Molecular Cues for Phenological Events in the Flowering Cycle in Avocado.

Reproductively mature horticultural trees undergo an annual flowering cycle that repeats each year of their reproductive life. This annual flowering cycle is critical for horticultural tree productivity. However, the molecular events underlying the regulation of flowering in tropical tree crops such as avocado are not fully understood or documented. In this study, we investigated the potential molecular cues regulating the yearly flowering cycle in avocado for two consecutive crop cycles. Homologues of flowering-related genes were identified and assessed for their expression profiles in various tissues throughout the year. Avocado homologues of known floral genes FT, AP1, LFY, FUL, SPL9, CO and SEP2/AGL4 were upregulated at the typical time of floral induction for avocado trees growing in Queensland, Australia. We suggest these are potential candidate markers for floral initiation in these crops. In addition, DAM and DRM1, which are associated with endodormancy, were downregulated at the time of floral bud break. In this study, a positive correlation between CO activation and FT in avocado leaves to regulate flowering was not seen. Furthermore, the SOC1-SPL4 model described in annual plants appears to be conserved in avocado. Lastly, no correlation of juvenility-related miRNAs miR156, miR172 with any phenological event was observed.

Getting to the root of grafting-induced traits.

Grafting is an ancient technique that involves the physical joining of genotypically distinct shoot and root systems, in order to achieve a desirable compound plant. This practice is widely used in modern agriculture to improve biotic and abiotic stress tolerance, modify plant architecture, induce precocious flowering and rejuvenate old perennial varieties, boost yield, and more. Beneficial new rootstock-scion combinations are currently identified through an inefficient trial and error process, which presents a significant bottleneck for the application of grafting to combat new environmental challenges. Identifying the mechanisms that underlie beneficial grafting-induced traits will facilitate rapid breeding and genetic engineering of new rootstock x scion combinations that exhibit superior performance across varying agricultural environments.

Scion control of miRNA abundance and tree maturity in grafted avocado.

BACKGROUND: Grafting is the common propagation method for avocado and primarily benefits orchard production by reducing the time to tree productivity. It also allows use of scions and rootstocks specifically selected for improved productivity and commercial acceptance. Rootstocks in avocado may be propagated from mature tree cuttings ('mature'), or from seed ('juvenile'). While the use of mature scion material hastens early bearing/maturity and economic return, the molecular factors involved in the role of the scion and/or rootstock in early bearing/reduced juvenility of the grafted tree are still unknown. RESULTS: Here, we utilized juvenility and flowering associated miRNAs; miR156 and miR172 and their putative target genes to screen pre-graft and post-graft material in different combinations from avocado. The abundance of mature miR156, miR172 and the miR156 target gene SPL4, showed a strong correlation to the maturity of the scion and rootstock material in avocado. Graft transmissibility of miR156 and miR172 has been explored in annual plants. Here, we show that the scion may be responsible for grafted tree maturity involving these factors, while the rootstock maturity does not significantly influence miRNA abundance in the scion. We also demonstrate that the presence of leaves on cutting rootstocks supports graft success and contributes towards intergraft signalling involving the carbohydrate-marker TPS1. CONCLUSION: Here, we suggest that the scion largely controls the molecular 'maturity' of grafted avocado trees, however, leaves on the rootstock not only promote graft success, but can influence miRNA and mRNA abundance in the scion. This constitutes the first study on scion and rootstock contribution towards grafted tree maturity using the miR156-SPL4-miR172 regulatory module as a marker for juvenility and reproductive competence.

Juvenility and Vegetative Phase Transition in Tropical/Subtropical Tree Crops.

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.