Ethylene Inhibits Root Elongation during Alkaline Stress through AUXIN1 and Associated Changes in Auxin Accumulation

Abstract

Soil alkalinity causes major reductions in yield and quality of crops worldwide. The plant root is the first organ sensing soil alkalinity, which results in shorter primary roots. However, the mechanism underlying alkaline stress-mediated inhibition of root elongation remains to be further elucidated. Here, we report that alkaline conditions inhibit primary root elongation of Arabidopsis (Arabidopsis thaliana) seedlings by reducing cell division potential in the meristem zones and that ethylene signaling affects this process. The ethylene perception antagonist silver (Ag(+)) alleviated the inhibition of root elongation by alkaline stress. Moreover, the ethylene signaling mutants ethylene response1-3 (etr1-3), ethylene insensitive2 (ein2), and ein3-1 showed less reduction in root length under alkaline conditions, indicating a reduced sensitivity to alkalinity. Ethylene biosynthesis also was found to play a role in alkaline stress-mediated root inhibition; the ethylene overproducer1-1 mutant, which overproduces ethylene because of increased stability of 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID SYNTHASE5, was hypersensitive to alkaline stress. In addition, the ethylene biosynthesis inhibitor cobalt (Co(2+)) suppressed alkaline stress-mediated inhibition of root elongation. We further found that alkaline stress caused an increase in auxin levels by promoting expression of auxin biosynthesis-related genes, but the increase in auxin levels was reduced in the roots of the etr1-3 and ein3-1 mutants and in Ag(+)/Co(2+)-treated wild-type plants. Additional genetic and physiological data showed that AUXIN1 (AUX1) was involved in alkaline stress-mediated inhibition of root elongation. Taken together, our results reveal that ethylene modulates alkaline stress-mediated inhibition of root growth by increasing auxin accumulation by stimulating the expression of AUX1 and auxin biosynthesis-related genes.

Ethylene mediates dichromate‐induced inhibition of primary root growth by altering AUX1 expression and auxin accumulation in Arabidopsis thaliana

“…These results indicate that ethylene participates actively in the regulation of auxin distribution and accumulation and that ethylene is regulating auxin‐mediated root growth inhibition under high Cr(VI) levels. In accordance to our current study, the effect of ethylene on the auxin levels under different abiotic stresses has been widely reported (Li, Xu, Liu, et al, ; Ruzicka et al, ; Sun et al, ). For further confirmation of this phenomenon, we exposed 7 days old seedlings of WT and aux1‐7 to ACC, Cr(VI), and ACC–Cr(VI) for 24 hr, followed by transfer of the treated seedlings to fresh ½ MS and measurement of the primary root elongation after 4 days.…”

“…Certainly, Cr(VI) down‐regulated the expression level of CYCA2‐1 , CYCB1‐1 , CYCD1‐1 , CYCD2‐1 , and E2Fb , which are cell cycle progressive elements, and induced the expression level of E2Fc gene that negatively regulates the cell division. These genes were investigated in response to other abiotic stresses, such as phosphate starvation, excess copper, alkaline stress, and toxic levels of Cr (Lei et al, ; Li, Xu, Liu, et al, ; Ortiz‐Castro et al, ; Yuan et al, ). Considering that phytohormone ethylene is a negative regulator of cell development, the induced quantity of ethylene is reported previously in the root of plants exposed to other abiotic stresses, such as Al and alkaline stress (Li, Xu, Liu, et al, ; Sun et al, ; Sun et al, ).…”

“…The auxin responsive synthetic promoter line DR5::GUS (Ulmasov, Murfett, Hagen, & Guilfoyle, ) was used to analyse auxin responses. The transgenic line AUX1::AUX1‐YFP (AUX1‐Yellow Fluorescent Protein ) was used to analyse AUX1 expression level in response to abiotic stresses (Li, Xu, Liu, et al, ; Swarup et al, ). Seeds of all mentioned lines were surface‐sterilized with 8% sodium hypochlorite for 10 min, followed by five times washing with sterilized distilled water under aseptic environment and incubated for 3 days at 4 °C.…”

“…To investigate the inhibitory effects of potassium dichromate [K 2 Cr2O 7 {Cr(VI)}] on root length, 7 days old seedlings of wild‐type (WT), etr1‐3 , ein2‐1 , eto1‐1 , and aux1‐7 were exposed to (0, 50, 100, and 200 μM) concentrations of Cr(VI) as potassium dichromate (Eleftheriou et al, ; Ma et al, ) in ½ MS for 24 hr. All treated plants were transferred to fresh ½ MS for another 4 days, increase in the primary root elongation after treatment was measured and analysed statistically (Li, Xu, Liu, et al, ; Yuan & Huang, ). Seven days old WT seedlings were treated with 10 μM of ACC, ACC–Cr(VI) [10 μM ACC + 100 μM Cr(VI)], IAA (indole‐3‐acetic acid), or IAA–Cr(VI) [10 μM IAA + 100 μM Cr(VI)] in ½ MS for 24 hr, all of the treated roots were transferred to fresh ½ MS for another 4 days, increase in the primary root elongation after treatment was measured and compared with control (7 days old seedlings exposed to ½ MS for 24 hr and transferred to fresh ½ MS for another 4 days).…”

“…The qRT‐PCR investigation was conducted in the 96‐well plate, with the following PCR settings: 600 s at 95 °C followed by 40 cycles of denaturation for 15 s at 95 °C, annealing for 15 s at 58 °C, and extension for 20 s at 72 °C, using SYBR green (Roche). The ACTIN2 (AT3G18780) and eukaryotic translation initiation factor 4A (AT3G13920) genes were used as reference genes, and the gene‐specific primers for all the above‐mentioned genes including reference genes are given in Table (Li, Xu, Liu, et al, ; Sun et al, ). The normalized expression levels of the genes were calculated according to the corresponding controls.…”