Transport of Salicylic Acid and Related Compounds - Salicylic Acid: Plant Growth and Development - page 50

applied to the foliage must cross at least once the plasma membrane whatever the
phloem loading mechanism, symplastic or apoplastic.
3.1.2 Phloem Transport of SA and Related Compounds
SA Phloem Transport in Relation to Biotic and Abiotic Stress
The first experiments on SA phloem transport were conducted with labeled mol-
ecules three decades ago. They failed to demonstrate an efficient long distance
transport of the label, probably because 14 C-SA is quickly metabolized and
sequestered in vacuoles (Bental and Cleland 1982 ).
A few years later, an increasing interest on SA phloem transport arose from the
demonstration that development of SAR in a cultivar of tobacco resistant to
tobacco mosaic virus (TMV) is accompanied by a dramatic increase in the level of
endogenous SA in infected leaves after TMV inoculation and also, to a lesser
extent, in uninfected upper leaves (Malamy et al. 1990 ). Furthermore, inoculation
of mature cucumber leaves with either the tobacco necrosis virus (TNV) or Col-
letotrichum lagenarium leads to a clear rise in SA in the phloem sap and the
development of SAR (Metraux et al. 1990 ). TMV infection of nearly fully
expanded leaves of tobacco plants also induces an increase in SA concentration in
the phloem sap (Yalpani et al. 1991 ). These data strongly suggest that SA, pre-
viously known as an exogenous inducer of resistance (White 1979 ) can also
function as a mobile signal that triggers local and systemic induction of PR-1
proteins and, possibly, some components of systemic acquired resistance (Metraux
et al. 1990 ; Yalpani et al. 1991 ).
Then SA phloem transport from inoculated mature leaves to systemically
protected apical organs was demonstrated. The first evidence came from in vivo
labeling with 18 O 2 of the SA synthesized in TMV-inoculated lower leaves of
tobacco (Shulaev et al. 1995 ). Spatial and temporal distribution of 18 O-SA indi-
cates that most of the SA molecules detected in the upper healthy tissues are 18 O-
labeled and have therefore been transported from the inoculated leaves. The
second evidence came from transport studies of 14 C-SA after injection of 14 C-
labeled benzoic acid into cucumber cotyledons inoculated with TNV (Molders
et al. 1996 ). Labeled SA moves in the phloem and is clearly detected in the upper
uninoculated leaf before the development of SAR. However, the specific activity
of 14 C-SA decreases indicating that, in addition to transport from the inoculated
cotyledons, the upper leaf produces SA in response to a primary signal. In this
regard, previous data from leaf removing and grafting experiments show that the
primary SAR inducing signal is not SA (Rasmussen et al. 1991 ; Vernooij et al.
1994 ). Studies are now focused on the mechanism of SAR activation. Several data
suggest that multiple signals are required among which methyl salicylate (Liu et al.
2011 ).