Abstract
In plants, juvenile to adult phase transition is regulated by the sequential activity of two microRNAs: miR156 and miR172. A decline in miR156 and increase in miR172 abundance is associated with phase transition. There is very limited information on phase transition in economically important horticultural tree crops, which have a significantly long vegetative phase affecting fruit bearing. Here, we profiled various molecular cues known to be involved in phase transition and flowering, including the microRNAs miR156 and miR172, in three horticultural tree crops: avocado (Persea americana), mango (Mangifera indica), and macadamia (Macadamia integrifolia). We observed that miR156 expression decreases as these trees age and can potentially be used as a juvenility marker. Consistent with findings in annual plants, we also observed conserved regulation of the miR156-SPL3/4/5 regulatory module in these genetically distant tree crops, suggesting that this pathway may play a highly conserved role in vegetative identity. Meanwhile, the abundance of miR172 and its target AP2-like genes as well as the accumulation level of SPL9 transcripts were not related with plant age in these crops except in avocado where miR172 expression increased steadily. Finally, we demonstrate that various floral genes, including AP1 and SOC1 were upregulated in the reproductive phase and can be used as potential markers for the reproductive phase transition. Overall, this study provides an insight into the molecular associations of juvenility and phase transition in horticultural trees where crop breeding and improvement are encumbered by long juvenile phases.
Highlights
The plant life cycle can be divided into three distinct growth phases – juvenile, adult, and reproductive – all of which involve temporal and spatially coordinated changes in various traits essential for plant survival and reproduction
To explore whether the transcript abundance of these two miRNAs changes with vegetative phase transition in horticultural tree crops, we confirmed their presence in mango, macadamia, and avocado using small RNA sequencing data
The quantitative real-time PCR (qRT-PCR) results in all three species revealed that, of the tissues tested, miR156 is most highly expressed in leaves of young seedlings (Figures 1A–C)
Summary
The plant life cycle can be divided into three distinct growth phases – juvenile, adult, and reproductive – all of which involve temporal and spatially coordinated changes in various traits essential for plant survival and reproduction. These phases may be thought of as developmental phases, with continuous development of new organs that possess different morphological features in each phase. Late rosette leaves exhibit adult traits having shorter petioles, serrated margins, and abaxial trichomes (Telfer et al, 1997; Huijser and Schmid, 2011; Tsukaya, 2013). Differences in juvenile and adult leaf morphology can be detected in some woody species including Acacia confuse, Acacia colie, Eucalyptus globulus, Quercus acutissima, and Hedera helix (Wang et al, 2011)
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