Chemical Inhibition of the Glycolate Pathway in Soybean Leaf Cells

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Abstract
Isolated soybean (Glycine max [L.] Merr.) leaf cells were treated with three inhibitors of the glycolate pathway in order to evaluate the potential of such inhibitors for increasing photosynthetic efficiency. Preincubation of cells under acid conditions in α-hydroxypyridinemethanesulfonic acid increased 14CO2 incorporation into glycolate, but severely inhibited photosynthesis. Isonicotinic acid hydrazide (INH) increased the incorporation of 14CO2 into glycine and reduced label in serine, glycerate, and starch. Butyl 2-hydroxy-3-butynoate (BHB) completely and irreversibly inhibited glycolate oxidase and increased the accumulation of 14C into glycolate. Concomitant with glycolate accumulation was the reduction of label in serine, glycerate, and starch, and the elimination of label in glycine. The inhibitors INH and BHB did not eliminate serine synthesis, suggesting that some serine is synthesized by an alternate pathway. The per cent incorporation of 14CO2 into glycolate by BHB-treated cells or glycine by INH-treated cells was determined by the O2/CO2 ratio present during assay. Photosynthesis rate was not affected by INH or BHB in the absence of O2, but these compounds increased the O2 inhibition of photosynthesis. This finding suggests that the function of the photorespiratory pathway is to recycle glycolate carbon back into the Calvin cycle, so if glycolate metabolism is inhibited, Calvin cycle intermediates become depleted and photosynthesis is decreased. Thus, chemicals which inhibit glycolate metabolism do not reduce photorespiration and increase photosynthetic efficiency, but rather exacerbate the problem of photorespiration.