We studied the temporal and spatial regulation of three mRNA sequence sets that are present exclusively, or at elevated levels, in the tobacco anther. One mRNA set accumulates in the tapetum and decays as the tapetum degenerates later in anther development. The second mRNA set accumulates after the tapetal-specific mRNAs, is localized within the stomium and connective, and also decays as these cell types degenerate during anther maturation. The third mRNA sequence set persists throughout anther development and is localized within most anther tissues. A tapetal-specific gene, designated as TA29, was isolated from a tobacco genome library. Runoff transcription studies and experiments with chimeric [beta]-glucuronidase and diphtheria toxin A-chain genes showed that the TA29 gene is regulated primarily at the transcriptional level and that a 122-base pair 5[prime] region can program the tapetal-specific expression pattern. Destruction of the tapetum by the cytotoxic gene had no effect on the differentiation and/or function of surrounding sporophytic tissues but led to the production of male-sterile plants. Together, our studies show that several independent gene expression programs occur during anther development and that these programs correlate with the differentiated state of specific anther cell types.

The term apomixis encompasses a suite of processes whereby seeds form asexually in plants. In contrast to sexual reproduction, seedlings arising from apomixis retain the genotype of the maternal parent. The transfer of apomixis and its effective utilization in crop plants (where it is largely absent) has major advantages in agriculture. The hallmark components of apomixis include female gamete formation without meiosis (apomeiosis), fertilization-independent embryo development (parthenogenesis), and developmental adaptations to ensure functional endosperm formation. Understanding the molecular mechanisms underlying apomixis, a developmentally fascinating phenomenon in plants, is critical for the successful induction and utilization of apomixis in crop plants. This review draws together knowledge gained from analyzing ovule, embryo, and endosperm development in sexual and apomictic plants. It consolidates the view that apomixis and sexuality are closely interrelated developmental pathways where apomixis can be viewed as a deregulation of the sexual process in both time and space.

We have cloned two genes, FIS1 and FIS2, that control both fertilization independent seed development and postpollination embryo development in Arabidopsis. These genes confer female gametophytic phenotypes. FIS2 encodes a protein with a C 2 H 2 zinc-finger motif and three putative nuclear localization signals, indicating that it is likely to be a transcription factor. FIS1 encodes a protein with homology to the Drosophila Polycomb group gene Enhancer-of-zeste and is identical to the recently described Arabidopsis gene MEDEA. FIS1 is a protein with a number of putative functional domains, including the SET domain present in Enhancer-ofzeste-related proteins. Comparison of the position of the lesions in the fis1 and medea mutant alleles indicates that fis1 is a null allele producing a truncated polypeptide lacking all the protein domains whereas the deduced protein from medea lacks only the SET domain. We present a model of the role of FIS1 and FIS2 gene products in seed development.Plants have two alternating life phases, a gametophytic phase giving rise to male and female gametes and, after gamete fusion during fertilization, a sporophytic phase initiating with the zygote that develops into the mature plant (1). In most diploid, sexually reproducing plants, seed development begins when the haploid egg and the homodiploid central cells of the female gametophyte are fertilized by two sperm cells, producing a diploid zygote and triploid endosperm, respectively. In apomictic plants, unreduced egg cells or diploid nucellar cells develop parthenogenetically to produce the zygote, and in some cases endosperm development is autonomous. To isolate genes that might control components of apomixis, we have isolated mutants of Arabidopsis in which some stages of seed development are initiated without pollination (2, 3). In fis1 and fis2, autonomous diploid endosperm development progresses to the cellularized stage, and occasionally zygote-like and early embryo-like structures form. In the fis3 mutant, endosperm development stops at the free-nuclear stage and autonomous embryo-like structures have not been seen (3). Another mutant, fie, also showing endosperm development without fertilization and mapping to chromosome 3, is likely to be an allele of fis3 (4). After pollination in fis1, fis2, and fis3 mutants, most embryos are arrested at the globular to torpedo stages. We suggested that the FIS genes define a complex that suppresses the development of the seed, including the embryo and endosperm, in the absence of fertilization, and after fertilization, plays a role in promoting embryo development (3). To further define the roles of these gene products in seed development we have cloned the FIS1 and FIS2 genes.In this paper we describe the FIS1 and FIS2 genes, their deduced products, and a model of their role in triggering fertilization-independent seed development and postpollination embryo development. Altered regulation of these genes and their homologs might play a role in the development of apomictic seeds. MATE...

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Genomic imprinting causes the expression of an allele depending on its parental origin. In plants, most imprinted genes have been identified in Arabidopsis endosperm, a transient structure consumed by the embryo during seed formation. We identified imprinted genes in rice seed where both the endosperm and embryo are present at seed maturity. RNA was extracted from embryos and endosperm of seeds obtained from reciprocal crosses between two subspecies Nipponbare (Japonica rice) and 93-11 (Indica rice). Sequenced reads from cDNA libraries were aligned to their respective parental genomes using single-nucleotide polymorphisms (SNPs). Reads across SNPs enabled derivation of parental expression bias ratios. A continuum of parental expression bias states was observed. Statistical analyses indicated 262 candidate imprinted loci in the endosperm and three in the embryo (168 genic and 97 non-genic). Fifty-six of the 67 loci investigated were confirmed to be imprinted in the seed. Imprinted loci are not clustered in the rice genome as found in mammals. All of these imprinted loci were expressed in the endosperm, and one of these was also imprinted in the embryo, confirming that in both rice and Arabidopsis imprinted expression is primarily confined to the endosperm. Some rice imprinted genes were also expressed in vegetative tissues, indicating that they have additional roles in plant growth. Comparison of candidate imprinted genes found in rice with imprinted candidate loci obtained from genome-wide surveys of imprinted genes in Arabidopsis to date shows a low degree of conservation, suggesting that imprinting has evolved independently in eudicots and monocots.