Interplay Between Environmental Signals and Endogenous Salicylic Acid Concentration - Salicylic Acid: Plant Growth and Development - page 65

constitutively high endogenous SA levels (cpr1-1, cpr5-1 and cpr6-1; Bowling
et al. 1994 , 1997 ; Clarke et al. 1998 ) exhibit the dwarf growth phenotype at low,
but not at higher light irradiances, relative to wild type line Col-0 (Mateo et al.
2006 ). Further, these A. thaliana mutants with high SA levels, when grown at a
lower light irradiance, had lower maximum efficiency and quantum yield of PSII,
as well as having reduced stomatal conductance and a lower accumulation of
photo-assimilates (Mateo et al. 2006 ). Thus, the negative effects of possessing high
SA levels are only expressed, in these mutant lines, under low irradiance levels.
Further, a mutant with constitutively reduced endogenous SA levels (sid2-2; Na-
wrath and Metraux 1999 ) did not show a dwarfing response, nor did it exhibit
reduced metabolic responses to low light irradiance levels (Mateo et al. 2006 ).
Therefore, low light irradiance-induced growth promotion is associated with low
SA levels and plants with high endogenous SA levels fail to etiolate under low
light irradiances, yet they have the same phenotype as wild type plants under high
irradiance light.
In tobacco (Nicotiana benthamiana L.) plants inoculated with Turnip mosaic
virus (TuMV), both low light irradiance and photosystem impairment can increase
the susceptibility of the host to TuMV infection (Manfre et al. 2011 ). Although
endogenous SA levels were not measured, exogenous SA application had no effect
on TuMV infection. Further, the expression of pathogen response-1 (pr-1) gene
was not affected by lower light irradiances or photosystem impairment (Manfre
et al. 2011 ). By implication, the reduction in light irradiance and associated
increased susceptibility of host to pathogen may not be mediated by lowered
endogenous SA levels. Such a conclusion is also supported by an earlier obser-
vation where the inoculation of A. thaliana with Pseudomonas syringae pv. ma-
culicola resulted in systemic acquired resistance (SAR) for plants grown at both
low and high light irradiances (Zeier et al. 2004 ). However, the higher light
irradiance did not result in an accumulation of endogenous SA or PR-1 protein
(Zeier et al. 2004 ).
In addition to light irradiance regulation of endogenous SA levels in plant
tissues, the quality of light perceived by a plant, via plant photoreceptors such as
phytochromes and cryptochromes, can also influence endogenous SA levels
(Kurepin et al. 2010a , 2012a ). For example, narrow-band far red (FR) light
[supplied by light emitting diodes (LED) as the dark period began] yielded up to a
5-fold increase in endogenous SA levels of sunflower hypocotyls, relative to white
(W) light or dark (D) treatments (Kurepin et al. 2010a ). Here, the FR-induced
increase in endogenous SA levels was positively associated with increased
hypocotyl elongation. In contrast, LED-produced red (R) light and blue (B) light
inhibited hypocotyl elongation relative to W or D treatments (Kurepin et al.
2010a ). That inhibition was associated with a decrease in endogenous SA levels,
relative to a W light treatment (but not a D treatment). Thus, it appears that the
endogenous SA content can also be regulated by light quality. That conclusion is
further supported by experiments where the effect of a change in R/FR ratio was
tested on endogenous SA accumulation. There, sunflower plants were grown at
both low and high light irradiances, each with varying R/FR ratios, and effects on