Preferentially enhancing anti-cancer isothiocyanates over glucosinolates in broccoli sprouts: How NaCl and salicylic acid affect their formation

Abstract

Broccoli (Brassica oleracea L. var. italica) sprouts contain glucosinolates (GLs) that when hydrolysed yield health promoting isothiocyanates such as sulforaphane (SF). SF content can be increased by salt (NaCl) stress, although high salt concentrations negatively impact plant growth. Salicylic acid (SA) treatments can attenuate the negative effects of salt on growth. To test whether sprout isothiocyanate content could be elevated without sprout growth being compromised, broccoli seed were germinated and grown for seven days in salt (0, 80 and 160 mM) alone and in combination with 100 μM SA. Increasing concentrations of salt lowered transcript accumulation of GL biosynthetic genes which was reflected in lowered content of Gluconapin, 4-methoxyglucobrassicin and neoglucobrassicin glucosinolates. Other glucosinolates such as glucoraphanin did not alter significantly. Salt (160 mM) increased transcript abundance of the GL hydrolytic gene MYROSINASE (BoMYO) and its cofactor EPITHIOSPECIFIER MODIFIER1 (BoESM1) whose encoded product directs MYROSINASE to produce isothiocyanate rather than nitrile forms. SF content was increased 6-fold by the 160 mM salt treatment, but the salt treatment reduced percentage seed germination, slowed seed germination, and reduced sprout hypocotyl elongation. This growth inhibition was prevented if 100 μM SA was included with the salt treatment. These findings suggest that the increase in SF production by salt occurs in part because of increased transcript abundance of genes in the hydrolytic pathway, which occurs independently of the negative impact of salt on sprout growth.