Molecular regulation of phase transition and flowering in tropical/subtropical tree crops

Abstract

Avocado (Persea americana), mango (Mangifera indica) and macadamia (Macadamia spp.) are important horticultural crops globally, especially in the tropics/subtropics. However, these tree crops experience a long juvenile phase, which is an impediment to crop breeding and productivity as fruit production depends on attaining reproductive phase.Phase transition in plants is the developmental switch from a juvenile state to a mature or reproductive state over time. Various studies in Arabidopsis and other plants have suggested a pivotal role for two highly conserved miRNAs: miR156 and miR172 in phase transition and flowering. Higher abundance of miR156 results in prolonged vegetative phase and delays flowering, while higher miR172 transcripts promote flowering. These act by downregulating members of the SQUAMOSA Promoter Binding Protein-like (SPL) and APETALA2-like (AP2) transcription factor gene families, respectively.There is very limited information on phase transition in economically important horticultural tree crops. This thesis explores the conservation of microRNAs and their target pathways in the phase-transition and yearly phenology of avocado, mango and macadamia for the first time. Chapter II profiles these molecular cues over tree development. Consistent with findings in annual plants, miR156 expression decreases as these trees age and can potentially be used as a juvenility marker. Conserved regulation of the miR156-SPL4 regulatory module across these genetically distant tree crops suggests this pathway may play a highly conserved role in vegetative identity in plants. In addition, various floral genes, including APETALA 1 (AP1) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) were upregulated in the reproductive phase and could be used as potential markers for the reproductive phase transition. Meanwhile, the abundance of miR172 and its target AP2-like genes were not related with plant age in these crops except in avocado where miR172 expression increased steadily.Graft transmissibility of miR156 and miR172 has been explored in annual plants. Grafting is the common propagation method in avocado and macadamia and primarily benefits orchard production by reducing the time to tree productivity through the use of mature scion material to hasten early bearing/maturity. Rootstocks, however, may be propagated from mature tree cuttings (‘mature’), or from seed (‘juvenile’). In Chapter III the molecular profiles of rootstock and scion, and possible influence of the rootstock on the maturity of the grafted tree were explored. Transcript abundance of miR156, miR172 and the miR156 target gene SPL4, was correlated to the maturity of the scion and rootstock material. The scion appeared to be responsible for grafted tree maturity involving these factors, while the rootstock maturity had minimal influence on miRNA abundance in the scion, or tree maturity. Furthermore, it was demonstrated that the presence of leaves on cutting rootstocks is essential for graft success and contributes towards inter-graft signalling involving the carbohydrate-marker TREHALOSE6-PHOSPHATE SYNTHASE 1 (TPS1).After transitioning to reproductive phase, horticultural trees enter an annual flowering cycle which lasts for several years. Here they undergo various physiological and developmental changes essential for successful flowering. Understanding the annual flowering cycle is critical for horticultural tree productivity. In Chapter IV, potential molecular cues regulating the yearly flowering cycle, or phenology, in avocado and macadamia were investigated for two consecutive crop cycles. Expression analysis suggested that known floral genes FLOWERING LOCUS T (FT), AP1, CONSTANS (CO), SEPALLATA 2 (SEP2/AGL4) and SEP3/AGL9 were upregulated at the time of floral induction and are potential candidate markers for floral initiation in these crops. On the other hand, Dormancy-associated MADS-box gene (DAM), which is associated with endodormancy, is downregulated at the time of floral bud break. A possible repression of FT through CO in avocado to regulate flowering is suggested. Furthermore, the SOC1-SPL4 model described in annual plants appears to be conserved in avocado and macadamia. Lastly, no correlation of miRNAs (miR156, miR172) with any phenological event was observed.Overall, this Ph.D. project generated valuable data to develop a better understanding of the molecular regulation of phase change and flowering in horticultural tree crops. Potential markers as well as a model for flowering and reproductive phase transition is presented for these crops. The generated data will be important in developing future strategies for managing these horticultural trees, and it will also serve as a base point for further research in this field.