Shade avoidance

The shadows flee or shade avoidance response (English shade avoidance or shade avoidance syndrome ) is comprised of various adjustments in the growth of a plant in response to shading by the surrounding vegetation. Their function is to maximize the luminous efficiency of photosynthesis. The most important signal for triggering the shadow escape the dominance of dark red against bright red light in the shaded area of larger plants. Depending on the plant species, ecotype and development stage, the shadows flee may have the following characteristics:

• Suppression of germination up to a thinning of the surrounding vegetation
• Stronger growth of the hypocotyl and delayed development of the cotyledons in seedlings
• Stronger growth in the amount of extension of the internodes
• Fewer side shoots ( apical dominance ) and especially with grasses such as wheat and corn less foothills
• Extension of the petioles
• Upward movement of the leaves ( hyponasty )
• Earlier flowering

Occurrence and function of the shadows flee

The shadow escape is observed particularly in those plants more pronounced that grow in lower vegetation outside dense forests (eg, meadows, pastures and fields). In forests, which form a more or less closed canopy, the shadows flee the plant is usually little or no pronounced because an increased height growth would enable more favorable light conditions for the photosynthesizing leaves. The plants concerned instead show an increased shade tolerance, that is a more effective utilization of sunlight, for example, by means of larger and thinner blades. As in dense forests also show plants in very sparsely vegetated areas at most a weak shadow escape, if a shading by other plants rarely happens and blow stronger winds, which tall shoots can not withstand. Is the shadow escape suppressed in non- shadow -tolerant plants, biomass and reproductive success can be greatly reduced: this effect, studies on mutants of Arabidopsis thaliana that are not as strong growth in the amount due to a disturbed transport of the plant hormone auxin during the shadow flight and less fruits ( form pods ). It is of agricultural interest that modern wheat varieties respond with mutual shading unlike older varieties with a reduction in grain number and partially reduced grain weight. Against this background must be taken into account when sowing to achieve high grain yields ample room. In addition, the shade avoidance response increases susceptibility to pathogens and herbivores.

Variability of the shadows flee

Not only in the wheat, but also in other plants, the features of the shade avoidance response are pronounced differently depending on the genetic background and environmental conditions within a species. The following examples are intended to illustrate this variability:

• Arabidopsis thaliana does not bloom during the shadows flee under long-day conditions (16 hours of light a day) before, but under short-day (8 hours of light per day).
• An ecotype of Stellaria longipes from a prairie vegetation grows in Canada shading by other plants shoot up significantly, while another ecotype from Canada, which occurs in the open alpine regions, no accelerated height growth shows.
• Populations of Impatiens capensis from a forest area in Rhode Iceland show, in contrast to populations outside the forest no distinct shadows flee.

Molecular Mechanisms

Basis to initiate the shade avoidance reaction is the conversion from an active to an inactive form of the red-light- sensitive protein phytochrome. In the shadow of a dark-red vegetation dominates over bright red light, causing the active Pfr form of phytochrome, which inhibits the shadows flee, growing in the inactive form Pr is converted. Inactivation of phytochrome causes the previously inhibited transcription factors PIF ( phytochrome Interacting Factors ) may exercise gene regulatory functions. Transcription factors induce inter alia genes of enzymes involved in the synthesis of auxin phytohormone. The associated increased auxin concentrations develop various effects that cause the symptoms of the shadows flee:

• In sperm auxin activates the gene for a transcription factor that inhibits the germination.
• In older seedlings and plant cell wall loosening enzymes ( pectin esterases and lyases ) are activated at the gene and protein level, so that the cells can zoom in and a visible growth in length occurs. Here, the activation results in the protein level of acidification of the cell wall, which causes an optimum pH of the enzymes.
• Increased auxin concentrations in the cells of the leaf underside cause there increased growth, so that the leaves move up ( hyponasty ).
• Auxin inhibits the development of side shoots ( apical dominance ) by the genes turning it off in the side buds for the synthesis of growth-promoting Zytokinins.
• The phytohormone induced genes of transcription factors in the apical meristem that trigger flower development.

Further studies show that even attenuated blue light and green light reinforced that dominates as far-red light in the shade of vegetation that can trigger symptoms of shadows flee. In these cases, the sets responsive to blue and green light protein crypto chrome the shade avoidance reaction.