Plant Biology Fall 2006

1. BISC 367 - Plant Physiology Lab Spring 2009 Plant Biology Fall 2006 • Notices: • The roots are looking really good! • Reading material: • Fukaki & Tasaki (2009) Hormone interactions during lateral root formation. Plant Mol. Biol. 69:437-449. • Ruzicka et al., (2007) Ethylene regulates root growth through effects on auxin biosynthesis and transport-dependent auxin distribution. Plant Cell 19: 2197-2212. • Ghassemian et al., (2000) Regulation of abscisic acid signaling by the ethylene response pathway in arabidopsis. Plant Cell 12:1117-1126. • Malamy (2005) Intrinsic and environmental response pathways that regulate root system architecture. Plant Cell & Environment 28:67-77. • Sharp (2002) Interactions with ethylene: changing views on the role of abscisic acid in root and shoot growth responses to water stress. Plant Cell and Environment 25:211-222.

2. Hormonal regulation of root system architecture • Root system architecture refers to the organization of the roots as determined by the degree of root branching. • Branching allows for root system proliferation to promote nutrient and water acquisition • Elongation growth occurs at the expense of root branching when soil conditions are not optimal for nutrient or water acquisition • allows roots to grow towards areas where water and nutrients are available • Root system architecture is influenced by water availability and nutrients (particularly N and P)

3. Hormone regulation of root system architecture • Root growth is influenced by hormones: • Auxin • Ethylene • Cytokinins • ABA All INHIBIT root growth! • Ethylene inhibits root growth via effects on auxin transport & synthesis • Cytokinins inhibit growth by increasing ET levels • ABA inhibits root growth by interacting with ET • Ability of ABA to inhibit root growth is dependent on ET signaling • Auxin inhibits root growth by ??????

4. Auxin is a growth promoting hormone - in stems • Auxin stimulates growth of excised stem sections • Outer stem tissues grow more rapidly in response to auxin • The growth response to auxin follows a classic hormone dose response curve • 10-5 to 10-4 M IAA is optimal for stimulating growth in stems • High levels of IAA inhibit growth b/c they stimulate ET synthesis • ET promotes lateral cell expansion • The [IAA] that stimulates stem growth inhibits roots growth • Very low [IAA] stimulates root growth

5. Root gravitropism is mediated by growth inhibition by auxin Gravity perception causes an asymmetric distribution of auxin whereby auxin is preferentially transported to the lower side of the root Differential transport of auxin drives many tropic responses (directional growth) • Roots grow downwards (positively gravitropic). Shoots are negatively gravitropic and grow up • Thigmotropism • Touch mediated growth • Roots grow around rocks in the soil • Shoots can coil around a solid support Auxin inhibits growth of lower side

6. ABA inhibits root growth Root Growth of Wild-Type and Ethylene Response Mutants Grown on Media Containing ABA • ET influences the ability of ABA to inhibit growth • ET signaling mutants are LESS SENSITIVE to the ability of ABA to reduce growth • Increasing ET synthesis INCREASES THE RESISTANCE of root growth to inhibition by ABA • Decreasing ET synthesis INCREASES THE SENSITIVITY of root growth to inhibition by ABA • Terminology: • LESS SENSITIVE - refers to a reduced response • MORE SENSITIVE – refers to an enhanced response • LESS RESISTANT – same as more sensitive • MORE RESISTANT – same as less sensitive ein2 etr1 WT Ghassemian, M., et al. Plant Cell 2000;12:1117-1126

7. ABA inhibits root growth Hypothetical Model for the Role of ABA and Ethylene in Regulating Root Growth in Arabidopsis • Part A. No ET • ABA inhibition of root growth occurs (at least in part) by signaling through the ETR1 (ET) response pathway • Mutations in the ET signaling pathway reduce the sensitivity of root growth to inhibition by ABA • Part B. ET is present • ABA is unable to use (or has restricted access to) the ET signaling pathway to reduce root growth • Mutations that increase ET synthesis increase the resistance of root growth to inhibition by ABA ein2 etr1 WT Ghassemian, M., et al. Plant Cell 2000;12:1117-1126

