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Introduction
'Penicillium. oxalicum' is species of fungi belongs to the genus penicillium, it has a world-wide distribution and is commonly found in soil, decomposing plant residues, oil seeds, cereals especially maize, yams, greenhouse cucumbers and other tropical and subtropical products (Currie & Thom, 1915). It grows fast in warm temperature (37℃), producing velvety-textured, heavy sporing colonies. The name P. oxalicum was proposed by James N. Currie and Charles Thom due to its ability to produce oxalic acid as a chief product of its secondary metabolism; besides that, P. oxalicum also forms toxic metabolites secalonic acid D and F, and roquefortine C. Previous studies have suggested that P. oxalicum plays an important role in biocontrol against tomato wilt and downy mildew disease.

History and taxonomy
Initially isolated from decaying straw-covered soil in 1979, ''Penicillium. oxalicum formerly classified as Penicillium decumbens'', is categorized as an anamorph species of the genus Penicillium, which link is one of the most common fungal genera occurring in omnivorous habitats. They are able to decompose organic matters, and play an important role in biological circulation of biomasses in nature, in particular, P. oxalicum secretes the enzyme -glucosidase essential for producing biofuel (Wang et al, 2017). In 1915, James N. Currie and Charles Thom proposed the name P. oxalicum due to the species’ ability to form a chief product of its secondary metabolism: the oxalic acid, and is considered to be one of the true oxalic acid ferments (Currie & Thom, 1915).

Growth and morphology
P.oxalicum is xerotolerant and therefore is adapted to a wide range of growth environment, however, when the osmotic potential is below -15.7 MPa, both growth and sporulation is limited. The optimum temperature for the growth of P. oxalicum is 25℃, at which its growth rate, germination and length of germ tubes are maximized (Pascual et al. 1997). Besides that, the optimum sporulation is between 15 to 30, when the temperature is lower than 10 and higher than 30, the growth rate is reduced and sporulation is inhibited. The optimum pH range for the growth of P. oxalicum lies between pH4-6, while the optimum pH range for the sporulation lies between pH7-8. In an alkaline environment (pH9), the growth of P. oxalicum is reduced (Pascual et al. 1997). P. oxalixum is characterized by its heavy-sporing, velvety-textured dark green colonies and large elliptical conidia that produce no exudates or odor. The morphology of penicilli produced abundantly is biverticillate and asymmetric, they are borne on top of the conidiophores 100 to 200 µm, aggregate densely and arise 3.5 to 4.5 µm from the substratum to create the velvety texture of the colony (Currie & Thom, 1915). The reverse side of the colony is generally observed as pale yellow, orange, pink, or uncolored in some strains. Conidia of the P. oxalixum are elliptical and smooth-walled, ranging from 4.5 to 6.5 µm by 3.0 to 4.0 µm, but mostly 5.0 to 5.5 µm by 3.0 to 3.5 µm (Currie & Thom, 1915). The fruiting structure of conidia consists of 2 to 3 branches forming a single verticil, bearing conidiiferous cells in small parallel whorls.

Physiology
P. oxalicum is known as the true ferment of oxalic acid. It also produces toxic metabolites such as secalonic acid D and F and roquefortine C; and secondary metabolites of unkown toxicity, meleagrin, oxaline, anthglutin, oxalicine and oxalic acid. Furthermore, this lygnocellulolytic enzyme producing fungus has significant economic value due to its ability to utilize lignocellulolytic biomass in agricultural residue as the substrate to produce fermentable sugar, which serves as an alternative for sustainable fuel (Liao et al. 2014).

Habitat and ecology
P. oxalicum is a soil-inhabiting species, it has a world wide geological distribution with a preference for warm and humid conditions. Although being originally isolated from soil and decomposing plant residues, it is also a very common species that can be found in pre-harvested and freshly-harvested maize. P. oxalicum has also been isolated from cereals including rice, spices and cheese, peanuts, fennel and yams，greenhouse cucumbers and other tropical and subtropical products.

Diseases and clinical demonstrations
Previous studies suggested that P. oxalicum secretes hepatotoxins secalonic acid D and F, along with roquefortine C (Frisvad & Filtenborg, 1989), and is pathogenic to greenhouse cucumbers；besides that, P. oxalicum is also involved in genital disease of water buffalo (Frisvad & samson, 1990), this pathogenicity of P. oxalicum is because of its ability to invade injured tissue of fleshy vegetables and animals. On the other hand, P. oxalicum is also known as a growth-promoting fungi widely used as biocontrol of tomato wiltBy applying conidia of P. oxalicum on the root or seedbed of the plant, an increased resistance against wilting is observed within the plant (Sabuquillo et al. 2006). Upon the application of P. oxalicum conidia, a reduction of downy mildew disease and an enhanced growth of the plant was also observed (Murali & Amruthesh, 2014).