Developmental Pathways Are Controlled by Networks of Interacting Genes

We have a great deal more to learn about the regulatory networks that control developmental pathways. However, several discoveries point to a model in which local and long-distance signaling events control the expression of genes that encode transcription factors. These transcription factors in turn determine the character or activities of a given tissue or cell. Often these mechanisms involve feedback loops in which two or more genes interact to regulate each other's expression. These interactions are seen most clearly in the case of the shoot apical meristem.

Expression of the KNOX gene STM (SHOOTMERIS-TEMLESS) is essential for the formation of the shoot apical meristem in the Arabidopsis embryo and for meristem function in the growing plant. STM is expressed throughout the apical dome of the vegetative meristem, except in the developing leaf primordia. Similarly, STM is expressed in the dome of the floral meristem, but it is silenced as floral organs appear. Two additional KNOX genes—KNAT1 and KNAT2—also are expressed in the apical meristem of Ara-bidopsis and participate in maintaining the meristem cells in an undifferentiated state.

Because cells actively divide in the early stages of leaf and floral organ primordia development, STM is not necessary for cell division. Rather KN1, STM, and their functional homologs maintain meristem identity by suppressing differentiation. Another gene, ASYMMETRIC LEAVES1 (AS1) promotes leaf development and is expressed in the primordia and young leaves of Arabidopsis (Figure 16.26) (Byrne et al. 2000). STM represses the expression of AS1, and AS1 in turn represses the expression of KNAT1 in the developing leaf primordia (Ori et al. 2000):

(A) Wild-type embryos

(A) Wild-type embryos

(B) stm mutant embryos

(B) stm mutant embryos

FIGURE 16.26 The meristem identity gene, STM, inhibits expression of the ASYMMETRIC LEAVES1 (AS1) gene, which promotes leaf development in Arabidopsis. Arrows point to the shoot apical meristem-forming region. (A) Expression of the STM gene is normally confined to the shoot apical meristem in the wild type, and it confers meristem identity on the vegetative meristem. In contrast, the AS1 gene is confined to leaf primordia and developing cotyledons in the wild type, as shown by in situ hybridization in embryos at two stages of development. (B) In stm mutants, expression of AS1 expands into the region that would normally become the shoot apical meristem. As a result, the apical meristem does not form. (From Byrne et al. 2000.)

STM-1 AS1-► Promotes leaf development

KNAT1-► Maintains meristem

The WUSCHEL (WUS) gene, which encodes another homeodomain transcription factor, is a key regulator of stem cell indeterminacy (Laux et. al. 1996). In plants with loss-of-function wus mutations, either an apical meristem is lacking entirely, or their stem cells are used up after they have formed a few leaves. The CLAVATA genes negatively regulate WUS expression. WUS expression is expanded in both clv1 and clv3 mutants (Figure 16.27). Conversely, WUS expression positively regulates CLV3 gene expression; (see Figure 16.24) (Brand et al. 2000).

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