Cyclin-dependent kinase inhibitors in maize endosperm and their potential role in endoreduplication

Two maize (Zea mays) cyclin-dependent kinase (CDK) inhibitors, Zeama; KRP;1 and Zeama; KRP; 2, were characterized and shown to be expressed in developing endosperm. Similar to the CDK inhibitors in Arabidopsis ( Arabidopsis thaliana) and tobacco ( Nicotiana tabacum), the maize proteins contain a carboxy-terminal region related to the inhibitory domain of the mammalian Cip/Kip inhibitors. Zeama; KRP; 1 is present in the endosperm between 7 and 21 d after pollination, a period that encompasses the onset of endoreduplication, while the Zeama; KRP; 2 protein declines during this time. Nevertheless, Zeama; KRP; 1 accounts for only part of the CDK inhibitory activity that peaks coincident with the endoreduplication phase of endosperm development. In vitro assays showed that Zeama; KRP; 1 and Zeama; KRP; 2 are able to inhibit endosperm Cdc2-related CKD activity that associates with p13(Suc1). They were also shown to specifically inhibit cyclin A1;3- and cyclin D5;1-associated CDK activities, but not cyclin B1;3/CDK. Overexpression of Zeama; KRP; 1 in maize embryonic calli that ectopically expressed the wheat dwarf virus RepA protein, which counteracts retinoblastoma-related protein function, led to an additional round of DNA replication without nuclear division.