Abstract
Lignin biosynthesis and its transcriptional regulatory networks have been studied in model plants and woody trees. However, lignification also occurs in some fleshy fruit and has rarely been considered in this way. Loquat ( Eriobotrya japonica ) is one such convenient tissue for exploring the transcription factors involved in regulating fruit flesh lignification. Firmness and lignin content of 'Luoyangqing' loquat were fund to increase during low-temperature storage as a typical symptom of chilling injury, while heat treatment (HT) and low-temperature conditioning (LTC) effectively alleviated them. Two novel EjMYB genes, EjMYB1 and EjMYB2, were isolated and were found to be localized in the nucleus. These genes responded differently to low temperature, with EjMYB1 induced and EjMYB2 inhibited at 0 °C. They also showed different temperature responses under HT and LTC conditions, and may be responsible for different regulation of flesh lignification at the transcriptional level. Transactivation assays indicated that EjMYB1 and EjMYB2 are a transcriptional activator and repressor, respectively. EjMYB1 activated promoters of both Arabidopsis and loquat lignin biosynthesis genes, while EjMYB2 countered the inductive effects of EjMYB1. This finding was also supported by transient overexpression in tobacco. Regulation of lignification by EjMYB1 and EjMYB2 is likely to be achieved via their competitive interaction with AC elements in the promoter region of lignin biosynthesis genes such as Ej4CL1.
Highlights
Lignin is important for plant secondary cell-wall formation, and lignin biosynthesis and the associated regulatory mechanisms have been studied in the model plant Arabidopsis, various crops, and woody trees (Goicoechea et al, 2005; Zhong and Ye, 2009; Vanholme et al, 2010; Zhao and Dixon, 2011)
Alignment analysis indicated that EjMYB2 and other repressor-type MYBs have a conserved motif, named an ERF-associated amphiphilic repression domain (EAR) (Legay et al, 2007), which is characteristic for repressor-type transcription factors (Fig. 2)
The full-length coding sequences of EjMYB1 and EjMYB2 without stop codes were constructed in pBI221. mCherry, which localizes to the nucleus, was co-transfected with 35S::EjMYB2-GFP and control vector pBI221 (35S)::EjMYB1–green fluorescent protein (GFP), 35S::EjMYB2–GFP, and empty vector pBI221 (35S::GFP)
Summary
Lignin is important for plant secondary cell-wall formation, and lignin biosynthesis and the associated regulatory mechanisms have been studied in the model plant Arabidopsis, various crops, and woody trees (Goicoechea et al, 2005; Zhong and Ye, 2009; Vanholme et al, 2010; Zhao and Dixon, 2011). Monolignols of lignin are synthesized via the phenylpropanoid pathway and oxidatively polymerized to lignin polymers. Genes encoding the enzyme cascade of the phenylpropanoid pathway have been widely characterized.
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