Plant Resistance to Insect Grazing : Proteinase Inhibitors (PI)

Most effective against proteinases which digest at serine a.a (trypsin).

Rapidly induced by wounding: mechanical / grazing/ nematode infection of roots…….

...and subsequently systemically expressed.

Effective against INSECT LARVAE / NEMATODES where serine proteinase's which are abundant in their digestive tracts.

Thus, previous wounding confers protection against further grazing

Proof: Trangenic tobacco plant leaves constitutively expressing PI's retarded larval development

Has an obvious biotechnological potential:

PI are induced during HR but are NOT SA inducible

-so are activated by a wound-specific systemic signal.

Systemic signaling following wounding.

The “successful” candidate will meet the following criteria.

(1) Exogenous treatment confer protection and induce proteinase inhibitors

(2) levels rise around lesion

(3) systemic dispersal

(4) Correlation of signal appearance with protection.

THE CANDIDATES:

(A) Polygalacturoic acids (PGAs): .......1,2,3 not 4,5

Radiolabelled PGA's did not move throughout the plant

........... therefore a purely local role.

(B) Abscisic Acid : ABA.......1,2,3 not 4,5

Role in the control of water relations:

e.g. ABA level rise during drought conditions.- > ABA closes stomata

In wound-responses:

(1) exogenous application of ABA can induce PI

(2) There is a six-fold rise in ABA concentration upon

wounding in tomato and potato leaves.

(3) In droopy tomato mutants, deficient in ABA

synthesis, PI genes are not induced.

No evidence for a role in systemic movement and other evidence suggests that the role of JA is purely local.

(C) Systemin: 1,2,3,4 and 5

· -18aa peptide isolated from wounded tomato leaves

a potent PI inducer.

· Only peptide shown to possess a hormone-like regulatory

function in plants.

· highly mobile : ¹⁴C-systemin applied to the wound-site was systemically mobilised via the phloem.

Systemin is derived from a larger molecule PROSYSTEMIN- 200aa.

Three other protein sequences are duplicated.

In animals: peptide hormones are derived from such a large precursor then processed to yield more than one biologically active peptide.

Transgenic evidence for systemin.

- antisense prosystemin gene fused to CaMV 35S

promoter, exhibited reduced (10-20%) systemic PI

induction.

- sense transgenes continuously synthesise PI proteins.

(D) Jasmonic acid.

Membrane derived signals in wounding

Membrane damage releases phospholipids mainly

LINOLENIC ACID

Octadecanoid pathway as LA is an C18:3 lipid

All downstream enzymes are constitutive : pathways are regulated at the levels of supply of phospholipids.

Converted to 13-hydroperoxylinolenic acid, by LIPOXYGENASE (LOX) – an enzyme which introduces a oxygen atom.

LOX activity increases following infection / wounding/ insect damage.

13-hydroperoxylinolenic acid serves as a branch point for the generation of at least three-signal molecules, via further oxidation

1) Traumatic acid -

First isolated directly from wounded leaves and shown to enhance cell-proliferation at the wound-site.

2) Hexanal and hexanols – also known as “green leaf

volatiles”

Saturated and unsaturated six carbon alcohols, aldehydes and esters.

Constitutively produced – may be stored in specialized glands or in trichomes.

Toxic products of fatty acid oxidation. produced in large quantities.

Active against fungi, bacteria, insects and possibly the host itself. May represent part of a cell-death mechanism

3) Jasmonic acid

Emerging as the major intracellular wounding signal.

Exogenous application of JA induces,

(1) proteinase inhibitors

(2) Phenylpropanoid enzyme genes

(3) extensins, glycine-rich proteins

JA has been recently reported as accumulating during HR.

In Nicotinia attenuta – JA increases the abundance of nicotine. This is naturally synthesized in the roots and mobilized systemically but is also synthesized in leaves following wounding.

This results in a 90% reduction in leaf loss with MeJA treatment to grasshopper (Trimerotrophs pallidipenni)

but at what cost?

· 6% of all nitrogen end—up in nicotine.

· nicotine is also effective against insect carnivores and parasitic wasps – so lose an trophic level in defence.

· Seed product was reduced by 26%

However, when considering the loss resulting from insect damage their was a net overall increase in plants able to produce viable seed.

- so JA defences are costly but worth it!

The role of Ethylene

:

(1) developmental - fruit ripening

(2) In defence. Application of ethylene induces

- PR protein genes.

- lignification, by activating phenylpropanoid enyzmes and

extensins.

Ethylene produced quickly after infection or wounding.

Proteinase Inhibitors and ethylene

Gassing with ethylene had no effect on PI induction.

BUT applying systemin and JA to suspension cultures cells induced both ethylene and proteinase inhibitors. ----????

Also : PI induction was blocked with inhibitors of ethylene synthesis AND reduced JA synthesis

how can you explain this?

Interplant acquired resistance

Signaling via gaseous signals

(A) Ethylene : exists exclusively as a gas...

(B) Methyl-jasmonate : volatile

Able to induce systemic PI expression.

Sagebrush produces copious amounts of jasmonic acid...and can induce PI genes in adjacent plants.

(C) Methyl-salicylate : volatile

detected during SAR

induces PR gene expression

Can induce SAR.

BUT An ironic situation –

It may be that you cannot have systemic resistance to pathogens cannot co-exist with systemic expression of PI.