8. ET inhibits root growth • ET inhibits root growth by stimulating auxin synthesis • DR5-GUS expression was increased by treatment with ACC (part C 1st 3 panels) • ET signaling is needed for ACC to increase DR5-GUS expression (part C, etr1 and ein2 panels) • ET signaling is not needed for auxin (NAA)-induced reduction in root growth • Remember: • ACC is applied to mimic an ET treatment • DR5-GUS is a marker for auxin Ruzicka, K., et al. Plant Cell 2007;19:2197-2212

9. ET inhibits root growth Model shows: • ET stimulates auxin biosynthesis and auxin moves toward the root tip • Auxin is transported basipetally and activates local auxin responses (via TIR1 – auxin receptor) to inhibit cell elongation • This accounts for most but not all ET effects on root growth Ruzicka, K., et al. Plant Cell 2007;19:2197-2212

10. Hormone regulation of root system architecture • Lateral root (LR) formation is also influenced by hormones: • Auxin • Ethylene • ABA • Cytokinins LRs arise from pericycle founder cells adjacent to xylem poles • Auxin moving acropetally stimulates pericycle founder cell division and LR primordium initiation • LR site position is determined in a region between the meristem and elongation zone Fukaki and Tasaka (2009) Plant Mol. Biol. 69: 437-449

11. Hormonal regulation of LR formation • Auxin is an important regulator of LR formation • Auxin transport, both acropetal and basipetal, is important for LR formation • Auxin affects LR initiation, primordium development and LR emergence • Cytokinins are negative regulators of LR formation • Cytokinis affect the pericycle cells and block the developmental program of LR formation • ABA is a negative regulator of LR formation • Exogenous ABA inhibits LR emergence prior to activation of the LR meristem • Auxin and ABA are believed to interact during LR development • ABA mediates the ability of nitrate to inhibit LR development • ET stimulates adventitious root formation but represses LR formation • Increased ET synthesis and enhanced ET signaling reduce LR formation • Decreased ET signaling increases LR formation • ET affects auxin synthesis, transport and signaling and may influence different aspects of LR formation DR5-GUS expression in developing LR

12. Root system architecture is plastic and influenced by the environment • In response to drought stress root elongation is maintained & LR formation is suppressed • This favours growth towards new soil environments • When roots are in better soil environment root proliferation (branching) occurs • ABA accumulates in the roots of drought-stressed plants and plays a role in maintaining growth.

13. ABA maintains root growth in water deficit stressed seedlings • Wild type and ABA-deficient seedlings were transferred to a low or high Yw growing medium • Growth was measured • In roots: • At low Yw the ABA-deficient mutant roots grew less than WT roots • ABA is involved in maintaining root growth at low Yw • ABA promotes osmotic adjustment in the root meristem • Maintains cell division and expansion • Changes in the cell wall (loosening) aid growth

14. ABA reduces shoot growth in water deficit stressed seedlings • Recall that in the shoot ABA inhibits growth: • The shoot of ABA-deficient seedlings grew more under low Yw than WT seedlings • ABA inhibits shoot growth in low Yw plants • Adaptive significance: • Increases root:shoot ratio in water deficit-stressed plants • Increases vol. of soil that can be explored for water uptake

15. Maize seedlings growing at low Yw From Spollen et al., Plant Phys. 122:967 (2000) ABA maintains root growth by suppressing ethylene production • FLU (fluridone) is an inhibitor of ABA biosynthesis • AOA, AVG & STS are inhibitors of ethylene action or biosynthesis • Inhibiting ethylene production or action in ABA-deficient seedlings restored growth in water deficit stressed seedlings • ABA suppresses ethylene production and thereby maintains root growth