Indoleacetic acid (IAA) is an important regulator that plays a crucial role in plant growth and responses to abiotic stresses. In the present study, a sand cultivation experiment was carried out to investigate the effects of IAA at different concentrations (0, 0.01, 0.1, 0.5, 1, and 2.5 mmol/L) on maize growth, root morphology, mineral elements (Ca, Mg) and Cd uptake under 20 mg/kg Cd stress. The results showed that 0.01 mmol/L is the optimal IAA concentration for enhancing the Cd tolerance of maize. Compared with the control treatment, 0.01 mmol/L IAA promoted maize growth, with significant increases in the height, shoot and root biomass by 34.6%, 25.0% and 16.3%; altered the root morphology, with increases in root length, root tip number, and root tip density by 8.9%, 31.4% and 20.7%, respectively; and enhanced the mineral element uptake of maize, resulting in significant increases in the Ca content in shoots and roots by 640.6% and 1036.4% and in the Mg content in shoots by 205.8%, respectively. In addition, 0.01 mmol/L IAA decreased the Cd content and uptake in the shoots by 51.9% and 39.6%, respectively. Furthermore, the Cd content and uptake exhibited a significant negative correlation with Ca content in roots and a significantly positive correlation with root morphology, and the Cd content in shoots was significantly and negatively correlated with root tip number. Thus, 0.01 mmol/L IAA was effective in enhancing the Cd tolerance and plant growth of maize.
Cadmium (Cd) is an unnecessary element for plants. After being absorbed by the soil, Cd is transported and taken up by plants [
Auxin is an important plant growth regulator that can promote cell division and elongation and the differentiation and formation of new organs, change the morphology of plant roots, and regulate the absorption of nutrients by plants. It plays an important role in plant Cd tolerance [
Maize has a large biomass, is easy to harvest, and is a widely planted crop worldwide [
The test material was a native low-Cd maize cultivar (Huidan No. 4) from Yunnan Province, China. The culture substrate was quartz sand (the main component of which is SiO2), with a particle size of 0.5–1 mm, white powder. IAA (3-indoleacetic acid, C10H9NO2) is a white crystal that is poorly soluble in water. IAA was purchased from Beijing Coollab Technology Co., Ltd. (China) (Coolaber RT Purity > 99%). The culture vessel was an uncovered cylindrical sealed tank with a diameter of 12 cm and glass bottles with a diameter of 6.5 cm and a height of 25 cm.
First, the seeds of Huidan No. 4 were surface-sterilized by immersion in 75% ethanol for 10 min and in 10% sodium hypochlorite for 10 min, followed by rinsing with water until colorless. The seeds were germinated in a petri dish (150 mm) at 25°C for 3 days. After germination to about 1 cm, two seedlings with uniform growth were transplanted to a glass bottle, which 0.4 kg quartz sand and 40 ml of 50% Hoagland nutrient solution were added. Seedlings were cultured for 14 days in a greenhouse (average temperature 25°C, average humidity 56%).
Second, after 14 days, two maize seedlings with the same growth were transplanted to a sealable tank. To each sealed tank, 1.475 kg of quartz sand and 100 ml of 50% Hoagland nutrient solution were added. The nutrient solution contained 20 mg/kg Cd and the corresponding concentration of IAA (0, 0.01, 0.1, 0.5, 1, 2.5 mmol/L IAA), Cd was added in the form of CdCl2·2.5H2O, 4 replicates per treatment. Seedlings were grown for 30 days and harvested in greenhouse.
