MicroRNA control of flowering and annual crop cycle in tropical/subtropical horticultural trees

The horticultural tree life cycle can be differentiated into two distinct growth stages – vegetative and reproductive. Unlike annual crops, horticultural tree crops enter into an annual flowering cycle once they attain the reproductive phase. Herein, they undergo various developmental changes including flowering, fruit set, fruit development, fruit ripening, fruit drop and vegetative growth. The annual crop cycle is critical for horticultural crops like Persea americana (avocado) and Macadamia integrifolia (macadamia) as their productivity depends upon successful flowering and fruit development. It is therefore important to understand flowering and the factors involved in the crop cycle in these horticultural trees. Various endogenous and exogenous factors are involved in flowering regulation. Among them a class of small non-coding RNAs known as microRNAs play a vital role. We investigated the expression of flowering-associated miRNAs in avocado and macadamia at distinctive time-points during their annual crop cycle. We looked at the expression of two miRNA that are known act antagonistically during flowering: the floral inhibitor miR156 and the floral promoter miR172, which regulates APETALA2 (AP2)-like transcription factors. Preliminary results from avocado showed no significant difference in the expression of miR156 during the crop cycle, while significant differences were observed for miR172. For both crops, abundance of miR172 was higher during flowering and initial fruiting but lower at fruit ripening and drop/harvest. These results will help us in understanding flowering regulation in horticultural trees by providing first insight into microRNAs and their putative targets in these crops