Shadow escape

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Juxtaposition of single and dense trees to demonstrate flight from shadow: In the forest, trees such as beech (top right) or birch (bottom right) compete for light and show apical dominance, which is characteristic of shadow flight. Free-standing specimens (left), however, are heavily branched.
Scheme for escaping from the shade: A: When escaping from the shade, seedlings show increased hypocotyl growth, hyponastia and impaired development of the cotyledons. B: Older plants show increased growth of the internodes and leaf stems, apical dominance (not shown here) and also hyponastia.
Triggering the flight from the shadows: By converting phytochrome into its inactive form, transcription factors (PIF) become active, which induce the synthesis of auxin. The phytohormone is responsible for the main symptoms of the shadow flight.

The Shadowprey or shade avoidance response (English shade avoidance or shade avoidance syndrome) includes various adjustments in the growth of a plant in response to shading by the surrounding vegetation. Their function is to maximize the light output for photosynthesis . The most important signal to trigger shadow escape is the dominance of dark red versus light red light in the shady surroundings of larger plants. Depending on the plant species, ecotype and stage of development, the shadow flight can have the following characteristics:

  • Suppression of germination until the surrounding vegetation clears up
  • Stronger growth of the hypocotyl and delayed development of the cotyledons in seedlings
  • Stronger growth in height through elongation of the internodes
  • Fewer side shoots ( apical dominance ) and especially in sweet grasses such as wheat and maize fewer runners
  • Extension of the petioles
  • Upward movement of the leaves ( hyponastia )
  • Earlier flowering time

Occurrence and function of shadow escape

The shadow flight is observed to a greater extent in plants that grow in lower vegetation outside of dense forests (e.g. meadows, pastures and fields). In forests, which form a more or less closed canopy, the shadow flight of the plants is usually hardly or not at all pronounced, because increased height growth would not allow more favorable light conditions for the photosynthesizing leaves. Instead, the affected plants show an increased shade tolerance, i.e. more effective use of sunlight, for example with the help of larger and thinner leaves. As in dense forests, plants in areas with very little vegetation show at most a weak shadow escape, if shading by other plants rarely occurs and stronger winds blow which tall shoots cannot withstand. If the shadow flight is prevented in plants that are not shadow-tolerant, biomass and reproductive success can be significantly reduced: Studies on mutants of Arabidopsis thaliana have shown this effect , which because of a disturbed transport of the phytohormone auxin cannot grow so much in height during shadow flight and has fewer fruits ( Pods). It is of agricultural interest that, when shaded from one another, modern wheat varieties react, in contrast to older varieties, with a reduction in the number of grains and sometimes with a reduced grain weight. Against this background, sufficient spacing must be taken into account when sowing in order to achieve high grain yields. In addition, the shadow avoidance reaction increases susceptibility to pathogens and herbivores.

Variability of shadow escape

Not only in wheat, but also in other plants, the characteristics of the shadow avoidance reaction are different depending on the genetic background and environmental situation within a species. The following examples are intended to illustrate this variability:

  • Arabidopsis thaliana blooms earlier during the shadow flight under long day conditions (16 hours of light per day), but not under short day conditions (8 hours of light per day).
  • An ecotype of Stellaria longipes from a Canadian prairie vegetation grows significantly in height when shaded by other plants, while another ecotype from Canada, which occurs in the open alpine regions, shows no accelerated growth in height.
  • Populations of Impatiens capensis from a wooded area on Rhode Island , unlike populations outside the forest, do not show pronounced shadow migration.

Molecular Mechanisms

The basis for triggering the shadow avoidance reaction is the conversion from an active to an inactive form of the red light-sensitive protein phytochrome . In the shade of a vegetation, dark red light dominates over light red light, whereby the active phytochrome form Pfr , which inhibits the escape from shadow, is increasingly converted into the inactive form Pr . The inactivation of the phytochrome means that the previously inhibited transcription factors PIF ( Phytochrome Interacting Factors ) can perform gene regulatory functions. The transcription factors induce u. a. Genes of enzymes that are involved in the synthesis of the phytohormone auxin. The associated increased auxin concentrations develop various effects that cause the symptoms of shadow flight:

  • In seeds, auxin activates the gene for a transcription factor that inhibits germination.
  • In seedlings and older plants, enzymes loosening the cell wall (pectin esterases and lyases) are activated at the gene and protein level, so that the cells can enlarge and a visible increase in length takes place. Activation at the protein level results from acidification of the cell wall, which brings about an optimum pH for the enzymes.
  • Increased auxin concentrations in the cells on the underside of the leaf cause increased growth there, so that the leaves move upwards (hyponastia).
  • Auxin inhibits the development of side shoots (apical dominance) by switching off the genes in the side buds for the synthesis of the growth-promoting cytokinin.
  • The phytohormone induces genes of transcription factors in the apical meristem that trigger flower development.