After the maize seedlings were harvested, the plant height was measured with a measuring tape, and then the seedlings were divided into shoots and roots. The samples were soaked in ethylenediaminetetraacetic acid solution (EDTA) for 15 min. The shoots and roots soaked in EDTA were rinsed with tap water and deionized water 3 times and then dried in an oven at 105°C for 30 min and then at 75°C. The dried shoots and roots were weighed to a constant weight to obtain the shoot and root biomass [
The root was rinsed with tap water and then with distilled water 2–3 times, and the water was absorbed with filter paper. The root morphology was scanned with an Epson Perfection V700 scanner (Seiko Epson, Japan), the scanned image was saved, and the root length, root surface area, root volume, root average diameter, root tip number, and branch number were analyzed with Win RHIZO PRO STD4800 (Regent, Canada) software [
According to the root length, root surface area, root tip number and branch number recorded by WinRHIZO and the root biomass, the specific root length (SRL), specific surface area (SRA), root tip density (RTD) and root branching intensity (RBI) of the root system were calculated. The calculation formula was as follows: specific root length (m/g) = root length (m)/biomass (g), specific surface area (cm2/g) = root surface area (cm2)/biomass (g), root tip density (No./cm) = root tip number(number)/root length (cm), branch density (No./cm) = branch number(number)/root length (cm) [
Plant samples were dried, ground and passed through a 100-mesh sieve. A 0.5-g sample was transferred to polyethylene digestion irrigation, concentrated HNO3 and H2O2 (5:3) were added to the sample, and the sample was digested in a pressure digestion tank and measured by flame atomic absorption spectrophotometry (AAS ICE 3000 Series, Thermo Scientific, Franklin, USA) [
Plant Cd uptake characteristics are expressed by the biological transfer factor (BTF) and the transfer factor (TF) [
The test data were the average of 4 replicates, and all data were sorted and analyzed in Excel. The least significant difference method (LSD) has a relatively simple method, in all possible pairwise comparison of mean of multiple treatments. LSD overcomes some drawbacks of the
Under 20 mg/kg Cd stress, 0.01 mmol/L IAA resulted in a significant increase in maize plant height and shoot and root biomass, which increased by 34.6%, 25.0%, and 16.3%, respectively. IAA (0.1 mmol/L) resulted in a significant increase in plant height by 21.4% but resulted in a significant decrease in root biomass by 20.4%. IAA (0.5 mmol/L) resulted in significant decreases in plant height and shoot and root biomass, which were decreased by 24.3%, 13.9%, and 18.4%, respectively. IAA at 1 and 2.5 mmol/L resulted a significant decrease in root biomass of 26.5% and 12.2%, respectively. Under Cd stress, 0.01 mmol/L IAA significantly promoted maize growth (
IAA concentration (mmol/L) | Plant height |
Shoot biomass |
Root biomass |
---|---|---|---|
0 | 23.33 ± 2.98 b | 0.36 ± 0.01 b | 0.49 ± 0.03 b |
0.01 | 31.42 ± 4.21 a | 0.45 ± 0.04 a | 0.57 ± 0.02 a |
0.1 | 28.33 ± 1.13 a | 0.38 ± 0.03 b | 0.39 ± 0.02 cd |
0.5 | 17.67 ± 1.60 c | 0.31 ± 0.02 c | 0.4 ± 0.0 cd |
1 | 27.08 ± 1.74 ab | 0.37 ± 0.02 b | 0.36 ± 0.03 d |
2.5 | 22.17 ± 2.62 b | 0.37 ± 0.03 b | 0.43 ± 0.01 c |
All values represent the means ± standard deviations; different lowercase letters within a column indicate significant differences based on one-way analysis of variance in SPSS 21 followed by the least significant differences at the 5% confidence level.
Under 20 mg/kg Cd stress, 0.01 mmol/L IAA resulted in a significant increase in root length of maize by 8.9%. IAA at 0.1 and 2.5 mmol/L resulted in significant decreases in root length of 14.4% and 12.8%, respectively. IAA at 0.01, 0.1, 0.5, 1, and 2.5 mmol/L resulted in significant decreases in root surface area and root volume; the root surface area was decreased by 11.1%, 20.2%, 12.5%, 16.9% and 17.8%, the root volume was decreased by 11.1%, 20.2%, 11.1%, 16.9%, 17.8%, and 0.5 mmol/L AA resulted in a significant decrease in the average root diameter by 20.0%. IAA (0.01 mmol/L) resulted in a significant increase in the number of root tips by 31.4%. IAA (1 mmol/L) resulted in a significant decrease in the number of root tips by 12.9%. There was no significant effect on the number of branches treated with IAA (
Under 20 mg/kg Cd stress, 0.5 and 1 mmol/L IAA resulted in significant increases in the specific root length of maize of 19.2% and 28.4%, respectively. IAA (0.01 mmol/L) resulted in a significant decrease in the specific surface area by 24.7%. IAA at 0.01, 0.1, and 2.5 mmol/L resulted in a significant increase in root tip density, which was increased by 20.7%, 13.7%, and 16.2%, respectively. IAA (2.5 mmol/L) resulted in a significant increase in root branch density by 16.8% (
