Plant responses to the Environment Plant Biology Basics Plant responses to the Environment Phototropism Photoperiodism Responses to Pathogen

Learning Objectives Plants have cellular receptors that allow them to detect and respond to external stimuli Plants produce hormones to coordinate physiological responses Phototropism and Auxins Photoperiodism and phytochromes Plant immune responses: MAMP receptors

Signal transduction pathways link signal reception to response REVIEW: Signal Transduction Pathways Plants have cellular receptors that detect changes in their environment. For a stimulus to elicit a response, certain cells must have an appropriate receptor. Stimulation of the receptor initiates a specific signal transduction pathway. CELL WALL CYTOPLASM 1 Reception 2 Transduction 3 Response Relay proteins and Activation of cellular responses second messengers Receptor Hormone or environmental stimulus Plasma membrane

Observation: Plants grow towards light Question: What part of the plant responds to light? Any response resulting in curvature of organs toward or away from a stimulus is called a tropism = a growth response. Phototropism= growth towards light RESULTS Light Tip removed Tip covered by opaque cap Tip covered by trans- parent cap Site of curvature covered by opaque shield Phototropic response only when tip is illuminated, Only the tip responds to light. CONCLUSION

How does the tip of the plant cause curving of the plant stem? RESULTS Phototropic Response is blocked by an impermeable barrier, but not by a permeable barrier. Conclusion: Hormones produced in the tip of the plant diffuse downward and coordinate the response in other parts of the organism. Any response resulting in curvature of organs toward or away from a stimulus is called a tropism = a growth response. Light Figure 39.5 What part of a grass coleoptile senses light, and how is the signal transmitted? Tip separated by gelatin (permeable) Tip separated by mica (impermeable)

RESULTS Excised tip placed on agar cube Auxin is produced by the tip and moves down the shaded side of the growing stem The stem bends towards the light because Auxin causes cell elongation on the shade side. Growth-promoting chemical diffuses into agar cube Agar cube with chemical stimulates growth Control (agar cube lacking chemical) has no effect Offset cubes cause curvature Control Figure 39.6 Does asymmetric distribution of a growth-promoting chemical cause a coleoptile to grow toward the light?

How does Auxin coordinate growth on the shade side of the plant in response to light? Auxins are plat hormones that promote elongation of stems Auxins are produced and released on the side of the plant in the shade. Auxin activates proton pumps in the plasma membrane to move H+ into the cell wall. The acidic pH in the cell wall activates enzymes that cut the carbohydrate cell wall. Turgor pressure on the cell wall causes the cell to elongate. The shade side elongates and the plant bends towards the light.

Enzymes cut carbohydrate cell wall Cell elongation in response to auxin: acid growth hypothesis 3 Enzymes cut carbohydrate cell wall 4 Cleaving allows turgor pressure to push cell wall outward. CELL WALL 2 Cell wall becomes more acidic. H2O Cell wall Plasma membrane Figure 39.8 Cell elongation in response to auxin: the acid growth hypothesis Auxin increases proton pump activity. 1 Nucleus Cytoplasm Plasma membrane Vacuole CYTOPLASM 5 Cell can elongate.

Photoperiodism: how do plants detect hours of light and dark? Plants have two types of light receptors Blue-light photoreceptors: control stomatal opening, and phototropism. Phytochromes: control flowering and seed germination.

Structure of a phytochrome Two identical subunits Chromophore Photoreceptor activity Figure 39.18 Structure of a phytochrome Kinase activity

Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses. Pr Pfr Red light Responses: seed germination, control of flowering, etc. Synthesis Far-red light Slow conversion in darkness (some plants) Enzymatic destruction Figure 39.19 Phytochrome: a molecular switching mechanism Phytochrome conversion marks sunrise and sunset, providing a biological clock with environmental cues for plants.

Photoperiodism and Responses to Seasons Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year. Photoperiodism is a physiological response to photoperiod. Some processes, including flowering in many species, require a certain photoperiod. Plants that flower when a light period is shorter than a critical length are called short-day plants. Plants that flower when a light period is longer than a certain number of hours are called long-day plants. Flowering in day-neutral plants is controlled by plant maturity, not photoperiod.

Photoperiodic control of flowering 24 hours Critical Night Length Flowering and other responses to photoperiod are actually controlled by night length, not day length. Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness. Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness. (a) Short-day (long-night) plant Light Flash of light Darkness Critical dark period (b) Long-day (short-night) plant Figure 39.21 Photoperiodic control of flowering Flash of light

Red light can interrupt the nighttime portion of the photoperiod. Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light. Pr Pfr Red light Responses: seed germination, control of flowering, etc. Synthesis Far-red light Slow conversion in darkness (some plants) Enzymatic destruction

Short-day (long-night) plant Long-day (short-night) plant Reversible effects of red and far-red light on photoperiodic response. 24 hours R RFR Figure 39.22 Reversible effects of red and far-red light on photoperiodic response RFRR RFRRFR Short-day (long-night) plant Long-day (short-night) plant Critical dark period

Plants use molecular recognition systems with systemic responses Plants have receptors for MAMPs similar to Toll like receptors in vertebrates. Plant MAMP receptors can recognize fungi, bacterial and viral specific proteins. Infection triggers chemical responses that destroy infected and adjacent cells, thus localizing the effects.