EFB530 Plant Physiology

Roles of auxin in growth and development

Auxin action

Auxin causes cell elongation in actively growing regions of shoots

• show this experimentally using coleoptile segments, submerged in auxin solution
• over time, see increase in length of those segments
• can generate a dose-response curve-at high concentrations, see inhibition

Roots are much more sensitive to auxin, elongation is inhibited at much lower concentrations

Elongation occurs when auxin induces loosening of the cell wall

• auxin induces enzymes that allow cell wall expansion by modifying hemicelluloses (XET enzyme) or a protein that loosens H-bonding between cellulose and hemi-cellulose (expansin protein)
• activation of H⁺-ATPase may be involved to make cell wall more acidic (acid-growth theory); higher [H+] can disrupt/loosen the associations between cellulose and hemicellulose

directional growth in response to the force of gravity

• positive gravitropism = toward the side where gravity is applied (in roots)
• negative gravitropism = curvature away from gravity (in shoots)

gravity sensing appears to occur in certain cells (statocytes) due to the force applied by statoliths = thought to be starch-filled plastids

curvature occurs (as with phototropism) due to accelerated growth on one side and reduced growth on the other side

Auxin in phototropism and gravitropism

Cholodny-Went theory = lateral distribution of auxin

• higher concentration of auxin on the side opposite the light stimulus for phototropism
• higher concentration of auxin on same side as gravity stimulus for gravitropism
• in shoots, this causes stimulation of elongation
• in roots this causes inhibition of elongation

can determine this using agar blocks at the ends of shoot segments for both photo- and gravitropism

• observe higher concentrations of auxin diffusing into blocks in the two sides of the segments

• IAA induces the expression of genes to produce new proteins
• function of many of these is not known

1) involved in apical dominance

• signal from the apical meristem that prevents growth of lateral buds, acting either as an inhibitor directly; or by control the allocation of resources to the tip and away from buds; or by maintaining high levels of ABA in buds (ABA is a growth repressor)

2) promotes fruit growth

• auxin is produced by seeds, stimulates the growth of fruit tissue around seed

3) promotes the formation of lateral roots

• even though primary root growth is inhibited by auxin, lateral roots are stimulated to grow by high levels of auxin
• new lateral roots can be induced to grow from stem sections by application of auxin (like Rootone)

4) delays leaf abscission

• production of IAA in vigorous, healthy leaves represses the hormone ethylene
• when the leaves senesce, IAA levels decline, removing the repression of ethylene, which induces formation of the abscission zone

5) promotes vascular differentiation

• IAA seems to regulate differentiation of cells into xylem cells, as demonstrated by rebuilding of xylem after wounding of the stem (can be induced by IAA) and by induction of differentiation of Zinnia mesophyll cells into xylem cells in culture

6) some synthetic auxins are powerful herbicides (2,4-D, dicamba, and 2,4,5-T)

• primary mode of action is not known, but seems to hyperactivate signalling pathways/regulation

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