Under 20 mg/kg Cd stress, 0.01, 0.1, 0.5, 1, and 2.5 mmol/L IAA resulted in significant increases in the Ca content of maize of 640.6%, 241.8%, 371.2%, and 387.5%. In addition, the root Ca content increased by 1036.4%, 1136.3%, 1265.4%, 902.1%, and 668.1%, respectively, and 0.01, 0.1, 0.5, 1, and 2.5 mmol/L IAA resulted in significant increases in shoot Mg content, of 205.8%, 126.2%, 162.9%, 156.5%, and 155.5%, respectively. IAA had no significant effect on the root Mg content. In general, under Cd stress, applying IAA can promote the absorption of Ca and Mg in maize (
Under 20 mg/kg Cd stress, 0.01 mmol/L IAA resulted in significant decreases in the Cd content, and the uptake of maize shoots decreased by 51.9% and 39.55%, respectively. IAA at 0.5 and 2.5 mmol/L resulted in significant increases in Cd content and shoot uptake. The Cd content increased by 141.5% and 36.3%, respectively, and the uptake increased by 108% and 41.9%, respectively. IAA at 0.1, 0.5, 1 and 2.5 mmol/L resulted in significant decreases in Cd content and uptake of maize roots; the Cd content decreased by 58.0%, 30.0%, 32.0%, and 47.4%, respectively, and the uptake decreased by 67.2%, 42.2%, 49.9%, and 53.4%, respectively. It can be seen that 0.01 mmol/L IAA can significantly reduce the Cd content and uptake of maize shoots (
Under 20 mg/kg Cd stress, 0.1, 0.5, 1 and 2.5 mmol/L IAA resulted in significant increases in the biological transport factor of maize Cd of 113.8%, 244.5%, 27.3% and 159.2%, respectively. IAA (0.1, 0.5 and 2.5 mmol/L) resulted in significant increases in transfer factor of 189.3%, 258.3% and 204.7%, respectively. In general, the application of IAA can promote maize Cd from root transfer shoots (
IAA concentration |
BTF | TF |
---|---|---|
0 | 0.10 ± 0.02 c | 0.14 ± 0.01 cd |
0.01 | 0.07 ± 0.01 c | 0.08 ± 0.01 d |
0.1 | 0.30 ± 0.04 a | 0.30 ± 0.04 b |
0.5 | 0.37 ± 0.04 a | 0.48 ± 0.04 a |
1 | 0.19 ± 0.05 b | 0.18 ± 0.04 c |
2.5 | 0.32 ± 0.07 a | 0.36 ± 0.06 b |
Different lowercase letters within a column indicate significant differences based on one-way analysis of variance in SPSS 21 followed by the least significant differences at the 5% confidence level.
Correlation analysis indicated that the Cd content of the shoots was significantly negatively correlated with the number of root tips. The Cd content of the roots was extremely negatively correlated with the IAA concentration, extremely positively correlated with root length and root surface area, and positively correlated with root volume. The Cd uptake of roots was extremely negatively correlated with IAA concentration, extremely positively correlated with root length and root surface area, and significantly positively correlated with root volume, root tip number and branch number (Table S1). In addition, the root Ca content was significantly negatively correlated with the Cd content (r = –0.490,
Growth status is an effective indicator of plant resistance. Cd can inhibit plant seed germination, photosynthesis, respiration, nutrient absorption, and other processes, cause plant metabolism disorders, slow growth, and reduce biomass [
The root is the first organ in which plants perceive soil abiotic stress. When roots are subjected to environmental stress, it usually manifests as a decrease in root biomass and a change in root morphology [
Both Ca and Mg are trace elements that are necessary for plant growth and are the same divalent ions as Cd, Ca and Mg have ionic radii similar to those of Cd, and the ions will compete for absorption and transport sites [
In this study, 0.01 mmol/L IAA resulted in significant decreases in the Cd content and uptake of maize shoots, while 0.5 and 2.5 mmol/L IAA resulted in significant increases in the Cd content and uptake of maize shoots. Correlation analysis indicated that the Cd content of the shoots was significantly negatively correlated with the number of root tips. This may be because under 0.01 mmol/L IAA, the number of root tips increased significantly, which promoted maize growth, diluted the Cd in the plant, increased the content of hemicellulose 1, and prevented the transfer of Cd from roots to shoots [
Biological transport factors and transfer factors are indicators to measure the ability of plants to transport heavy metals from roots to shoots after absorbing heavy metals [
Under 20 mg/kg Cd stress, 0.01 mmol/L IAA changed maize root morphology and promoted mineral element uptake in maize, decreased the Cd content and uptake in shoots, and promoted maize growth. In conclusion, the application of optimal concentrations can help maize adapt to cadmium stress. Therefore, this study may help to better understand the role of phytohormones in promoting Cd tolerance in host plants.