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← Theoretical B

Q37 — Auxin Transport and Apical Dominance

Theoretical B Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Plant Hormones

Auxin promotes and cytokinins repress apical dominance. To further understand how auxin and its transport exert apical dominance, scientists treated nodal stem sections containing axillary buds with auxin from either end (depicted as A and B in Figure 1) in the presence or absence of the auxin transport inhibitor NPA, and measured the auxin content in buds and stems separately. For stems, auxin content was measured at the opposite end to the treatment end (Table 1).

Auxin content in buds — End A Auxin content in buds — End B Auxin content in stems — End A Auxin content in stems — End B
− NPA 14.2 ± 0.9 15.8 ± 2.4 2221.0 ± 232.3 13.4 ± 0.5
+ NPA 11.9 ± 0.8 13.5 ± 0.2 31.5 ± 2.93 16.3 ± 0.7

Table 1. Ends A and B indicate auxin-injection points. Values are in counts per minute ± standard error.

Next, they observed how grafting between Col (wild-type) and axr1-mut (an auxin-insensitive mutant that can still produce auxin) affects stem branching (Figure 2).

Finally, they tested how tissue-specific expression (compare Tables 2 and 3) of the AXR1 (auxin-sensitive) gene affects stem branching in transformed axr1-mut mutants.

Two-panel figure: Figure 1 is a schematic of a stem segment with a bud growing from its middle and ends labeled B (left) and A (right); Figure 2 is a bar chart of stem branching for four graft combinations — Col/Col, Col/axr1-mut, axr1-mut/Col, and axr1-mut/axr1-mut (shoot/root). Figure 1. Schematic of the nodal stem section, with the axillary bud and the two auxin-injection ends, A and B, labeled. Figure 2. Stem branching for each shoot/root graft combination between wild-type (Col) and the auxin-insensitive axr1-mut mutant.

Sample Stem branching
Wild-type (untransformed) 16 ± 0.5
Mutant axr1-mut (untransformed) 26 ± 2.0

Table 2. Influence of mutations on stem branching.

Epidermis Cortex Phloem Xylem Pith Stem branching
O O O O O 18 ± 1.0
O X X X X 25 ± 1.0
X X O X X 25 ± 1.0
X X X O X 16 ± 0.5

Table 3. AXR1-expressing tissues in transformed axr1-mut mutants. O: AXR1 is expressed, X: AXR1 is NOT expressed.

On your answer sheet, indicate “T” for true statements and “F” for false ones.

A. End A is closer to the shoot apical meristem than end B.
B. Auxin does not enter into the bud to inhibit axillary bud growth.
C. Based on the data, auxin potentially suppresses bud growth by inhibiting cytokinin biosynthesis in the root.
D. The xylem is critical in mediating the inhibitory action of auxin on bud growth.
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Question reproduced from IBO 2024, Theoretical Exam Part B, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF