Functional Redundancy and New Roles for Genes of the Autonomous Floral-Promotion Pathway      

Abstract

The early-flowering habit of rapid-cycling accessions of Arabidopsis (Arabidopsis thaliana) is, in part, due to the genes of the autonomous floral-promotion pathway (AP). The AP promotes flowering by repressing expression of the floral inhibitor FLOWERING LOCUS C (FLC). AP mutants are therefore late flowering due to elevated levels of FLC, and this late-flowering phenotype is eliminated by loss-of-function mutations in FLC. To further investigate the role of the AP, we created a series of double mutants. In contrast to the phenotypes of single mutants, which are largely limited to delayed flowering, a subset of AP double mutants show a range of defects in growth and development. These phenotypes include reduced size, chlorophyll content, growth rate, and fertility. Unlike the effects of the AP on flowering time, these phenotypes are FLC independent. Recent work has also shown that two AP genes, FCA and FPA, are required for the repression and, in some cases, proper DNA methylation of two transposons. We show that similar effects are seen for all AP genes tested. Microarray analysis of gene expression in AP single and double mutants, however, suggests that the AP is not likely to play a broad role in the repression of gene expression through DNA methylation: very few of the genes that have been reported to be up-regulated in DNA methylation mutants are misexpressed in AP mutants. Together, these data indicate that the genes of the AP play important and sometimes functionally redundant roles in aspects of development in addition to flowering time.A fundamental question in biology is how differentiating cells make decisions between alternative fates. The change from vegetative to reproductive development in Arabidopsis (Arabidopsis thaliana) is an attractive model for studying the factors that regulate such developmental transitions. Early in the life cycle, the undifferentiated stem cells of the shoot apical meristem give rise to organ primordia that will produce the vegetative portions of the plant (e.g. leaves). Later, the shoot apical meristem switches to producing primordia that will form the reproductive organs (e.g. flowers). Proper timing of this transition is critical for successful reproduction. Therefore, plants have evolved mechanisms to regulate flowering in response to both endogenous and environmental factors (Boss et al., 2004). These mechanisms help to ensure that plants flower at a suitable time in their development and a favorable time of year.

Noncanonical Translation Initiation of theArabidopsisFlowering Time and Alternative Polyadenylation Regulator FCA

“…Consistent with this, the first IRES identified in plant cells was in mRNA encoding the maize (Zea mays) heat shock protein, Hsp101, and this accounts for its cap-independent translation during heat stress (Dinkova et al, 2005). FCA functions to regulate flowering time by repressing FLC, but we also know it plays other roles in regulating the expression of genome sequences that are also targets of RNA-mediated chromatin silencing and likely acts somewhat redundantly with other autonomous pathway components to regulate a wider set of gene targets (Veley and Michaels, 2008). Therefore, continued translation of FCA through mitosis may be required to prevent changes in otherwise epigenetically silenced loci, or translation regulation may affect FCA function in stress-dependent conditions, distinct from its role in flowering.…”

The Timing of Flowering

“…One thing that has recently become clear is that the function of many of the so-called autonomous-pathway genes is not restricted to the regulation of flowering time. Although the phenotypes of most autonomous-pathway single mutants are largely limited to delayed flowering, some autonomous-pathway double mutants show strong pleiotropic phenotypes and many autonomous-pathway mutants show defects in gene silencing (Baurle et al, 2007;Veley and Michaels, 2008). Interestingly, these loss-of-genesilencing phenotypes are correlated with changes in DNA methylation at the affected loci.…”

Crosstalk between Cold Response and Flowering in Arabidopsis Is Mediated through the Flowering-Time Gene SOC1 and Its Upstream Negative Regulator FLC

“…Thus, it is likely that these two genes affect the intermittent cold-sensing pathway via separate mechanisms. However, both fve and gi showed epistatic interaction with fpa in a double mutant analysis (Koornneef et al, 1998;Veley and Michaels, 2008). Thus, there is still an open possibility that these three genes participate in the same intermittent coldsensing pathway for flowering, which is independent of SOC1.…”