Abstract
HSFA9 is a seed-specific transcription factor that in sunflower (Helianthus annuus) is involved in desiccation tolerance and longevity. Here we show that the constitutive overexpression of HSFA9 in tobacco (Nicotiana tabacum) seedlings attenuated hypocotyl growth under darkness and accelerated the initial photosynthetic development. Plants overexpressing HSFA9 increased accumulation of carotenoids, chlorophyllide, and chlorophyll, and displayed earlier unfolding of the cotyledons. HSFA9 enhanced phytochrome-dependent light responses, as shown by an intensified hypocotyl length reduction after treatments with continuous far-red or red light. This observation indicated the involvement of at least two phytochromes: PHYA and PHYB. Reduced hypocotyl length under darkness did not depend on phytochrome photo-activation; this was inferred from the lack of effect observed using far-red light pulses applied before the dark treatment. HSFA9 increased the expression of genes that activate photomorphogenesis, including PHYA, PHYB, and HY5. HSFA9 might directly upregulate PHYA and indirectly affect PHYB transcription, as suggested by transient expression assays. Converse effects on gene expression, greening, and cotyledon unfolding were observed using a dominant-negative form of HSFA9, which was overexpressed under a seed-specific promoter. This work uncovers a novel transcriptional link, through HSFA9, between seed maturation and early photomorphogenesis. In all, our data suggest that HSFA9 enhances photomorphogenesis via early transcriptional effects that start in seeds under darkness.
Highlights
Plants use sunlight as the ultimate source of energy and as one of the most important environmental cues
POR is essential for the light-induced completion of the biogenesis of the photosynthetic membranes (Reinbothe et al, 1999); the results shown in Fig. 1 suggest that HSFA9 might favour photomorphogenesis
We propose that HSFA9—a seed-specific transcription factors (TFs)—mediates a transcriptional link between seed maturation and photomorphogenesis
Summary
Plants use sunlight as the ultimate source of energy and as one of the most important environmental cues. Seedlings that germinate under soil or litter adopt a dark-grown developmental programme called skotomorphogenesis, which stimulates the elongation of the embryonic stem, or hypocotyl, and represses cotyledon unfolding and chloroplast development. When emerging from the ground or exposed to light, seedlings switch to a light-grown developmental programme called photomorphogenesis, which restrains hypocotyl growth and stimulates cotyledon unfolding and the development of photosynthetically active. These regulators include photomorphogenesis repressors, such as the PIF and CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1) genes, and activators, such as ELONGATED HYPOCOTYL 5 (HY5) and HYH (Leivar and Monte, 2014; Wu, 2014)
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