A Growth Regulatory Loop That Provides Homeostasis to Phytochrome A Signaling[W]

Abstract

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.

Signalling for developmental plasticity

“…In agreement with this view, the effect of enhanced PKS1 levels is larger in the pks2 background (i.e. in the absence of the negative regulation imposed by PKS2) [52]. These loops are therefore a source of homeostasis.…”

OWL1: An Arabidopsis J-Domain Protein Involved in Perception of Very Low Light Fluences

“…These include FHY1 and FHY1-LIKE, two proteins necessary for the import of phyA into the nucleus upon light activation (see Supplemental Figure 1 online; Zeidler et al, 2004;Hiltbrunner et al, 2005Hiltbrunner et al, , 2006Zhou et al, 2005;Rosler et al, 2007;Genoud et al, 2008) and some members of the PKS1 and SPA1 protein family. The cytoplasmic PKS1 and 2 proteins, which can interact with phyA, show enhanced phyA-mediated VLFR and are also involved in blue light-dependent phototropism (Fankhauser et al, 1999;Lariguet et al, 2003). SPA1, which together with COP1 is needed for the degradation of several light signaling factors, is also important for VLFR as the hypersensitive phenotype of spa1 can also be observed under very low fluences (Hoecker et al, 1999;Fittinghoff et al, 2006).…”

“…Pks1 and 2 mutants show enhanced phyA-mediated VLFR but are also involved in blue light-mediated phototropism (Lariguet et al, 2003(Lariguet et al, , 2006. Although GIGANTEA (GI) was first characterized as playing a role in the circadian clock and deetiolation under red light conditions, recent work showed that this protein is also important for hypocotyl growth, cotyledon opening, and germination under VLF conditions (Oliverio et al, 2007).…”

An anionic class III peroxidase from zucchini may regulate hypocotyl elongation through its auxin oxidase activity

“…X-Gluc (5-bromo-4-chloro-3-indolyl-beta-D-glucuronic acid, cyclohexylammonium salt) was purchased from X-GLUC DIRECT (WakeWeld, UK). Staining was performed as described elsewhere (Lariguet et al 2003). BrieXy, 72-hold seedling grown vertically in the dark were soaked in cold (4°C) acetone 90%, then rinsed and stained (5 min at 4°C and 12 h at 37°C) in a solution containing 2 mM XGluc, 2 mM ferrocyanide, 2 mM ferricyanide and 10 mM sodium phosphate buVer.…”