Abstract
Gravity and light are major factors shaping plant growth. Light perceived by phytochromes leads to seedling deetiolation, which includes the deviation from vertical hypocotyl growth and promotes hypocotyl phototropism. These light responses enhance survival of young seedlings during their emergence from the soil. The PHYTOCHROME KINASE SUBSTRATE (PKS) family is composed of four members in Arabidopsis (Arabidopsis thaliana): PKS1 to PKS4. Here we show that PKS4 is a negative regulator of both phytochrome A- and B-mediated inhibition of hypocotyl growth and promotion of cotyledon unfolding. Most prominently, pks4 mutants show abnormal phytochrome-modulated hypocotyl growth orientation. In dark-grown seedlings hypocotyls change from the original orientation defined by seed position to the upright orientation defined by gravity and light reduces the magnitude of this shift. In older seedlings with the hypocotyls already oriented by gravity, light promotes the deviation from vertical orientation. Based on the characterization of pks4 mutants we propose that PKS4 inhibits changes in growth orientation under red or far-red light. Our data suggest that in these light conditions PKS4 acts as an inhibitor of asymmetric growth. This hypothesis is supported by the phenotype of PKS4 overexpressers. Together with previous findings, these results indicate that the PKS family plays important functions during light-regulated tropic growth responses.
Highlights
Gravity and light are major factors shaping plant growth
To PKS1 and PKS2, PKS4 interacts with phytochrome A (phyA) and phytochrome B (phyB) in yeast two-hybrid analysis and in vitro pull-down assays (Supplemental Fig. S1)
To investigate whether the expression of PKS4 is regulated by light during deetiolation, 4-d-old seedlings were transferred from darkness to different light conditions (Supplemental Fig. S2)
Summary
Light perceived by phytochromes leads to seedling deetiolation, which includes the deviation from vertical hypocotyl growth and promotes hypocotyl phototropism. We show that PKS4 is a negative regulator of both phytochrome A- and B-mediated inhibition of hypocotyl growth and promotion of cotyledon unfolding. Based on the characterization of pks mutants we propose that PKS4 inhibits changes in growth orientation under red or far-red light. The phytochromes, which sense red and far-red light, are important for several developmental transitions and for adaptive responses such as the shade-avoidance syndrome or the modulation of tropic growth responses (Chen et al, 2004; Iino, 2006; Mathews, 2006). The perception of gravity typically directs root growth downwards while shoots grow upwards These tropic growth responses maximize the acquisition of essential resources such as water, minerals, and light The most apical part of the hypocotyl goes from positive gravitropism in darkness (to maintain the apical hook) to negative gravitropism as the hook opens in response to light (MacDonald et al, 1983)
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