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The 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., (© 2011 The Authors. The Plant Journal © 2011 Blackwell Publishing Ltd.)

In transverse orientation, maize (Zea mays) roots are composed of a central stele that is embedded in multiple layers of cortical parenchyma. The stele functions in the transport of water, nutrients, and photosynthates, while the cortical parenchyma fulfills metabolic functions that are not very well characterized. To better understand the molecular functions of these root tissues, protein- and phytohormone-profiling experiments were conducted. Two-dimensional gel electrophoresis combined with electrospray ionization tandem mass spectrometry identified 59 proteins that were preferentially accumulated in the cortical parenchyma and 11 stele-specific proteins. Hormone profiling revealed preferential accumulation of indole acetic acid and its conjugate indole acetic acid-aspartate in the stele and predominant localization of the cytokinin cis-zeatin, its precursor cis-zeatin riboside, and its conjugate cis-zeatin O-glucoside in the cortical parenchyma. A root-specific beta-glucosidase that functions in the hydrolysis of cis-zeatin O-glucoside was preferentially accumulated in the cortical parenchyma. Similarly, four enzymes involved in ammonium assimilation that are regulated by cytokinin were preferentially accumulated in the cortical parenchyma. The antagonistic distribution of auxin and cytokinin in the stele and cortical parenchyma, together with the cortical parenchyma-specific accumulation of cytokinin-regulated proteins, suggest a molecular framework that specifies the function of these root tissues that also play a role in the formation of lateral roots from pericycle and endodermis cells.

In an approach to study lateral root development in monocots, genome-wide searches for homologs of the Gibberellic Acid Stimulated Transcript-like (GAST-like) gene family in rice (Oryza sativa) and maize (Zea mays) were carried out. Six novel GAST-like genes in rice and 10 members of the gene family in maize, which were designated ZmGSL (for Z. mays Gibberellic Acid Stimulated-Like), were identified. The ZmGSL family encodes small proteins of 75 to 128 amino acids, which are characterized by a conserved 59 to 64 amino acid C-terminal domain. Within this domain, 17 amino acids, including 12 cysteines, are perfectly conserved. The transcript of the ZmGSL1 gene is differentially spliced into the alternative variants ZmGSL1a and ZmGSL1b, the latter of which is translated into a premature protein that lacks the C-terminal domain. The presence of an additional N-terminal cleavable signal sequence in eight of the 10 ZmGSL proteins suggests that they are secreted into the extracellular matrix. In-depth root-specific gene expression analyses carried out in the wild type and the lateral root mutants lrt1 and rum1 suggest a role for ZmGSL genes in early lateral root development, which is likely regulated by gibberellic acid. Expression patterns of ZmGSL1a and ZmGSL1b propose antagonistic functions of these splice variants during early lateral root formation.

Maize has a complex root system composed of different root types formed during different stages of development. The rtcs (rootless concerning crown and seminal roots) mutant is impaired in the initiation of the embryonic seminal roots and the post-embryonic shoot-borne root system. The primary root of the mutant shows a reduced gravitropic response, while its elongation, lateral root density and reaction to exogenously applied auxin is not affected. We report here the map-based cloning of the RTCS gene which encodes a 25.5 kDa LOB domain protein located on chromosome 1S. The RTCS gene has been duplicated during evolution. The RTCS-LIKE (RTCL) gene displays 72% sequence identity on the protein level. Both genes are preferentially expressed in roots. Expression of RTCS in coleoptilar nodes is confined to emerging shoot-borne root primordia. Sequence analyses of the RTCS and RTCL upstream genomic regions identified auxin response elements. Reverse transcriptase-PCR revealed that both genes are auxin induced. Microsynteny analyses between maize and rice genomes revealed co-linearity of 14 genes in the RTCS region. We conclude from our data that RTCS and RTCL are auxin-responsive genes involved in the early events that lead to the initiation and maintenance of seminal and shoot-borne root primordia formation.

This study comprises a comprehensive gene expression analysis of the root tip specific maize gene ZmGrp3. In the first part of this paper expression of ZmGrp3 was studied in maize inbred lines. First, RNA in situ hybridization experiments confined the expression of ZmGrp3 to the columella and the epidermis of all embryonic and postembryonic root types. Second, Northern-blot analyses of the maize root initiation mutants rtcs and lrt1 revealed that the ZmGrp3 gene is not expressed prior to root initiation, thus providing a novel marker for this developmental process. Finally, a comprehensive expression profiling in 42 tissues via the Lynx MPSS system revealed almost exclusive expression of ZmGrp3 in maize roots. In the second part of this survey, ZmGrp3 expression was assayed in maize hybrids. In this context, a novel approach to quantify allele-specific contribution to gene expression in maize hybrids was developed. This assay combines RT-PCR amplification of polymorphisms between two alleles and subsequent quantification of allele-specific gene expression via a combination of didesoxyterminator assays and capillary electrophoresis. Allelic expression of the ZmGrp3 gene in six reciprocal hybrids generated from three ZmGrp3 alleles was analyzed via a new statistical mixed model approach.