<p>There are 42 members of the homeodomain-leucine zipper (HDZip) family of transcription factors in <i>Arabidopsis thaliana</i>. This thesis focuses on the functional analysis of one member of this family, ATHB16, and on the biochemical properties of HDZip proteins. </p><p>To assess the function of the <i>ATHB16</i> gene, the expression of <i>ATHB16</i> was altered in transgenic Arabidopsis plants by using sense and antisense RNA constructs under the control of the 35S promoter. The reciprocal phenotypic effects associated with elevated and reduced levels of <i>ATHB16</i> expression suggested that, in wild-type plants, ATHB16 acts as a mediator of blue and red light effects on the regulation of plant growth and the timing of the floral transition. </p><p>In wild-type Arabidopsis, expression of <i>ATHB16</i> is high in leaves, intermediate in adult roots and inflorescences, and low in stems and siliques. The expression of <i>ATHB16</i> in the root is markedly increased in response to exogenous abscisic acid (ABA) treatment, but is reduced in the ABA response mutants <i>abil </i>and <i>abi2</i>, suggesting that <i>ATHB16</i> may be involved in ABA signal transduction. This hypothesis was corroborated by observations of alterations in sensitivity to ABA inhibition of root growth in seedlings of a T-DNA insertion mutant of <i>ATHB16</i> and of transgenic plants with elevated <i>ATHB16</i> levels. </p><p>HDZip proteins bind DNA as dimers. DNA-binding studies showed that different HDZip proteins interact with very similar target sequences <i>in vitro</i> and that they selectively form heterodimers with each other. For example, it was demonstrated that ATHB16 can heterodimerize with ATHB6 and ATHB7 in yeast and with ATHB5 <i>in vitro</i>, suggesting that ATHB16 may interact with other HDZip proteins in Arabidopsis. This interaction may have functional significance, since it may provide a mechanism for the plant to integrate different input signals, like light of different spectral qualities and water availability in the regulation of its growth. </p>

There are 42 members of the homeodomain-leucine zipper (HDZip) family of transcription factors in Arabidopsis thaliana. This thesis focuses on the functional analysis of one member of this family, ATHB16, and on the biochemical properties of HDZip proteins. To assess the function of the ATHB16 gene, the expression of ATHB16 was altered in transgenic Arabidopsis plants by using sense and antisense RNA constructs under the control of the 35S promoter. The reciprocal phenotypic effects associated with elevated and reduced levels of ATHB16 expression suggested that, in wild-type plants, ATHB16 acts as a mediator of blue and red light effects on the regulation of plant growth and the timing of the floral transition. In wild-type Arabidopsis, expression of ATHB16 is high in leaves, intermediate in adult roots and inflorescences, and low in stems and siliques. The expression of ATHB16 in the root is markedly increased in response to exogenous abscisic acid (ABA) treatment, but is reduced in the ABA response mutants abil and abi2, suggesting that ATHB16 may be involved in ABA signal transduction. This hypothesis was corroborated by observations of alterations in sensitivity to ABA inhibition of root growth in seedlings of a T-DNA insertion mutant of ATHB16 and of transgenic plants with elevated ATHB16 levels. HDZip proteins bind DNA as dimers. DNA-binding studies showed that different HDZip proteins interact with very similar target sequences in vitro and that they selectively form heterodimers with each other. For example, it was demonstrated that ATHB16 can heterodimerize with ATHB6 and ATHB7 in yeast and with ATHB5 in vitro, suggesting that ATHB16 may interact with other HDZip proteins in Arabidopsis. This interaction may have functional significance, since it may provide a mechanism for the plant to integrate different input signals, like light of different spectral qualities and water availability in the regulation of its growth.