''Alternaria brassicicola'' is a fungal

necrotrophic A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from t ...

plant pathogen Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomyc ...

that causes black spot disease on a wide range of hosts, particularly in the genus of ''

Brassica ''Brassica'' () is a genus of plants in the cabbage and mustard family (Brassicaceae). The members of the genus are informally known as cruciferous vegetables, cabbages, or mustard plants. Crops from this genus are sometimes called ''cole cr ...

'', including a number of economically important crops such as cabbage, Chinese cabbage, cauliflower, oilseeds, broccoli and canola. Although mainly known as a significant plant pathogen, it also contributes to various respiratory allergic conditions such as asthma and rhinoconjunctivitis. Despite the presence of mating genes, no sexual reproductive stage has been reported for this fungus. In terms of geography, it is most likely to be found in tropical and sub-tropical regions, but also in places with high rain and humidity such as

Poland Poland, officially the Republic of Poland, is a country in Central Europe. It is divided into 16 administrative provinces called voivodeships, covering an area of . Poland has a population of over 38 million and is the fifth-most populous ...

. It has also been found in

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the nort ...

and

Israel Israel (; he, יִשְׂרָאֵל, ; ar, إِسْرَائِيل, ), officially the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, label=none, translit=Medīnat Yīsrāʾēl; ), is a country in Western Asia. It is situated ...

. Its main mode of propagation is vegetative. The resulting

conidia A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to the ...

reside in the soil, air and water. These spores are extremely resilient and can

overwinter Overwintering is the process by which some organisms pass through or wait out the winter season, or pass through that period of the year when "winter" conditions (cold or sub-zero temperatures, ice, snow, limited food supplies) make normal activi ...

on crop debris and overwintering herbaceous plants.

Growth and morphology

The

of ''A. brassicicola'' are abundant in the outdoor environment from the months of May to late October in the northern hemisphere, peaking in June and again in October. The conidia are dark brown and smooth-walled, up to 60 x 14µm. The conidia are cylindrical to oblong in shape and are muriform and produced in chains of 8-10 spores. They are firmly attached to

conidiophores A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an Asexual reproduction, asexual, non-motility, motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are al ...

that are olive-brown,

septate In biology, a septum (Latin for ''something that encloses''; plural septa) is a wall, dividing a cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate. Examples Human anatomy * Interatr ...

, and growing to an upper range of 100-200 µm, although this overall length may vary. Conidia are borne in continuous, chain-like structure, but branching at the base has also been observed. Although conidia can be spread by rain, the most common means of spread is through the air. The fungus grows on

epidermal The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water relea ...

leaf wax of plants, particularly those in the

Brassicaceae Brassicaceae () or (the older) Cruciferae () is a medium-sized and economically important family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family. Most are herbaceous plants, while some are shrubs. The le ...

, and prefers an environment with high humidity and temperature range of . Macroscopically, the

mycelium Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrate (biology), substrates. A typical single ...

exhibits a range of colour: unpigmented when young, to olive-grey, grey-black at maturity. Colonies of ''A. brassicicola'' tend to be dark brown or black in colour.

Research history

Historically, much of the early research concerning the fungus was based on plant defense mechanisms. However, once its

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...

was sequenced, efforts shifted to identifying the

genes In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

involved in host-parasite interaction. One of the pioneers for genetic research into ''Alternaria brassicicola'' was the Lawrence group at Virginia Bioinformatics Institute and the Genome Center at Washington University. The most common media used for ''A. brassicicola'' growth are PDA (

potato dextrose agar Potato dextrose agar (BAM Media M127
from the U.S.

) and V8 juice-agar.

In vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...

and under optimal conditions, colonies grow rapidly and appear dark green or white-grey. Spontaneous sporulation occurs at 25ºC in darkness on PDA medium.

Growth cycle

Hours after inoculation: *2h: Conidia swells *3h: Germ tube formation observed at the apical or middle cells of conidia *8h: Vesicle of dissolved contents moves from conidial cell to germ tube *20h: Infection of the host cell *48h: Mycelial network develops on the surface *72h: Many chains of conidia can be seen

Pathogenesis and infection

There are three main sources of infection: nearby infected seeds, spores from plant debris in the topsoil and Brassica weeds, and spores moved by wind and air from farther away. Infected leaves can spread their spores up to a diameter of 1800m. There are also three major entry points to the host cell:

penetration, stomatal penetration and penetration through an insect. Contact with the host cell triggers the release of various cell wall degrading enzymes which allow the fungus to attach itself to the plant and begin degradation. The suggested mode of attack is through host-specific toxins, primarily

AB toxins The AB toxins are two-component protein complexes secreted by a number of pathogenic bacteria, though there is a pore-forming AB toxin found the eggs of a snail. They can be classified as Type III toxins because they interfere with internal cell ...

