Abscisic acid‐mediated modifications of radial apoplastic transport pathway play a key role in cadmium uptake in hyperaccumulator Sedum alfredii

Abstract

Abscisic acid (ABA) is a key phytohormone underlying plant resistance to toxic metals.However, regulatory effects of ABA on apoplastic transport in roots and consequences for uptake of metal ions are poorly understood. Here, we demonstrate how ABA regulates development of apoplastic barriers in roots of two ecotypes of Sedum alfredii and assess effects on cadmium (Cd) uptake. Under Cd treatment, increased endogenous ABA level was detected in roots of nonhyperaccumulating ecotype (NHE) due to up-regulated expressions of ABA biosynthesis genes (SaABA2, SaNCED), but no change was observed in hyperaccumulating ecotype (HE). Simultaneously, endodermal Casparian strips (CSs) and suberin lamellae (SL) were deposited closer to root tips of NHE compared with HE. Interestingly, the vessel-to-CSs overlap was identified as an ABA-driven anatomical trait. Results of correlation analyses and exogenous applications of ABA/Abamine indicate that ABA regulates development of both types of apoplastic barriers through promoting activities of phenylalanine ammonialyase, peroxidase, and expressions of suberin-related genes (SaCYP86A1, SaGPAT5, and SaKCS20). Using scanning ion-selected electrode technique and PTS tracer confirmed that ABA-promoted deposition of CSs and SL significantly reduced Cd entrance into root stele. Therefore, maintenance of low ABA levels in HE minimized deposition of apoplastic barriers and allowed maximization of Cd uptake via apoplastic pathway.

Evolution of Abscisic Acid Signaling for Stress Responses to Toxic Metals and Metalloids

“…Application of ABA further enhanced Cd tolerance and accumulation activity in the Cd-hyperaccumulating ecotype (HE) of Sedum alfredii ( Lu et al, 2020b ). Interestingly, elevated levels of endogenous ABA accompanied by up-regulated SaNCED and SaABA2 was observed in the non-hyperaccumulating ecotype (NHE) subjected to Cd treatment compared to those in HE, restricting radial transport of Cd toward root vascular tissues ( Tao et al, 2019 ; Lu et al, 2020b ).…”

“…Application of ABA promotes Cd resistance and mobility from root to shoot in the Cd-hyperaccumulating ecotype (HE) of Sedum alfredii by inducing the transcription of SaHMA2 , SaHMA3 , and SaHMA4 ( Lu et al, 2020b ). Interestingly, more endogenous ABA was generated in the non-hyperaccumulating ecotype (NHE) subjected to Cd treatment and induced the expression of lignin- and suberin-related biosynthetic enzymes in NHE roots to limit Cd radial transport towards the stele as well as accumulation in the shoot ( Tao et al, 2019 ). Moreover, some membrane transporters are indirectly involved in ABA-related Cd tolerance.…”

Hybridization With an Invasive Plant of Xanthium strumarium Improves the Tolerance of Its Native Congener X. sibiricum to Cadmium

“…Plant endogenous ABA content and gene expression were affected by heavy metal stress, and ABA content was positively correlated with plant tolerance to heavy metals ( Shi W. G. et al, 2019 ; Hu B. et al, 2020 ). Studies on the response of plants such as Arabidopsis , Solanum tuberosum , Sedum alfredii , Malus hupehensis to heavy metal Cd have shown that ABA plays a positive role in alleviating the accumulation and toxicity of heavy metals and metalloid compounds ( Stroiński et al, 2013 ; Sheng et al, 2018 ; Han et al, 2019 ; Tao et al, 2019 ; Zhang W. et al, 2019 ). Three major pathways involved in the detoxification of toxic metals can be triggered by ABA, inhibiting the uptake ( Zhang L. et al, 2019 ; Pan et al, 2020 ), altering the translocation from root to shoot ( Pompeu et al, 2017 ; Shi W. G. et al, 2019 ; Hza et al, 2020 ), and promoting the conjugation with chelators ( Lu et al, 2020 ).…”

The Mechanism of Metal Homeostasis in Plants: A New View on the Synergistic Regulation Pathway of Membrane Proteins, Lipids and Metal Ions

“…ABA alters the movement of Cd and Pb from underground to aboveground plant tissues in a variety of ways [89]. First of all, ABA is capable of promoting the alteration of apoplastic root barriers to inhibit the radial transport of Cd to the stele [90]. Second, ABA inhibits Cd and Pb transport from roots to aboveground tissues by regulating stomatal closure [91].…”