Presentation on theme: "A Seed Maturation and Antistress Signal"— Presentation transcript:
1 A Seed Maturation and Antistress Signal Abscisic Acid:A Seed Maturation and Antistress Signal
2 Contents Occurrence, Chemical Structure, and Measurement of ABA Biosynthesis, Metabolism, and Transport of ABADevelopmental and Physiological Effect of ABA
3 Abscisic Acid (ABA)Physiologists suspected that the phenomena of seed & bud dormancy were caused by inhibitory compounds.The experiments led to the identification of a group of growth-inhibiting compounds, including dormin
4 Upon discovery that dormin was chemically identical to a substance that promotes the abscission of cotton fruits, abscissin IIThe compound was renamed abscisic acid (ABA)
5 The chemical structures of the S (counterclockwise array) and R (clockwise array) forms of cis-ABA, the (S)-2-trans form of ABA, and lunularic acid. The numbers in the diagram of (S)-cis-ABA identify the carbon atoms.
6 Occurrence, Chemical Structure, and Measurement of ABA ABA is a ubiquitous plant hormone in vascular plants.
7 It has been detected in mosses but appears to be absent in liverworts.
8 ABA has been detected in living tissues from the root cap to the apical bud. It is synthesized in almost all cells that contain chloroplasts or amyloplasts.
10 The chemical structure of ABA determines its physiological activity ABA is a 15-C compound that resembles the terminal portion of some carotenoid molecules.Nearly all the naturally occurring ABA is in the cis form
26 ABA concentrations in tissues are highly variable ABA biosynthesis and concentrations can fluctuate dramatically in specific tissues during development or in response to changing environmental conditions. of ABA and exhibit vivipary.
42 That ABA is redistributed in the leaf in this way without any increase in the total ABA level. Therefore, the increas in xylem sap pH may function as a root signal that promotes early closure of the stomata.
43 Developmental and Physiological Effects of ABA ABA plays primary regulatory roles in the initiation and maintenance of seed and bud dormancy and in plant’s response to stress.
44 ABA regulates seed maturation Seed can be divided into three phases of approximately equal duration:
45 1. During the first phase, which is characterized by cell division and tissue differentiation
46 2. During the second phase, cell divisions cease and storage compounds accumulate.
47 3. During the final phase, the embryo becomes tolerant to dessication, and the seed dehydrates, losing up to 90% of it water.
48 As a consequence of dehydration, metabolism comes to a halt and enters a quiescent (resting) state.
49 The latter two phases result in the production of viable seeds with adequate resources to support germination and the capacity to wait weeks to years before resuming growth.
50 ABA inhibits precocious germination and vivipary ABA added to the culture medium inhibits precocious germination.
51 Further evidence for the role of ABA in preventing precocious germination has been provided by genetic studies of vivipary.
52 In maize, several viviparous mutants have been selected in which the embryos germinate directly on the cob while still attached plant.
53 Several of these mutants are ABA-deficient (vp2, vp5, vp7, vp9, vp14); one is ABA-insensitive (vp1)
54 ABA promotes seed storage reserve accumulation and desiccation tolerance During mid – to late embryogenesis, when seed ABA levels are highest, seeds accumulate storage compounds that will support seedling growth at germination.
55 As maturing seeds begin to lose water, specific mRNA encoding so-called late-embrogenesis-abundant (LEA) proteins thought to be involved in desiccation tolerance accumulate in embryos.
56 ABA not only regulates the accumulation of storage proteins and desiccation protectants during embryogenesisIt can also maintain the mature embryo in a dormant state until environment are optimum for growth.
57 The seed coat or the embryo can cause dormancy During seed maturation, the embryo enters a quiescent phase in response to desiccation.Seed germination can be defined as the resumption of growth of the embryo of the mature seed.
58 In many cases a viable seed will not germinate even if all the necessary environmental conditions for growth are satisfied.This phenomenon is termed seed dormancy.
59 COAT-IMPOSED DORMANCY There are five basic mechanisms of coat-imposed dormancy.Prevention of water uptake.Mechanical conatraint.Interference with gas exchange.Retention of inhibitors.Inhibitor production.
60 EMBRYO DORMANCYA dormancy that is intrinsic to the embryo and is not due to any influence of the seed coat or other surrounding tissues.In some cases, embryo dormancy can be relieved by amputation of the cotyledons
62 Embryo dormancy is thought to be due to the presence of inhibitors, especially ABA, as well as the absence of growth promoters, such as GA.
63 PRIMARY VERSUS SECONDARY SEED DORMANCY Different types of seed dormancy also can be distinguished on the basis of the timing of dormancy onset rather than the cause of dormancy.
64 Seeds that are released from the plant in a dormant state are said to exhibit primary dormancy. Seeds that are released from the plant in a nondormant state, but that become dormant if the conditions for germination are unfavorable, exhibit secondary dormancy.
76 ABA exerts this inhibitory effect via at least two mechanisms one direct and indirect: VP1, a protein originally identified as an activator of ABA-induced gene expression, acts as a transcriptional repressor of some GA-regulated genes.ABA repression the GA-induced expression of GAMYB, a transcription factor that mediates the GA induction of α-amylase expression.
77 ABA closes stomata in response to water stress Elucidation of the roles of ABA in freezing, salt, and water stress led to the characterization of ABA as a stress hormone.
