Auxins as regulators of growth and development of cereal crops under abiotic stresses: Quick notes

Biosynthesis, Signalling and Transport

  • Auxins: key endogenous regulators of growth; main natural auxin is indole-3-acetic acid (IAA). Other active auxins include 4-Cl-IAA, IBA, and phenylacetic acid (PAA). IAA is widespread across organisms.
  • IAA biosynthesis: occurs via two pathways:
    • Trp-independent: older, baselines auxin production; enzymes include Nonhebel and other contributors.
    • Trp-dependent: major pathway in cereals via the two-step TAA/YUC process. IPyA is produced by TAA, then YUC oxidatively decarboxylates IPyA to IAA. The direction of IPyA formation/conversion can be modulated by substrate availability and enzymes such as VAS1.
  • Conjugation and storage: most IAA is conjugated for inactivation or storage (glucose, amino acids, peptides, and other moieties). IAA conjugates can be hydrolyzed to release active IAA; common forms include IAA–glucose and amino acid conjugates (e.g., Asp-IAA, Leu-IAA).
  • Catabolism: primary inactivation products include OxIAA (2-oxindole-3-acetic acid) and methyl esters via IAMT1; glycosylation is common in cereals (UDP-glucosyltransferases).
  • Transport and distribution: IAA is transported via apoplast and vacuoles, with polar transport establishing local maxima/minima essential for morphogenesis. Major transporters: AUX/LAX (influx), PIN (efflux/polar transport), and ABCB family (multifunctional, including intracellular/local transport).
  • Transport rates in cereals:
    • coleoptiles: 8-15 ext{ mm h}^{-1}
    • roots: 1-2 ext{ mm h}^{-1}
  • Local maxima/minima control morphogenesis; expression/localization of PIN, AUX/LAX, and ABCB transporters modulate auxin flow (examples: TaPIN1-6, ZmPIN9, OsPIN9, ZmPIN10 family).
  • Metabolism and transport interplay:
    • IAA conjugates serve as transport forms, temporary storage, or storage for later release.
    • In cereals, ester-linked IAA–sugar conjugates can supply IAA during germination; hydrolases (esterases) release active IAA when needed.

Two main biosynthesis and regulation pathways

  • Trp-independent pathway provides baseline auxin in growth; active under various conditions.
  • Trp-dependent pathway (TAA/YUC) dominates in cereals during key developmental stages (embryogenesis, seed germination, flowering, damage recovery).
  • Enzyme localization spans chloroplasts, cytoplasm, ER membranes, mitochondria; active IAA and metabolites are stored/transported in apoplast and vacuoles.

Functional activity of auxins in cereals

  • Auxin influences embryogenesis, organ formation, and overall plant architecture; outcomes depend on concentration and spatial distribution, plus tissue sensitivity.
  • In monocots, auxins regulate roots, shoots, leaves, vascular tissues, panicle and spikelet development.
  • Polar auxin transport (PINs) shapes developmental patterns:
    • overexpression of ZmPIN1a in maize increases lateral root density and alters shoot architecture.
    • OsAUX1/TAUX1-like transporters affect lateral root initiation; OsPID modulates floral organ development via auxin transport/signaling.
  • Auxin drives cell elongation by activating plasma membrane H+-ATPases, contributing to organogenesis and growth.

Interaction with other phytohormones

  • Auxin interacts with cytokinin, abscisic acid (ABA), salicylic acid (SA), gibberellins (GA), brassinosteroids, and ethylene to regulate tillering, flowering, and inflorescence architecture.
  • Examples of crosstalk:
    • LT1 lowers auxin and increases cytokinin, promoting branching/tillering; cytokinin degradation enzymes (CKX) are suppressed via LT1.
    • Auxin–SA antagonism affects tillering; auxin gradients influence leaf/root development.
    • ABA and GA biosynthesis/signaling interplay with auxin to modulate stem and leaf development; in barley, VRS-related genes connect hormone gradients with inflorescence patterning.
    • Ethylene can modulate auxin-driven gravitropism by affecting auxin biosynthesis.
  • Hormone balance in cereals (barley, wheat, maize, rice) shapes ear/inflorescence development and grain yield via auxin-cytokinin-GA-ABA interactions.
  • IPT genes (cytokinin biosynthesis) and their regulation by auxin influence tiller development (e.g., OsIPT regulation by IAA).

