Differences in Cadmium Accumulation, Detoxification and Antioxidant Defenses between Contrasting Maize Cultivars Implicate a Role of Superoxide Dismutase in Cd Tolerance.

Cadmium (Cd), a readily absorbed and translocated toxic heavy metal, inhibits plant growth, interrupts metabolic homeostasis and induces oxidative damage. Responses towards Cd-stress differ among plant cultivars, and the complex integrated relationships between Cd accumulation, detoxification mechanisms and antioxidant defenses still need to be unraveled. To this end, 12 Egyptian maize cultivars were grown under Cd-stress to test their Cd-stress tolerance. Out of these cultivars, tolerant (TWC360 and TWC321), moderately sensitive (TWC324) and sensitive (SC128) cultivars were selected, and we determined their response to Cd in terms of biomass, Cd accumulation and antioxidant defense system. The reduction in biomass was highly obvious in sensitive cultivars, while TWC360 and TWC321 showed high Cd-tolerance. The cultivar TWC321 showed lower Cd uptake concurrently with an enhanced antioxidant defense system. Interestingly, TWC360 accumulated more Cd in the shoot, accompanied with increased Cd detoxification and sequestration. A principal component analysis revealed a clear separation between the sensitive and tolerant cultivars with significance of the antioxidant defenses, including superoxide dismutase (SOD). To confirm the involvement of SOD in Cd-tolerance, we studied the effect of Cd-stress on a transgenic maize line (TG) constitutively overexpressing AtFeSOD gene in comparison to its wild type (WT). Compared to their WT, the TG plants showed less Cd accumulation and improved growth, physiology, antioxidant and detoxification systems. These results demonstrate the role of SOD in determining Cd-tolerance.

Al exposure increases proline levels by different pathways in an Al-sensitive and an Al-tolerant rye genotype.

Aluminium (Al) toxicity limits crop productivity, particularly at low soil pH. Proline (Pro) plays a role in protecting plants against various abiotic stresses. Using the relatively Al-tolerant cereal rye (Secale cereale L.), we evaluated Pro metabolism in roots and shoots of two genotypes differing in Al tolerance, var. RioDeva (sensitive) and var. Beira (tolerant). Most enzyme activities and metabolites of Pro biosynthesis were analysed. Al induced increases in Pro levels in each genotype, but the mechanisms were different and were also different between roots and shoots. The Al-tolerant genotype accumulated highest Pro levels and this stronger increase was ascribed to simultaneous activation of the ornithine (Orn)-biosynthetic pathway and decrease in Pro oxidation. The Orn pathway was particularly enhanced in roots. Nitrate reductase (NR) activity, N levels, and N/C ratios demonstrate that N-metabolism is less inhibited in the Al-tolerant line. The correlation between Pro changes and differences in Al-sensitivity between these two genotypes, supports a role for Pro in Al tolerance. Our results suggest that differential responses in Pro biosynthesis may be linked to N-availability. Understanding the role of Pro in differences between genotypes in stress responses, could be valuable in plant selection and breeding for Al resistance.

Salicylic acid confers resistance against broomrape in tomato through modulation of C and N metabolism.

It is well known that parasitic weeds such as Orobanche (broomrape) significantly decrease crop growth and yield. Although hormonal priming is a well-known inducer of plant resistance against broomrapes (Orobanche spp.), the metabolic events associated with such resistance are poorly understood. Therefore, the current work was undertaken to elucidate the role of SA in inducing tomato resistance against Orobanche, considering its impact on carbon and nitrogen metabolism of the host. Total carbon and nitrogen and levels of carbon (sugars, organic acids and fatty acids) and nitrogen (amino acids and polyamines)-containing metabolites as well as the activities of some key enzymes involved in their metabolic pathways were evaluated. Broomrape infection significantly disrupted C/N ratio in the host roots. On contrary, SA treatment markedly induced accumulation of sugars, organic acids, fatty acids, amino acids as well as polyamines in healthy plants. Under broomrape challenge, SA mitigated the infection-induced growth inhibition by improving the level of nitrogen-containing osmoprotectants (proline, arginine and some polyamines). However, a decrease was observed in some C and N assimilates which are well known to be potentially transferred to the parasite, such as sucrose, asparagine, alanine, serine and glutamate. Interestingly, SA treatment induced the catapolism of polyamines and fatty acids in the host root. Accordingly, our study suggests that SA-induced resistance against broomrape relies on the rational utilization of C and N assimilates in a manner that disturbs the sink strength of the parasite and/or activates the defense pool of the host.

Maize roots and shoots show distinct profiles of oxidative stress and antioxidant defense under heavy metal toxicity.

Heavy metal accumulation in agricultural land causes crop production losses worldwide. Metal homeostasis within cells is tightly regulated. However, homeostasis breakdown leads to accumulation of reactive oxygen species (ROS). Overall plant fitness under stressful environment is determined by coordination between roots and shoots. But little is known about organ specific responses to heavy metals, whether it depends on the metal category (redox or non-redox reactive) and if these responses are associated with heavy metal accumulation in each organ or there are driven by other signals. Maize seedlings were subjected to sub-lethal concentrations of four metals (Zn, Ni, Cd and Cu) individually, and were quantified for growth, ABA level, and redox alterations in roots, mature leaves (L1,2) and young leaves (L3,4) at 14 and 21 days after sowing (DAS). The treatments caused significant increase in endogenous metal levels in all organs but to different degrees, where roots showed the highest levels. Biomass was significantly reduced under heavy metal stress. Although old leaves accumulated less heavy metal content than root, the reduction in their biomass (FW) was more pronounced. Metal exposure triggered ABA accumulation and stomatal closure mainly in older leaves, which consequently reduced photosynthesis. Heavy metals induced oxidative stress in the maize organs, but to different degrees. Tocopherols, polyphenols and flavonoids increased specifically in the shoot under Zn, Ni and Cu, while under Cd treatment they played a minor role. Under Cu and Cd stress, superoxide dismutase (SOD) and dehydroascorbate reductase (DHAR) activities were induced in the roots, however ascorbate peroxidase (APX) activity was only increased in the older leaves. Overall, it can be concluded that root and shoot organs specific responses to heavy metal toxicity are not only associated with heavy metal accumulation and they are specialized at the level of antioxidants to cope with.