Further studies show that weakened blue light and intensified green light, which dominates like dark red light in the shade of vegetation, can also trigger symptoms of shadow escape. In these cases, the protein cryptochrome, which reacts to blue and green light, triggers the shadow avoidance reaction .

literature

Individual evidence

  1. ^ J Casal: Shade Avoidance . In: The Arabidopsis Book , 10, 2012, p. E0157. PMC 3350169 (free full text), doi: 10.1199 / tab.0157
  2. Jump up ↑ DH Keuskamp, ​​S Pollmann, LACJ Voesenek, AJM Peeters, R Pierik: Auxin transport through PIN-FORMED 3 (PIN3) controls shade avoidance and fitness during competition . In: Proc. Natl. Acad. Sci. USA , 2010, doi: 10.1073 / pnas.1013457108
  3. ↑ CC Ugarte, SA Trupkin, H Ghiglione, G Slafer, JJ. Casal: Low red / far-red ratios delay spike and stem growth in wheat . In: J Exp Bot. , 61 (11), 2010, pp. 3151-3162
  4. ↑ DH Keuskamp, R Sasidharan, R Pierik: Physiological regulation and functional significance of shade avoidance responses to neighbors . In: Plant Signal Behav , 56, 2010, pp. 655-662, doi: 10.4161 / psb.5.6.11401 .
  5. ^ AC Wollenberg, B Strasser, PD Cerdán, RM Amasino): Acceleration of flowering during shade avoidance in Arabidopsis alters the balance between FLOWERING LOCUS C-mediated repression and photoperiodic induction of flowering . In: Plant Physiol. , 2008, 148 (3), pp. 1681–1894, PMC 2577263 (free full text)
  6. ↑ R Sasidharan, CC Chinnappa, LA Voesenek, R. Pierik: The regulation of cell wall extensibility during shade avoidance: a study using two contrasting ecotypes of Stellaria longipes . ( Memento of the original from September 24, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In: Plant Physiol. , 2008, 148 (3), pp. 1557-1569. @1@ 2Template: Webachiv / IABot / www.plantphysiol.org
  7. ^ EJ von Wettberg, J Schmitt: Physiological mechanism of population differentiation in shade-avoidance responses between woodland and clearing genotypes of Impatiens capensis . In: American Journal of Botany , 2005, 92, pp. 868-874, PMID 21652468
  8. ↑ XD Liu, H Zhang, Y Zhao, ZY Feng, Q Li, HQ Yang, S Luan, JM Li, ZH He: Auxin controls seed dormancy through stimulation of abscisic acid signaling by inducing ARF-mediated ABI3 activation in Arabidopsis . (PDF) In: Proc Natl Acad Sci USA , 2013, doi: 10.1073 / pnas.1304651110
  9. ↑ BB Lippincott, JA Lippincott: Auxin-induced hyponasty of the leaf blade of Phaseolus vulgaris . In: Am J Bot , 1971, 58, pp. 817-826.
  10. ↑ M Tanaka, K Takei, M Kojima, H Sakakibara, H Mori: Auxin controls local cytokinin biosynthesis in the nodal stem in apical dominance . In: Plant J. , 2006, 45, pp. 1028-1036, PMID 16507092
  11. ↑ N Yamaguchi, MF Wu, CM Winter, MC Berns, S Nole-Wilson, A Yamaguchi, G Coupland, BA Krizek, D Wagner: A molecular framework for auxin-mediated initiation of flower primordial . In: Dev Cell. , 2013, 24 (3), pp. 271–282
  12. ↑ T Zhang, SA Maruhnich, KM Folta: Green Light Induces Shade Avoidance symptom . In: Plant Physiol. , 2011, 157, pp. 1528-1536.