Characterization of the Floral Induction Mechanism of Longan

Proper interpretation of environmental stimuli leading to the transition from vegetative growth to reproductive development is essential for plant survival.  Furthermore, effecting this change can become a challenge for the reliable production of horticultural crops due to variable environmental conditions or when crops are grown in different climactic zones.  Hawai’i has several unique advantages for US domestic agriculture, including bountiful sunshine and rainfall, yet Hawai’i agriculture must compete with abundant foreign competition.  For this reason, there is a major impetus to cultivate high-value crops for the fresh market, and two of the crops being tested are lychee and longan, both subtropical fruit crops originating from Asia.  A major challenge to developing these crops and others in Hawai’i is unreliable flowering caused by mild and variable winters, which in turn causes unreliable yield.  Flowering pathways have been extensively characterized in the annual dicot Arabidopsis, but understanding floral transition in perennial trees in temperate or tropical zones is exceedingly challenging.  Three major pathways leading to floral transition have been described in Arabidopsis, one responds to shifts in day-length, another senses temperature changes, and the third ‘autonomous’ pathway causes floral transition regardless of light or temperature stimuli.  Limited genetic analyses from poplar and apple have indicated a role for orthologs of the Arabidopsis FLOWERING LOCUS T (FT) protein in floral transition in these species. Defoliation and girdling experiments in lychee have implicated a leaf-derived signal initiated by a chilling response that is essential for flower induction in this species.  In longan, a close relative to lychee, natural flower induction is believed to also be a response to a temperature stimulus.  However, longan has an interesting and unique additional phenomenon regarding floral induction:  in longan, application of potassium chlorate to the root systems of trees causes widespread flowering of treated trees within approximately seven weeks of treatment.  The discovery of chlorate induced flowering has enabled off-season production of longan.  The molecular-genetic mechanism underlying chlorate induced flowering of longan remains unknown.  In order to determine whether this is a novel floral induction pathway or utilizes components of known pathways, I have cloned putative orthologs of known floral signal integrator genes from the longan transcriptome for expression analysis.   The transcript abundance of these floral identity genes are analyzed over time following chlorate application, as well as during natural (temperature-induced) flowering.