| Manganese/iron plaque is a kind of manganese and iron oxide/hydroxide and amorphous colloidal red-brown substances precipitated on the root surfaces of aquatic plant species, such as Oryza sativa, which is always seemed as the crucial mechanism for submergence stress adaptation attributed to the long time plants evolution. It is manganese/iron plaque’s special electrochemical characteristics, the strong sequestration capacity of cations and anions from soil solutions that brings about large amounts of beneficial ecological and environmental effects. However, since the present study about manganese/iron plaque was mainly focused on plant nutrition effects of nutrient uptake by whole plant body and ecological protective effects of alleviating or preventing plants from harmful heavy metal toxicities, the investigations about connections between plant hormones and manganese/iron plaque is still in a blank field. In our experiment, manganese plaque was chosen for object to primarily reveal that both gaseous plant hormone ethylene and growth promoted hormone gibberellins involved in complete submergence-induced manganese plaque formed on rice root surfaces and affected each other in regulating plaque amounts. While effects of gibberellins and Mn+/Fe+ induction treatments on nutrient uptake/translocation in rice seedlings were also tested at the same time. This study could provide some rational references to found chemical control measures for common cultivated rice and increase rice yield in those high-frequently flooded areas. The results showed as follows:1. Firstly we investigated enhancement of submergence on manganese plaque amounts in rice root surfaces under the complete submerged conditions and the results showed that exogenous ethylene precursor,1-aminocyclopropane-l-carboxylic acid (ACC) pretreatments for 12h significantly increased the amounts of rice roots in two Kimura B nutrient solution containing different Mn2+concentrations (0.5mg·L-1 and 17 mg·L-1) through a time-course manner. And a dose-response of manganese plaque contents could also be observed in the 34 mg·L-1 Mn2+ nutrient solution group. While reverse effects were found in the aminoethoxyvinyl glycine (AVG) pretreatment groups. Then we gave four-leaf rice seedlings grown in 0.5mg·L-1 Mn2+ Kimura B nutrient solution a 96h-period completely submerged stress and found manganese plaque contents increased dramatically from 6h to 96h. The subsequent submergence test under a series of concentrations of Mn2+ in Kimura B nutrient solution during 96h showed that the completely submerged treatment significantly raised manganese plaque amounts in every nutrient solution groups in comparison with no stress treated samples. In addition, the gene expression level of members from ACC synthase (OsACS1, OsACS2, OsACS4, OsACS5 and OsACS6) and ACC oxidase family was significantly increased during the 96h complete submergence. At last, the AVG pretreatment group was found less manganese plaque deposed on the root surfaces comparing with the distilled water pretreated group in 17mg·L-1 Mn2+ Kimura B nutrient solution after 96h-period submergence. These results indicated that both complete submergence and ethylene increased manganese plaque amounts, and the effect of complete submergence on manganese plaque may act through the stress hormone ethylene.2. Then we detected changes of manganese/iron plaque amounts on rice root surfaces after exogenous gibberellins treatments and found that treatment with exogenous gibberellic acids (GA3) made the rice root color from white to reddish-brown and increased manganese plaque but decreased iron plaque in Kimura B nutrient solution with 0.5 mg·L-1, 34 mg·L-1 and 84 mg·L-1 Mn2+ or 1.2 mg-L"1,17 mg·L-1 and 42 mg·L-1 Fe2+. In addition, strong does and time response were detected in the nutrient solution group containing 0.5 mg·L-1 Mn+ and 1.2 mg·L-1 Fe2+. The similar effect of GA3 was also found in scanning electron microscopy (SEM) and energy dispersive X-ray spectrometric microanalysis (EDX). However, the inhibitor of GA3, S3307, reversed the effect of GA3. The action of GA3 or S3307 on Kimura B nutrient solution with no Mn2+ group (0 mg·L-1) equaled Fe2+ contained nutrient solution treatments and quite the reverse was true for no Fe2+ group (0 mg·L-1) to Mn2+ contained nutrient solutions. We also tested iron and manganese amounts on other rice varieties such as Nipponbare (japonica rice), S (japonica rice), R (japonica rice), Zhenshan97A(indica rice, male sterile line) and Zhenshan97B (indica rice, maintainer line) after 72h-period exogenous GA3 treatments, and found that changes of S rice on iron and manganese plaque was the same as Nanjing44. While effects of GA3 on the other varieties suggested that both manganese and iron plaque contents increased after hormone treatments. These results showed that the regulation of gibberellins on manganese/iron plaque amounts were existed widely in rice plants but a diversity of effects was also revealed among different rice varieties.3. Thirdly we studied the relationship between gibberellins, ethylene and submergence in manganese plaque formation enhancement under complete submerged or no stress treatments. The results indicated that the ethylene precursor (ACC) plus gibberellins (GA3) treatments significantly deepened the reddish-brown to become the dakest one on rice root surfaces among three groups, ACC, GA3 and ACC plus GA3. Then we found expression of OsGA20ox2 dramatically increased 2.5-4 times after 12-24h period complete submergence treatments. The expression of OsGA2ox3 on the other hand, suggested a slightly rise after 12 h of complete submergence and came to 1.7-1.5 times of control group from 48 to 96h. 0.18mM GA3 with 5μM AVG pretreated rice roots obviously formed less manganese plaque than GA3 pretreatment alone in each Kimura B nutrient solution (0.5 mg-L-1 Mn2+, 4.25mg-L-1 Mn2+ and 17 mg·L-1 Mn2+, pH 5.0) group. The same situation could also be found on ACC+S3307 and single ACC lines. These results indicated that ethylene and gibberellins coordinately regulated amounts of manganese plaque precipitated on rice root surfaces.4. Finally we tested changes of nutrient contents in rice seedlings under exogenous gibberellins or Mn2+/Fe2+ induced treatments and discovered that exogenous 0.18mM GA3 foliar spary for 72h increased uptake of Fe, Mn, P, K and Mg nutrients in any nutrient solution group, in comparison with no GA3 treatment and could also promote translocation of Fe from root to shoot. Large amounts of manganese plaque induced by high Mn solution groups (34mg·L-1 and 84 mg·L-1) significantly raised Mn, P, K and Mg uptake in plants, but declined Fe contents, in comparison with Kimura B nutrient solution treatment (0.5 mg-L"1 Mn2+ and 1.2 mg·L-1 Fe2+). In addition, manganese plaque also increased translocation of K from root to shoot. While much contents of iron plaque by high Fe solution induction groups (17 mg·L-1 and 42 mg·L-1) significantly increased Fe and P contents in plants, but decreased Mn and K contents, compared with Kimura B nutrient solution treatment (0.5 mg·L-1 Mn2+ and 1.2 mg·L-1 Fe2+). Iron plaque also enhanced translocation of Fe or K from root to shoot but possessed no power on Mg element. These results indicated that there existed an enhancement of Mn, P, K, Mg uptake by both GA3 and manganese plaque and Fe, P uptake by both GA3 and iron plaque in rice seedlings. The investigation seemed to provide some rational references to achieve development and popularity on new chemical compound products composed of hormone regulators and manganese or iron fertilizer mixture in the future agricultural production.In consideration of rice yield decrease primarily due to the submergence stress in flooded area and the special ecological and environmental protective effects of manganese /iron plaque deposited on rice root surfaces during stress procedures, this study can be used as a medium or bridge to strengthen the combination of plant biology, environment science/eeology and also to improve the study object systemic and offset limitations of single subject in the future days. In addition, the study can also make sense to enhance rice plants submergence adaptability and further optimize the present chemical control strategies in order to guide rice agriculture production increase in those flooding areas. |
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