Pests and diseases pose a threat to food security, which is nowadays aggravated by climate change and globalization. In this context, agricultural policies demand innovative approaches to more effectively manage resources and overcome the ecological issues raised by intensive farming. Optimization of plant mineral nutrition is a sustainable approach to ameliorate crop health and yield. Zinc is a micronutrient essential for all living organisms with a key role in growth, development, and defense. Competition for Zn affects the outcome of the host–attacker interaction in both plant and animal systems. In this review, we provide a clear framework of the different strategies involving low and high Zn concentrations launched by plants to fight their enemies. After briefly introducing the most relevant macro- and micronutrients for plant defense, the functions of Zn in plant protection are summarized with special emphasis on superoxide dismutases (SODs) and zinc finger proteins. Following, we cover recent meaningful studies identifying Zn-related passive and active mechanisms for plant protection. Finally, Zn-based strategies evolved by pathogens and pests to counteract plant defenses are discussed.

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The role of defence gene expression triggered by Cd toxicity in the plant's response to Botrytis cinerea was investigated in Arabidopsis thaliana Columbia 0. Silicon (0 or 1.5 mM) and Cd (0, 1 or 10 μM) were supplied to 3-month-old solution-cultured plants. After 3 days, half of the plants of each treatment were inoculated with Botrytis. Supplied Cd concentrations were below the toxicity threshold and did not cause shoot growth inhibition or evidence of oxidative stress, while Botrytis infection severely decreased plant growth in all treatments. The expression of marker genes PR1 and BGL2 for the salicylic acid (SA) and the PDF1.2 for the jasmonic acid-ethylene (JA-ET) signalling pathways was enhanced in 10 μM Cd-treated non-infected plants. Twenty hours after inoculation, PDF1.2 expression showed a strong increase in all treatments, while enhanced PR1, BGL2, and CHIB expression was only found 7 days after infection. A great synergistic effect of Cd and Botrytis on PDF1.2 expression was found in 10 μM Cd-treated plants. Silicon decreased PR1, BGL2, and CHIB, while increasing PDF1.2 expression, which indicates its role as a modulator of the signalling pathways involved in the plant's response to fungal infection. Botrytis growth decreased in 10 μM Cd-treated plants, which could be due to the combined effects of Cd and Botrytis activating the SA and JA-ET-mediated signalling pathways. Taken together, our results provide support for the view that Cd concentrations close to the toxicity threshold induce defence signalling pathways which potentiate the plant's response against fungal infection.

Two tree medics contrasting in salt tolerance, Medicago arborea and Medicago citrina, were compared to evaluate the relative importance of abscisic acid on leaf growth and stomatal responses to salt stress. Plants were grown for 30 d in solution culture with 1, 50, 100 or 200 mM NaCl. Salinized plants of M. citrina had lower Na+ and Cl- uptake and maintained better leaf growth than M. arborea. In M. citrina, stomatal conductance was only slightly affected by salt and, in consequence, the salt treatment had no significant influence, neither on the CO2 fixation rate nor the transpiration rate in these plants. Moreover, leaf photosynthetic pigments and soluble protein in M. citrina were increased by the presence of NaCl, while a decrease of both parameters with salt was found in M. arborea. However, leaf and xylem ABA increased only in salt-treated M. citrina, while no differences were found among treatments in M. arborea. The role of ion compartmentation, gas exchange parameters and ABA concentrations in relation to salt tolerance in M. arborea and M. citrina is discussed.