A growth regulatory loop that provides homeostasis to phytochrome a signaling.

Phytochrome kinase substrate1 (PKS1) is a cytoplasmic protein that interacts physically with, and is phosphorylated by, the plant photoreceptor phytochrome. Here, we show that light transiently increases PKS1 mRNA levels and concentrates its expression to the elongation zone of the hypocotyl and root. This response is mediated by phytochrome A (phyA) acting in the very low fluence response (VLFR) mode. In the hypocotyl, PKS1 RNA and protein accumulation are maintained only under prolonged incubation in far-red light, the wavelength that most effectively activates phyA. Null mutants of PKS1 and its closest homolog, PKS2, show enhanced phyA-mediated VLFR. Notably, a pks1 pks2 double mutant has no phenotype, whereas overexpression of either PKS1 or PKS2 results in the same phenotype as the pks1 or pks2 single null mutant. We propose that PKS1 and PKS2 are involved in a growth regulatory loop that provides homeostasis to phyA signaling in the VLFR. In accordance with this idea, PKS1 effects are larger in the pks2 background (and vice versa). Moreover, the two proteins can interact with each other, and PKS2 negatively regulates PKS1 protein levels specifically under VLFR conditions.