Regulating Growth and Development: The Plant Hormones

Home > Biology 103: Botany > Exam Three

Learning Objectives

1. Identify which hormone(s) function in which physiological process.
2. Understand the mechanism of hormone action.

Solutions

1. Auxin and brassinosteriods regulate cell differentiation and elongation, and are transported polarly. Cytokinins are antagonists for auxin, and delay leaf senescence. Ethylene is a gaseous hormone which controls ripening. Abscisic acid (ABA) promotes dormancy and stomata closing. Gibberellic acid (GA) is an antagonist for ABA, and promotes cell division and elongation.
2. The mechanism of hormonee action is 3 main steps:
1. The hormone binds to the receptor (it does NOT enter the cell)
2. Signal transduction cascade
3. Cellular response (usually gene expression, but not always)

Notes

Hormones

• Chemical signals
• Coordinate growth and development between cells, tissues, and organs
• Function at very small concentrations
• Often made in different part of plant from where they function; roots produce ABA that signal stomata to close and conserve water

Auxin

• Derived from the Greek word for increase-helpful
• Growth towards light, or phototropism, is a result of differential cell elongation and auxin
• Unique in that it is polarly transported; all others diffuse away from where they are generated in all directions; auxin is controllable and goes in a direction
• Starts within cells (polarity if from apex to base
• Moves in a pronated form (IAAH) via passive diffusion or with the help of influx carrier proteins
• Deprotonated back to IAA- when within cell because of pH difference; transported back down to bottom of plant
• Can only leave where there are efflux carriers
• Efflux carriers: transmembrane proteins
• Can be moved through endocytosis, where part of the plasma membrane is pinched of, brought into the cell, and then fused to a new a new location (process will be revisited)
• Polar transportation and changing direction of flow is done often in response to environmental signals.
• Functions:
• Vascular tissue differentiation
• ex: wounded plant with severed vascular tissue
• Ground tissue on either side made of parenchyma cells that can dedifferentiate; under the influence of auxin, can redifferentiate into new vascular tissue
• Auxin sent to wound site and it is able to turn parenchyma into new vascular tissue
• Apical Dominance
• Inhibits lateral branches
• Made mostly at shoot apical maristem (very top of plant), closely connected to polarity
• Made in relatively large quantities and transported down the stem to the roots
• Gradient is created with more at the top and less at the bottom
• Lots of auxin inhibits lateral branching, with the least inhibition that farthest from apical meristem
• Does the exact opoosite in the roots; promotes lateral roots and inhibits lateral shoot and branches

Cytokinins

• Promotes cell divisions
• Antagonists of auxin
• Promotes lateral branch growth
• Inhibits lateral root growth
• Delay leaf senescense
• Whether or not lateral branches grow depends on both auxin and cytokinin levels
• Senescence: normal part of plant life; chlorophyll and other nutrients are transferred to other tissues and older plant leaves senesce.
• Plants can be "transgenetically" modified to express cytokinens and hold on to older leaves

Ethylene

• Exists as a gas
• Functions:
• Fruit ripening
• Leaf abscission
• As a gas, one plant produces ethylene and can influence plants around it, acting as a signal to neighboring plants
• "One bad apple ruins the bunch" is because of ethylene
• Different fruits ripena at different times (one the vine versus after picking); bananas with unripened fruits placed into a paper bag will make the ripen faster
• Leaf abscission: leaf drops off plant

Abscisic Acid (ABA)

• Functions:
• Promotes dormancy
• Stomata closing
• Seeds have mutation that reduces sensitivity to ABA, so they don't stay dormany
• Without ABA, corn kernels will germinate while still on the cob

Gibberellic Acid (GA)

• Functions:
• Stimulate cell division and elongation
• Break dormancy
• Antagonist to ABA
• Dwarf corn has a mutation in GA biosysthesis pathway; when added exogenously, the corn grows tall

Brassinosteriods

• Functions:
• Cell division and elongation
• Vascular differentiation
• Relatively new discovery
• When added to mesophylls, cells started building secondary cel walls, indicating the presence of xylem and phloem
• Similar to auxin in terms of vascular differentiation

Mechanism for Hormone Action

1. Hormones bind to receptors (they do not directly act on the cell)
2. Signal transduction cascade
3. Cellular response

• Changes in gene expression is not the only cell response; there can be a variety of responses
• Side note: calcium (Ca+) is a common secondary messenger for hormones