The significance of glucosinolates for sulfur storage in Brassicaceae seedlings

Abstract

Brassica juncea seedlings contained a twofold higher glucosinolate content than B. rapa and these secondary sulfur compounds accounted for up to 30% of the organic sulfur fraction. The glucosinolate content was not affected by H2S and SO2 exposure, demonstrating that these sulfur compounds did not form a sink for excessive atmospheric supplied sulfur. Upon sulfate deprivation, the foliarly absorbed H2S and SO2 replaced sulfate as the sulfur source for growth of B. juncea and B. rapa seedlings. The glucosinolate content was decreased in sulfate-deprived plants, though its proportion of organic sulfur fraction was higher than that of sulfate-sufficient plants, both in absence and presence of H2S and SO2. The significance of myrosinase in the in situ turnover in these secondary sulfur compounds needs to be questioned, since there was no direct co-regulation between the content of glucosinolates and the transcript level and activity of myrosinase. Evidently, glucosinolates cannot be considered as sulfur storage compounds upon exposure to excessive atmospheric sulfur and are unlikely to be involved in the re-distribution of sulfur in B. juncea and B. rapa seedlings upon sulfate deprivation.

Keywords: Brassicaceae; glucosinolate; hydrogen sulfide; myrosinase activity and expression; sulfur deficiency; sulfur dioxide; sulfur storage.

FIGURE 1

Impact of H₂S, SO₂ and sulfate deprivation on total sulfur, sulfate and organic sulfur content of shoots and roots of B. juncea and B. rapa. For experimental details, see legends of Table 1. The sulfate and organic sulfur fraction is presented in gray and white, respectively. Data represent the mean of two experiments with three measurements with six to nine plants in each (±SD). Different letters indicate significant differences between treatments (P < 0.01, Student’s t-test).

FIGURE 2

Impact of H₂S, SO₂ and sulfate deprivation on glucosinolate content of shoots and roots of B. juncea and B. rapa. For experimental details, see legends of Table 1. Data represent the mean of two experiments with three measurements with nine plants in each (±SD). Different letters indicate significant differences between treatments (P < 0.01, Student’s t-test).

FIGURE 3

Impact of H₂S, SO₂ and sulfate deprivation on transcript levels of myrosinase in shoots of B. juncea and B. rapa. For experimental details, see legends of Table 1. Relative gene expression of myrosinase was determined by qRT-PCR and the mRNA levels were compared to actin. Data on relative expression in each treatment represent the mean of three measurements with three shoots in each (±SD). Different letters indicate significant differences between treatments (P < 0.01, Student’s t-test).