The Relationships Between Physico-Chemical Characteristics Of Rare Earth Elements And Photosynthesis As Well As Their Underlying Mechanisms

Rare earth fertilizers have been widely used in China for nearly 40 years. It has been demonstrated in many experiments that rare earth elements (REEs) are able to promote the growth and development of plants, enhance photosynthesis and increase the yield of crops. However, the mechanisms underlying the biological effects of REEs on plants still remain elusive. Among all the questions, the effects of REEs on photosynthesis, is the most puzzling one. Specifically, very little is known about the relationships between the physiological effects and the unique characteristics of REEs. Thus, we selected three representative REEs, Lanthanum, Neodymium and Cerium (La³⁺, Ce³⁺ and Nd³⁺), to study the effects and mechanisms of REEs on photosynthesis. This study focuses on the energy transfer processes of photosynthesis——from light energy to electrical energy then active chemical energy, and stable chemical energy in the end. We also compare the different effects imposed by three REEs treatments to clarify the mechanism how unique characteristics of REEs, such as 4f electron characteristic, alternation valence and the similarity to the Ca²⁺, affect photosynthesis. This study will not only provide insights into photosynthesis mechanism study, but lay a solid ground for the efficient use of REEs in agriculture。1) REEs are able to promote the light action of spinach, enhance the light absorption, transfer and distribution efficiency. Treated by La³⁺, Ce³⁺ and Nd³⁺, the characteristic absorbance peaks of the chloroplast are enhanced and blue shifted as well. The ratio of Soret band intensity and Q band intensity is also increased, indicating the ability to catch light of the chloroplast pigments is enhanced, especially in short wavelength. Fluorescence spectra further indicate that the light absorbed by chlorophyll b (chl b) and carotene could efficiently transfer to chlorophyll a (chl a) of photosystemâ…¡(PSâ…¡). The results of Dual-PAM-100 showed that REEs treatments are able to promote both the photochemical activity and electron transfer ratio of two photosystems. However, PSâ…¡activity is increased more efficiently. Based upon the chlorophyll fluorescence, the photo protective abilities of two photosystems are also increased, thus enhance the tolerance of light. The facts that the whole chain electron transport activity, PSâ…¡, PSâ… DCPIP photo reduction and oxygen evolution of the chloroplast are all increased by treatments with REEs, further indicate that REEs are able to efficiently distribute the light energy and trigger the excitation energy mostly to PSâ…¡. Among these three REEs, the effect of the Ce³⁺ treatment is the best, which is followed by the Nd³⁺, and the La³⁺ treatment is not as effective as other two elements. 4f electron characteristic and alternation valence of REEs might contribute to these observed differences, for Ce³⁺ has 1 4f electron and can change to +4, Nd³⁺ has 3 4f electrons but no alternation valence, whereas La has neither 4f electron nor alternation valence.2) REEs are able to promote the PSâ…¡activity by regulating LHCâ…¡(Light harvesting complexâ…¡). A suitable concentration of REEs treatment is able to change the redox state of PQ pool,whereby dephosphorylating LHCâ…¡and changing the photosystem to stateâ… , and to certain degree enhancing the photoabsorption cross section and light absorption of PSâ…¡. The real-time PCR experiments showed another mechanism of REEs to LHCâ…¡. The results showed that REEs are able to enhance expression of Lhcâ…¡b of arabidopsis and significantly increase LHCâ…¡content on the thylakoid membranes, thus inducing the LHCâ…¡to trimer formation and adjusting the grana distribution of the thylakoid membranes. This changes the electron transfer ratio of two photosystems and triggers the excitation energy mostly to PSâ…¡. The effects of the three REEs are as follows : Ce³⁺>Nd³⁺>La³⁺>control.3) REEs are able to promote carbon assimilation. The La³⁺,Ce³⁺,Nd³⁺ treatments enhance the carboxylase activity of Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) and increase the dry matter content of spinach. It is resulted from the formation of the Rubisco- Rubisco activase super complex by REEs treatment. A higher molecular weight protein is found during the purifying procedure of Rubisco in REEs-reated spinach. Its spectral characteristic, sulfhydryl groups and secondary structure are quite different from the pure Rubisco. SDS-PAGE analysis showed that this higher molecular weight protein contains not only 52 kD and 14 kD subunits of Rubisco, but 2 extra isoforms at 41 and 45 kD as well, which was supposed to be the isoforms of Rubisco activase. The native-PAGE result suggested the higher molecular weight protein is the Rubisco- Rubisco activase super complex and the molecular weight is about 1200 kD (a 560 kD Rubisco holoenzyme plus 16 isoforms of Rubisco activase). It was examined with the antibodies of both Rubisco and Rubisco activase in Western-blotting. This is the first evidence to confirm the formation of the Rubisco- Rubisco activase super complex, which is the hypothesis for activating the carboxylase activity of Rubisco in vivo.4) The mechanism for REEs to induce the formation of the Rubisco- Rubisco activase super complex may be complicated. RT-PCR, Northern blotting and real-time PCR strongly indicate that the mRNA levels of the Rubisco and Rubisco activase were enhanced by the REEs treatment. However, rbca is highly expressed which may increase both the protein content of Rubisco activase and the ratio of the Rubisco activase to Rubisco. It is still the Ce that has the strongest influence on the gene expression, then follwed by Nd, and La is the least. These results suggested that 4f electron characteristics and alternation valence of REEs have a close relationship with carbon assimilation. The second mechanism to form the super complex is found by EXAFS. Nd is coordinates with Rubisco by 4 Nd-N(O)bond(2.46 (A|°))and 2 Nd-S bond(3.47 (A|°)) in vivo, which suggests REEs ion may provide the extra linkage between Rubisco and Rubisco activase, which stabilize the super complex structure. The in vitro study also showed that that Ce³⁺ could directly bind Rubisco, coordinates with 8 oxygen atoms in first shells and 6 oxygen atoms in second shells, change the spectra characteristic of Rubisco, which enhances the enzyme activity.5) Ce³⁺ has similar chemical property to Ca²⁺, but the charge and potential energy in Ce³⁺ are higher than Ca²⁺. So Ce³⁺ could not only occupy a Ca²⁺ position, but also substitute for bound Ca²⁺. Thus Ce ³⁺ are known a"supercalcium". It was showed that Ce ³⁺ could relieve calcium-deficiency symptoms in spinach. Ce treatment could raise the dry weight and fresh weight of calcium deficient spinach, increase the chlorophyll content and photosynthetic rate, and the oxygen release rate and enhance the activities of Rubisco and the nitrogen metabolism related enzymes system. Ce treatment could also enhance the activities of the antioxidation system both in the normal and calcium-deficiency condition. Specifically, the activities of superoxide dismutase (SOD), catalase(CAT) and peroxidase(POD) are enhanced and the membrane permeability, while reactive oxygen species (ROS) and malond ialdehyde(MDA) are reduced, which would maintain photosynthesis. The in vitro study showed that Ce could bind the Ca-depleted PSâ…¡particle, coordinate with oxygen atoms and partially recover the activity of Ca-depleted PSâ…¡. It also suggests that Ce³⁺ could replace Ca²⁺.