Aluminium (Al) toxicity is widely considered to be the most important growth‐limiting factor for plants in strongly acid soils (pH<5.0). The inhibition of root elongation in three varieties of maize (Zea mays L. vars Clavito, HS701b and Sikuani) was followed over the first 48 h of Al treatment, and during the initial 10 h elongation was determined on an hourly basis. The silicon (Si)‐induced amelioration of Al toxicity was investigated by pre‐treating seedlings for 72 h in nutrient solutions with 1000 μM Si before transfer into solutions with 0, 20 or 50 μM Al (without Si). Plants were either grown in complete low ionic strength nutrient solutions (CNS) or in low salt solutions of 0.4 mM CaCl₂ (LSS). In addition, the role of root exudation of organic compounds as a mechanism of Si‐induced alleviation of Al toxicity was investigated. Aluminium‐induced inhibition of root elongation in the maize var. HS701b was observed within 1 h of Al exposure. After a lag time of at least 8 h, Si‐induced alleviation of Al toxicity was observed in this variety when grown in LSS. In the Al‐resistant var. Sikuani, Al‐resistance was only observed after exposure to 50 μM Al, and not after exposure to 20 μM Al, suggesting that there exists a threshold Al concentration before the mechanisms of Al resistance are activated. Aluminium stimulated root exudation of oxalic acid in all three varieties, but exudate concentrations did not increase with either Al resistance or with Si pretreatment. Aluminium and Si triggered release of catechol and of the flavonoid‐type phenolics: catechin, and quercetin. In the Al‐resistant variety, Sikuani, Al‐exposed plants pretreated with Si exuded up to 15 times more phenolics than those plants not pretreated with Si. The flavonoid‐type phenolics, to date unconsidered, appear to play a role in the mechanism(s) of Si‐induced amelioration of Al toxicity.

The short-term influence (5–180 min) of 50 μM Al on cell division was investigated in root tips of two Zea mays L. varieties differing in Al-resistance. The incorporation of bromodeoxyuridine into S-phase nuclei was visualized by immunofluorescence staining using confocal laser fluorescence microscopy. In Al-sensitive plants 5 min Al exposure was enough to inhibit cell division in the proximal meristem (250–800 μm from the tip). After 10 or 30 min with Al only, a few S-phase nuclei were found in the cortical initials. By contrast, cell division was stimulated in the distal elongation zone (2.5–3.1 mm). After 180 min the protrusion of an incipient lateral root was observed in this zone. These observations suggest a fast change in cell patterning rather than a general cariotoxic effect after exposure to Al for a short time. No such changes were found in Al-resistant maize. This is the first report showing such fast Al-induced alterations in the number and the position of dividing cells in root tips. The observation that similar changes were induced by a local supply of naphthylphthalamic acid to the distal transition zone suggests that inhibition of auxin transport plays a role in the Al-induced alteration of root cell patterning.

Bicarbonate plays a fundamental role in the cell pH status in all organisms. In autotrophs, HCO₃⁻ may further contribute to carbon concentration mechanisms (CCM). This is especially relevant in the CO₂-poor habitats of cyanobacteria, aquatic microalgae, and macrophytes. Photosynthesis of terrestrial plants can also benefit from CCM as evidenced by the evolution of C₄ and Crassulacean Acid Metabolism (CAM). The presence of HCO₃⁻ in all organisms leads to more questions regarding the mechanisms of uptake and membrane transport in these different biological systems. This review aims to provide an overview of the transport and metabolic processes related to HCO₃⁻ in microalgae, macroalgae, seagrasses, and terrestrial plants. HCO₃⁻ transport in cyanobacteria and human cells is much better documented and is included for comparison. We further comment on the metabolic roles of HCO₃⁻ in plants by focusing on the diversity and functions of carbonic anhydrases and PEP carboxylases as well as on the signaling role of CO₂/HCO₃⁻ in stomatal guard cells. Plant responses to excess soil HCO₃⁻ is briefly addressed. In conclusion, there are still considerable gaps in our knowledge of HCO₃⁻ uptake and transport in plants that hamper the development of breeding strategies for both more efficient CCM and better HCO₃⁻ tolerance in crop plants.

A relationship between aluminum (Al) toxicity, endocytosis, endosomes and vesicle recycling in the root transition zone has recently been demonstrated. Here the importance of filamentous actin (F-actin)-based vesicle trafficking for Al tolerance has been investigating in maize varieties differing in their Al sensitivities. More Al was internalized into root tip cells of the Al-sensitive variety 16×36 than in the Al-tolerant variety Cateto. The actin cytoskeleton and vesicle trafficking were primary targets for Al toxicity in the root tips of the sensitive variety. Visualization of boron-cross-linked rhamnogalacturonan II (RGII)-containing brefeldin A (BFA) compartments revealed that Al inhibited the formation of these compartments, especially in variety 16×36. The time sequence of Al effects on pectin recycling matches the growth effects of Al in this sensitive variety. These results support the hypothesis that Al binding to pectin-rich cell walls can contribute to reversible inhibition of root elongation. Al-induced alterations on F-actin were most evident in the central part of the transition zone of Al-sensitive 16×36, where Al was localized inside the nucleoli. In relation to this observation, a role for symplastic Al in both irreversible growth inhibition and amelioration of BFA-induced inhibition of root elongation is discussed.