Multicellular living organisms and unmodified parts thereof and – Plant – seedling – plant seed – or plant part – per se – Higher plant – seedling – plant seed – or plant part
Reexamination Certificate
2000-02-25
2003-08-19
Nelson, Amy J. (Department: 1638)
Multicellular living organisms and unmodified parts thereof and
Plant, seedling, plant seed, or plant part, per se
Higher plant, seedling, plant seed, or plant part
C800S301000, C800S317000, C800S279000, C435S320100, C435S419000, C536S023600
Reexamination Certificate
active
06608245
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to the field of molecular biology. In particular, the invention pertains to genes which confer resistance on plants to Verticillium species.
BACKGROUND OF THE INVENTION
Verticillium wilt is a common vascular disease that causes severe yield and quality losses in many crops. The disease is caused by fungi of the genus Verticillium, most commonly
Verticillium albo
-
atrum
Reingke & Berthier or
Verticillium dahliae
Kleb. The relationship between
V. albo-atrum
and
V. dahliae
has been the subject of debate (see, generally, Domsch et al., 1980). They have previously been considered representatives of a single variable species, but they have more recently been viewed as distinct species.
V. albo
-
atrum
was first identified as a causal agent of Verticillium wilt in potato, but is now known also to cause wilt in hop, tomato, alfalfa, strawberry, sainfoin, runner bean, broad bean, pea, clover and cucumber. Vascular infection of these hosts by
V. albo
-
atrum
leads to wilt, with or without obvious flaccidity, and, commonly, browning of the infected xylem stems.
V. dahlia
is far more common than
V. albo
-
atrum
, but causes disease symptoms that are less severe than those caused by
V. albo
-
atrum
infection. Further, in the above-mentioned hosts,
V. dahlia
is usually less virulent than
V. albo
-
atrum. V. dahlia
is known to infect canola, cotton, dahlia, mint species, vine, tomato, potato, eggplant, olive, pistachio, stone fruits, Brussels sprouts, groundnut, horse radish, tobacco, red pepper, strawberry, and other plant species. Symptoms of
V. dahlia
infection usually include flaccidity or chlorosis of leaves, followed by permanent wilting.
In a recent survey in North America, Verticillium wilt was ranked as the most important disease of both seed and commercial potato crops, and the second greatest constraint on tuber yield. Pathogen-mediated reductions in net photosynthesis, transpiration, and increased leaf temperature cause premature foliage senescence and yield loss. Disease symptoms in potato include wilting and leaf chlorosis and necrosis, while the tubers of infected plants develop necrosis in the vascular tissue that reduces tuber quality, in particular for the manufacture of french fries and chips. Abiotic factors such as moisture stress and high temperatures accelerate development of visual disease symptoms. Studies have demonstrated a synergistic interaction between root-lesion nematodes (
Pratylenchus penetrans
) and Verticillium wilt, further complicating disease control.
Management strategies for the control of Verticillium wilt include soil fumigation, crop rotation, and development of resistant cultivars. Of these strategies, only disease resistance is effective. While several recent potato cultivar releases have some resistance to the pathogen, the major potato varieties grown in North America are susceptible. The genetic mechanisms of resistance towards fungi of the taxon Verticillium spp. vary from polygenic in strawberry and alfalfa, to a dominant single gene in cotton, sunflower, and tomato. Conclusions from studies with tetraploid
Solanum tuberosum
subsp. tuberosum L. suggest that inheritance is polygenic and complex. Screening for resistance in the non-cultivated diploid tuber-bearing wild Solanum species has identified resistance but not immunity, and a recent study concluded that resistance is polygenic and genetically complex. Thus, incorporation of resistance into new potato cultivars by classical breeding techniques is difficult and inefficient. It would therefore be desirable to develop alternative approaches to obtaining plants that are resistant to Verticillium wilt.
Plants have natural defenses which prevent infection of the plant by viruses, bacteria, fungi, nematodes and insects. As plants do not have a circulatory system, each plant cell must have a preformed or inducible defensive capacity. Recently, disease resistance genes, which confer resistance to specific pathogens, have been identified in various plants. It is believed that the mechanism of resistance may differ depending on the mode of pathogen attack.
Necrotrophy, biotrophy and hemibiotrophy are the three principal modes of pathogen attack on plants (see, generally, Hammond-Kosack et al., 1997). Necrotrophs first kill host cells, then metabolize the cell contents. Necrotrophs often have a broad host range, and cause host cell death with toxins or enzymes targeted to certain substrates. Plant resistance to necrotrophs may therefore be achieved through loss or alteration of the target of the toxin in the plant, or through detoxification of the toxin produced by the plant. An example of a plant disease resistance gene of the latter type Hm1, isolated from maize, and which confers resistance to the leaf spot fungus
Cochliobolus carbonum
. Hm1 encodes a reductase enzyme which is thought to inactivate the
C. carbonum
toxin.
Flor (1949) developed the classic “gene-for-gene” model for plant pathogen resistance. Flor proposed that for incompatibility (i.e. resistance) to occur, complementary pairs of dominant genes are required, one in the host and the other in the pathogen. Loss or alteration of either the plant resistance (“R”) gene or the pathogen avirulence (“Avr”) gene results in compatibility (i.e. disease). It is thought that R genes encode proteins that can recognize Avr-gene-dependent ligands. The simplest possibility is that the Avr-gene-dependent ligand binds directly to the R gene product. Following binding, the R gene product is believed to activate downstream signaling cascades to induce defense responses, such as the hypersensitive response, which causes localized plant cell death at the point of pathogen attack, thereby depriving the pathogen of living host cells. Downstream signaling components may include kinase and phosphatase cascades, transcription factors, and reactive oxygen species (see, generally, Hammond-Kosack et al., 1997).
Hammond-Kosack et al. (1997) summarize the five classes of known R genes, classified according to predicted features of the R gene product.
The first class is composed of R genes that encode detoxifying enzymes. An example is Hm1 from maize, discussed above, which confers resistance to
Cochliobolus carbonum
(Johal et al., 1992).
R genes of the second class encode intracellular serine/threonine-specific protein kinases. An example is Pto, isolated from tomato, and which confers resistance to
Pseudomonas syringe
pv. tomato (Martin et al., 1993).
The third class of R genes is divided into two subclasses. The first encode intracellular proteins with an amino terminal leucine zipper domain, a nucleotide binding site (“NBS”) domain, and a leucine rich repeat (“LRR”) domain. Examples include RPS2 of Arabidopsis, which confers resistance to
Pseudomonas syringe
pv. tomato (Bent et al., 1994; Mindrinos et al., 1994), and I2, from tomato, which confers resistance to
Fusarium oxysporum
(Ori et al., 1997). R genes of the second subclass encode intracellular proteins with an amino terminal domain having homology with Drosophila Toll protein, and NBS and LRR domains. Examples include R gene N of the tobacco plant, which confers resistance to tobacco mosaic virus (Whitman et al., 1994), and L6 of flax, which confers resistance to
Melampsora lini
(Lawrence et al., 1995).
R genes of the fourth class encode proteins having an extracellular LRR, a single membrane spanning region, and a short cytoplasmic carboxyl terminus. An example is Cf9 of tomato, which confers resistance to
Cladosporium fulvum
(Jones et al., 1994).
The fifth class encompasses R genes encoding proteins having an extracellular LRR, a single membrane spanning region, and a cytoplasmic kinase domain. An example is R gene Xa21 of rice, which confers resistance to
Xanthomonas oryzae
pv. oryzae (Song et al., 1995).
Certain isolated R genes have been introduced into susceptible plants, resulting in transgenic, disease-resistant plants. For instance, U.S. Pat. No. 5,859,339 teaches transformation of susceptible rice plants wi
Hachey John
Kawchuk Lawrence M.
Kulcsar Frank
Lynch Dermot R.
Greenlee Winner and Sullivan PC
Her Majesty the Queen in right of Canada as represented by the
Kubelik Anne
Nelson Amy J.
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