, that induce cell death by

apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...

. This results in what look like dents and lesions in the host plant. These are brown, concentric circles with a yellow tinge at the circumference, usually about 0.5-2.5cm in diameter.

Necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...

can generally be observed within 48 hours of infection. The spores can reside on the external seed coat of infected seeds, but the

can also penetrate under the seed coat, where it has the ability to remain viable for several years. Occasionally, it can even penetrate the

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...

tissue. The primary mode of transmission is through contaminated seed. Also, the infection is not limited to specific areas of the host plant; it can spread all over and even cause

damping off Damping off (or damping-off) is a horticultural disease or condition, caused by several different pathogens that kill or weaken seeds or seedlings before or after they germinate. It is most prevalent in wet and cool conditions. Symptoms There ar ...

of the seedlings at a relatively early stage. It also affects the host species at various developmental stages. As mentioned above, seedlings exhibit dark stem lesions followed by damping off. Velvety, black spots, resembling soot, can be observed on older plants. Pathogenesis is affected by factors such as: temperature, humidity, pH, reactive oxidation species, host defense molecules.

Genes

Out of the 10,688 predicted

from the ''A. brassicicola'' genome, 139 encode small secretion proteins that may be involved in pathogenesis, 76 encode

lipases Lipase ( ) is a family of enzymes that catalyzes the hydrolysis of fats. Some lipases display broad substrate scope including esters of cholesterol, phospholipids, and of lipid-soluble vitamins and sphingomyelinases; however, these are usually tr ...

and 249 encode glycosyl hydrolases that are important for

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...

digestion, potentially damaging host cells. In contrast, mutations in genes such as ''AbHog1'', ''AbNPS2'', and ''AbSlt2'' affect cell wall integrity and make the fungus more susceptible to host defenses. Currently, research is being done to identify the gene(s) responsible for encoding a

transcription factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...

, Bdtf1, important for the detoxification of host

metabolites In biochemistry, a metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...

Biochemistry

The most common toxin studied for ''A. brassicicola'' is the AB toxin, said to be connected to the

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...

pathogenicity In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

and host range for the fungus. It is most likely produced during conidial germination and probably linked to the ability of the fungus to infect and colonize Brassica leaves However, recent studies have explored new potential metabolites. For example, this fungus also produces

histone deacetylase inhibitors In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn ar ...

, but these do not have a significant impact on lesion size. Some studies show only a 10% reduction in virulence. Furthermore,

alternariol Alternariol is a toxic metabolite of ''Alternaria'' fungi. It is an important contaminant in cereals and fruits. Alternariol exhibits antifungal and phytotoxic activity. It is reported to inhibit cholinesterase enzymes.tenuazonic acid Tenuazonic acid is a mycotoxin produced by ''Alternaria'' species. It is a powerful eukaryotic protein synthesis inhibitor. It is a tetrameric acid that is ubiquitous in biological environments and prevents the release of newly synthesized prote ...

seem to affect mitochondrial-mediated apoptosis pathways and protein synthesis respectively (in the host cell), but again, not to a significant degree. Some

cytokines Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autocrin ...

have been linked with the discolouration associated with ''A. brassicicola'' infection. Cell wall degrading enzymes like

and cutinases are also linked to its pathogenicity, but more evidence of their efficacy is required. One important transcription factor is AbPf2. It regulates 6 of the 139 genes encoding small secretion proteins and may have a role in pathogenesis, specifically cellulose digestion.

Treatments

In order to protect their crops, many individuals pre-treat their seeds with

fungicides Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...

