Abstract
We explored the interplay between ethylene signals and the auxin pool in roots exposed to high salinity using Arabidopsis thaliana wild-type plants (Col-0), and the ethylene-signaling mutants ctr1-1 (constitutive) and ein2-1 (insensitive). The negative effect of salt stress was less pronounced in ctr1-1 individuals, which was concomitant with augmented auxin signaling both in the ctr1-1 controls and after 100 mM NaCl treatment. The R2D2 auxin sensorallowed mapping this active auxin increase to the root epidermal cells in the late Cell Division (CDZ) and Transition Zone (TZ). In contrast, the ethylene-insensitive ein2-1 plants appeared depleted in active auxins. The involvement of ethylene/auxin crosstalk in the salt stress response was evaluated by introducing auxin reporters for local biosynthesis (pTAR2::GUS) and polar transport (pLAX3::GUS, pAUX1::AUX1-YFP, pPIN1::PIN1-GFP, pPIN2::PIN2-GFP, pPIN3::GUS) in the mutants. The constantly operating ethylene-signaling pathway in ctr1-1 was linked to increased auxin biosynthesis. This was accompanied by a steady expression of the auxin transporters evaluated by qRT-PCR and crosses with the auxin transport reporters. The results imply that the ability of ctr1-1 mutant to tolerate high salinity could be related to the altered ethylene/auxin regulatory loop manifested by a stabilized local auxin biosynthesis and transport.
Highlights
Saline soils present a serious agricultural constraint especially in coastal areas and in regions with industrial pollution or intensive plant breeding
We evaluated the changes of the expression in the efflux carriers PIN-FORMED1 (PIN1), PIN-FORMED 2 (PIN2), and PIN-FORMED3 (PIN3) as well as the influx carriers AUXIN RESISTANT 1 (AUX1) and LIKE AUX1 3 (LAX3) under high salinity, as it has been already demonstrated that some of them were involved in auxin–ethylene interactions [31]
Disturbed root bending upon gravitropic stimulation is indicative of defects in auxin-dependent processes such as vesicle trafficking and cytoskeletal organization [40,41].The root elongation and gravitropic response on high salinity substrate outlined the differential reaction of the constitutive ethylene-signaling mutant ctr1-1 to salt stress compared to the wild type and ein2-1 (Figure 1a)
Summary
Saline soils present a serious agricultural constraint especially in coastal areas and in regions with industrial pollution or intensive plant breeding. Moderate salt stress could remain undetected since it causes no apparent injuries other than restricted growth. This means that high salinity has a negative effect on the signaling cascades involved in the regulation of plant growth and development. As a major stress hormone, ethylene causes growth reduction primarily due to the inhibition of cell expansion which is an adaptive response to the adverse environment [3,4]. Several studies have demonstrated that plants exposed to salt stress show induced ethylene biosynthesis and have enhanced ethylene signaling maintaining both shoot and primary root growth [5,6,7,8,9,10]. The existing experimental evidence for organ-specific regulatory mechanisms in response to salt stress [12] calls for the elucidation of ethylene signal input at an organ-specific basis
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