- A tryptophan auxotroph in corn showed 50 times more accumulation of IAA then the normal. Probable explanation for this is
(1) There exist another tryptophan independent pathway for IAA synthesis
(2) IAA is probably inhibited by feedback mechanism
(3) IAA was not oxidized
(4) Deconjugation of ester linked IAA does not take place
Concept and Correct Option
A tryptophan auxotroph of maize (corn) that cannot synthesize normal levels of Trp was found to accumulate about 50 times more total IAA (free plus conjugated) than the wild type.
This paradox is best explained if IAA can be synthesized via a pathway that does not require tryptophan as an intermediate (Trp‑independent pathway).
Therefore, option (1) is correct: there exists another tryptophan‑independent pathway for IAA synthesis.
Explanation of Each Option
(1) There exist another tryptophan independent pathway for IAA synthesis – Correct
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Genetic and isotope‑labeling studies in maize and Arabidopsis show that IAA can be produced from early shikimate‑pathway intermediates such as indole or indole‑3‑glycerol phosphate, bypassing free tryptophan.
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In Trp auxotrophs, blockage of Trp synthesis can relieve feedback inhibition on upstream steps, enhancing flux into this Trp‑independent route; IAA synthesis continues or even increases despite low Trp, explaining the 50‑fold IAA accumulation.
(2) IAA is probably inhibited by feedback mechanism – Incorrect
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A feedback inhibition of IAA biosynthesis by IAA would reduce auxin levels, not increase them 50‑fold.
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Evidence instead suggests feedback control of Trp biosynthesis and of some Trp‑dependent IAA routes, but that does not explain a dramatic overaccumulation of IAA in a Trp‑deficient mutant.
(3) IAA was not oxidized – Unlikely/Incorrect as main explanation
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Oxidative degradation is one route of IAA turnover, but blocking oxidation alone would not specifically link to a tryptophan auxotroph phenotype.
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Moreover, in maize and other species, much of the IAA pool is controlled by conjugation–deconjugation rather than just oxidation.
(4) Deconjugation of ester linked IAA does not take place – Incorrect in this context
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Many maize tissues store IAA as ester‑linked conjugates that can be hydrolyzed to release free IAA when needed.
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If deconjugation did not occur, free IAA levels would be lower, not 50‑fold higher. In the mutant cited in the literature, the large “total IAA” pool is largely conjugated, consistent with enhanced synthesis rather than blocked deconjugation.
Short SEO‑Style Overview
In maize, discovery of a Trp‑auxotrophic mutant with 50× more IAA than normal provided strong evidence for a tryptophan‑independent IAA biosynthesis pathway. This pathway draws from shikimate‑pathway intermediates upstream of Trp and continues to produce auxin even when Trp synthesis is impaired.
Recognizing both Trp‑dependent and Trp‑independent routes is essential for understanding auxin homeostasis, mutant phenotypes, and how plants flexibly regulate growth via IAA.
