Plants are extraordinary chemical factories producing an overwhelming variety of natural compounds referred to as specialized metabolites. These metabolites constitute the language of plants and enable them to interact with the surroundings e.g. by attracting beneficial insects, repelling herbivores or enable plantplant communication. Cyanogenic glucosides (CNglcs) are nitrogenous defense compounds that are widely distributed in the plant kingdom. CNglcs exert their activity by releasing hydrogen cyanide (HCN) upon herbivory – a process referred to as cyanogenesis. Beside this well-established function, a growing body of literature suggests that CNglcs possess significant roles in scavenging reactive oxygen species, modulating plant growth and stress response, transporting nutrients and as a storage reserve of reduced nitrogen.
In Sorghum bicolor (L.) Moench the recovery of nitrogen from the CNglc dhurrin has been recently proposed to occur via two possible recycling pathways, which circumnavigate the release of the toxic intermediate hydrogen cyanide (HCN). The products of these pathways are reduced nitrogen in form of ammonia, and a variety of dhurrin derivatives. These metabolites are hypothesized to be important plant components serving fundamental functions at certain stages of plant development or under specific environmental conditions such as water limitation. However, very little is known about these possible physiological functions and their metabolic network in the plant. In this PhD thesis, the roles of dhurrin derivatives and their dynamic metabolic relationship is investigated at multiple spatial scales, using sorghum as the main model system.
The research presented creates new knowledge with respect to the multifunctionality of dhurrin, dhurrin derivatives and recycling pathways. For example, this thesis demonstrates that accumulation of dhurrin and its derivatives is substantially affected by tissue types, changes in the surrounding environment and through plant ontogeny. Specifically, water limitation induces accumulation of certain dhurrin derivatives in specific tissue types, pointing towards their possible function.
Further investigation on the bioactivity of these metabolites revealed them to be potentially involved in plant growth and stress response. Furthermore, by using a highly advanced metabolite bio-imaging technique the physiological functions and metabolic relationships between dhurrin and its derivatives were investigated by studying the localization of their accumulation in different tissues. The cellular localization across different tissue types confirms that dhurrin provides the first line of chemical defense against herbivore attack in sorghum and creates new hypotheses regarding the role of dhurrin recycling. In addition, determination of dhurrin subcellular localization with high spatial resolution overhaul the classical theory of its tissue compartmentalization, proposing its storage by chemical stabilization. The research presented in this PhD thesis is a significant advance in the understanding of the endogenous recycling of CNglcs and brings further evidence that CNglcs are more than just defense compounds.