Overexpression of AGAMOUS-LIKE 28 (AGL28) promotes flowering by upregulating expression of floral promoters within the autonomous pathway

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Abstract

MADS box genes are known to perform important functions in the development of various plant organs. Although the functions of many MADS box genes have previously been elucidated, the biological function of the type I MADS box genes remains poorly understood. In order to understand the function and regulation of the type I MADS box genes, we conducted molecular genetic analyses of AGL28, a member of the Mα class of type I genes. AGL28 was expressed in vegetative tissues in a photoperiod-independent manner, but not within the reproductive apex. This indicates that AGL28 plays a role in the vegetative phase. Overexpression of AGL28 caused precocious flowering via the upregulation of the expression of FCA and LUMINIDEPENDENS (LD), both floral promoters within the autonomous pathway. However, the loss of AGL28 function did not result in any obvious flowering time phenotype, which suggests that AGL28 may perform a redundant function. Collectively, our data suggest that AGL28 is a positive regulator of known floral promoters within the autonomous pathway in Arabidopsis.

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Materials and methods

Plant materials and measurement of flowering time. Wild-type Arabidopsis (ecotype Columbia) was employed in this study, unless otherwise noted. The plants were grown in Sunshine Mix 5 (Sungro Horticulture, Quincy, MI, USA) under long-day (16-h-light/8-h-dark at 120 μmol/m2/s) or short-day conditions (8-h-light/16-h-dark at 120 μmol/m2/s) at 23 °C. For RNA sampling in the circadian rhythm experiment, plants grown for 9 days under long-day conditions were transferred to continuous light conditions.

Phylogenetic analysis of AGL28 with type II MADS box genes that control flowering time

In order to characterize the phylogeny of the MADS box genes involved in the control of flowering time in Arabidopsis, we generated a multiple sequence alignment of the MADS domain of AGL28 and the type II MADS box flowering time genes (Fig. 1A). The MADS box domains of the type II MADS proteins were found to be highly homologous to each other. However, the MADS domain of AGL28 was less homologous to those of the type II MADS box proteins. This suggests a high degree of sequence divergence,

Acknowledgments

We are grateful to Dr. Chae Eun Lim for her help with the phylogenetic analyses. S.K. Yoo was supported by the BK21 program. This study was supported by a grant from the Crop Functional Genomics Center to J.S.L., by the Plant Signaling Network Research Center, and by a grant from the Korea Research Foundation (KRF-2002-015-CS0038) to J.H.A.

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    Abbreviations: AG, AGAMOUS; AGL, AGAMOUS-LIKE; AP1, APETALA1; AP3, APETALA3; FBP11, FLORAL BINDING PROTEIN 11; FLC, FLOWERING LOCUS C; FLF, FLOWERING LOCUS F; FLM, FLOWERING LOCUS M; FT, FLOWERING LOCUS T; LD, LUMINIDEPENDENS; MAF, MADS AFFECTING FLOWERING; PI, PISTILLATA; SOC1, SUPPRESSOR OF OVEREXPRESSION OF CO 1; SVP, SHORT VEGETATIVE PHASE.

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