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Some plant cytoplasms express novel mitochondrial genes that cause male sterility. Nuclear genes that disrupt the accumulation of the corresponding mitochondrial gene products can restore fertility to such plants. The Texas (T) cytoplasm mitochondrial genome of maize expresses a novel protein, URF13, which is necessary for T cytoplasm-induced male sterility. Working in concert, functional alleles of two nuclear genes, rf1 and rf2 , can restore fertility to T cytoplasm plants. Rf1 alleles, but not Rf2 alleles, reduce the accumulation of URF13. Hence, Rf2 differs from typical nuclear restorers in that it does not alter the accumulation of the mitochondrial protein necessary for T cytoplasm-induced male sterility. This study established that the rf2 gene encodes a soluble protein that accumulates in the mitochondrial matrix. Three independent lines of evidence establish that the RF2 protein is an aldehyde dehydrogenase (ALDH). The finding that T cytoplasm plants that are homozygous for the rf2-R213 allele are male sterile but accumulate normal amounts of RF2 protein that lacks normal mitochondrial (mt) ALDH activity provides strong evidence that rf2 -encoded mtALDH activity is required to restore male fertility to T cytoplasm maize. Detailed genetic analyses have established that the rf2 gene also is required for anther development in normal cytoplasm maize. Hence, it appears that the rf2 gene was recruited recently to function as a nuclear restorer. ALDHs typically have very broad substrate specificities. Indeed, the RF2 protein is capable of oxidizing at least three aldehydes. Hence, the specific metabolic pathway(s) within which the rf2 -encoded mtALDH acts remains to be discovered. INTRODUCTIONMaternally inherited cytoplasmic male sterility (CMS) occurs in many plant species and is widely used to facilitate the production of hybrid seed because it eliminates the need for emasculation by hand. Mitochondrial defects account for all instances in which the nature of the lesion responsible for CMS has been identified (reviewed in Mackenzie et al., 1994;Schnable and Wise, 1998). In many species, the deleterious effects of these mitochondrial defects can be avoided or overcome by the action of nuclear genes, termed nuclear restorers. However, the specific mechanisms by which restoration can occur are only poorly understood.The male-sterile Texas (T) cytoplasm (cms-T) was used to produce ‫ف‬ 85% of U.S. hybrid maize seed until the 1970 epidemic of southern corn leaf blight (Ullstrup, 1972;Pring and Lonsdale, 1989). cms-T maize is highly sensitive to a hostselective toxin (T toxin) produced by race T of Cochliobolus heterostrophus , the causal organism of southern corn leaf blight (Hooker et al., 1970; Comstock and Scheffer, 1973; Yoder, 1973). The genomes of T cytoplasm mitochondria contain a unique mitochondrial gene, urf13 , which encodes the URF13 protein. URF13 accumulates in the inner membrane of the mitochondria (Forde and Leaver, 1980; Dewey et al., 1986;Wise et al., 1987a;Hack et al., 1991;Korth et al., 1991;...

Potato (Solanum tuberosum) homeobox 1 (POTH1) is a class I homeobox gene isolated from an early-stage tuber cDNA library. The RNA expression pattern of POTH1, unlike that of most other class I knotted-like homeobox genes, is widespread in the cells of both indeterminate and differentiated tissues. Using in situ hybridization, POTH1 transcripts were detected in meristematic cells, leaf primordia, and the vascular procambium of the young stem. Overexpression of POTH1 produced dwarf plants with altered leaf morphology. Leaves were reduced in size and displayed a "mouse-ear" phenotype. The mid-vein was less prominent, resulting in a palmate venation pattern. The overall plant height of overexpression lines was reduced due to a decrease in internode length. Levels of intermediates in the gibberellin (GA) biosynthetic pathway were altered, and the bioactive GA, GA 1 , was reduced by one-half in sense mutants. Accumulation of mRNA for GA 20-oxidase1, a key biosynthetic enzyme, decreased in overexpression lines. In vitro tuberization was enhanced under both short-and long-day photoperiods in several POTH1 overexpression lines. Sense lines produced more tubers at a faster rate than controls. These results imply that POTH1 mediates the development of potato by acting as a negative regulator of GA biosynthesis.Homeobox genes, a family of transcription factors highly conserved in animals, plants, and yeast (Chan et al., 1998), are implicated in the control of cell fate. The Antennapedia homeobox gene in fruitfly (Drosophila melanogaster), for example, specifies leg identity while inhibiting the formation of an antenna (Mann and Chan, 1996). Ectopic expression of the eyeless gene in the wing imaginal disc tissue of fruitfly embryos causes a normal eye to form on the wings (Halder et al., 1995). The first plant homeobox gene to be discovered was knotted1 (kn1) from maize (Zea mays; Vollbrecht et al., 1991). Dominant gain-offunction mutations of Kn1 formed knot-like structures along lateral veins. These knots were composed of cells that continued to divide rather than differentiate normally (Vollbrecht et al., 1991;Smith et al., 1992), indicating that kn1 is involved in regulating cell fate (Clark et al., 1996; Kerstetter et al., 1997; Chan et al., 1998).Knotted-like homeobox (knox) genes have been isolated from several plant species (for review, see Reiser et al., 2000) and can be divided into two classes based on expression patterns and sequence similarity (Kerstetter et al., 1994). Class I knox genes have high similarity to the kn1 homeodomain and generally have a meristem-specific mRNA expression pattern. Class II knox genes usually have a more widespread expression pattern. Knox genes are members of the three amino acid loop extension (TALE) superclass of homeobox genes (Bü rglin, 1997). The TALE superclass includes members from plants, animals, and fungi and is characterized by the addition of three amino acids, Pro-Tyr-Pro (PYP), between helices 1 and 2 of the homeodomain. Knox genes share conserved regions outside of...

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Tobacco floral nectaries undergo changes in form and function. As nectaries change from green to orange, a new pigment is expressed. Analysis demonstrated that it is β-carotene. Plastids undergo dramatic changes. Early in nectary development, they divide and by stage 9 (S9) they are engorged with starch. About S9, nectaries shift from quiescent anabolism to active catabolism resulting in starch breakdown and production of nectar sugars. Starch is replaced by osmiophilic bodies, which contain needle-like carotenoid crystals. Between S9 and S12, amyloplasts are converted to chromoplasts. Changes in carotenoids and ascorbate were assayed and are expressed at low levels early in development; however, following S9 metabolic shift, syntheses of β-carotene and ascorbate greatly increase in advance of expression of nectar redox cycle. Transcript analysis for carotenoid and ascorbate biosynthetic pathways showed that these genes are significantly expressed at S6, prior to the S9 metabolic shift. Thus, formation of antioxidants β-carotene and ascorbate after the metabolic shift is independent of transcriptional regulation. We propose that biosynthesis of these antioxidants is governed by availability of substrate molecules that arise from starch breakdown. These processes and events may be amenable to molecular manipulation to provide a better system for insect attraction, cross pollination, and hybridization.