1. Root acclimation enhances zinc tolerance in tobacco plants
Nadia Bazihizina1, Cosimo Taiti1, Lucia Marti1, Ana Rodrigo-Moreno1, Christiana Giordano2, Stefania Caparrotta1, Massimo Gori1, Elisa Azzarello1, Stefano Mancuso1
1 LINV - Department of Agrifood Production and Environmental Sciences – University of Florence, Viale delle Idee 30, Sesto F.no, Florence, Italy
2 Centro di Microscopie Elettroniche “ Laura Bonzi” (Ce.M.E.), ICCOM, CNR, Via Madonna del Piano, Sesto F.no, Florence, Italy
Tobacco plants were grown to evaluate long- and short-term responses to Zn2+ stress in acclimated and non acclimated plants. In the long-term experiment, plants were exposed to: 4 (control), 30, 250 and 500 μM ZnSO4. One week prior to the addition of 250 and 500 μM ZnSO4, with half of the plants were exposed to 30 μM ZnSO4 (i.e. acclimation) meanwhile the remaining were left in a control solution. Short-term experiments focused on the responses to 250 μM ZnSO4 with or without acclimation.
Acclimation occurs during plant ontogeny and describes enhanced stress tolerance as a result of physiological, biochemical and molecular adjustments within the plant (Pandolfi et al. 2012). Although examples of acclimation processes in plants, such as cold acclimation, are well accepted, plant acclimation to heavy metal is still a controversial topic and very little information is available on the putative mechanism(s) underlying the increased tolerance.
Acclimation improved growth after 3 week of treatments
Control
After 3 weeks of Zn2+ treatments, in acclimated plants both leaf photosynthetic pigments and leaf area mass remained similar or decreased slightly compared to values in control plants, meanwhile with 250 and 500 μM ZnSO4 both parameters declined by more than 50%. With 250 μM ZnSO4, the acclimation process led to a substantial increase in total plant dry mass.
Acclimation reduced root damage both in cortical cells and in the central cylinder. By contrast, with no acclimation the presence of elevated Zn2+ severely damaged cortical cells; with 500 μM ZnSO4 roots presented damaged cells in the central cylinder with collapsed cytoplasmic organelles. * *
Auxin Localization
No acclimation
Acclimation
250 μM Zn2+ * * *
500 μM Zn2+
To unravel mechanism(s) underlying enhanced Zn2+-tolerance in acclimated plants, we looked at roots responses after the addition of 250 μM Zn2+
Root cortical cells
membrane potential
Zinc localization in root cortical cells
Expression in roots of tobacco ortholog of Arabidopsis thaliana MTP1 gene
No acclimation
Metal Tolerance Proteins (MTPs) are vacuolar metal transporters that regulate translocation of metal ions towards the vacuoles (Arrivault et al. 2006)
Increased expression of the gene in acclimated plants and following Zn2+ exposure
Given homology with MTP3 (At3g58810), can it be an MTP3 ortholog? *
No acclimation
Acclimation * *
Time 0 *
Acclimation
Conclusions
24 h
with 250
μM Zn2+
Acclimation to Zn2+ stress improved plant growth, and this was associated, in the short-term, with an improved root membrane functionality
Improved root membrane functionality was probably depended on the higher Zn2+-sequestration in the vacuoles.
Although MTPs genes have not been characterized in tobacco, results suggest their involvement in vacuolar sequestration of Zn2+; as they are energized by proton gradients, these results would also explain root hyperpolarization in acclimated plants following Zn2+ addition.
Acclimation with 30 μM ZnSO4 was associated with a transient transient hyperpolarization of membrane potentials following Zn2+ addition and a more negative membrane potentials in cortical cells in the following 24 h
Acclimation with 30 μM ZnSO4 was associated with enhanced vacuolar sequestration, suggesting an improved detoxifying mechanism
Arrivault S, Senger T, Kramer U The Arabidopsis metal tolerance protein AtMTP3 maintains metal homeostasis by mediating Zn exclusion from the shoot under Fe deficiency and Zn oversupply. Plant Journal 46:
Pandolfi C, Mancuso S, Shabala S Physiology of acclimation to salinity stress in pea (Pisum sativum). Environmental and Experimental Botany 84: