Abstract
Carrageenans are a collective family of linear, sulphated galactans found in a number of commercially important species of marine red alga. These polysaccharides are known to elicit defense responses in plant and animals and possess anti-viral properties. We investigated the effect of foliar application of ι-, κ- and λ-carrageenans (representing various levels of sulphation) on
Arabidopsis thaliana
in resistance to the generalist insect
Trichoplusia ni
(cabbage looper) which is known to cause serious economic losses in crop plants. Plants treated with ι- and κ-carrageenan showed reduced leaf damage, whereas those treated with λ- carrageenan were similar to that of the control. In a no-choice test, larval weight was reduced by more than 20% in ι- and κ- carrageenan treatments, but unaffected by λ-carrageenan. In multiple choice tests, carrageenan treated plants attracted fewer
T. ni
larvae by the fourth day following infestation as compared to the control. The application of carrageenans did not affect oviposition behaviour of
T. ni
. Growth of
T. ni
feeding on an artificial diet amended with carrageenans was not different from that fed with untreated control diet. ι-carrageenan induced the expression of defense genes;
PR1
,
PDF1.2
, and
TI1
, but κ- and λ-carrageenans did not. Besides
PR1
,
PDF1.2
, and
TI1
, the indole glucosinolate biosynthesis genes
CYP79B2
,
CYP83B1
and glucosinolate hydrolysing QTL,
ESM1
were up-regulated by ι-carrageenan treatment at 48 h post infestation. Gas chromatography-mass spectrometry analysis of carrageenan treated leaves showed increased concentrations of both isothiocyanates and nitriles. Taken together, these results show that carrageenans have differential effects on
Arabidopsis
resistance to
T. ni
and that the degree of sulphation of the polysaccharide chain may well mediate this effect.