[Effect and Mechanism of Chlamydomonas reinhartii Living Cell Agents in Alleviating Cd Stress on Wheat Seedlings].

Cadmium （Cd） pollution not only leads to the reduction in crop yields but also migrates and accumulates through the food chain, thus posing a threat to human health. It is an important issue to reduce Cd uptake and enrichment in crops and increase crop resistance to Cd stress for sustainable development of green-health agriculture. To establish a novel agronomic technique using microalgae to control Cd pollution and improve crop stress resistance, the microalga Chlamydomonas reinhardtii and wheat （Triticum aestivum）were employed for systematic investigation so as to the elucidate effects and mechanism of microalgae in blocking Cd migration and enhancing crop resistance to Cd stress. The test crop materials were the winter wheat variety JM182 and spring wheat variety JC6. Hydroponic experiments were used to simulate Cd stress at two dosages, 50 mg·L^-1 （Cd50） and 100 mg·L^-1 （Cd100）. Two different dosages of C. reinhardtii living cell agents, i.e., the microalgal cell cultures with D₆₈₀ = 1.0 （A1.0） and D₆₈₀ = 2.0 （A2.0）, were prepared using conventional microalgal-cultivation methods and subsequently used to treat wheat seedlings under Cd stress. Several physiological and biochemical parameters were determined for wheat seedlings under two doses of Cd stress, respectively, including growth features, photosynthesis, cellular antioxidant enzyme activity, Cd uptake, transportation and accumulation, and transcriptions of the genes associated with heavy metal transportation. The results showed that Cd stress crucially inhibited the growth of wheat seedlings. However, the application of C. reinhardtii living cell agents significantly increased the contents of photosynthetic pigments （chlorophyll a, chlorophyll b, and carotenoids） and activated the antioxidant enzyme system activities （SOD, POD, CAT, GSH, and APX）. The microalgal living cell agents also reduced the damage of Cd stress on wheat seedling growth. Moreover, both doses （A1.0 and A2.0） of C. reinhardtii living cell agents downregulated the expression of the genes related to heavy metal absorption and transportation （TaHMA2, TaHMA3, TaNramp1,and TaLCT1）. Analysis of Cd contents indicated that Cd stress resulted in Cd accumulation in wheat seedlings, with a higher level of Cd in JC6 than in JM182 under both doses of Cd stresses. Cd50 and Cd100 stresses led to Cd levels up to 185.01 mg·kg^-1 and 342.11 mg·kg^-1 in JC6 wheat seedlings and 176.76 mg·kg^-1 and 317.65 mg·kg^-1 in JM182, respectively. Notably, the addition of the microalgal living cell agent significantly reduced Cd enrichment in wheat seedlings of both varieties under Cd stresses. Compared to the Cd level in JC6 wheat seedlings under Cd50 stress, the Cd accumulation level in roots and stem-leaf parts of A2.0-treated JC6 wheat seedlings was reduced by 76.80% and 66.91%, respectively, followed by a 27.58% diminution of Cd transport rate from roots toward stem-leaf parts. Collectively, microalgal living cell agents could remarkably reduce Cd uptake, accumulation, and transportation from roots to stem-leaf organs of wheat seedlings by down-regulating gene expressions involved in heavy metal absorption and transportation. The microalgal living cell agent can also activate the antioxidant system and increase photosynthesis, thus mitigating Cd toxicity and promoting the growth and development of wheat seedlings. The present findings provide a scientific basis and new strategy for using microalgal living cell agents as bio-fertilizers or bio-stimulants to impede migration of heavy metal pollutants and enhance plant resistance to heavy metal stresses.