Endogenous auxins under abiotic stress

  • Global climate stress reduces endogenous auxin in many cereals, though tissue- and species-specific responses occur.
  • Temperature: high heat reduces IAA in spikelets, disrupting flower differentiation; cold can alter auxin homeostasis and stabilizes some responses via dehydrins and phenolics.
  • Drought: drought stress often lowers IAA in shoots/roots, with changes in the IAA/CK/GA balance and ABA increases.
  • Heavy metals (Cd, Zn, Ni, Cd, Al, As): generally depress auxin levels or disrupt transport/signaling; some contexts show localized IAA accumulation in roots under Al stress or NH4+ conditions, but severe pollution reduces IAA and growth.
  • Transport and signaling changes under stress: ZmPIN, ZmPILS, ZmLAX, ZmABCB members show organ-specific regulation under drought, salinity, cold; OsPIN3t overexpression can confer drought resistance; GH3 family (e.g., OsGH3-2) modulates auxin/ABA levels affecting drought/cold tolerance.
  • Cross-talk with ABA is common under stress; carotenoid/biosynthesis status can influence IAA levels and stress responses.
  • Example patterns across cereals: drought/cold increase or decrease of auxin transporter gene expression; Al stress affects basipetal vs acropetal transport; IAA distribution shifts correlate with tolerance phenotypes.

Exogenous auxins in regulation of stress resistance

  • Exogenous IAA and auxin precursors improve tolerance to various abiotic stresses:
    • Drought: IAA boosts antioxidant activity, carbohydrate/protein content, and biomass; reduces lipid peroxidation/ROS.
    • Salinity: IAA with kinetin increases grain yield; seed/seedling treatments improve germination and ion balance; exogenous IAA can increase endogenous IAA in grains, promoting cell division and expansion.
    • Heat/anoxia: IAA can improve seedling growth and grain traits under stress conditions; foliar application can improve photosynthetic pigment content and stress resilience.
    • Heavy metals: exogenous IAA/IBA upregulate auxin biosynthesis genes and improve tolerance to Cd, Pb, Ni; priming seeds with IAA mitigates Cd stress and supports growth/yield.
  • Exogenous auxin transport inhibitors (e.g., 1-NOA, NPA) can reduce stress resilience by limiting auxin distribution; applying auxin (NAA, IAA) can compensate for this under Cd stress.
  • IAA and IBA can mitigate Al toxicity by modulating root pH and proton pumps; exogenous NO interacts with auxin to modify root growth and stress responses (NO can enhance or constrain auxin transport depending on context).
  • Silicon and nitric oxide can work with auxin signaling to enhance arsenic resistance; combined NO/auxin treatments can boost root/shoot growth and antioxidant capacity under HM stress.
  • Overall: exogenous auxins support stress tolerance, biomass accumulation, and yield, often via antioxidant protection, improved ion homeostasis, and modulation of hormone crosstalk; benefits can depend on concentration, application method (seed priming vs foliar spray), and environmental context.

Practical implications and applications

  • Exogenous auxins (IAA, IBA, NAA) provide a tool to enhance cereal resilience to drought, salinity, heat, cold, and heavy metal stress.
  • Seed priming with auxin precursors (e.g., tryptophan + IAA) can improve germination under salinity and improve subsequent growth by modulating SA and other hormones.
  • Foliar sprays can increase spikelet numbers, grain yield, and stress tolerance under saline or metal-polluted conditions.
  • Biotechnological strategies targeting auxin biosynthesis (TAA/YUC, GH3 family, PIN-like transporters) offer routes to improve drought, heat, and nutrient use efficiency through optimized auxin gradients.

Conclusion

  • Auxin is a central regulator integrating abiotic stress signals with growth and development in cereals.
  • Auxin biosynthesis, signaling, transport, and interactions with other hormones shape stress responses and yield outcomes.
  • Exogenous auxins and biosynthetic modulation hold promise for improving cereal performance under environmental stress, supporting sustainable agriculture.
  • Ongoing research should refine application strategies and dissect transporter- and hormone-network dynamics to maximize benefits under diverse field conditions.
Note on key figures and terms
  • Major pathways: Trp-dependent TAA/YUC; Trp-independent baseline synthesis.
  • Key transporters: PIN family (PIN1/6/9 etc.), AUX/LAX, ABCB; ER-localized PILS as intracellular transporters; LAZY1 involvement in gravitropism and shoot architecture.
  • Common conjugates and catabolites: IAA–Sugar conjugates, IAA–amino acid conjugates; OxIAA and related forms; IAMT1 and DAO1 enzymes regulate inactivation and oxidation.
  • Typical transport rates (as above) and organ-specific expression patterns underline the importance of spatial regulation for cereal development and stress adaptation.