2019 ASHS Annual Conference

Juvenility in Citrus is generally long, usually requiring 4-10 years to achieve flowering and fruit set. In Citrus, the FLOWERING LOCUS T (FT) gene is supposed to be mobile and responsible for flowering. We have produced transgenic Carrizo citrange rootstocks that express the transformed FT gene, resulting in early flowering and fruiting within 18 months. These transgenic plants were propagated using two different methods: 1. An in vitro micropropagation method; and 2. A rooted cutting method. The in vitro method uses MT or MS basal media supplemented with 1-2 mg/l BA with 1500 mg/l malt extract for optimal shoot multiplication. The rooted cutting method uses 1.6% or 3% IBA to achieve optimal rooting. We are testing 6 transgenic lines that test positive for the FT gene; all of these lines flowered and fruited in the greenhouse. To determine if the early flowering trait could be transmitted through a graft union to juvenile scions, we grafted the transgenic Carrizo lines with selected scions from the UF-CREC breeding program, including juvenile triploid mandarin hybrids and diploid grapefruit cybrids. A few of the juvenile scions have quickly flowered and fruited as a result. This suggests successful movement of the FT protein through the graft union into the scion, initiating the early flowering. However, following initial flowering, sustained flowering in the grafted scions has not been observed, although it does continue in ungrafted transgenic Carrizo rootstocks. We have concluded that early flowering in the grafted juvenile citrus scions was because of the movement of FT protein from the rootstock to the scion, but further work is necessary to determine the sustained presence of the FT protein in the scion. Molecular analyses are underway to validate movement of the FT protein across the graft union, and to gain a better understanding of the longer-term effects on the scion.