Plants often grow in densely populated environments in which access to sunlight is a precious commodity. In order to maximize opportunities for photosynthesis, plants have evolved complex biochemical sensory pathways that initiate shade avoidance syndrome (SAS). SAS results in growth away from shade and is of critical importance to maintaining a plant’s competitive edge. The thale cress has evolved dual, redundant sensory mechanisms for avoiding shade. The phyB (phytochrome B) SAS pathway depends on detecting a decrease in the ratio of red light to far-red light (R:FR). In plants growing in the canopy above a thale cress, chlorophyl absorbs red light and cell walls scatter far-red light. Thus, a decrease in the R:FR ratio (the plant is exposed to less red light) is a good signal for the thale that it is in the shade of the canopy and should begin SAS processes to "escape". When thale cress leaves absorb more far-red light, the phytochrome molecule changes shape leading to a cascade of reactions that produce proteins related to growth away from the shade. A parallel system is triggered by a reduction in blue light. Chlorophyll from competing plants strongly absorbs blue light, so a reduction in blue light is a reliable indication that sunlight is being blocked.Edit Summary
“A prime example of morphological plasticity is the shade avoidance syndrome (SAS). SAS responses typically include increased elongation of the stem and petioles, leaf hyponasty, reduced branching and phototropic orientation of the plant shoot towards gaps in the canopy…The best characterized of these signals is the red/far red ratio (R:FR; 660–670/725–735 nm), which decreases in response to canopy density because of the strong absorption of red light by chlorophyll and scattering of far-red photons by cell walls and other plant constituents.” (Keller et al. 2011:195-6)
“Phytochrome B (phyB) is the major photoreceptor that senses a reduction in the R:FR ratio, and controls the initial appearance of SAS phenotypes…SAS responses to low R:FR (increased petiole elongation and leaf hyponasty) depend on increased auxin biosynthesis through the TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1 (TAA1) pathway, and polar auxin transport by the auxin efflux carrier PIN3. Increased gibberellin (GA) production, perhaps in response to increased auxin, is also required for the expression of petiole elongation responses to low R:FR…Increased GA production triggers degradation by the 26S proteasome of DELLA proteins, a group of five nuclear proteins that redundantly repress growth. DELLA proteins bind to and inactivate PHYTOCHROME INTERACTING FACTORS (PIFs), which are growth-promoting transcription factors involved in the elicitation of SAS responses to low R:FR…plants can present strong SAS-like responses to blue light attenuation…blue light responses required cry1, and were mediated through hormonal pathways that showed only limited overlap with the pathways activated in response to phyB Pfr depletion by low R:FR ratios…PIF4 and PIF5, which are basic/helix-loop-helix (bHLH) transcription factors known to mediate morphological responses to phyB inactivation, are also required for the elicitation of the SAS phenotype in response to blue light attenuation.” (Keller et al. 2011:196)
“Arabidopsis rosettes can display very marked SAS responses to blue light attenuation, including pronounced hyponasty and a reduced L:P ratio…this response is predominantly mediated by cry1…The two SAS responses characterized in this study (hyponasty and altered leaf morphology), although concomitantly displayed in response to competition signals (low R:FR or low blue light), appeared to be controlled by different mechanisms.” (Keller et al.
“[W]hereas DELLA degradation appears to play a significant role in mediating SAS responses to low R:FR ratios in seedlings and rosette plants, it was not activated by blue light attenuation, and not directly involved in the production of the SAS response.” (Keller et al. 2011:201)
“[B]oth PIF4 and PIF5 need to be present for full expression of the hyponastic response to blue light attenuation…There is no evidence of cry-induced degradation of PIFs, or of physical interactions between crys and PIFs…A potential mechanism whereby blue light attenuation activates PIF4 and PIF5 might involve a related bHLH factor, HFR1…Communication theory maintains that redundancy is a requirement for the reliable transmission and processing of information in noisy environments.” (Keller et al. 2011:203)
“[P]lants use low R:FR and blue light as partially redundant signals of the proximity of competitors…signals, perceived by phyB and cry1, respectively, also activate separate signaling networks that show limited overlap with regard to the molecular players involved. Parallel control pathways frequently converge in major regulatory nodes…PIF4 and PIF5 are critical hubs in the core SAS pathway, which integrate information from the principal light signaling routes that control adaptive plasticity in plant canopies. ” (Keller et al. 2011:204)