Brassinosteroids Mitigate Cadmium Effects in Arabidopsis Root System without Any Cooperation with Nitric Oxide

Abstract

The heavy metal cadmium (Cd) affects root system development and quiescent center (QC)-definition in Arabidopsis root-apices. The brassinosteroids-(BRs)-mediated tolerance to heavy metals has been reported to occur by a modulation of nitric oxide (NO) and root auxin-localization. However, how BRs counteract Cd-action in different root types is unknown. This research aimed to find correlations between BRs and NO in response to Cd in Arabidopsis’s root system, monitoring their effects on QC-definition and auxin localization in root-apices. To this aim, root system developmental changes induced by low levels of 24-epibrassinolide (eBL) or by the BR-biosynthesis inhibitor brassinazole (Brz), combined or not with CdSO4, and/or with the NO-donor nitroprusside (SNP), were investigated using morpho-anatomical and NO-epifluorescence analyses, and monitoring auxin-localization by the DR5::GUS system. Results show that eBL, alone or combined with Cd, enhances lateral (LR) and adventitious (AR) root formation and counteracts QC-disruption and auxin-delocalization caused by Cd in primary root/LR/AR apices. Exogenous NO enhances LR and AR formation in Cd-presence, without synergism with eBL. The NO-signal is positively affected by eBL, but not in Cd-presence, and BR-biosynthesis inhibition does not change the low NO-signal caused by Cd. Collectively, results show that BRs ameliorate Cd-effects on all root types acting independently from NO.

Convergence between Development and Stress: Ectopic Xylem Formation in Arabidopsis Hypocotyl in Response to 24-Epibrassinolide and Cadmium

“…No pericycle cell proliferation, or very reduced, occurred both in the controls (untreated seedlings) and in Cd-alone-treated seedlings ( Figure 2 A and Figure 3 A, and Insets), without significant differences between the two treatments ( Figure 4 ). The combination of Cd with the highest eBL concentration (1 µM) resulted in a very stunted seedling growth ( Figure S1 ), confirming previous results [ 34 ]. For this reason, the histological analysis was carried out on the seedlings treated with Cd combined with 1 nM or 10 nM eBL and not with eBL at 1 µM concentration.…”

“…Seeds of A. thaliana (L.) Heynh ecotype Columbia (Col) were stratified and sterilized according to Della Rovere and co-workers [ 3 ] and then sown in vitro on a medium containing half-strength Murashige and Skoog [ 52 ] basal salt mixture, 0.5% sucrose and 0.8% agar at pH 5.8. To the control medium (Hormone-free, HF), either 24-epibrassinolide (eBL, Sigma-Aldrich-E1641, Saint Louis, MO, USA) at 1 nM, 10 nM, and 1 µM or 60 µM CdSO 4 (Cd) [ 38 ], alone or combined with 1 nM, 10 nM, and 1 µM eBL, was added [ 34 ]. The Cd concentration was selected based on previous results showing its effects on morphogenesis when applied alone [ 38 , 53 ] or in combination with eBL in seedlings of the same plant [ 34 ].…”

“…To the control medium (Hormone-free, HF), either 24-epibrassinolide (eBL, Sigma-Aldrich-E1641, Saint Louis, MO, USA) at 1 nM, 10 nM, and 1 µM or 60 µM CdSO 4 (Cd) [ 38 ], alone or combined with 1 nM, 10 nM, and 1 µM eBL, was added [ 34 ]. The Cd concentration was selected based on previous results showing its effects on morphogenesis when applied alone [ 38 , 53 ] or in combination with eBL in seedlings of the same plant [ 34 ]. Plates (12 cm × 12 cm; 10 seeds per plate) were placed in a vertical position and kept in a growth chamber at 22 ± 2 °C, 70% humidity.…”

“…For example, their application increases tolerance to Cadmium (Cd), e.g., in tomato [ 32 ], rice [ 33 ], and Arabidopsis [ 25 ]. In the latter, it has been recently demonstrated that exogenous eBL nullifies the negative effects of Cd on the apical structure of the primary root and strongly increases lateral and adventitious root formation [ 34 ].…”

Cadmium treatment induces endoplasmic reticulum stress and unfolded protein response in Arabidopsis thaliana

“…Our data on BRI1, the brassinosteroid receptor which folds under ERQC control, confirm that BRI1 is an important regulator of plant growth under Cd 2+ stress and the normal BRI1 glycoprotein levels observed in bzip28/bzip60 could at least in part explain the better fitness of this mutant under Cd 2+ stress. This is in agreement with a recent paper showing that brassinosteroid treatment increases Cd 2+ tolerance (Della Rovere et al, 2022).…”