The Study Of Factors And Mechanism Of Cadmium Accumulation In Lentinus Edodes

Lentinula edodes has high nutritive value, and tastes deliciously. It is called "king of mushrooms", "natural health food", "summit of plant food" and "God’s Food". So it’s very popular for most consumers. But Lentinula edodes especially accumulate Cadmium (Cd) in the growth process and that’s caused worry about quality safety of Lentinula edodes products. Thus, reducing Cd content in Lentinula edodes is urgently. In order to ensure quality safety of Lentinula edodes products, we investigate main source, bioconcentration factor, accumulation rule and mechanism of Cd in Lentinula edodes under Cd stress, which can offer theoretical reference for controlling Cd content in Lentinula edodes, screening and breeding low accumulation Cd strain of Lentinula edodes.We mainly gain four aspects of resaults of this experiment as follows:1. Cd content was detected in cultivation substrate, fruiting body of different strain Lentinula edodes and different flushes, and different morphological portions. The results were showed as follows:First, Background Cd content in wheat bran was highest, and was 3.10 times as much as that in sawdust. But sawdust content accounted for the majority of cultivation substrate. It was 3.10 times more than that in wheat bran. So, Cd in Lentinula edodes came mainly from sawdust and wheat bran; Second, in different morphological portions, Cd content accumulated from cultivation substrate was very different. And the accumulation ability was the strongest in the gill, then in the cap, and weakest in the stipe. Third, Cd content in the first flush and second flush fruiting body was high, and low in the third and forth flush fruiting body. Forth, under normal growth conditions that was adding no any Cd to the cultivation substrate, Cd bioconcentration factor of wuxiang,208 and 808 strains of Lentinula edodes were 18.38,29.41,15.15 respectively. The Cd accumulation ability of different strains were ranked as follow:208>wuxiang> 808.2. The Cd accumulation rule of fruiting body of Lentinula edodes was studied by adding Cd to cultivation substrate with a series of Cd concentration and statistical method. The results showed that, the Cd accumulation regression equation of wuxiang,208 and 808 strain of Lentinula edodes were Y= 16.114X1.0025,Y=14.272X08243. Y=13.095 X 092 respectively. The coefficient correlation R2 was 0.9933,0.9755 and 0.9965 respectively. According to 《THE INDUSTRIAL STANDARD OF THE PEOPLE’S REPUBLIC OF CHINA NY/T 749-2003 Green food-Edible mushroom》 and 《THE INDUSTRIAL STANDARD OF THE PEOPLE’S REPUBLIC OF CHINA NY 5095-2002 Non-pollution food-Lentinula edodes》, the limited value of Cd concentration was 1.0 mg/kg in dry fruiting body of Lentinula edodes. So, the limited values of Cd concentration in cultivation substrate were 0.062、0.040、0.061 mg/kg for cultivating wuxiang,208 and 808 strain of Lentinula edodes respectively.3. In order to investigate accumulation mechanism and detoxification mechanism of Cadmium (Cd) in Lentinula edodes, copper (Cu), we studied the change of copper (Cu) content, organic acids content, phytochelatins (PCs) content and transcript level of metallothioneins (MTs) genes in the fruiting body of Lentinula edodes when adding Cd to cultivation substrate with a series of Cd concentration. We found that:First, Cd in the cultivation substrate can promoted Cu accumulation in Lentinula edodes. And the Cd accumulation mechanism may be similar to Cu accumulation mechanism, or there existed synergistic effect between Cd accumulation and Cu accumulation in Lentinula edodes. Second, organic acids content almost didn’t change when Cd content was low in cultivation substrate. But organic acids were induced synthesis largely with high Cd concentration in cultivation substrate. Third, low concentration Cd could largely induced expression of two metallothionein (MT) genes (Lemtl and Lemt2), that could result in more MTs synthesis. Forth, Cd couldn’t induced PCs synthesis in Lentinula edodes. So, Lentinula edodes could accumule and detoxicate Cd coordinately by organic acids path and metallothioneins path. Metallothioneins pathway mainly played a role at low levels of Cd concentration in Lentinula edodes, and organic acids pathway play a role at high levels. Moreover, there might exist the similar pathway or cross pathway between Cd accumulation and Cu accumulation in Lentinula edodes.4. In order to reduce Cd content in Lentinula edodes, some measures could be taken by comprehensive analysis of Cd accumulation factor as follows:First, controled cadmium content in Lentinula edodes from the source, so we should select raw material with no Cd or very low Cd content as cultivation substrate, especially selecting sawdust and wheat bran with no Cd pollution; Second, selected the strains which gill accounted for relatively small proportion in the entire mushroom fruiting bodies; Third, selected the strains of which Cd accumulation ability was relatively weak, for instance 808 strain; Forth, screened MT genes mutant strain, in order to decrease Cd accumulation ability, but other functions are normal. Fifth, selected the strains of which organic acids content was low.This research relative systemically studied macro aspects of the Cd accumulation factors. Meanwhile, it investigate micro aspects of Cd accumulation mechanism such as biochemical level and molecular level. All those could provide theoretical reference for screening and breeding low cadmium accumulation varieties.However, this research also existed shortage. For example, this experiment used HPLC method for detecting PCs. PCs contents might so low in the fruiting body of Lentinula edodes that exceed the measurement range of HPLC method. So, more sensitive methods such as hygroplasm could be used to further validate PCs whether or not existing in Lentinula edodes. Two MT genes sequence fragments were from EST. we also didn’t amplify the whole genes sequence including promoter sequence and make a functional complementation. In addition, it provided theoretical reference for screening and breeding low cadmium accumulation varieties in perspective of detoxification and accumulation, but didn’t study how to let cell resist the absorption of cadmium or cadmium not pass through cell membranes and then enter to cells. And so on. So, relative studies needed to be further carried out.