Signal Tansduction Kel 4

PLANT PHYSIOLOGY“Signal Transduction”

By:Dias Rindang Windari (4401411039)

Kinaseh (4401411041)Lembayung Windi P (440141047)

Dewi Setiyana (4401411058)

Created by Kel. 4 Plant Physiology

Signal Transduction• Signal Transduction in plants has long fascinated researchers• Charles Darwin performed pioneering studies on phototropic

growth responses in canary grass and oat seedling tissues (Darwin 1881)

• Since these classis studies, researches have demonstrated that plant perceive many environmental and physiological signals in order to fine-tune their growth and development

Signal Receptor Signal transduction Respone


Signal Transduction in Plant and Animal Cell

Plants and animals have similar transduction components



ANIMAL FIBROBLAST GROWTH FACTOR

Created by Kel. 4 Plant PhysiologyCreated by Kel. 4 Plant Physiology

PLANT BRASSINOSTEROID PATHWAY

Receptor Kinases Can Initiate A Signal Transduction Cascade


Receptor kinase must modify the activity of other proteins in order to trigger an intracellular signal transduction cascade. A target protein can be phosphorylated at

various amino acid residues (serine, threonine, tyrosine or histidine).

The intracellular target proteins are often protein kinases themselves, and they may be activited or inactivated by phosphorylation. This mecanism for modulating kinase

activity is common in signal transduction pathway.

• In addition to kinase, enzymes that remove phosphate group from protein, protein

phosphatases, play important roles within signal transduction pathway. This binding occurs independently of the presence or

absence of ABA. • In the absence of ABA, PP2C blocks SnRK2 kinase activity by removing phosphate groups

from a region within the kinase domain termed the activation loop (FIGURE 14.3A).


• When ABA is present, its receptor binds directly to PP2C proteins (Park et al. 2009), which prevents PP2C from dephosphorylating the kinase domain of SnRK2. SnRK2 proteins are then free to phosphorylate target proteins, including specific transcription factors that activate ABA-responsive gene expression (FIGURE 14.3B) .

• ABA signal transduction is therefore based on reversing the balance between PP2C protein phosphatase and SnRK2 kinase activities.


• In summary, plants and animals employ reseptors in conjunction with kinase and phosphatase transduction components in a number of important signaling pathway.


Plant Signal transduction components have evolved from both prokaryotic and eukaryotic ancestors


• The function of the sensor protein is to receive the input signal and to pass the signal on to the response regulator , which brings about the cellular response, typically gene expression. Sensor proteins have two domai, an input domain, which receives the environmental signal, and a transmitter domain, which transmits the signal to the response regulator.


• The response regulator protein also has two domains, a receiver domain, which receives the signal from the transmitter domain of the sensor protein, and an output domain, such as a DNA-binding domain, which brings about the response.

• The signal is passed from transmitter domain to receiver domain via protein phosphorylation. Transmitter domains have the ability to phosphorylate themselves, using ATP, on a specific histidine residue near the amino terminus

• The phosphate is then transferred to a spesific aspartate residue causes the response regulator to undergo a conformational change that usually results in the activation of gene expression.

• The hormone cytokinin is perceived via a phosphorelay that is organized in a similar way to the bacterial two-component systems (see figure 14.4; Chapter 21).


Signals are perceived at many locations with plant cells


Plant signal transduction often involves inactivation of repressor proteins


Protein degradation is a common feature in plant signaling pathways


• these include pathways for important plant signals such as jasmonate and giberellins. JA is associated with defense responses against herbivores and necrotrophic pathogens, while GA regulates important growth processes such as seed germination, stem and root elongation, and determination of leaf size and shape.

• Bioactive gibberellins act in large part by mediating the rapid turnover of della repressor proteins. della proteins restrain plant growth, which GA over comes by promoting destruction of the della proteins. GA does this by binding its receptor GID1, triggering the ubiquitin-dependent degradation of della repressor proteins. della degradations results in the release and activation of transcription factors, thus triggering changes in gene expression. to date, five della genes have been identified in arabidopsis: repressor of gal-3

