Peptide Hormones

BY

SONU KUMARI

2011A39M

Plant peptide hormones

Within the last decade many families of polypeptide hormones with signaling roles in defense, growth and development, mitogenicity, and reproduction have been identified in plants.

Peptide signaling plays a significant role in various aspects of plant growth and development.

These regulatory signals include families such as systemin family of defense hormones found in the Solanaceae family, SCR, the male determinant of self-incompatibility in Brassica, CLV3, a regulator of shoot apical meristem development, PSK, a small polypeptide involved in mitogenesis of plant cells grown in culture, and RALF, a ubiquitous polypeptide in plants affecting growth and development.

Specific receptors for various peptides have been identified as being membrane-localized receptor kinases, the largest family of receptor-like molecules in plant

Signaling peptides include members of the following protein families.

1. Systemin 2. CLV3/ESR-related ('CLE') peptide family3. ENOD404. Phytosulfokine(PSK) 5. POLARIS(PLS) 6. Rapid Alkalinization Factor (RALF)7. SCR/SP118. ROTUNDIFOLIA4/DEVIL1 (ROT4/DVL1) 9. Inflorescence deficient in abscission (IDA)

Systemin It is a small polypeptide functioning as a long-distance signal

to activate chemical defenses against herbivores. Systemin activates processes which help tomato deter insect

herbivores, such as this hornworm

It was the first plant hormone proven to be a peptide. Systemin induces the production of protein defense compound

called protease inhibitors. Systemin was first identified in tomato leaves.

It was found to be an 18-amino acid peptide processed from the C-terminus of a 200-amino acid precursor, which is called prosystemin.

Systemin is biologically active at subnanomolar concentrations, specifically processed from near the C-terminus of prosystemin, mobile in the plant vascular tissue,

It activates rapidly an intracellular signaling cascade leading to defense gene activation.

However, in contrast to animal polypeptide hormones, the systemin precursor is not synthesized through the secretory pathway, but in the cytoplasm

Model for the activation of defense genes in tomato in response to wounding and insect attack. After wounding, systemin is released from its precursor prosystemin by proteolytic processing. Systemin subsequently binds a membrane-bound receptor to initiate an intracellular signaling cascade, including the activities of a MAP kinase, a phospholipase, a calcium dependent protein kinase, an extracellular alkalinization, and the release of linolenic acid from membranes. Linolenic acid is converted to jasmonic acid, a messenger for early defense gene activation. Catalytic activity of polygalaturonase, an early gene, leads to generation of hydrogen peroxide acting as a second messenger for late gene activation. R, receptor; MAPK, MAP kinase; Ca2+PK; calcium dependent protein kinase; PLA2, phospholipase A2; LA, linolenic acid; JA, jasmonic acid; pm, plasma membrane.

CLV3/ESR-related ('CLE') peptide family CLV3 encodes a small secreted peptide that functions as a

short range ligand to the membrane-bound CLV1 receptor like kinase that together with CLV2 (a receptor-like protein) function to maintain stem cell homeostasis in Arabidopsis shoot apical meristems.

Although the maize embryo-surrounding region protein (ESR) and CLV3 are very different, they are both members of the CLE peptide family given that they share a short conserved 14-amino acid sequence at the carboxy terminal region.

To date, more than 150 CLE signaling peptides are identified. This proteolytically processed bioactive region is important for both promoting and inhibiting cellular differentiation in both apical and cambial meristems.

ENOD40 It is an early nodulin gene, hence ENOD, that

putatively encodes two small peptides, one of 12 and the other of 18 amino acid residues.

Controversy exists on whether the mRNA or peptides themselves are responsible for bioactivity.

Both peptides have been shown "in vivo" to bind to the 93 kDa subunit of sucrose synthase, an essential component in sucrose metabolism.

Sucrose degradation is a key step in nitrogen fixation, and is a pre-requisite for normal nodule development.

ENOD40 affects phytohormone cross-talkDuring the interaction between legumes and rhizobia, the nodule is formed on the roots of leguminous plants to host the bacteria.Upon infection with rhizobia, differentiated cortical cells are triggered to dedifferentiate, divide, and give rise to the nodule primordium. After the bacteria have reached and entered the primordium cells, the primordium develops into a nodule and the bacteria differentiate into their endosymbiotic form, the bacteroids. The developmental process is coordinated by an intricate network of signaling pathways. In addition to signaling molecules like Nod-factors released by the bacteria and plant factors like the hormones ethylene, auxin and cytokinin, a specific set of plant genes is induced during the early stages of nodulation. These are known as the early nodulin (ENOD) genes, and some are thought to have a regulatory role during nodule organogenesis.

