Aluminium stress tolerance by Citrus plants: a consolidated review

Abstract

Aluminium, a metallic element abundant in soils as aluminosilicates minerals, poses a toxic threat to plants, particularly in acidic soil conditions, thereby affecting their growth and development. Given their adaptability to diverse soil and climate conditions, Citrus plants have gained significant attention regarding their tolerance to Aluminium toxicity. In the North-eastern region of India, where soils are often slightly acidic with elevated aluminium levels, Citrus species are predominantly found. Understanding the tolerance mechanisms of these Citrus fruits and screening wild Citrus species for their adaptability to abiotic stresses is crucial for enhancing fruit production. Numerous investigations have demonstrated that Citrus species exhibit remarkable tolerance to aluminium contamination, surpassing the typical threshold of 30% incidence. When cultivated in acidic soils, Citrus plants encounter restricted root growth and reduced nutrient and moisture uptake, leading to various nutrient deficiency symptoms. However, promisingly, certain Citrus species such as Citrus jambhiri (Rough lemon), Poncirus trifoliata, Citrus sinensis, and Citrus grandis have shown considerable aluminium tolerance. This comprehensive review delves into the subject of aluminium toxicity and its implications, while also shedding light on the mechanisms through which Citrus plants develop tolerance to this element.

Keywords: Aluminium toxicity; Citrus species; Molecular; Stress.

Fig. 1

The longitudinal root section providing some valuable visual insights into the morphological and structural changes induced by aluminum stress in plant roots, highlighting the detrimental effects of aluminum toxicity on root architecture and cellular integrity (Adapted from Pandey 2014)

The figure provides visual comparisons between control conditions (no aluminum treatment) and aluminum-treated conditions for both Citrus shoots and roots, highlighting the detrimental effects of aluminum toxicity on the growth of Citrus plants (A: Control, B: Al treated) and roots (C: Control, D: Al treated)

Fig. 3

The figure is clearly providing a comprehensive understanding of the mechanisms involved in aluminum tolerance via the ABA signalling pathway offering insights into the adaptive strategies employed by plants to cope with aluminum toxicity. The ABA signalling pathway leads to the increased synthesis and secretion of organic acids, such as citrate and malate, from the roots. The citrate and malate organic acids chelate aluminum ions in the rhizosphere, reducing root absorption. Despite these adaptive responses, aluminum stress still negatively impacts plant physiology by diminishing carboxylation, suppressing photosynthesis, increasing ROS production, damaging cell membranes, and obstructing ATP synthesis [Aluminum-activated malate transporter (ALMT) and Aluminum-activated citrate transporter (AACT): These transporters are known to mediate the efflux of malate and citrate from roots in response to aluminium stress]

Fig. 4

This figure provides an elaborate explanation of the various exclusion and tolerance mechanisms employed by Citrus plants to combat aluminum toxicity at the cellular level, offering insights into the intricate regulatory processes involved in plant adaptation to environmental stressors. (Adapted from Yan et al. 2022)

Fig. 5

A visual representation depicting hormone signalling pathways under aluminium stress, encompassing the diverse roles of auxins, cytokinins, ethylene and their interactions in coordinating adaptive responses and stress tolerance mechanisms (Adapted from Liu et al. 2022)