Hypersensitive

the state of being extremely or excessively sensitive. It often refers to an extreme reaction by a plant to an invading pathogen in which the plant tissue around infected sites dies in order to prevent further spread of the infection.
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Hypha (pl. Hyphae)

a single tubular thread-like filament of a fungal mycelium. The hypha is the basic structural unit of a fungus.

I

Inoculate

to introduce a microorganism or virus into an environment (i.e. an organism or culture medium) suitable for its growth; to insert a pathogen into healthy tissue.

Inoculum (pl. Inocula)

a pathogen or its parts which can cause infection when transferred to a favourable location; the population of microorganisms introduced in an inoculation.

L

Latent Infection

where the host is infected with a pathogen but does not show any symptoms.

Lignin

a complex polymer deposited in some cell walls of vascular plants. Lignin is one of the main constituents of secondary walls and wood. It gives compressive strength and rigidity to the cell wall as well as making it impermeable to water.

Lignituber

a structure formed by the deposition of lignin around the tip of a fungal hypha penetrating a host plant cell. The formation of a lignituber is an active defense mechanism and its function is to inhibit the growth of the hypha into other cells.
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Lumen

central cavity of a cell, vessel or other structure.

M

Mesophyll

the photosynthetic tissue of a leaf, located between the upper and lower epidermis. Mesophyll is commonly differentiated into palisade parenchyma and spongy parenchyma.

Messenger RNA

a form of RNA that carries information to direct the synthesis of protein.

Mosaic

patchy variation of normal green colour in leaves, usually light and dark green mosaic, symptomatic of many viral diseases.
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Mycelium (pl. Mycelia)

a mass of hyphae that forms the body (thallus) of a fungus.

Multigene Family

a set of genes that, due to their high degree of sequence similarity, are believed to have evolved from a single ancestral gene.

N

Necrosis

the death of cells, often accompanied by black or brown darkening of the tissue.

Necrotroph

an organism (parasite) that causes the death of host tissues as it grows through them, obtaining its energy from the dead cells. Compare with biotroph.

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O

Obligate Parasite

an organism that is only capable of living as a parasite in association with its host plant. The term is synonymous with biotroph.

P

Papilla

a hump or swelling. In plant pathology, papilla refers to the accumulation of material between the host plant's cell wall and cell membrane at the point of fungal penetration. A papilla is usually composed of silicon, lignin and proteins.

Parasite

an organism or virus living in or on another organism (host) from which it obtains its nutrient supply. A parasite is not necessarily a pathogen.

Parenchyma

the soft tissue comprised of living, thin-walled cells of variable size and form. Parenchyma cells are the most abundant cell type in plants.
In leaves, parenchyma is differentiated into two forms:

- Palisade parenchyma: the tissue found beneath the upper epidermis of leaves, composed of elongate, tubular cells aligned perpendicular to the leaf surface. Palisade parenchyma cells generally contain an abundance of chloroplasts.

- Spongy parenchyma: the tissue typically found between the palisade parenchyma and the lower epidermis of leaves. Spongy parenchyma consists of loosely arranged, irregularly shaped cells containing chloroplasts and is interspersed with intercellular spaces.

Pathogen

a disease causing organism or agent.

Pathogenesis

the sequence of processes in disease development that describes a pathogen's association with its host. The sequence begins with initial contact between the pathogen and host and ends when the pathogen is no longer associated with that host (i.e. when the host/pathogen dies or the pathogen moves to another host).

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