Ch. 39 – Plant Responses; essential knowledge 2.E.2a and 2.E.2b (1-2) Big Idea 2

Explain what you think happened, and give evidence to support your statement.

Growth in Animals Animals grow throughout the whole organism many regions & tissues at different rates

Growth in Plants Specific regions of growth: meristems stem cells: perpetually embryonic tissue regenerate new cells apical shoot meristem growth in length primary growth (at top and roots) apical root meristem primary growth lateral meristem growth in girth secondary growth

Apical meristems shoot root

Root structure & growth protecting the meristem

Growth in woody plants Woody plants grow in height from tip Primary xylem Growth in woody plants Woody plants grow in height from tip primary growth apical meristem Woody plants grow in diameter from sides secondary growth lateral meristems vascular cambium makes 2° phloem & 2° xylem cork cambium makes bark Primary phloem Epidermis Lateral meristems Secondary xylem Primary phloem Primary xylem Secondary phloem Annual growth layers Bark

Secondary growth Secondary growth growth in diameter thickens & strengthens older part of tree cork cambium makes bark growing ring around tree vascular cambium makes xylem & phloem

Vascular cambium Phloem produced to the outside Why are early & late growth different? Vascular cambium Phloem produced to the outside Xylem produced to the inside bark phloem cork cambium xylem late vascular cambium early last year’s xylem

Woody stem How old is this tree? cork cambium vascular cambium late early 3 2 1 xylem phloem bark

Tree trunk anatomy tree girdling What does girdling do to a tree? Aaaargh! Murderer! Arborcide! Tree trunk anatomy tree girdling What does girdling do to a tree?

Where will the carving be in 50 years?

Explain what you think happened, and give evidence to support your statement.

Overview – plant hormones Plants don’t have things like a brain, nervous system, muscles – things that help them respond to their environment They use something similar to our endocrine system – they send out hormones Hormones are chemical signals that coordinate different parts of an organism These hormones diffuse throughout the plant

Plant Hormones cont. 5 types of plant hormones we’ll be discussing: Auxin Cytokinins Gibberellins Abscisic acid Ethylene *there’s lots of others

Auxin (IAA) Effects controls cell division & differentiation phototropism growth towards light asymmetrical distribution of auxin cells on darker side elongate faster than cells on brighter side (loosens the cellulose wherever it’s found) apical dominance (the growth of the top shoot impedes the growth of other lower shoots, so one shoot becomes the “leader”)

Cytokinins Location: roots (and actively growing tissues) Function: root growth and differentiation; cell division; affects apical dominance As a plant grows, it grows both up and out (lateral growth) Increases width to increase amount of light To do this, you need to make new cells Cytokinins are a hormone that causes cells to divide It helps plants respond to light (if there are no barriers, auxin can allow it to grow straight up – if there is a barrier in the way, then cytokinins kick in, cause lateral growth around the barrier, and then auxins allow more upward growth) Common in forest competition

Gibberellins Family of hormones Effects over 100 different gibberellins identified Effects stem elongation fruit growth seed germination plump grapes in grocery stores have been treated with gibberellin hormones while on the vine

Abscisic acid Location: leaves, stems, roots, green fruit Function: inhibits growth; closes stomata during stress; maintains dormancy Responsible for opening and closing stomata Opposite of gibberelins Is triggered when the amount of daylight drops off (looks at amount and quality of light) Causes leaves to fall off, stops fruit growth and formation, stops transpiration, etc Basically it causes everything to slow down and enter into dormancy (think of scissors – cuts everything off)

Abscisic acid (ABA) high concentrations of abscisic acid germination only after ABA is inactivated or leeched out survival value: seed will germinate only under optimal conditions light, temperature, moisture

Ethylene Hormone gas released by plant cells Effects fruit ripening One bad apple spoils the whole bunch… Ethylene Hormone gas released by plant cells Effects fruit ripening leaf drop like in Autumn apoptosis Have you ever noticed that fruit on a tree all ripen at about the same time? That’s because as some fruit ripens, that ethylene gas spreads to the other fruits and causes them to ripen as well This helps attract things like birds and humans that come and eat the fruit and spread the seeds

Fruit ripening Adaptation Mechanism hard, tart fruit protects developing seed from herbivores ripe, sweet, soft fruit attracts animals to disperse seed Mechanism triggers ripening process breakdown of cell wall softening conversion of starch to sugar sweetening positive feedback system ethylene triggers ripening ripening stimulates more ethylene production

Apoptosis in plants Many events in plants involve apoptosis What is the evolutionary advantage of loss of leaves in autumn? Many events in plants involve apoptosis response to hormones ethylene auxin death of annual plant after flowering senescence differentiation of xylem vessels loss of cytoplasm shedding of autumn leaves The loss of leaves each autumn is an adaptation that keeps deciduous trees from desiccating during winter when the roots cannot absorb water from the frozen ground. Before leaves abscise, many essential elements are salvaged from the dying leaves and are stored in stem parenchyma cells. These nutrients are recycled back to developing leaves the following spring. Fall color is a combination of new red pigments made during autumn and yellow and orange carotenoids that were already present in the leaf but are rendered visible by the breakdown of the dark green chlorophyll in autumn. Photo: Abscission of a maple leaf. Abscission is controlled by a change in the balance of ethylene and auxin. The abscission layer can be seen here as a vertical band at the base of the petiole. After the leaf falls, a protective layer of cork becomes the leaf scar that helps prevent pathogens from invading the plant (LM).

