The maize mutant narrow sheath fails to establish leaf margin identity in a meristematic domain.

The maize mutant narrow sheath (ns) displays a leaf shape and plant stature phenotype that suggests the preprimordial deletion of a leaf domain. The ns mutant phenotype is inherited as a recessive, duplicate-factor trait, conditioned upon homozygosity for each of the two unlinked mutations narrow sheath-1 (ns1) and narrow sheath-2 (ns2). Mutant leaves are missing a large domain including the leaf margin, and mutant internodes are shortened on the marginal side of the stem. This domain deletion extends from the internode to beyond the longitudinal mid-length of the blade, and corresponds to an alteration in the organization of a specific region of the shoot apical meristem. The premargin region of mutant founder cells fail to down-regulate expression of Knox genes, markers of nonleaf meristematic identity. Our results indicate that leaf domains may acquire identity in the meristem itself, and that the subdivision of preprimordial developmental fields into differential domains is a common feature of both plant and animal organogenesis.

Ectopic expression of the knox homeo box gene rough sheath1 alters cell fate in the maize leaf.

Rough sheath1 (Rs1) is a dominant mutation that alters cell fate and causes unregulated cell division and expansion in the maize leaf. A knox (Kn1 like-homeo box gene) sequence closely linked to the Rs1-O mutation was cloned and shown by transposon mutagenesis to encode the rs1 gene. The deduced amino acid sequence of the RS1 protein is highly similar to KN1 in the homeo domain but contains a unique amino-terminal region. rs1 is expressed in the shoot apical meristem in a circular pattern preceding leaf initiation, but is not detectable in leaf primordia or mature leaves in normal plants. Rings of rs1 expression subtend leaf insertion sites in the shoot, and lateral organ primordia in inflorescence and floral meristems. The timing and position of rs1 expression in meristems suggests a possible role for rs1 in patterning the placement of lateral organs along the axis of the shoot. In contrast to wild type, rs1 is expressed in early leaf primordia of Rs1 mutant plants, suggesting that ectopic expression causes the mutant phenotype. Ectopic expression in Rs1-O plants suggests the ligular [corrected] region is more competent to respond to rs1 than blade or sheath tissues.

Specific DNA alterations associated with the environmental induction of heritable changes in flax.

Several flax varieties have been shown to undergo environmentally induced heritable changes resulting in stable lines termed genotrophs. The most notable of these is the variety Stormont Cirrus, also termed "plastic" or Pl. A number of morphological, biochemical and genetic differences are associated with environmental induction of heritable changes in flax. We have studied 5S rDNA alterations as a model system for understanding environmental induction of heritable changes in flax. This paper reports the isolation of a flax 5S rRNA gene variant which identifies genotroph specific restriction fragment length polymorphisms (RFLPs) in flax. Restriction fragment patterns for several enzymes were observed in both large and small genotrophs which consistently differed from the progenitor, Stormont Cirrus. Identical RFLP profiles for all restriction endonucleases tested were observed in four small genotrophs produced from separate environmental induction experiments. Comparison between Stormont Cirrus and these small genotrophs showed at least six differing bands in addition to several high molecular weight polymorphisms. Genetic data indicate that the polymorphisms were all produced from a repetitive 5S rRNA gene cluster at a single chromosomal locus. Similar, but not identical, polymorphisms are also detected in other flax varieties and Linum species suggesting that the induced variation is related to that which occurs naturally. The results are evidence that a specific set of DNA alterations occur in association with the induction of heritable changes in flax. This is the first genetic marker which is altered to an identical state in one type of genotroph. The results are discussed with respect to mechanisms for environmentally induced heritable change in plants.

Chromosomal and molecular analysis of 5S RNA gene organization in the flax, Linum usitatissimum.

The 5S rRNA genes (5S DNA) comprise up to 3% of the flax genome. Large copy-number changes in 5S DNA have been observed in flax genotrophs. We have characterized the chromosomal and molecular organization of this large gene family. In situ hybridization studies indicate the 5S DNA is distributed over many chromosomes, unlike most plants studied to date. Eleven genomic clones were isolated and characterized. All but one of the clones contain both 5S DNA and non-5S DNA. The homology of the 5S DNA of each clone, to a previously isolated flax 5S plasmid clone (pBG13), was determined. Five groups of 5S DNA were identified based on shared identity and repeat unit size. Group-1 and group-2 clones are the most abundant in terms of genomic representation. The remaining groups are significantly different from the previously described flax 5S DNA and are in low representation in comparison to group-1 and group-2 5S DNA. The results establish the presence of several groups of 5S DNA which are distributed over many chromosomes. The extent of identity shared among these groups to pBG13, indicates a high degree of divergence between the different groups.