Abstract
Plant phytochromes are known as autophosphorylating serine/threonine protein kinases. However, the functional importance of their kinase activity is not fully elucidated. Previously, the kinase activity is shown to be necessary for the function of Avena sativa phytochrome A (AsphyA) using transgenic plants with mutants displaying reduced kinase activity, such as K411L and T418D. In this study, we isolated and analyzed two AsphyA mutants, K411R and T418V, that showed increased kinase activity. Transgenic phyA-201 plants with these mutants showed hypersensitive responses to far-red (FR) light, such as shorter hypocotyls and more expanded cotyledons than those of control plant (i.e., transgenic phyA-201 plant with wild-type AsphyA). Contrary to the mutants with reduced kinase activity, these mutants accelerated FR-induced phosphorylation and subsequent degradation of phytochrome-interacting factor 3 (PIF3) in Arabidopsis. Moreover, elongated hypocotyl 5 (HY5), a critical positive regulator of photoresponses in plants, accumulated in higher amounts in the transgenic plants under FR light than in the control plant. In addition, PIF1 degradation was accelerated in the transgenic plants. Consequently, the transgenic plants exhibit higher germination frequencies than the control plant. Collectively, our results demonstrate that the AsphyA mutants with increased kinase activity are hyperactive in plants, supporting a positive relationship between the kinase activity of phytochromes and photoresponses in plants.
Highlights
Phytochromes are red (R) and far-red (FR) photoreceptors that regulate various plant photoresponses, such as seed germination and seedling de-etiolation (Legris et al, 2019; Tripathi et al, 2019)
We provided evidence that phytochromes function as protein kinases in plant light signaling by showing that transgenic plants with Avena sativa phytochrome A (AsphyA) mutants displaying reduced kinase activity exhibited hyposensitive responses to FR light (Shin et al, 2016)
The protein kinase activity assays in a time-dependent manner showed an increase in phytochrome-interacting factor 3 (PIF3) phosphorylation by the K411R and T418V mutants, compared with that by wild-type AsphyA (Supplementary Figure 2A)
Summary
Phytochromes are red (R) and far-red (FR) photoreceptors that regulate various plant photoresponses, such as seed germination and seedling de-etiolation (Legris et al, 2019; Tripathi et al, 2019) In higher plants, they are encoded by small gene families, for example, five members (phyA to phyE) in Arabidopsis thaliana (Mathews, 2010). PIF1 is rapidly phosphorylated under FR and R light before being degraded, in which phyA plays a dominant role in regulating the PIF1 degradation following initial light exposure (Shen et al, 2008) In addition to this regulation, phytochromes inhibit the activity of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and SUPPRESSOR OF PHYA-105 (SPA) complex that functions as an E3 ligase for photomorphogenesis-promoting transcription factors, especially ELONGATED HYPOCOTYL 5 (HY5) (Lu et al, 2015; Sheerin et al, 2015; Podolec and Ulm, 2018). A principal regulatory mechanism of phytochromes is proposed as the transcriptional regulation of photoresponsive genes via removal of negative regulators such as PIFs and accumulation of positive regulators such as HY5
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