Chitin, a polymer of N-acetyl-β-D-glucosamine, is widely found in insect carapace and the cell walls of pathogenic bacteria . The cell wall determines the shape and strength of the pathogen cells and is a key determinant of cell morphology development. As one of the main components of the cell wall, chitin plays a very important role in the growth and development of pathogenic bacteria as well as defense against external stress . The degradation product of chitin is chitin oligosaccharide, which can induces the natural immunity of plants . Previous study suggested that the perception of the chitin oligosaccharides contributes to enhance disease resistance against the rice blast fungus in rice .
Chitinase is an enzyme system that uses chitin as a substrate and hydrolyzes it to N-acetol oligosaccharide and glucose. Chitinase is widely present in various organs of higher plants and can be rapidly produced and accumulated after infection by pathogens . The induction of pathogenic bacteria can enhance the activity of chitinase, which then inhibit spore germination and mycelial growth, and even directly degrade the chitin of the fungal cell wall . Abiotic stresses such as heavy metals and high temperatures can also induce the expression of chitinase gene and promote their accumulation [7, 8]. Margispinheiro et al. (1994) examed the expression pattern of chitinases in bean in response to various stresses and showed that salicylic acid (SA) and heat shock treatment can induce the expression of chitinase . Liu et al. (2005) reported that a class IV chitinase gene was induced and chitinase was accumulated in both susceptible and slow-canker-growth resistant seedlings of western white pineafter infection by Cronartium ribicola . Chitinase genes were also induced during the infection by Botryosphaeria berengeriana and Aphis citricota in Malus hupehensis .
Cabbage (Brassica oleracea var. capitata L.) is one of the most important leafy vegetables belonging to Brassica oleracea species cultivated worldwide, with a yield of 28.4 tons per hectare .
Fusarium wilt (FW) is a fungal disease caused by Fusarium oxysporum, which can infect many different types of crops such as tomatoes, cucumbers, bananas, cotton, flax and chrysanthemums . The specialization type of cabbage wilt pathogen is Fusarium oxysporum f. sp. Conglutinans. This type include two races No. 1 and No. 2, among which the race 1 is the main race in the world . F. oxysporum can break through the barrier of the cell wall of the root of the crop by secreting cell wall degrading enzyme (CWDE) . It then secretes toxins that destroy the structure of the host cell and impede the normal metabolic processes of the host plant . In addition, pathogenic bacteria can also adjust their own metabolic reactions and secreted proteins to adapt to the surrounding environmental conditions, so that they can complete the infection more effectively .
black spot (BS) disease is one of the important diseases of cruciferous vegetables, mainly caused by Alternaria. The spores of Alternaria brassicae adhere to the host with the help of lipase and recognize the leaves with waxy matter. Then, with the help of several serine esterases, mycelium was able to penetrate the host epidermis . The invasion of pathogenic bacteria will lead to the change of metabolite content in the crops. Doughty et al. (1991) reported that the contents of glucosinolate in rapeseed increased after being infected by Alternaria brassicae . Chawla et al. (2001) found that the infection of pathogenic bacteria will induce an increase of polyphenol oxidase in Brassica juncea .
Plasmodiophora brassicae Wor. is a soil-borne biotrophic pathogen, leading to clubroot in Brassica crops and then cause severe crop yield loss . The life cycle of P. brassicae contained two stages: primary infection stage and secondary infection stage. Firstly, resting spores release primary zoospores, which can penetrates through the cell wall of the root hairs and form primary plasmodia therein. Subsequently, primary plasmodia form secondary resting zoospores then infected cortical cells of the main roots, followed by gall formation in the tissues. The plasmodia finally develop into a new generation of resting spores, followed by their release back into soil as survival structures. The rotten swollen root finally releases the resting spores into the soil, thus the life history was completed .
Cabbage powdery mildew (PM), caused by Erysiphe cruciferarum, is also a common disease in cruciferous crops. The disease is transmitted mainly through the air. The mycelia of pathogenic bacteria can be attached to the epidermal tissue of the plant, and then insert a sucker into the tissue to absorbed nutrient. The typical symptoms of the disease are chlorosis and yellowing of the leaves, and even plant death.
downy mildew (DM) is another common fungal disease of cruciferous crops, mainly caused by Hyaloperonospora brassicae . Pathogenic spores can overwinter in the soil and wait for suitable environmental conditions to germinate and infect new plants. It will cause a white mold layer on the back of the infected plant's leaves, where with more stomata and is conducive to the reproduction of pathogen . which will then turn yellow-brown. In the later stages of the disease, the leaves gradually turn yellow-brown and die.
Chitinase can protect plants from a variety of pathogenic bacteria. Ntui et al. (2011) increased tobacco resistance to FW by transferring chitinase genes into tobacco . The same results were also confirmed in tomatoes . Similarly, transgenic grapes carrying the wheat chitinase gene have increased resistance to DM . Marchant et al. (1998) reports that expression of the chitinase transgene reduced the severity of blackspot development by 13–43% in rose . Ludwig-Müller et al. (1994) reports that the activity of chitinase in infected Chinese cabbage was significantly enhanced . Similarly, Chen et al. (2018) found that the expression of chitinase reduced the symptoms of clubroot in Chinese cabbage . In addition, exogenous application of chitinase to barley can also inhibit the proliferation of PM pathogens.
Although the function of chitinase has been analyzed in various plant species, such as tomato , potato , rice , Brassica juncea , wheat  and apple , the role of chitinase gene family in cabbage with FW, BS, clubroot, PM and DM are currently unavailable.
In this study, 20 chitinase genes were identified in cabbage and their chromosome location, gene structure, colinearity relationships, evolution and the cis-acting regulatory elements in promoters were further analyzed. The expression patterns of the chitinase family genes responses to FW, BS, clubroot, PM and DM were also investigated. Our results will promote to understand the chitinase functions associated with various diseases response, as well as inform the genetic improvement of cabbage.