Preliminary Study On The Effect And Mechanism Of Rare Earth Element Activated Root Cell Endocytosis On Photosynthesi

The widely application of rare earth elements(REEs)has resulted in the excessive accumulation of REEs in the environment and being absorbed by edible plants,which enters human body through the food chain and endangers our health.Therefore,it is urgent to establish a standard limitation of REEs in edible plants.Our previous study discovered that REEs can break the endocytic inertia of leaf cells,which is a result of long-term evolution,and initiate the endocytosis in leaf cells,which further improves the chlorophyll content and promote the growth and development of plants.Biological research shows that the growth and development of plants must undergo three major parts:seed germination(underground),morphogenesis and photosynthesis(aboveground),and follow the rule of "thick with leaves and deep-rooted".Based on these,two questions have been raised.Can REEs directly activate the endocytosis of root cells?What are the effects of activated endocytosis in root cells on the growth and development of plants?In this paper,in order to study the effects and mechanism of the endocytosis in root cells activated by REEs on photosynthesis,lanthanum[La(Ⅲ)]and cerium[Ce(Ⅲ)]were selected as representatives of REEs and Arabidopsis thaliana was chosen as model plants.The main results are as follows:(1)REEs can activate the endocytosis in root cells and promote the photosynthesis in leaf cells.First,after REEs(≤55μmol·L-1)act on plant roots,the endocytosis in root cells was activated and was increased with the increasing REEs concentration.Secondly,the growth and development of leaf cells were promoted after the endocytosis of root cells was activated by REEs.And the enlargement of leaf cells was positively correlated with the endocytosis activation in root cells.Finally,the enhancement of root-cell endocytosis could enhance stomatal conductance of leaves,increase the maximum photon yield of photosystem Ⅱ(PS Ⅱ),improve the photochemical efficiency,enhance the photochemical electron transport;reduce the intercellular concentration of CO2 and decrease the light energy heat dissipation of the central pigment molecules of PS Ⅱ,and then macroscopically promote the photosynthesis of plants.(2)REEs regulate the morphogenesis of plants through activating the endocytosis of plant roots in which the adapter protein 2(AP2)serves as an aptamer,and then affects the photosynthesis.First,the results showed that the endocytosis in ap2μ-1 mutant root cells was lower than that in wild-type Arabidopsis thaliana(Col-0)after REEs act on the roots,indicating that REEs mainly activate endocytosis in which AP2 served as an aptamer,and AP2μ subunit played an important role in this pathway.Secondly,the morphogenetic results discovered that under a certain degree of endocytosis,the variation of the morphogenesis regulators,such as CONSTITUTIVE PHOTOMORPHOGENIC 1(COP1),CONSTITUTIVE PHOTOMORPHOGENIC 9(COP9),PHYTOCHROME-INTERACTING FACTOR 4(PIF4),ELONGATED HYPOCOTYL 5(HY5)and HY5 HOMOLOG(HYH)were increased with activation of endocytosis in root cells of dark-grown Col-0;yet there were not any significant changes in these morphogenetic regulators in dark-grown ap2μ-1 mutant.These results indicated that REEs[≤ 30 μmol·L-1 La(Ⅲ)or≤45μmol·L-1 Ce(Ⅲ)]affected the morphogenesis of plants through activating endocytosis in root cells in which AP2 served as an aptamer.Thirdly,the results of detecting endogenous ethylene level showed that the endogenous ethylene level was increased with the enhancement of REE-activated endocytosis in dark-grown Col-0;yet the endogenous ethylene level did not show any significant change in dark-grown ap2μ-1 mutant,indicating that REEs regulate the ethylene biosynthesis in dark-grown plants via activating endocytosis in root cells in which AP2 serves as an aptamer,and further regulate the morphogenesis of plants and then affect the photosynthesis of plants.Finally,with the increase of REEs concentration[≥45 μmol·L-1 La(Ⅲ)or ≥ 55 μmol·L1 Ce(Ⅲ)],intensely REEs-activated endocytosis led to abnormal changes in COP1,COP9,PIF4,HY5 and HYH,which might result from the large consumption of energy and damage on plasma membrane caused by excessive REEs-activated endocytosis.(3)The dynamic detection of the effects of REEs-activated endocytosis of leaf cells on photosynthesis in plants.First,the electrochemical study showed that the amount of vitronectin-like protein(VN),the first line for defending plant cells from the attack of exogenous hazardous materials,on the surface of leaf-cell plasma membrane was increased with the increasing REEs concentration[<55 μmol·L-1 La(Ⅲ)or<80μmol·L-1 Ce(Ⅲ)](without causing visible damage to plant cells).And based on this,an electrochemical sensor was constructed for detecting the VN content on the surface of leaf-cell plasma membrane,and important results were obtained.First,the impedance was in a stable linear relationship with REEs concentration.Secondly,REEs can initiate endocytosis of leaf cells and the intensity of REEs-initiated endocytosis in leaf cells was increased with the increasing REEs concentration.Third,with the increase of REEs-initiated endocytosis,the chlorophyll content in leaves was increased first and then decreased.In addition,the chlorophyll content in leaves began to decrease when the VN content on the leaf-cell plasma membrane increase by 9 times.Therefore,the effects of REEs-activated endocytosis in leaf cells on photosynthesis and the invisible damage on plants caused by REEs can be monitored via detecting the dynamic changes in VN content on the surface of leaf-cell plasma membrane.In summary,REEs can regulate the plant morphogenesis through activating endocytosis in root cells in which AP2 serves as an aptamer,thereby regulating plant photosynthesis and ultimately affecting plant growth and development.The effects of REEs on photosynthesis can be dynamically monitored via detecting the changes in VN content on leaf-cell plasma membrane.The results in this paper can provide references for illuminating the mechanism of REEs affecting photosynthesis,establishing a standard for the limit concentration of REEs in edible plants,assessing the effects of REEs on the growth and development of plants and the security of plants.