One of the earliest activated ENOD genes is ENOD40. The spatio-temporal ENOD40 expression pattern is closely associated with the nodule developmental program. Strikingly, ENOD40 is also expressed in tissues not related to symbiosis indicating that the role of ENOD40 is probably not restricted to nodulation. ENOD40 genes are present in non-legumes, suggesting that ENOD40 has a general role in plant development. Almost 40 different ENOD40 homologs have been identified in species across the plant kingdom. ENOD40 is a common gene and could be involved in regulation of plant growth and development. ENOD40 could act on the hormone status of a cell, thus participating in regulation of the (cellular) responses to hormones.

Phytosulfokine (PSK):-

Phytosulfokines are plant hormones that belong to the growing class of plant peptide hormones.

Phytosulfokines are sulfated growth factors strongly promoting proliferation of plant cells in culture.

It was first identified as a "conditioning factor" in asparagus and carrot cell cultures.

The bioactive five amino acid peptide (PSK) is proteolytically processed from an ~80 amino acid precursor secreted peptide. PSK has been demonstrated to promote cellular proliferation and transdifferentiation.

It has been demonstrated that PSK binds to a membrane bound LRR receptor like kinase (PSKR).

POLARIS (PLS) The PLS peptide has a predicted length of 36 amino

acids however possesses no secretion signal, suggesting that it functions within the cytoplasm.

The PLS peptide itself has not yet been biochemically isolated, however loss-of-function mutants are hypersensitive to cytokinin with reduced responsiveness to auxin.

Developmentally it is involved in vascularization, longitudinal cell expansion and increased radial expansion.

Rapid Alkalinization Factor (RALF) It is 49 amino acid peptide that was identified whilst purifying

systemin from tobacco leaves, it causes rapid medium alkanalization and does not activate defence responses like systemin.

Tomato RALF precursor cDNA encodes a 115 amino acid polypeptide containing an amino-terminal signal sequence with the bioactive RALF peptide encoded at the carboxy terminus.

It is not known how mature RALF peptide is produced from its precursor, but a dibasic amino acid motif (typical of recognition sites of processing enzymes in yeast and animals) is located two residues upstream from the amino terminus of mature RALF.

RALF has been identified to bind to potential membrane bound receptors complex containing proteins 25 kDa and 120 kDa in size.

Arabidopsis has 34 genes encoding proteins related to rapid alkalinization factor (RALF), a peptide growth factor. One of those genes (AtRALF23) is significantly downregulated by brassinolide (BL)

Brassinosteroids (BRs) repress the expression of AtRALF23 (At3g16570).(a) Two-week-old Col-0 (WT) or bes1-D mutant seedlings were treated without or with 1 μm brassinolide (BL) for 3 h. The expression of AtRALF23 was determined by quantitative real-time PCR.(b) Histochemical staining and (c) quantitative 4-methylumbelliferyl β-d-glucuronide (MUG) assays of AtRALF23 promoter:GUS seedlings germinated and grown for 7 days in the presence or absence of 50 nm BL. Two biological replicates with two technical repeats each were used to determine the averages and standard deviations of the gene expression and quantitative MUG assays.

(a) Root growth in 5-day-old wild-type and AtRALF23-mycox

seedlings grown on agar in the light (50 μmol m−2 s−1).

(b) Hypocotyl elongation in 5-day-old seedlings grown in low light (15 μmol m−2 s−1).

(c) Hypocotyl elongation assay in which wild-type and AtRALF23-mycox seeds were

geminated and grown in the absence or presence of 50 nm BL for 5 days under normal lighting conditions (50 μmol m−2 s−1). Hypocotyl lengths were measured in 10–20 seedlings, and averages and standard deviations were calculated.

(d) Rosette development in 3-week-old plants grown in soil, and

(e) (e) growth of 5-week-old plants in soil.

AtRALF23-myc

overexpression Reduces

Arabidopsis growth.

SCR/SP11 These are small polymorphic peptides produced by the tapetal

cells of anthers and is involved in self-incompatibility of Brassica species.

This secreted polypeptide is between 78 and 80 amino acid residues in length.

Unlike other peptide hormones, no further post-translational processing occurs, except for the removal of the N-terminal signal peptide.

SCR/SP11 like other small peptide hormones binds to a membrane bound LRR receptor like kinase (SRK).

ROTUNDIFOLIA4/DEVIL1 (ROT4/DVL1)

The ROT4 and DVL1 are peptides of 53 and 51 amino acids respectively, which have a high degree of sequence homology. They are two members of 23 member peptide family. ROT4 and DVL1 are involved in regulating polar cell proliferation on the longitudinal axis of organs.

Inflorescence deficient in abscission (IDA)• A family of secreted peptides identified to be

involved in petal abscission. The peptides are 77 amino acids in length and possess an amino-terminal secretions signal.

• Like the CLE peptide family these proteins have a conserved carboxy-terminal domain that is bordered by potentially cleavable basic residues. These proteins are secreted from cells in the floral abscission zone.

• Studies suggest that the HAESA membrane-associated LRR-RLK is likely to be this peptide's receptor as it too is expressed in the zone of floral organ abscission.

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