METABOLISM OF AROMATIC COMPOUNDS IN HEALTHY AND RUST-INFECTED PRIMARY LEAVES OF WHEAT: I. STUDIES WITH14CO2, QUINATE-U-14C, AND SHIKIMATE-U-14C AS PRECURSORS

Abstract

Healthy and stem rust infected leaves of two near-isogenic lines of wheat were allowed to metabolize C14O2, quinate-U-C14; or shikimate-U-C14 for 22 h in the light. Quinate-U-C14 and shikimate-U-C14 were interconvertible but differed in their efficiency as precursors of phenylalanine and tyrosine, suggesting that an alternative pathway, not via shikimate, exists from quinate to phenylalanine in wheat leaves. Activity from the cyclites was not incorporated into tryptophan during the metabolic period. Evidently, synthesis of aromatic amino acids in wheat leaves is not restricted to the "classic" shikimate pathway. Infection with rust led to an increase of the carbon flow from CO2 to shikimate and to higher levels of both quinate and shiMmate. This trend was more pronounced in susceptible than in resistant leaves. Moreover, utilization of quinate and shikimate was increased in infected susceptible leaves but was not altered significantly in infected resistant leaves. Resistant and susceptible reacting leaves differed in the distribution of activity derived from quinate-U-C14 and shikimate-U-C14. With shikimate-U-C14 as precursor, resistant reacting leaves accumulated activity in unidentified components of the amino acid fraction (Amberlite IR-120 eluate). Susceptible reacting leaves accumulated less activity in these compounds, and healthy leaves contained only traces of activity in them. When either quinate-U-Cl4 or shikimate-U-Cl4 was metabolized by healthy leaves, more activity was recovered in insoluble esters (of ferulate and p-coumarate) than in soluble esters. Resistant reacting leaves accumulated still more activity in the insoluble esters, whereas susceptible reacting leaves contained a proportionately higher amount of activity in soluble esters. Compared with healthy leaves, susceptible reacting leaves always contained less activity, and resistant reacting leaves more activity, in the non-hydrolyzable, insoluble residue.