78 Biosynthesis of ABA is very effective in causing stomatal closure.
80 ABA promotes root growth and inhibits shoot growth at low water potentials ABA has different effects on the growth of roots and shoots, and the effects are strongly dependent on the water status on the plant
85 ABA promotes leaf senescence independently of ethylene ABA is clearly involved in leaf senescence, and through its promotion of senescence it might indirectly increase ethylene formation and stimulate abscission.
87 Leaf segments senesce faster in darkness than in light, and turn yellow as a result of chlorophyll breakdown.
88 ABA greatly accelerates the senescence of both leaf segments and attached leaves.
89 ABA accumulates in dormant buds ABA was originally suggested as the dormancy-inducing hormone because it accumulates in dormant buds and decrease after tissue exposed to low temperatures.
90 Later studies showed that the ABA content of buds does not always correlate with degree of dormancy.
91 Although much progress has been achieved in elucidating the role of ABA in seed dormancy by the use of ABA-deficient mutants, progress on the role of ABA in bud dormancy has lagged because of the lack of a convenient genetic system.
92 Analyses of traits such as dormancy are complicated by the fact that they are often controlled by the combined action of several genes, resulting in a gradation of phenotypes refereed to as quantitative traits.
94 ABA regulates ion channels and the PM-ATPase in guard cell Stomatal close by the cell itself turgor pressureTriggered by two factors* ABA-induced plasma membrane depolarization* Increase [Ca2+] in cytosolAnathor factor contribute the depolrization is thePlasma membrane H+-ATPase (PM-ATPase)
95 23.7 ABA inhibition of blue light–stimulated proton pumping by guard cell protoplasts pp4e-fig jpg
96 K+ channel is keep opening where plasma membrane is hyperpolarizedThus, [Ca2+] and pH affect guard cell plasmamembrane in two ways(1) Inhibiting inward of K+ channel and protonpump on plasma membrane(2) Activation the K+ efflux channel
97 ABA may be percesived by both cell surface intracellular receptor Efforts to identify ABA receptors have employed bothbiochemical and genetic approach
98 Three experiments support an intracellular location for the ABA receptorABA supplied directly and continuously tothe cytosol via a “patch pipette” inhibitedboth inward K+ channels and S-typeanion channels, which are required forstomatal opening (Schwartz et al. 1994;schroeder et al. 2001)
99 Extracellar application of ABA was nearly twice as effective at inhibiting stomatal opening at pH6.15, when it is fully protonated and readily taken up by guard cells, as it was at pH8, when it is largely dissociated to the anionic form that does not readily cross membranes(Anderson et al. 1994)
100 Microinjection of an inactive, “caged” form of ABA into guard cells of Commelina resulted in stomatal closureafter the stomata were treated briefly with UVirradiation to activate the hormone --- that is, release itfrom its molecular cage. (Allen et al. 1994) Contralguard cells injected with a nonphotolyzable form of thecaged ABA did not close after UV irrdiation.
101 23.8 Stomatal closure induced by UV photolysis of caged ABA in the guard cell cytoplasm (Part 1) pp4e-fig jpg
102 23.8 Stomatal closure induced by UV photolysis of caged ABA in the guard cell cytoplasm (Part 2) pp4e-fig jpg
103 Determine the ABA-bind proteins with ABA itself oranti-idiotypic antibodies* Flowering Time Control Protein A (FCA) / ABA , Flowing Locus Y (FY) mechanism
104 23.9 A model of ABA interaction with one of its receptors, FCA, in Arabidopsis pp4e-fig jpg
105 ABA signaling involves both calcium-dependent and calcium-independent pathway ABA are transient membrane depolarization caused bythe net influx of positive charge and transient increasesin the cytosolic calcium concentration.ABA stimulates elevation in the concentration ofcytosolic Ca2+ by inducing both membrane channel andinternal compartments, such as vacuole.
106 23.10 Simultaneous measurements in a guard cell of broad bean (Vicia faba) pp4e-fig jpg
107 23.11 Time course of the ABA-induced increase in guard cell cytosolic Ca2+ concentration pp4e-fig jpg
108 23.12 ABA-induced calcium oscillations in Arabidopsis guard cells expressing yellow cameleon Measured by the use of microinjected calcium-sensitive ratiometric fluorescent dyespp4e-fig jpg
109 In addition to increasing the cytosolic calcium concentration, ABA causedan alkalinization of the cytosol fromabout pH7.67 to pH7.94. The increasein cytosolic pH has been shown toincrease the activity of the K+ effluxchannels
110 ABA seems to be able to act via one or more calcium-independentpathways. A raise in cytosolic pHcan lead to the activation of outwardK+ channels, and one effectof theabi1 mutation is to render these K+channels insensitive to pH.
111 23.13 Sphingosine kinase catalyzes the phosphorylation of sphingosine pp4e-fig jpg
112 23.14 Simplified model for ABA signaling in stomatal guard cells pp4e-fig jpg
113 ABA signaling involves ATP concentration Protein kinase inhibitorsalso block ABA-inducedstomatal closing.
114 Other regulators also influence the ABA response Phosphatase, GeneExpression, Secondarymessenger, Ethylene(so as other plant hormone),Stress, RNA regulator (suchas miRNA) and so on.