Abstract
Plants produce various l-tyrosine (Tyr)-derived compounds that are critical for plant adaptation and have pharmaceutical or nutritional importance for human health. Tyrosine aminotransferases (TATs) catalyze the reversible reaction between Tyr and 4-hydroxyphenylpyruvate (HPP), representing the entry point in plants for both biosynthesis of various natural products and Tyr degradation in the recycling of energy and nutrients. To better understand the roles of TATs and how Tyr is metabolized in planta, here we characterized single and double loss-of-function mutants of TAT1 (At5g53970) and TAT2 (At5g36160) in the model plant Arabidopsis thaliana As reported previously, tat1 mutants exhibited elevated and decreased levels of Tyr and tocopherols, respectively. The tat2 mutation alone had no impact on Tyr and tocopherol levels, but a tat1 tat2 double mutant had increased Tyr accumulation and decreased tocopherol levels under high-light stress compared with the tat1 mutant. Relative to WT and the tat2 mutant, the tat1 mutant displayed increased vulnerability to continuous dark treatment, associated with an early drop in respiratory activity and sucrose depletion. During isotope-labeled Tyr feeding in the dark, we observed that the tat1 mutant exhibits much slower 13C incorporation into tocopherols, fumarate, and other tricarboxylic acid (TCA) cycle intermediates than WT and the tat2 mutant. These results indicate that TAT1 and TAT2 function together in tocopherol biosynthesis, with TAT2 having a lesser role, and that TAT1 plays the major role in Tyr degradation in planta Our study also highlights the importance of Tyr degradation under carbon starvation conditions during dark-induced senescence in plants.
Highlights
Plants produce various L-tyrosine (Tyr)-derived compounds that are critical for plant adaptation and have pharmaceutical or nutritional importance for human health
These results indicate that TAT1 and TAT2 function together in tocopherol biosynthesis, with TAT2 having a lesser role, and that TAT1 plays the major role in Tyr degradation in planta
Sequencing analyses showed that tat1-1 and tat2-1 carried a T-DNA insertion in the second and third exons, respectively, whereas the T-DNA insertions of tat1-2 and tat2-2 were in first intron and 40 nucleotides upstream of the transcription start site, respectively, of the corresponding locus (Fig. 2A)
Summary
To investigate the roles of Arabidopsis TAT1 and TAT2 in vivo, two independent T-DNA insertion lines of TAT1 and TAT2 (tat, tat, tat, and tat2-2) were isolated from SALK T-DNA knockout collections [58]. A prolonged dark treatment was conducted using 2-week-old seedlings of WT, tat, and tat mutants grown in 1/2 MS medium (Fig. 5C), and the levels of chlorophylls, Tyr, and sucrose were analyzed (Fig. 5D). No significant differences were observed between WT and tat2 These results suggest that tat mutants have reduced degradation of Tyr and depleted carbon source during the dark-induced senescence. To test this hypothesis further, the MS medium was supplemented with different concentrations of sucrose during the dark treatment. TAT1, but not TAT2, plays the major role in degradation of Tyr into the TCA cycle
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