Interplay Between Environmental Signals and Endogenous Salicylic Acid Concentration - Salicylic Acid: Plant Growth and Development - page 66

hypocotyl growth and endogenous SA concentrations were measured. A decrease
in the R/FR ratio (i.e. an increase in FR radiation relative to R irradiance) from a
high R/FR ratio to a normal R/FR ratio, and then to a low R/FR ratio yielded a
gradual increase in endogenous SA levels. This, in turn, was positively associated
with increased hypocotyl elongation (Kurepin et al. 2010a ). For another species, S.
longipes, using both sun and shade ecotypes, decreasing the R/FR ratio from high
to normal had no effect on endogenous SA levels. However, the further decrease in
R/FR ratio from normal to a low R/FR ratio significantly increased endogenous SA
levels in the shade ecotype plants (Kurepin et al. 2012a ). Again, there was a clear
positive association between the changes in endogenous SA levels and shoot
growth, as only shade (but not sun) ecotype plants increased their shoot growth in
response to decreases in the R/FR ratio (Kurepin et al. 2012a ).
Therefore, light irradiance (a sum of total light that a plant can absorb, i.e.
photosynthetically active radiation) and light quality (a manipulation of individual
light wavelengths, especially R and FR) have different effects on endogenous SA
levels (and also on growth). Further, SA levels are associated with growth changes
in a different manner, i.e. a decrease in light irradiance causes an increase in shoot
elongation, but that increased growth is associated with decreased endogenous SA
levels. However, when shoot elongation is induced by a decrease in R/FR ratio,
endogenous SA levels rise. Does endogenous SA, then, play any role in these light-
induced growth responses? There are reports that exogenously applied SA pro-
motes stem elongation in bean (Phaseolus vulgaris L.; Hegazi and El-Shraiy 2007 )
and applied SA also promotes the growth of isolated stem segments of Ullucus
tuberosus (Caldas) plants (Handro et al. 1997 ). In contrast, there are numerous
examples where high concentrations of exogenously applied SA have inhibited
shoot growth, while lower doses of SA promoted it (Hayat et al. 2010 ). For
sunflower hypocotyls, exogenous SA applied at higher concentrations inhibited
growth at lower light irradiances, whereas lower concentrations had no effect on
growth (Kurepin et al. 2010a ). Similar results (different responses from low versus
high SA concentrations) were obtained when sunflower hypocotyl growth was
measured under different R/FR ratios across a range of exogenously applied SA
doses (Kurepin et al. 2010a ).
To summarize, the light irradiance and light quality effects on endogenous SA
levels are generally quite different than is seen for other classes of plant hormones
that have a long and proven history of causally regulating plant shoot growth. In
fact, for each of sunflower and A. thaliana, both light irradiance and light quality
signaling are known to modify endogenous hormone levels. More specifically,
they increase gibberellin, auxin and cytokinin levels, decrease ethylene evolution
and generally have a nil effect on abscisic acid and brassinosteroid levels (Kurepin
et al. 2007a , b , c , 2010b , 2011a , b , 2012b , c ). Additionally, only gibberellins, auxin
and ethylene have been shown to directly regulate stem elongation growth
increases in response to low light irradiance and low R/FR ratio signals for both
sunflower and A. thaliana (Kurepin et al. 2007a , b , c , 2011a , b , 2012b , c ). Since a
low R/FR ratio increases endogenous SA levels in sunflower hypocotyls, while
low light irradiance has the exact opposite effect, it does not seem reasonable to