We describe lacerata (lcr) mutants of Arabidopsis, which display various developmental abnormalities, including postgenital organ fusions, and report cloning of the LCR gene by using the maize transposon Enhancer͞Suppressor-mutator (En͞Spm). The pleiotropic mutant phenotype could be rescued by genetic complementation of lcr mutants with the wild-type LCR gene. The LCR gene encodes a cytochrome P450 monooxygenase, CYP86A8, which catalyzes -hydroxylation of fatty acids ranging from C12 to C18:1, as demonstrated by expression of the gene in yeast. Although palmitic and oleic acids were efficient substrates for LCR, 9,10-epoxystearate was not metabolized. Taken together with previous studies, our findings indicate that LCR-dependent -hydroxylation of fatty acids could be implicated in the biosynthesis of cutin in the epidermis and in preventing postgenital organ fusions. Strikingly, the same pathway seems to control trichome differentiation, the establishment of apical dominance, and senescence in plants.T he epidermis of plants is a composite tissue that comprises several cell types. Some of these, such as stoma cells, trichomes, and papilla cells, can be easily distinguished from the predominating pavement cells by their characteristic morphological features. Other cell types are not readily distinguishable, although they apparently perform specific functions. One example of this is given by epidermal cells on the adaxial side of carpels, which exhibit a unique contact response during elaboration of the pistil and are able to adhere and redifferentiate into parenchymatous cells. Another unique feature of these epidermal cells is their ability to adhere to the growing pollen tube and guide it to the embryo sac (1, 2). In contrast to animals, where selectively established cell adhesions are common and play an enormous role in development (3, 4), examples of regular cell adhesions in higher plants are rare, and indeed may be restricted to the processes cited above. In particular mutants in several plant species fusions of organs occur during development of the shoot, in a process that resembles the regular fusion of carpels. It is not yet known whether the same molecular mechanisms underlie all instances of cell fusions. By comparison with the epidermis cells of fused carpels, epidermis cells at sutures in fusion mutants do not alter their normal anticlinal plane of division and do not redifferentiate in response to the adhesion. Cell differentiation, however, is affected in at least two fusion mutants. The epidermis of crinkly4 (cr4) maize plants contains enlarged, occasionally spherical, cells, which can divide periclinally to give rise to multilayered sectors (5). In the fiddlehead ( fdh) mutant of Arabidopsis, the epidermis of rosette leaves displays a 2-fold reduction in the number of trichomes (6). These findings indicate a link between the altered cell differentiation in the epidermis and the fusion of organs in the mutants.By using transposon tagging, FDH and CR4, two genes that result in organ fusions when m...

SummaryA male sterile mutant with a defect in anther dehiscence was identified in an Arabidopsis thaliana population mutagenized with the Zea mays transposon En-1/Spm. Mutants produce viable pollen that can fertilize when released mechanically from the anthers. Mutant stamens are of normal size and shape, but lack cell wall fortifications in the endothecial cell layer of the anther, which are required for the dehiscence process. The mutant phenotype was shown to be caused by a transposon insertion in AtMYB26, disrupting the putative DNA-binding domain of this R2R3-type MYB transcription factor. RT-PCR revealed that expression of AtMYB26 is restricted to inflorescences. Sterility was shown to be stable under several environmental conditions. The high stability of the sterile phenotype, together with the fact that pollen is functional, makes AtMYB26 and its orthologs a valuable tool for manipulating male fertility in higher plants.

SummaryThe psae1-1 mutant of Arabidopsis was identi®ed on the basis of a decrease in the effective quantum yield of photosystem II, among a collection of plants subjected to transposon tagging with the Enhancer element. The steady-state redox level and the rate of re-oxidation of P700 are signi®cantly altered in psae1-1 mutants. The responsible mutation was localised to psaE1, one of two Arabidopsis genes that encode subunit E of photosystem I. An additional mutant allele, psae1-2, was identi®ed by reverse genetics. In wild-type plants, the psaE1 transcript is expressed at a higher level than psaE2 mRNA. In the mutants, however, the E1 transcript was barely detectable, and was expressed only in small groups of wild-type cells resulting from somatic reversions. As a consequence, the amount of PsaE protein present in the mutant is signi®cantly reduced. Concomitantly, the levels of other stromal photosystem I subunits (PsaC and PsaD) are also affected. Mutant plants showed a marked increase in light sensitivity and photoinhibition. Additional effects of the psae1 mutation include light green pigmentation, an increase in chlorophyll¯uorescence and a decrease of approximately 50% in growth rate under greenhouse conditions.

Expression of defense-associated genes was analyzed in leaf tissues of near-isogenic resistant and susceptible barley cultivars upon infection by Rhynchosporium secalis. The genes encoding pathogenesis-related (PR) proteins PR-1, PR-5, and PR-9 are specifically expressed in the mesophyll of resistant plants, whereas a germin-like protein (OxOLP) is synthesized in the epidermis irrespective of the resistance genotype. Restriction-mediated differential display was employed to identify additional epidermis-specific genes. This resulted in the detection of another PR gene, PR-10, along with a lipoxygenase gene, LoxA, and a gene of unknown function, pI2-4, which are specifically induced in the epidermis of resistant plants. The gene encoding a putative protease inhibitor, SD10, is preferentially but not exclusively expressed in the epidermis. The fungal avirulence gene product NIP1 triggers the induction of the four PR genes only. At least two additional elicitors, therefore, must be postulated, one for the unspecific induction of OxOLP and one for the resistance-specific induction of LoxA, pI2-4, and SD10. PR-10 expression can be assumed to be the consequence of NIP1 perception by epidermis cells. In contrast, gene expression in the mesophyll is likely to be triggered by an as yet unknown signal that appears to originate in the epidermis and that is strongly amplified in the mesophyll.

Mutants obtained by insertional mutagenesis are widely used for determining gene‐phenotype relationships. In Arabidopsis thaliana, several populations mutagenized either by T‐DNA or transposon insertion are available for screening for knockout mutants in genes of interest. We have so far screened our Arabidopsis population mutagenized with the Zea mays transposon En‐1/Spm for insertion mutations in 718 genes, using PCR on DNA pools. Although successful, this common approach is too time consuming for use in systematic screening of all 25 498 predicted genes of the Arabidopsis genome. We therefore investigated the use of DNA arrays for the direct identification of mutants in our population. All transposon‐flanking regions from individual plants are amplified by PCR and subsequently spotted at high density onto nylon membranes. A single hybridization experiment with a gene‐specific probe then allows one to identify candidate mutant plants. The efficiency of each separate step was determined and optimized. Screening of filters representing 2880 plants for insertions in 144 genes and subsequent investigation of some of the potential insertion mutants suggest that an overall screening efficiency of 50 % is attained.