A role of hormones, particularly abscisic acid (ABA) has been well established in both bud and seed dormancy ( Rohde and Bhalerao, 2007 Horvath et al., 2008 Graeber et al., 2012) and commonalities in genes that regulate bud and seed dormancy were reported in peach ( Leida et al., 2012a Wang et al., 2016). Recent studies in Populus revealed striking similarities between genes and pathways that regulate flowering time in annuals and growth and dormancy cycles in perennials, as reviewed by Ding and Nilsson (2016) homologs of circadian-clock and photoperiod pathway components phytochromes and CONSTANS ( CO) impact on growth cessation, homologs of key flowering genes FLOWERING LOCUS T ( FT), bZIP transcription factor FD and floral meristem identity gene APETALA1 ( AP1) have all been shown to regulate growth, while a homolog of TERMINAL FLOWER1 ( TFL1) affects chilling-requirement and dormancy release. These buds shift to the ecodormant state after satisfying the chilling requirement, where buds are capable of resuming growth ( Lang et al., 1987). Subsequently, internal signals in the bud are induced to prevent its growth during the endodormant state ( Lang et al., 1987 Horvath et al., 2003). External environmental factors such as cold or drought stress trigger the transition to dormant phase, resulting in shoot growth cessation and bud set.
The annual growth cycle of trees in temperate zones consists of dormant and active growth phases. The association of MdSVPa and MdSVPb expression with floral bud development in the low fruiting ‘Off’ trees of a biennial bearing cultivar ‘Sciros’ suggested the SVP genes might also play a role in floral meristem identity. Consistent with this, ectopic expression of MdDAMb and MdSVPa in ‘Royal Gala’ apple plants resulted in delayed budbreak and architecture change due to constrained lateral shoot outgrowth, but normal flower and fruit development. In contrast, the expression of MdDAMb, MdSVPa and MdSVPb suggested a role in maintaining bud dormancy. The expression of MdDAMa and MdDAMc indicated they may play a role in triggering autumn growth cessation. Here, we identified and characterized three DAM-like ( MdDAMs) and two SHORT VEGETATIVE PHASE-like ( MdSVPs) genes from apple ( Malus × domestica ‘Royal Gala’).
MADS-box genes with similarity to Arabidopsis SHORT VEGETATIVE PHASE ( SVP) have been implicated in regulating flowering and growth-dormancy cycles in perennials. The onset of winter triggers an endodormant state preventing bud growth and, once a chilling requirement is satisfied, these buds enter an ecodormant state and resume growing. The annual growth cycle of trees is the result of seasonal cues. 4Department of Biochemistry, University of Otago, Dunedin, New Zealand.3School of Biological Sciences, University of Auckland, Auckland, New Zealand.
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