Mutations of Arabidopsis thaliana that transform leaves into cotyledons

Abstract

We describe mutations of three genes in Arabidopsis thaliana-extra cotyledon1 (xtc1), extra cotyledon2 (xtc2), and altered meristem programming1 (amp1)-that transform leaves into cotyledons. In all three of these mutations, this transformation is associated with a change in the timing of events in embryogenesis. xtc1 and xtc2 delay the morphogenesis of the embryo proper at the globular-to-heart transition but permit the shoot apex to develop to an unusually advanced stage late in embryogenesis. Both mutations have little or no effect on seed maturation and do not affect the viability of the shoot or the rate of leaf initiation after germination. amp1 perturbs the pattern of cell division at an early globular stage, dramatically increases the size of the shoot apex and, like xtc1 and xtc2, produces enlarged leaf primordia during seed development. These unusual phenotypes suggest that these genes play important regulatory roles in embryogenesis and demonstrate that the development of the shoot apical meristem and the development of the embryo proper are regulated by independent processes that must be temporally coordinated to ensure normal organ identity.Embryogenesis in seed plants consists of several discrete processes (1-4). During the early, morphogenetic phase of embryogenesis, the primary axis of the plant is established, one or more cotyledons are formed, and the primordia of the root and shoot systems are initiated. The embryo subsequently undergoes a process of maturation in which it acquires the capacity for autonomous growth, accumulates nutritional reserves, becomes desiccation tolerant, and then enters a period of dormancy. Upon germination, the seedling uses its stored reserves, and the shoot and root meristems resume growth. Recent work has shown that although these events usually occur in a predictable temporal sequence, they are not part of a linear developmental pathway (4). For example, in several species, precocious germination leads to the simultaneous expression of genes that are normally expressed only during embryo maturation and genes that are normally expressed only during germination (reviewed in ref. 1). Similarly, the leafy cotyledon1-2 mutation in Arabidopsis causes plants to express germination-specific genes without completely suppressing the expression of a storage protein gene that is normally expressed during the maturation phase of embryogenesis (5). Conversely, the raspberry mutant of Arabidopsis thaliana is blocked at a globular stage of embryogenesis, but expresses genes that are normally activated much later (6).To study how morphogenetic events are temporally coordinated during embryogenesis, we screened for mutations of A. thaliana that mimicked the cotyledon-leaf phenotype of precociously germinated Brassica napus embryos (7, 8). We identified three mutations that have this phenotype. Here we describe the effect of these mutations on the morphogenesis of the embryo and provide evidence that the effect of these mutations on leaf identity is the result of a change i...

AMP1andMPantagonistically regulate embryo and meristem development inArabidopsis

“…Except for the oversized shoot meristem and frequent supernumerary cotyledons, the architecture of amp1 late-stage embryos is remarkably normal (Fig. 1Y) (Conway and Poethig, 1997). These features suggest that the mutant phenotype is primarily a consequence of the increased cell numbers in early pro-embryos.…”

Dual Effects of miR156-TargetedSPLGenes andCYP78A5/KLUHon Plastochron Length and Organ Size inArabidopsis thaliana

“…These plants, which produced no seeds, were small with compacted rosette leaves and an increased leaf initiation rate. The seedlings had three or four cotyledons (see Supplemental Figure 4 online), similar to amp1 mutants, which also have an embryonic phenotype reminiscent of cyp78a5 cyp78a7 double mutants (Chaudhury et al, 1993;Conway and Poethig, 1997). In summary, these results indicate that CYP78A5 and CYP78A7 play redundant roles in regulating relative growth of the shoot apical meristem and the rest of the plant.…”

“…The phenotype of embryos that lack activity of both CYP78A5 and its most similar Arabidopsis homolog, CYP78A7, is reminiscent of amp1 mutants (Chaudhury et al, 1993;Conway and Poethig, 1997). The cyp78a double mutant phenotype appears to be stronger than that of amp1 mutants, being often embryonic lethal and always causing several cotyledons to develop in surviving seedlings.…”

“…Inactivation of several genes, including rice (Oryza sativa) PLASTOCHRON1 (PLA1), maize (Zea mays) TERMINAL EAR1 (TE1) and its rice ortholog PLA2, and ALTERED MERISTEM PROGRAM1 (AMP1) in Arabidopsis thaliana, causes a shortening of the plastochron (Conway and Poethig, 1997;Itoh et al, 1998;Veit et al, 1998;Helliwell et al, 2001;Miyoshi et al, 2004;Kawakatsu et al, 2006). By contrast, phytochrome b (phyb) and serrate (se) mutants in Arabidopsis suffer from delayed initiation of new leaves (Reed et al, 1993;Clarke et al, 1999;Prigge and Wagner, 2001).…”

In planta functions of the Arabidopsis cytokinin receptor family

“…One function of the SAM is to produce leaf primordia. Typically, plants with an enlarged SAM have a shorter interval of leaf production, or more rapid plastochron, than plants with a smaller SAM (5,30,31). As expected from the reduced SAM, the triple mutants had a prolonged plastochron with respect to WT plants.…”