SUMMARYThe maize (Zea mays L.) rum1-R (rootless with undetectable meristems 1-Reference) mutant does not initiate embryonic seminal roots and post-embryonic lateral roots at the primary root. Map-based cloning revealed that Rum1 encodes a 269 amino acid (aa) monocot-specific Aux/IAA protein. The rum1-R protein lacks 26 amino acids including the GWPPV degron sequence in domain II and part of the bipartite NLS (nuclear localization sequence). Significantly reduced lateral root density (approximately 35%) in heterozygous plants suggests that the rum1-R is a semi-dominant mutant. Overexpression of rum1-R under the control of the maize MSY (Methionine SYnthase) promoter supports this notion by displaying a reduced number of lateral roots (31-37%). Functional characterization suggests that Rum1 is auxin-inducible and encodes a protein that localizes to the nucleus. Moreover, RUM1 is unstable with a half life time of approximately 22 min while the mutant rum1-R protein is very stable. In vitro and in vivo experiments demonstrated an interaction of RUM1 with ZmARF25 and ZmARF34 (Z. mays AUXIN RESPONSE FACTOR 25 and 34). In summary, the presented data suggest that Rum1 encodes a canonical Aux/IAA protein that is required for the initiation of embryonic seminal and post-embryonic lateral root initiation in primary roots of maize.

HighlightMaize lateral root primordia 1 encodes an auxin-inducible transcriptional activator confined to root primordia that is repressed by direct promoter binding of the Aux/IAA protein ROOTLESS WITH UNDETECTABLE MERISTEM 1.

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HighlightThe maize Aux/IAA protein RUM1 and its homeolog RUL1 display conserved biochemical properties. The specific interaction of RAP1 with RUM1 but not RUL1 suggests differences in their molecular interactions.