Transport of Salicylic Acid and Related Compounds - Salicylic Acid: Plant Growth and Development - page 49

Fig. 1 SA biosynthesis and transport between cell compartments. ISC isochorismate pathway;
PHE phenylalanine pathway; SAG salicylic acid glucoside; HAEC high affinity efflux carrier;
LAEC low affinity efflux carrier. (1) From Dean and Mills ( 2004 ); (2) from Dean et al. ( 2005 );
(3) and (4) from Chen et al. ( 2001 ) and Kawano et al. ( 2004 )

energized by the protonmotrice force (Giaquinta 1977 ; Delrot and Bonnemain
1981 ). These species are characterized by a low or very low plasmodesmatal
density between the companion cell—sieve element complex and the neighboring
cells, a high expression of the plasma membrane H + -ATPase in the companion
cells and a loading inhibition by pCMBS (Bourquin et al. 1990 ; Bouchepillon et al.
1994 ; DeWitt and Sussman 1995 ; Turgeon 2010 ). On the other hand, especially in
many tree species, phloem loading is passive, driven by a downhill concentration
of nutrients from the mesophyll to the companion cell—sieve element complex.
These species are characterized by a high symplastic connectivity between
mesophyll and conducting cells (symplastic loading) and non-inhibition of phloem
loading by pCMBS (Gamalei 1989 ; Van Bel 1993 ; Turgeon 2010 ).
Consequently, phloem loading of endogenous hormones depends on the leaf
anatomy. It may be apoplastic or symplastic according to the characteristics of the
symplastic continuity between the companion cell—sieve element complex and
the neighboring cells. However, when hormone biosynthesis occurs in companion
cells as it is the case for jasmonic acid, the hormone may move directly into the
sieve-tubes via the branched plasmodesmatal connections for long distance

transport (Champigny and Cameron 2009 ). By contrast, synthetic hormones