Identification of quantitative trait loci (QTL) affectingendoreduplication and characterization of cyclin dependent kinase inhibitorsin developing maize (Zea mays L.) endosperm

Abstract

Endoreduplication is a process of genome duplication without mitosis. Although endoreduplication is common among plants and animals, the molecular mechanisms involved with this process are not fully understood. Two strategies were used to determine the genetic components involved with endoreduplication. One, based on QTL analysis and the other strategy involved identifying cell cycle candidate genes, and characterizing them in regard to endoreduplication. To map genes influencing endoreduplication, four backcross populations were created from crosses between a high (Sg18) and a low (Mo17) endoreduplication inbred and their F1 (Sg18 x Mo17) progeny. In all, fourteen quantitative trait loci were identified that affect the degree of endoreduplication in maize endosperm. Six QTLs were mapped with a triploid mode of inheritance in the endosperm. Two QTLs were mapped with parent-of-origin effect inheritance. Six QTLs were mapped using a model that considers genetic interaction between embryo and endosperm. Previous studies with maize endosperm showed that accumulation of a cyclin-dependent kinase (CDK) inhibitor is coincident with the onset of endoreduplication, but the identity of this inhibitor is unknown. We therefore tried to determine if cyclin-dependent kinase inhibitor (CKI) activity is required for the occurrence of endoreduplication in maize endosperm. The expression of two maize CKI genes, Zeama;CKI;1 and Zeama;CKI;2, were characterized in developing endosperm. The accumulation of Zeama;CKI;1 RNA is not developmentally regulated, and its expression encompasses the period in which endoreduplication takes place. In contrast, Zeama;CKI;2 gene expression is developmentally regulated in the endosperm, since its protein level decreases after 13 DAP. Both proteins were able to inhibit the maize Cdc2/CDK kinase activity associated with p13 Suc1. They also specifically inhibited cyclin A1;3/ and cyclin D5;1/-associated CDK activities, but not cyclin B1;3/CDK. Although Zeama;CKI;1 was found to be associated with the endosperm CKI activity, it did not account for all of the CDK inhibitor. Over-expression of ZeamaCKI1 in maize embryonic calli that ectopically expressed the wheat dwarf virus RepA protein, which counteracts retinoblastoma-related protein (RBR) function in the cell cycle, led to an additional round of DNA replication without nuclear division. However, a role for Zeama;CKI;1 in endoreduplication could not be demonstrated in maize endosperm.