As leaf lettuce matures, a tall flowering shoot extends beyond the basal edible leaves. After the plant bolts like this, it no longer produces broad, tasty leaves. Sup[pose you want to prevent bolting so that you can harvest lettuce longer. You may want to prevent the plant from synthesizing which hormone?

Answer: Gibberellins

Why would fruit ripen if it was put in a bag with other ripening fruit?

Ethylene would speed the ripening process

A graduate student growing plant cells in a laboratory dish wants them to divide. Therefore, The student treats them with __________

Answer: cytokinins

Tropism Plant hormones can control many processes Any response resulting in movement toward or away from a stimulus is called a tropism, which is often caused by hormones We’ll be talking about different types of tropism

Tropism Phototropism – when a plant grows toward light (positive phototropism) Light cues many key events in plant growth and development Effects of light on plant morphology are called photomorphogenesis Negative photoropism – when parts grow away from the light (like roots – that uses gravitropism) Remember – uses auxin to do this, which is cool because it can move without any muscles

Tropism - Stimuli besides light Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms Response to gravity is known as gravitropism Roots show positive gravitropism (grow towards gravity) Stems show negative gravitropism (grow against gravity)

Tropism - Stimuli besides light cont. Thigmotropism is growth in response to touch It occurs in vines and other climbing plants The term thigmomorphogenesis refers to changes in form that result from mechanical disturbances Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls

Timing and control Cyclical responses to environmental stimuli are called circadian rhythms and are about 24 hours long Circadian rhythms can be entrained to exactly 24 hours by the day/night cycle Animals (like humans) have a circadian rhythm (jet lag is when your rhythm becomes upset)

Plant Responses to Light Plants can detect direction, intensity, & wavelenth of light Regulate seed germination, shade avoidance Etiolation, detiolation

Timing and Control Photoperiod-the relative lengths of night and day Photoperiodism is the response of a plant to the amount of daylight hours the plant is subjected to light (helps it detect the time of year – also used by animals to help with hibernation and migration).

Timing and control cont. 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 (like a poinsetta) Plants that flower when a light period is longer than a certain number of hours are called long-day plants In the 1940s, researchers discovered that flowering and other responses to photoperiod are actually controlled by night length, not day length

Timing and control cont. Photoperiodism – senses the amount of light throughout the day Uses a phytochrome, which is a pigment (like chlorophyll) found in the plant and only absorbs red light; sometimes it will change it’s shape to absorb a different shade of red, and by the amount of phytochromes that have changed their shape, they can tell what time of the day it is. (leads to de-etiolation, or greening) Phytochrome+circadian clock = plant can tell what time of year it is based on amount of light helps it determine the season. This can determine if it survives or not.

Timing and Control Cont. Plants use phototropism and photoperiodism (a type of circadian rhythm) Bacteria use something called quarum sensing (https://www.youtube.com/watch?v=TVfmUfr8VPA) *essential knowledge 2.E.2

Environmental Stresses Environmental stresses have a potentially adverse effect on survival, growth, and reproduction They can have a devastating impact on crop yields in agriculture

Flooding (O2 deprivation): Drought (H2O deficit): close stoma release abscisic acid to keep stoma closed Inhibit growth roll leaves  reduce SA & transpiration deeper roots Flooding (O2 deprivation): release ethylene  root cell death  air tubes formed to provide O2 to submerged roots

Excess Salt: Heat: Cold: cell membrane – impede salt uptake produce solutes to ↓ψ - retain H2O Heat: evap. cooling via transpiration heat shock proteins – prevent denaturation Cold: alter lipid composition of membrane (↑unsat. fatty acids, ↑fluidity) increase cytoplasmic solutes antifreeze proteins

Control root (aerated) Experimental root (nonaerated) LE 39-28 Vascular cylinder Air tubes Epidermis 100 µm 100 µm Control root (aerated) Experimental root (nonaerated)

Defense against pathogens Plants counter external threats with defense systems that deter herbivory (getting eaten) and prevent infection or combat pathogens Plants counter excessive herbivory with physical defenses such as thorns and chemical defenses such as distasteful or toxic compounds Some plants even “recruit” predatory animals that help defend against specific herbivores

Defense against pathogens cont. A plant’s first line of defense against infection is its “skin,” the epidermis or periderm If a pathogen penetrates the dermal tissue, the second line of defense is a chemical attack that kills the pathogen and prevents its spread (similar to humans) This second defense system is enhanced by the inherited ability to recognize certain pathogens

Pathogens cont. A virulent pathogen is one that a plant has little specific defense against An avirulent pathogen is one that may harm but not kill the host plant

Pathogens Cont. A hypersensitive response against an avirulent pathogen seals off the infection and kills both pathogen and host cells in the region of the infection (similar to what happens when we try to kill cancer cells with radiation)