2018 ASHS Annual Conference

Avocado trees flower in response to periods of low temperature but little is known regarding the gene-environment interaction associated with floral development. In this research, temporal expression profiles of the floral promoter genes FLOWERING LOCUST (FT), LEAFY (LFY), FRUITFUL (FUL) and APETALA2 (AP2) and downstream genes associated with avocado floral organ identity, APETALA3 (AP3), PISTILLATA (PI), AGAMOUS1 (AG1) and AGAMOUS3 (AG3), were quantified in buds of ‘Hass’ avocado trees (3 years from budding) maintained under warm temperature (WT) (30 °C, 14-h day/20 °C, 10-h night) for 14 weeks relative to those of trees subjected to 8 weeks of low temperature (LT) (14 °C, 10-h day/10 °C, 14-h night) followed by 6 weeks of WT. Only LT-treated trees flowered (week 14). All trees were maintained under WT for 5 months prior to initiation of the experiment in July. At this time, buds of all trees expressed LFY, FUL, and AP2, with FT, AP3 and PI mRNA at detectable levels, suggesting the possibility that all buds were induced to flower but not determined (irreversiblly commitment to floral development). By week 8 of LT treatment, bud expression of LFY, FUL, and AP2 increased to levels significantly greater than that of WT-treated trees. Two weeks after transfer of LT-treated trees to WT, bud expression of FT significantly increased followed by activation of the downstream genes AP3, PI, AG1 and AG3 by week 12. In contrast, for WT trees, bud expression of FT, AP3, and PI remained at the limit of detection, with AG1 and AG3 below the limit of detection through week 14. Taken together, our results support that LT directly or indirectly up regulated expression of the floral promoter genes FT, LFY, FUL, and AP2, which activated the downstream floral organ identity genes and resulted in flowering. The results further suggest that bud expression levels of LFY, FUL and AP2 in week 8 of LT treatment were sufficient to confer bud determinacy, since transfer of the trees from LT to WT did not prevent flowering. In light of the fact that bud expression of FT, AP3, PI, AG1 and AG3 did not occur until after transfer of the LT-treated trees to WT, our results further suggest that LT serves as both a promoter and inhibitor of flowering, preventing floral organogenesis until the low temperature stress was removed and warm temperature prevailed.

Key words: Floral gene-environment interaction, floral promoter genes, floral organ identity genes, floral organogenesis, ‘Hass’ avocado