Effects of temperature, soil moisture and light intensity on the temporal pattern of floral gene expression and flowering of avocado buds (Persea americana cv. Hass)

Abstract For ‘Hass’ avocado (Persea americana Mill.), low temperature is known to induce flowering, but effects of other environmental factors remain equivocal. In addition, documentation of interactions among environmental factors, floral gene expression and subsequent flower development is limited. Thus, in this research, the effects of environmental factors on the temporal expression patterns of genes related to flowering time, floral meristem identity and floral organ identity were quantified in buds relative to inflorescence number. ‘Hass’ avocado trees were subjected to four different environmental conditions: optimal growth condition (OGC) (warm temperature, well-watered, photosynthetic active radiation > 900 μmoles m–2 s–1), low temperature (LT), low soil moisture (LSM) or low light intensity (LLI) for 8 weeks, followed by OGC for 6 weeks. Bud expression of homologs of Arabidopsis thaliana flowering genes FLOWERING LOCUS T (FT), LEAFY (LFY), APETALA1 (AP1), APETALA2 (AP2), APETALA3 (AP3), PISTILLATA.1 (PI.1), AGAMOUS.1 (AG.1) and AGAMOUS.3 (AG.3) were quantified over time. Only buds of LT-treated trees flowered. At the start of the experiment (week 0), buds of all trees expressed PaLFY, PaAP1 and PaAP2. After 4 weeks of treatment, bud PaAP1 expression was greater than in week 0 and continued to increase through week 8 only in LT-treated trees, whereas by week 8 PaLFY expression increased in LT- and LSM-treated trees. By week 10 (2 weeks after transfer to OGC), bud PaFT expression increased only in LT-treated trees, followed by activation of downstream floral organ identity genes PaAP3, PaPI.1, PaAG.1 and PaAG.3 by week 12. For OGC-, LSM- and LLI-treated trees, bud PaAP1, PaFT, PaAP3, PaPI.1 and PaAG.1 expression remained low, with PaAG.3 transcripts undetected at week 12. These trees did not flower. Among environmental factors tested, only LT increased bud expression of both PaLFY and PaAP1, which likely conferred floral meristem determinacy since transfer of LT-treated trees to OGC did not prevent flowering but activated PaFT, downstream floral organ identity genes, and flowering. The results suggest ‘Hass’ avocado floral development is promoted by low temperature-dependent floral induction and warm temperature-dependent floral organogenesis.