. The most widespread active ingredients in these fungicides are

Iprodione Iprodione is a hydantoin fungicide and nematicide. Application Iprodione is used on crops affected by Botrytis bunch rot, Monilinia fructicola, Brown rot, Sclerotinia and other fungal diseases in plants. It is currently applied in a variety of cr ...

and Strobilurins. In 1995, it was reported that Iprodione most likely acts by mutating two

histidine Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the de ...

residues in the target site of enzymes. Ultimately, it inhibits germ tube growth. However, the ubiquitous use of fungicides has resulted in the fungus growing increasingly resistant. Thus, different, non-chemical approaches have been explored. People have tried to develop resistant

crops through breeding. However, this has proved challenging due to the difficulty of transferring genes from wild-type to cultivated strains, resulting in genetic bottlenecks. It is further complicated by the probability that resistance seems to be a

polygenic A polygene is a member of a group of non-epistatic genes that interact additively to influence a phenotypic trait, thus contributing to multiple-gene inheritance (polygenic inheritance, multigenic inheritance, quantitative inheritance), a type of ...

trait. There are also some ''Brassica'' plants that have developed resistance to the pathogen naturally. High phenolase activity, high leaf sugar, and thicker wax layers reduce water-borne spore germination. It has been shown that the presence of

camalexin Camalexin (3-thiazol-2-yl-indole) is a simple indole alkaloid found in the plant ''Arabidopsis thaliana'' and other crucifers. The secondary metabolite functions as a phytoalexin to deter bacterial and fungal pathogens. Structure The base struct ...

in the host plant helps it to disrupt pathogen development. For example, an Arabidopsis mutant in the ''pad-3'' gene that does not produce camalexin is more susceptible to infection. Varying levels show differing levels of resistance. Another suggestion put forth is crop debris management. The aim is to minimize exposure of the crop plants to spores present in the soil by using crop rotation and weed control. Biological approaches have also been studied. One approach has been to use antagonistic fungi such as ''

Aureobasidium pullulans ''Aureobasidium pullulans'' is a ubiquitous and generalistic black, yeast-like fungus that can be found in different environments (e.g. soil, water, air and limestone). It is well known as a naturally occurring epiphyte or endophyte of a wide r ...

'' & ''

Epicoccum nigrum ''Epicoccum nigrum'' is a species of fungus in the phylum Ascomycota. A plant pathogen and endophyte, it is a widespread fungus which produces coloured pigments that can be used as Fungicide, antifungal agents against other pathogenic fungi. The ...

'' to subdue the effect of ''A. brassicicola''. The plants '' C. fenestratum'' and ''

Piper betle The betel (''Piper betle'') is a vine of the family Piperaceae, which includes pepper and kava. The betel plant is native to Southeast Asia. It is an evergreen, dioecious perennial, with glossy heart-shaped leaves and white catkins. Betel plant ...

'' also show potent fungicidal activity towards ''A. brassicicola'' both in vitro and under greenhouse conditions. These levels are comparable to Iprodione. The active compound,

berberine Berberine is a quaternary ammonium salt from the protoberberine group of benzylisoquinoline alkaloids found in such plants as ''Berberis vulgaris'' (barberry), ''Berberis aristata'' (tree turmeric), ''Mahonia aquifolium'' (Oregon grape), ''Hydra ...

, affects cell wall integrity and

ergosterol Ergosterol (ergosta-5,7,22-trien-3β-ol) is a sterol found in cell membranes of fungi and protozoa, serving many of the same functions that cholesterol serves in animal cells. Because many fungi and protozoa cannot survive without ergosterol, the ...

biosynthesis. Ethanol extracts from the dried roots of ''

Solanum nigrum ''Solanum nigrum'', the European black nightshade or simply black nightshade or blackberry nightshade, is a species of flowering plant in the genus ''Solanum'', native to Eurasia and introduced in the Americas, Australasia, and South Africa. Ripe ...

'' (black nightshade), traditionally used as herbal remedies in places ranging from the Far East to

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

and

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

, show promising anti-fungal activity as well. They seem to suppress conidial germination, possibly by interfering with the AB toxin.

Economic impact

As mentioned previously, ''Alternaria brassicicola'' causes severe black spot diseases in a number of ecologically important crops. Often, it occurs in conjunction with ''

Alternaria brassicae ''Alternaria brassicae'' is a plant pathogen able to infect most ''Brassica'' species including important crops such as broccoli, cabbage and oil seed rape. It causes damping off if infection occurs in younger plants and less severe leaf spot ...

''. However, it is the more dominant invasive species. These infections lead to a significant loss in viable seeds and produce. The resulting lesions greatly reduce available photosynthetic area, leading to wilt and plant death. Crops like infected cabbages do not last long during storage or transportation. In some cases, yield reductions can be as high as 20-50%. The lack of ability to use fungicides makes it challenging to sustain organic crops in a cost-effective way.

References

{{Taxonbar, from=Q5234097 brassicicola Fungal plant pathogens and diseases Eudicot diseases Fungi described in 1947