Cross protection occurs when infection of a plant with a mild or attenuated virus strain protects the plant from later infection by a more severe strain of the same virus. This was first demonstrated in 1929 with tobacco mosaic virus. It has since been used successfully to control diseases caused by citrus tristeza virus and papaya ringspot virus. There are two main mechanisms of cross protection - coat protein-mediated resistance, which involves blocking virus uncoating or replication, and RNA-mediated resistance, where excess mild strain RNA hybridizes to block replication of the challenge virus. While cross protection has proven effective for some diseases, there are also limitations such as yield loss, incomplete protection, and genetic instability of the protector virus.

1. Cross protection
N. H. SHANKAR REDDY
Ph.D., Plant Pathology
Annamalai University

2. • Infection of a susceptible plant with a mild or attenuated strain of virus helps to prevent
against the later systemic infection of more severe strain of the same virus is called “cross
protection”.
(or)
• Cross-protection is a natural phenomenon whereby tolerance or resistance of a plant to one
virus strain is induced by systemic infection with a second.
• The phenomenon was first demonstrated by McKinney (1929), who observed that in
tobacco plants systemically infected with a “light green strain” of Tobacco mosaic virus
(TMV: Genus Tobamovirus)
• Later Salaman (1933) demonstrated that an avirulent strain of Potato virus X (PVX:
Genus Potexvirus) provided protection against superinfection with a virulent strain of PVX
in potato
• Plants may be purposely infected with a mild strain as a protective measure
against severe disease.

3. • Mild strains of tomato mosaic virus are particularly helpful in controlling infection
by severe strains in tomato plants
• Protection by CTV with mild strain is also helpful in protecting citrus crops
against severe strains of Citrus tristeza virus.
• The naturally occurring satellite in CMV strains (CM-S) has been used as a
biological control agent to protect tomato plants against disease induced by severe
strains of CMV.

4. • Webb et al. (1952) showed that cross protection against the phloem-limited virus,
Potato leaf roll virus (PLRV: Genus Polerovirus) could be achieved by infection
with the aphid vector.
• The first demonstrations of virus-disease control by mild strains were done with
Citrus tristeza virus (CTV: Genus Closterovirus) (Grant and Costa, 1951), and
Cacao swollen shoot disease (Posnette and Todd, 1955).

5. • Various terms were used to describe this phenomenon, including “acquired
immunity”, “antagonism”, “cross immunization”, “induced immunity”,
“interference”, “pre-immunity” “protection inoculation”.
• The term “cross-protection” (Matthews, 1949) seemed most appropriate, since it
described an aspect of the phenomenon concerned with disease control in crops
(Fulton, 1986; Lecoq, 1998)

6. Mechanism of cross protection
• Coat-protein (CP)-mediated resistance and
• RNA-mediated resistance.

7. i. Coat-protein (CP)-mediated resistance
• CP-mediated resistance has been shown to be involved in cross protection
between strains of TMV and Cucumber mosaic virus (CMV: Genus
Cucumovirus)
• CP-mediated resistance in transgenic plants depends on the expression level of the
transgene CP, and a higher level of transgene expression elicits better protection.
In general, CP-mediated resistance is broken by a high level of challenge virus
• The most commonly proposed model for CP-mediated resistance is based on
prevention of the uncoating of the challenge virus as it enters the plant cell, which
interferes with the translation and replication processes

8. ii. RNA-mediated resistance
• Protector virus produces excess progeny positive-sense RNA, which hybridizes to
the first minus-strand RNA of the challenge virus, thereby blocking further
replication and translation of the incoming virus

9. Current uses of cross-protection
• Protection of orchard crops. There are only two examples of cross protection of
major economic importance in orchards that are in use today: against CTV, PRSV,
CMV and ZYMV
• CTV In several regions of the world CTV was the most disastrous epidemic
disease in citrus orchards, and protection by graft inoculation (budding) with mild
naturally occurring strains provided good protection
• PRSV This virus, identified about 30 years ago, is the most destructive papaya
disease worldwide, and has become a limiting factor in production. Cross-
protection against PRSV has been achieved successfully and is widely used in
Taiwan, Thailand, Mexico, Florida and Hawaii, with several different mutated
mild viruses

10. Successful application of cross protection
• Citrus tristeza virus
• Tomato mosaic
OTHER DISEASES AS CANDIDATES FOR CONTROL BY CROSS PROTECTION
• Coconut swollen shot virus - Mild strains were less readily transmitted by insect
vectors than severe strains
• Papaya Ringspot
• Passion Fruit Woodiness - mild isolates of this virus by grafting from stem tips in
summer, protected from severe strains of the woodiness virus
• Avocado Sun Blotch
• Concave Gum and Psoriasis of Citrus - Mild strains of these viruses have been
reported to protect against budding inoculations with more severe strains
• Stone and Pome Fruit Virus Diseases

11. Limitation in cross protection
• Loss of yields as a result of the mild strain infection in certain physiological
instances.
• Incomplete protection and breakdown of protection
• A strain that is mild in one crop might be severe in another
• Difficulty in restricting the protector virus to the treated field, because of natural
vectors.
• Unavailability of mild strains of practical value.
• Synergism and ruinous interactions with other viruses
• Genetic instability of the protector virus because of mutation or recombination

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