Abstract
Shortening the juvenile stage in citrus and inducing early flowering has been the focus of several citrus genetic improvement programs. FLOWERING LOCUS T (FT) is a small phloem-translocated protein that regulates precocious flowering. In this study, two populations of transgenic Carrizo citrange rootstocks expressing either Citrus clementina FT1 or FT3 genes under the control of the Arabidopsis thaliana phloem specific SUCROSE SYNTHASE 2 (AtSUC2) promoter were developed. The transgenic plants were morphologically similar to the non-transgenic controls (non-transgenic Carrizo citrange), however, only AtSUC2-CcFT3 was capable of inducing precocious flowers. The transgenic lines produced flowers 16 months after transformation and flower buds appeared 30–40 days on juvenile immature scions grafted onto transgenic rootstock. Gene expression analysis revealed that the expression of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and APETALA1 (AP1) were enhanced in the transgenics. Transcriptome profiling of a selected transgenic line showed the induction of genes in different groups including: genes from the flowering induction pathway, APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) family genes, and jasmonic acid (JA) pathway genes. Altogether, our results suggested that ectopic expression of CcFT3 in phloem tissues of Carrizo citrange triggered the expression of several genes to mediate early flowering.
Highlights
Shortening the juvenile stage in citrus and inducing early flowering has been the focus of several citrus genetic improvement programs
Several studies have suggested that orthologs of the A. thaliana FLOWERING LOCUS T (FT) (AtFT) protein are a primary component of the flowering signal cascade in different p lants[3,16,18,19,20,21,22,23]
When compared to other methods such as the maximum-parsimony or the neighbor-joining, results were almost identical. Both the CcFT1 and CcFT3 sequences were homologous to the CiFT1 and CiFT3, were closely located and in the same clade, along with the CiFT3 (Fig. 2)
Summary
Shortening the juvenile stage in citrus and inducing early flowering has been the focus of several citrus genetic improvement programs. Two populations of transgenic Carrizo citrange rootstocks expressing either Citrus clementina FT1 or FT3 genes under the control of the Arabidopsis thaliana phloem specific SUCROSE SYNTHASE 2 (AtSUC2) promoter were developed. In the genetic improvement of citrus by conventional breeding or biotechnology, the long juvenile phase is one of the major challenges encountered towards rapid evaluation of fruit quality[1] Shortening this phase has been the focus of several s tudies[2,3,4]. In Arabidopsis, four major regulatory pathways for flowering have been discovered including: the photoperiod pathway, autonomous floral initiation, the vernalization pathway, and those regulated by gibberellins These pathways can be negatively or positively regulated by transcription factors, repressors, endogenous signal cascades, and/or environmental signals, which converge to regulate floral meristem identity genes that promote flowering[9,10,11]. Grafting experiments in tomatoes demonstrated that the signal was transmissible from transgenic plants expressing SFT under 35S promoter to non-transgenic sft mutant plants[24]
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