TYPE II MADS-BOX GENES ASSOCIATED WITH POPLAR APICAL BUD DEVELOPMENT AND DORMANCY

MADS-box genes encode a group of transcription factors that regulate the development of vegetative and reproductive organs in plants. Little is known about the role of MADS-box genes in tree development. To advance the understanding of MADS-box genes in tree vegetative bud development and dormancy, 57 putative type II MADS-box genes representing 14 functional classes were identifi ed from the JGI Populus trichocarpa genome database. DNA sequencing of cDNAs for the Poplar type II MADS-box genes revealed that a signifi cant number of these genes possess different intron-exon structures than those predicted in the JGI Poplar genome database. Furthermore, splicing variants were detected for approximately 20% of the type II MADS-box genes. Using RT-PCR analysis, transcripts of all 14 classes were detected in vegetative (shoot apices, leaf, bark, xylem, and root) and/or fl oral tissues. These results indicate that these genes may play specifi c regulatory roles in specifi c developmental process in poplar. Such knowledge could be useful for selecting candidate genes for functional studies in poplar development. Introduction Transcription factors are important regulators of cellular processes and mediate responses to biotic and abiotic stimuli. MADS-box genes encode a family of transcription factors present in plants, animals, and fungi. MADS-box genes are known to be involved in the regulation of diverse developmental processes in fl owering plants including reproductive (fl ower, seed, fruit) and vegetative (root, leaf) development and differentiation (Table 1-1B). Many Arabidopsis loss-of-function mutants (agamous, agl24, ap1, ap3, cal, pi, fl c, ful, maf1, maf2, sep1, sep2, sep3, sep4, soc1, svp, etc.) have been studied in detail to understand their regulatory roles. For example, fl oral homeotic genes such as AP1, PI, AP3, AG, and SEP1–4 35 identify fl oral organs (Ng et al. 2001; Theissen 2001; Theissen et al. 2000), some MADS-box genes govern reproductive development (Ng et al. 2001; Theissen 2001), and fl oweringtime genes such as FLC, AGL24, SVP and SOC1 repress or promote the fl oral transition mediated by internal or environmental factors (plant age, day-length, cold) (Hartmann et al. 2000; Sheldon et al. 2002; Sheldon et al. 2000). MADS-box genes have also been shown to be involved in regulating embryo, pollen, endosperm, seed, fruit, root, guard cells, and leaf development (Alvarez-Buylla et al. 2000a; Theissen et al. 2000). The fl oral-quartet-model describes how combinations of class A, B, C, D and E genes form specifi c tetramers involved in the regulation of fl oral organ identifi cation (sepals, petals, stamens, carpels, ovules) (Figure 1-2A) (Krizek et al. 2005; Theissen et al. 2001). The evolutionary history of the MADS-box gene family in plants is characterized by duplication events and subsequent divergence and fi xation (Alvarez-Buylla et al. 2000b; Litt et al. 2003). All MADS-box genes originated from a common ancestor and have diverged into two large groups, type I and type II, by gene duplication that occurred prior to divergence of plants and animals (Alvarez-Buylla et al. 2000b). The type II MADS-box genes comprise the MEF2-like genes in animals and fungi and the MIKC-type genes in plants. MIKC-type proteins contain 4 major domains: a highly conserved MADS-box domain of ~55–60 amino acids which includes the DNA-binding domain and dimerization motif, a less-well-conserved intervening (I) domain (~30 amino acids) that appears to be involved in DNA binding specifi city and dimerization, a well-conserved keratin-like coiled-coil (K) domain (~70 amino acids) which contains a protein-protein interaction region, and a variable length C-terminal (C) domain which is involved in transcriptional activation, posttranslational modifi cation, or protein-protein interaction (Alvarez-Buylla et al. 2000b; Hileman et al. 2006; Kaufmann et al. 2005; Nam et al. 2003; Vandenbussche et al. 2003). Type I MADS-box genes contain MADS-domain but lack the K-domain and experienced faster birth-and-death evolution than type II MADS-box genes (Nam et al. 2004). The biological functions of type I MADS-box genes are poorly understood due to lack of mutant phenotypes.