Transport of Salicylic Acid and Related Compounds - Salicylic Acid: Plant Growth and Development - page 52

area (PSA) which is defined as the sum of surfaces of polar atoms in a molecule,
usually oxygen and nitrogen and the hydrogens bonded to these atoms (Ertl et al.
2000 ), and the number of hydrogen bond donors (HBD), i.e., a hydrogen atom
attached to a relatively electronegative atom (Winiwarter et al. 2003 ). To diffuse
significantly through the animal tissue barrier, the drugs must meat several criteria:
MW \ 500 Da, Log D \ 5, PSA \ 60 Å 2 (blood-brain barrier) or \140 Å 2
(intestinal barrier) and HBD \ 5 (Lipinski et al. 1997 ; Palm et al. 1997 , 1998 ;
Winiwarter et al. 2003 ; Bhal et al. 2007 ). The first two parameters (MW and Log
D) are approximately the same for plant membranes. No data are available about
the PSA and HBD values limits but it may be suggested that PSA3140 Å 2 and
HBD [ 5 are poorly consistent with diffusion through cell plant membranes. Then,
according the ACD Log D Sol Suite software predictions, all the molecules of
Table 1 meet these criteria except SAG (PSA = 137 Å 2 , high HBD) which is
exclusively under its hydrophilic anionic form in the cytosol. This is why SAG
accumulation in vacuole requires the involvement of intrinsic proteins of the
tonoplast (Fig. 1 ).
Most of the molecules of Table 1 are predicted to be more or less phloem
mobile according to the Kleier's model (Fig. 2 ) except methyl salicylate (j), tri-
fluoroethylsalicylate (q) a salicylate derivative recently synthesized (Qian et al.
2006 ) and 3,5-dichlorosalicylic acid due to its Log K ow value. SAG (i) which is
near the boundary between poorly mobile and non mobile molecule areas (Fig. 2 )
is not present in detectable amounts in the phloem sap of tobacco infected with
TMV (Enyedi et al. 1992 ). The ability of the dihydroxyderivatives of benzoic acid
to diffuse through the plasma membrane to accumulate in the phloem is probably
very low taking into account their presence almost exclusively under the anionic
form in the apoplast and a D Log D pH = 5.0-8.0 from 0 (2,6-dihydroxybenzoic acid,
g) to about 0.4 (2,3-dihydroxybenzoic acid, e, and 2,5-dihydroxybenzoic acid, f)
(Table 1 ). Similarly, according to ACD Log D software predictions, acibenzolar-
S-methyl (t) and its metabolite 1,2,3-benzothiadiazole-7-carboxylic acid (u) can-
not diffuse easily from apoplast to symplast (Table 1 ).
While jasmonic acid and azelaic acid, two putative primary signals in SAR
although the former is a matter of debate (Truman et al. 2007 ; Jung et al. 2009 )
exhibit physicochemical properties perfectly suitable for long distance transport
within the sieve tubes (Chollet et al. unpublished data), MeSA (Vapor Pressur-
e = 0.0700 Torr at 25 C) does not meet the required criteria for such translo-
cation. MeSA can diffuse through the plasma membrane taking into account its
MW, PSA and HBD values but similarly in both directions because it is not
ionizable at biological pH values (D Log D pH = 5.0-8.0 = 0) (Table 1 ). Its con-
centration in the phloem sap is extremely low (Park et al. 2007 ). Therefore, this
rises an intriguing problem to solve about the pathway and mechanisms of MeSA
transport because SAR clearly requires both MeSA biosynthesis in the primary
infected leaf and MeSA esterase activity which converts MeSA to SA within the
uninoculated systemic tissues (Park et al. 2007 ; Liu et al. 2011 ).