We report mutants in Arabidopsis thaliana (fertilization-independent seed: fis) in which certain processes of seed development are uncoupled from the double fertilization event that occurs after pollination. These mutants were isolated as ethyl methanesulfonate-induced pseudo-revertants of the pistillata phenotype. Although the pistillata (pi) mutant has short siliques devoid of seed, the fis mutants in the pi background have long siliques containing developing seeds, even though the f lowers remain free of pollen. The three fis mutations map to loci on three different chromosomes. In fis1 and fis2 seeds, the autonomous endosperm nuclei are diploid and the endosperm develops to the point of cellularization; the partially developed seeds then atrophy. In these two mutants, proembryos are formed in a low proportion of seeds and do not develop beyond the globular stage. When FIS͞fis plants are pollinated by pollen from FIS͞FIS plants, Ϸ50% of the resulting seeds contain fully developed embryos; these seeds germinate and form viable seedlings (FIS͞FIS). The other 50% of seeds shrivel and do not germinate; they contain embryos arrested at the torpedo stage (FIS͞fis). In normal sexual reproduction, the products of the FIS genes are likely to play important regulatory roles in the development of seed after fertilization.Arabidopsis seed, like the seed of other angiosperms, is a product of double fertilization, in which one of the two sperm cells fertilizes the haploid egg cell, giving rise to a diploid embryo, and the other sperm cell fertilizes the polar nuclei in the central cell, giving rise to the triploid endosperm (1). As the embryo and the endosperm develop, the ovule enlarges into a seed; the maternal tissues of the inner and outer integuments surrounding the embryo sac form the seed coat.We have proposed a strategy for identifying genes that uncouple components of seed development from the fertilization process (2). Inactivation of genes that normally repress seed development may lead to precocious seed development without fertilization. In many apomictic plants, seed development does occur without fertilization or with only partial fertilization (3). In autonomous apomixis, seed development occurs without pollination and thus without fertilization of either the egg cell or the polar cell. In pseudogamous apomixis, pollination is required; in some cases, the pollination event results in fertilization of the polar cell but not of the egg cell. Apomixis has been described in a close relative of Arabidopsis, Arabis holboellii (3), and some genetic data support a one or two gene control of apomixis (3), so we reasoned that a mutational approach in Arabidopsis might detect mutants displaying some components of apomixis.For the isolation of these mutants, we used stamenless pistillata (pi) (4). If pi plants are not pollinated, the siliques remain short; they only elongate when seed is formed. We identified mutants in which the siliques elongated without pollination (5), and recently Ohad et al (6) described ...

amp1, a mutant of Arabidopsis thaliana has a phenotype altered in three different aspects of plant development; spatial pattern, photomorphogenetic growth, and initiation of flowering. While fewer than 0.1% of the seedlings of wild‐type plants are non‐dicot as many as 20% of the seedlings of the amp1 mutant are tricot or tetracot. The rate of leaf initiation is faster and vegetative phyllotaxy is altered in amp1. When grown in the dark amp1 seedlings show morphogenetic properties similar to light‐grown wild‐type plants: they do not form an apical hook, have hypocotyls shorter than wild‐type plants and form etiolated true leaves. amp1 mutant flowers significantly earlier than congenic Amp1 plants. The mutant has six times more cytokinin than wild‐type suggesting that endogenous cytokinin levels might play an important role in mediating these different developmental processes. AMP1 might code for a negative regulator of cytokinin biosynthesis, or may be required for the degradation of cytokinin.

We investigated the uptake and distribution of AI in root apices of near-isogenic wheat (Triticum aesfivum L.) lines differing in AI tolerance at a single locus (Altl: aluminum tolerance). Seedlings were grown in nutrient solution that contained 100 p~ AI, and the roots were subsequently stained with hematoxylin, a compound that binds AI in vitro to form a colored complex. Root apices of Alsensitive genotypes stained after short exposures to AI (10 min and 1 h), whereas apices of AI-tolerant seedlings showed less intense staining after equivalent exposures. Differential staining preceded differences observed in either root elongation or total AI concentrations of root apices (terminal 2-3 mm of root). After 4 h of exposure to 100 p~ AI in nutrient solution, AI-sensitive genotypes accumulated more total AI in root apices than AI-tolerant genotypes, and the differences became more marked with time. Analysis of freeze-dried root apices by x-ray microanalysis showed that AI entered root apices of AI-sensitive plants and accumulated in the epidermal layer and in the cortical layer immediately below the epidermis. Long-term exposure of sensitive apices to AI (24 h) resulted in a distribution of AI coinciding with the absence of K. Quantitation of AI in the cortical layer showed that sensitive apices accumulated 5-to 10-fold more AI than tolerant apices exposed to AI solutions for equivalent times. These data are consistent with the hypothesis that Altl encodes a mechanism that excludes AI from root apices.A1 toxicity is one of the major factors that limit plant growth in many acid soils (Wright, 1989). The primary effect of A1 is to inhibit root growth in Al-sensitive genotypes with subsequent effects on nutrient and water uptake (Foy, 1983). Root elongation is affected within hours of A1 exposure (Wallace et al., 1982), and, as in many plant species, tlie primary site of A1 toxicity in wheat (Triticum aestivum L.) appears to be the root apex (Bennet and Breen, 1991). have shown that in wheat and maize, root elongation is inhibited only when apices are exposed to Al, whereas selectively exposing the remainder of the root does not inhibit elongation. Hematoxylin, a stain for Al, stains root apices of Al-sensitive wheat genotypes more intensely than root apices of Al-tolerant genotypes, but the remainder of the root shows the same degree of staining in different genotypes (Polle et al., 1978;Wallace et al., 1982), indicating that tolerance might be a property of the root apex.Differential uptake of A1 into roots could account for differences in tolerance between genotypes, but conflicting results have been reported regarding differences in A1 uptake in roots of different wheat genotypes. Some of these conflicting results appear to be due to the size of the root portion analyzed and the time of exposure to Al. Recently RincÓn and Gonzales (1992) showed that an Al-sensitive wheat cultivar accumulated more A1 in its root apices (2 mm terminus of root) than an Al-tolerant cultivar, which is consistent with the above discus...