Question

How does the photo-reversibility of phytochromes relate to the mechanism of plants' response to the shade?

Answer 1

Model of the Shade Avoidance Syndrome Allowing Plants to Escape from Competition

Plants use different photoreceptor systems to detect unfavorable low light conditions caused by shading.

(A) Arabidopsis thaliana, the model plant used in this study, encodes five phytochromes (phyA–phyE), and phyB is the main phytochrome involved in SAS (Casal, 2013). PhyB measures the red:far-red (R:FR) ratio, which is strongly lowered under foliar shade. Under such conditions, most of the phyB is converted to its inactive red-light absorbing form (PrB), releasing the inhibitory effect of its active far-red absorbing form (PfrB) on the activity of PIF7 and other PIFs. PIF7 induces genes (YUCCAs) required for the biosynthesis of the phytohormone auxin from the precursor tryptophan (Trp) in the leaves. Auxin is then transported to the stem through efflux carriers (PINs), where it promotes elongation growth.

(B) The cryptochrome 2 (cry2) blue light photoreceptor binds to PIF4 and PIF5 on the promoter regions of shade-induced genes, potentially regulating their expression.

(C) The inhibitory role of cry1 on PIF4 and PIF5 is suppressed under low blue light. PIF-induced genes encode cell-wall-remodeling enzymes (such as expansins, XTH), allowing extension growth of the stem in a process that also requires auxin. Phytochrome- and cryptochrome-mediated SAS results in similar morphological changes but through different mechanisms.

Participation of hormones in the shade-avoidance syndrome. Exposure of plants to low R:FR ratios increases the production of auxin through the TAA1-dependent biosynthetic pathway. Auxin is then responsible for the activation of CKX6 (shown in pink) in the vasculature, and the consequent reduction in CKs concentration and arrest of leaf growth associated with SAS. Detection of low R:FR ratios also triggers the rapid upregulation of expression of several genes, like bHLH and HDzip transcription factors, which interdependently regulate their downstream targets, here represented by auxin responsive (ARE) genes, E-box containing genes (EBG), and genes without E-boxes in their promoters (NEG). The stability of PIF transcription factors is also regulated by light in a PhyB-dependent manner, and their DNA-binding activity is high in this conditions thanks to the degradation of DELLA proteins under canopy-simulating low R:FR ratios. Shading also decreases blue light irradiance, which in turns allows an increase in gaseous ethylene under the canopy and promotes growth. Genes are shown as boxes with black arrows, and hormones are highlighted in bold