Abstract
Ethylene is required for climacteric fruit ripening. Inhibition of ethylene biosynthesis genes, 1-aminocyclopropane-1-carboxylate (ACC) synthase and ACC oxidase, prevents or delays ripening, but it is not known how these genes are modulated during normal development. LeHB-1, a previously uncharacterized tomato homeobox protein, was shown by gel retardation assay to interact with the promoter of LeACO1, an ACC oxidase gene expressed during ripening. Inhibition of LeHB-1 mRNA accumulation in tomato fruit, using virus-induced gene silencing, greatly reduced LeACO1 mRNA levels, and inhibited ripening. Conversely, ectopic overexpression of LeHB-1 by viral delivery to developing flowers elsewhere on injected plants triggered altered floral organ morphology, including production of multiple flowers within one sepal whorl, fusion of sepals and petals, and conversion of sepals into carpel-like structures that grew into fruits and ripened. Our findings suggest that LeHB-1 is not only involved in the control of ripening but also plays a critical role in floral organogenesis.
Highlights
The gaseous hormone ethylene regulates many aspects of plant growth and development, including ripening, senescence, abscission, and responses to biotic and abiotic stresses (Abeles et al, 1992)
We report here on the identification of a cDNA clone for a previously uncharacterized HD-Zip protein from tomato, LeHB-1, that binds to LeACO1 promoter fragments containing the putative HD protein binding sequences
Tomato LeHB-1 was identified as the closest match to AtHB-1, and we isolated the corresponding expressed sequence tag (EST) clone, referred to here as LeHB-1, by RT-PCR using the primers LeHB-1F1/ LeHB-1R1 corresponding to the coding sequence (Table 1)
Summary
The gaseous hormone ethylene regulates many aspects of plant growth and development, including ripening, senescence, abscission, and responses to biotic and abiotic stresses (Abeles et al, 1992). HD-Zip homeobox proteins are defined by the conserved homeodomain (HD) and adjacent leucine zipper motifs (Sessa et al, 1993) They are unique to plants, but are related to other eukaryotic HD proteins (Henriksson et al, 2005). The knotted-like homeobox (KNOX) proteins function in shoot apical meristems through regulating the production of gibberellin (GA) and cytokinin (Ori et al, 1999) In both Arabidopsis and tobacco, the KNOX proteins directly repress transcription of genes encoding GA 20-oxidases, the enzymes that encode the last step in GA biosynthesis (Hay et al, 2002; Jasinski et al, 2002; Sakamoto et al, 2001)
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