APETALA2 (AP2) plays a central role in the establishment of the floral meristem, the specification of floral organ identity, and the regulation of floral homeotic gene expression in Arabidopsis. We show here that in addition to its functions during flower development, AP2 activity is also required during seed development. We isolated the AP2 gene and found that it encodes a putative nuclear protein that is distinguished by an essential 68-amino acid repeated motif, the AP2 domain. Consistent with its genetic functions, we determined that AP2 is expressed at the RNA level in all four types of floral organs--sepals, petals, stamens, and carpels--and in developing ovules. Thus, AP2 gene transcription does not appear to be spatially restricted by the floral homeotic gene AGAMOUS as predicted by previous studies. We also found that AP2 is expressed at the RNA level in the inflorescence meristem and in nonfloral organs, including leaf and stem. Taken together, our results suggest that AP2 represents a new class of plant regulatory proteins that may play a general role in the control of Arabidopsis development.

The mechanisms by which plants modulate their growth rate in response to environmental and developmental conditions are unknown, but are presumed to involve specialized regions called meristems where cell division is concentrated. The possible role of cell division in influencing meristem activity and overall plant growth rate is controversial, with a prevailing view that cell division is secondary to higher order meristem controls. Here we show that a reduction in the length of the cell-cycle G1 phase and faster cell cycling occur when the rate of cell division in transgenic tobacco plants is increased by the plant D-type cyclin CycD2 (ref. 8). The plants have normal cell and meristem sizes, but elevated overall growth rates, an increased rate of leaf initiation and accelerated development in all stages from seedling to maturity. We conclude that cell division is a principal determinant of meristem activity and overall growth rate, and propose that modulation of plant growth rate is achieved through regulation of G1.

Little is known about the signals that govern the network of meristem and organ identity genes that control f lower development. In Arabidopsis, we can induce a heterochronic switch from f lower to shoot development, a process known as f loral meristem reversion, by manipulating photoperiod in the f loral homeotic mutant agamous and in plants heterozygous for the meristem identity gene leafy. The transformation from f lower to shoot meristem is suppressed by hy1, a mutation blocking phytochrome activity, by spindly, a mutation that activates basal gibberellin signal transduction in a hormone independent manner, or by the exogenous application of gibberellins. We propose that LFY and AG play an important role in the maintenance of f lower meristem identity and that f loral meristem reversion in heterozygous lfy and in ag f lowers is regulated by a phytochrome and gibberellin signal transduction cascade.Plant growth and morphogenesis is controlled by meristems, organized tissues containing pluripotent stem cells whose identities and activities are regulated by intrinsic and environmental signals. In Arabidopsis, the shoot apical meristem undergoes two phases, vegetative and inflorescence; both phases are characterized by reiterative and indeterminate patterns of growth and organogenesis (1). The vegetative meristem produces a compact rosette consisting of a short stem and a variable number of leaves. By contrast, the inflorescence meristem produces an elongated stem punctuated by narrow cauline leaves, lateral secondary shoots, and flowers that are derived from the flanks of the inflorescence meristem. Although closely related spatially and by cell lineage to the inflorescence meristem, the floral meristem proceeds along a determinate developmental pathway producing four compact whorls of organs (four sepals, four petals, six stamens, and two carpels). At the completion of organogenesis, the floral meristem is thought to be depleted or its activity is suppressed (2, 3). The transition from vegetative to inflorescence shoot meristem is controlled by environmental signals including photoperiod and temperature (4-6), by intrinsic growth regulators such as the gibberellins (6-8), and by a system of flowering time genes (9). The inflorescence meristem in turn produces an indeterminate number of floral meristems. Genetic and molecular studies have shown that the establishment of floral meristem identity is governed by a network of genes including APETALA1 (AP1), APETALA2 (AP2), CAULI-FLOWER (CAL), CLAVATA1 (CLV1), CLAVATA3 (CLV3), and LEAFY (LFY) (3,(10)(11)(12)(13)(14)(15)(16)(17)(18), and several models have been proposed to explain how these genes function together (17,19,20). By comparison, much less is known about the signals and genes required for the maintenance of flower meristem identity.Recently several genes have been implicated in the maintenance of flower meristem identity including LFY and AGA-MOUS (AG) (3,15,17,(21)(22)(23). The LFY gene encodes a novel polypeptide that is reported to have DNA bin...

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We have isolated two nitrate reductase genes and their corresponding cDNAs from Arabidopsis thaliana. Sequences of the two cDNAs, when compared to a sequence of a barley cDNA clone, confirm their identity as nitrate reductase clones and show that they are closely related. The two genes have been mapped using restriction fragment length polymorphisms; gNR2 is close to the previously identified chl‐3 locus and is probably identical to it, while gNR1 maps to a new locus (NIA1) on chromosome 1, near gl‐2.