A. Auxin response


B. Jasmonate response


C. Gibberellin response


Several plant hormone receptors are part of SCF ubiquitination. Auxin, jasmonate (JA), and gibberellins (GA) signal by promoting interaction between components of the SCF ubiquitination machinery and repressor proteins operating in each hormon transduction pathway. Auxin (A) and JA (B) directly promote interaction between the SCFTIR1 and SCFCOI1 complexes and the AUX/IAA and JAZ repressors, respectively. in contrast, GA (C) additionally requires an adaptor protein, GID1, to form the complex between SCFSLY1 and DELLA proteins. Addition of multiple ubiquitins (polyubiquitin) marks these repressor proteins for degradation. This triggers the activation of ARF, MYC2, and PIF3/4 transcription factors, resulting in auxin-, JA-, and GA- induced changes in gene expression


Several plant hormone receptors encode components of the ubiquitination machinery

The auxin receptor is composed of two proteins: the SCF complex component TIR1 and the repressor protein AUX/IAA. Ubiquitin moieties are first activated by E1 ligase and added to target proteins by E2 ligase. TIR1 recruits AUX/IAA proteins to the SCFTIR1 complex in an auxin-dependent manner. once recruited by auxin, AUX/IAA proteins are ubiquitinated by the E3 ligase activity of the SCFTIR1 complex, which marks the protein for destruction by the 26s proteasome


Inactivation of repressor proteins results in a gene expression response

• In the case of auxin, AUX/IAA repressor proteins are targeted for degradation, thereby

activating auxin response factor (ARF)-dependent gene expression.


• Feedback regulation represent another key mechanism employed to attenuate a response. For example, the AUX/IAA genes, which encode the AUX/IAA auxin represor proteins, have auxin response element binding sites located in their promoter regions(Hagen and Guilfoyle 2002). Thus, AUX/IAA proteins can bind to the promoters of their own genes and repress their own expression.

• Hormone signaling pathways are often subject to several loops of negative feedback regulation. This is nicely illustrated by the GA pathway(figure 14.10). Bioactive GA (GA4 in this example) is synthesized through complex biosynthetic pathway that involves multiple enzyme-catalyzed reaction (hedden and phillips 2000). The last two enzymes in this pathway are encoded by members of the GA20ox and GA3ox gene families, whose expression is inhibited by GA.Thus GA acts to slow down its own synthesis.

plants have evolved mechanisms for switching off or attenuating signaling responses


The GA response is regulated by a series of feedback mechanisms involving components of both GA signal transduction and GA biosynthesis. GA20 ox and GA3ox genes encode enzymes that catalyze the last steps of the GA biosynthetic pathway, while GA2ox catalyzes the breakdown of the bioactive GA ,GA4. GID1 encodes the GA receptor that, following ligand binding, recruits DELLA repressor proteins to the SCF SLY1 complex for ubiquitination, triggering their degradation. in the absence of GA, the DELLA proteins positively regulate GID1, GA20ox,and GA3ox (plus signs) and negatively regulate GA2ox (minus signs). Conversely, both bioactive GA and the GID1 receptor enhance DELLA repressor degradation (plus signs), while GA2ox blocks DELLA repressor degradation (minus signs).


Cross- regulation allows signal tranduction pathways to be integrated


Within plant cells, Signal transduction pathways never function in isolation, but operate as part of complex web of signaling interactions. This realization helps us understand why plant hormones often exhibit agonistic (addtive or positive) or antagonistic (inhibitory or negative) interactions with other signals.

Examples include the antagonistis interaction between GA and ABA in the contol of seed germination. The interaction between signaling pathway has been termed cross-regulation and several categories have been proposed. (figure 14.11)

Primary cross-regulation involves distinct signaling pathway regulating a shares transduction

Secondary croos-regulation involves the output of one signaling pathway regulating the abundance or perception of a second signal

Tertiary cross-regulation involves the output of two distinct pathways exerting influences on one another


• The control of cell elongation in the Arabidopsis hypocotyl by light and GA represent an example of negative primary cross-regulation.

• In this case, the light and GA pathways share common down-stream signaling components- the closely related transcription factors PIF3 and PIF4- that act to stimulate cell elongation. The accumulation of PIF3/4 is regulated in opposite ways by the two signals : positively by GA and Negatively by light.


Signal Transduction in Space and Time• Plant signal transduction occurs over a wide range

of distances.• Example : Radial pattering in the Arabidopsis

primary root is regulated by the transcription regulator SHORT-ROOT .( FIGURE 14.13)

• The genes SHR is transcribed and translated in cells of the stele, at the center of the root( Figure 14.13 A and C)


The SHR protein then move via plasmadesmata to cells in the endodermis, where it activates the expression of cell fate

regulator such as SCARECROW (SCR)



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