**1. Introduction**

Rice (*Oryza sativa*) is the second most abundant cereal crop and fulfils the nutritional needs of at least 50% of the global population [1]. Soil contamination with environmental pollutants (mainly of anthropogenic origin) such as heavy metals (HMs) has caused severe rice yield losses in Asian countries, including China [2]. Rice plants taking up HMs from contaminated soils and their depositing in plant parts, mainly grains, leads to reduction in crop yields [3]. Chromium (Cr) is the seventh most hazardous metal, which usually exists in Cr+3 and Cr+6 forms, while Cr+6 imposes more lethal symptoms on plants, relative to Cr+3 [4]. After taking up Cr ions from soils, plants generally accumulate an excessive amount of Cr in their tissues, severely hindering their growth and development [5–8]. At morpho-physiological, biochemical, metabolic and cellular levels, Cr inhibits the rate of seed germination, plant growth indices (length/height and biomass), photosynthesis process, and accumulation of mineral nutrients [1,7], induces oxidative stress, viz., lipid

**Citation:** Basit, F.; Bhat, J.A.; Ulhassan, Z.; Noman, M.; Zhao, B.; Zhou, W.; Kaushik, P.; Ahmad, A.; Ahmad, P.; Guan, Y. Seed Priming with Spermine Mitigates Chromium Stress in Rice by Modifying the Ion Homeostasis, Cellular Ultrastructure and Phytohormones Balance. *Antioxidants* **2022**, *11*, 1704. https://doi.org/10.3390/ antiox11091704

Academic Editor: Juan B. Barroso

Received: 8 July 2022 Accepted: 22 August 2022 Published: 30 August 2022

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peroxidation, and reactive oxidative species (ROS), damages the inside of cellular membranes and ultra-structures [8,9], and provokes disorganization in antioxidant enzyme activities [10]. Plants boost their internal antioxidant defense system, which includes enzymatic and non-enzymatic antioxidants, by minimizing the overproduction of ROS and the induced oxidative cellular damage [7,11–13].

Polyamines (PAs) are aliphatic compounds and osmo-protectants, with low molecular weight, located within plant cells, and control diverse plant functioning such as regulation of seed germination, embryogenesis, flower expansion, fruit development and ripening, against both normal and stress conditions [6,14]. Moreover, PAs improve the plant's tolerance capacity by enhancing the endogenous accumulation of PAs under various environmental stresses, i.e., water-deficit [15], salt [16], chilling [17], heat [18] and heavy metals [19]. Previous findings have revealed that exogenous applications of PAs improved plants' growth indices by improving levels of chlorophyll pigments, photosystem II, the antioxidant enzymatic defense system and membrane protection, and reducing the HM-induced cellular oxidative damage, as noticed under cadmium [20], lead [21] and chromium [22] stress. Furthermore, PAs such as spermine (SPM) contribute to regulation of cellular functioning, cell division and phytohormonal signal transductions in cadmium and copper exposed wheat leaves [23]. The key mechanism is that the exogenous supply of PAs stimulates the endogenous PA content, which may help plants boost their intrinsic immunity in response to outside environmental stressors, improving the morpho-physiological and biochemical attributes of mung bean [20], wheat [24], rice [25], and maize [22]. However, the contribution of spermine when used as a priming agent in the detoxification of Cr in rice plants has been less commonly investigated. Therefore, the current study was performed to obtain insights into the protective roles of SPM in the alleviation of Cr stress in sensitive (CY927) and tolerant (YLY689) rice cultivars by targeting plant growth traits, photosynthetic apparatus (chlorophyll pigments and photosystem II), nutrients uptake, Cr-accumulation, oxidative damage, antioxidant enzyme activities, endogenous phytohormone production, and membrane or cellular ultra-structures. These biomarker studies may help plant scientists fully understand the mechanistic approaches utilized by SPM to detoxify Cr in rice, and possibly other cereals grown in HM-polluted soils.
