cyclins in the cytoplasm. In the resumption of cell division, both in lateral-root formation and in wound response, high nuclear and low cytoplas mic accumulation of cyclin ZmCycB2;1 was the first visible sign of cell ded ifferentiation, implying a role for cyclin ZmCycB2;1 in the G(0)-G(1) phase transition. Next, cytoplasmic accumulation of cyclin ZmCycA1;1, followed b y a rearrangement of cortical microtubules, was observed and since both the cyclins ZmCycA1;1 and ZmCycB2;1 were found at places of high tubulin conce ntration, they may function in the microtubule rearrangement for cell divis ion. When the nuclei of dedifferentiating cells had visibly enlarged, all c yclins and Cdc2a accumulated there, possibly contributing to DNA replicatio n and preparation for mitosis. Later, presumably during G(2) phase, cytopla smic accumulation was observed for Cdc2a at low levels, as observed in GL p hase cells of the primary meristem, and for cyclins ZmCycB1;1 and ZmCycB1;2 accumulation was observed above the levels found in undisturbed meristems, suggesting special contributions to late dedifferentiation processes in bo th wound-induced and lateral meristems.