ABSTRACT
Understanding the molecular mode(s) of plant tolerance to heat stress (HS) is crucial since HS is a potential threat to sustainable agriculture and global crop production. Polyamines (PAs) seem to exert multifaceted effects in plant growth and development and responses to abiotic and biotic stresses, presumably via their homeostasis, chemical interactions and contribution to hydrogen peroxide (H2O2) cellular 'signatures'. Downregulation of the apoplastic POLYAMINE OXIDASE (PAO) gene improved thermotolerance in tobacco (Nicotiana tabacum L.) transgenics. However, in the present work we show that transgenic tobacco plants with antisense-mediated S-ADENOSYL-L-METHIONINE DECARBOXYLASE silencing (AS-NtSAMDC) exhibited enhanced sensitivity and delayed responses to HS which was accompanied by profound injury upon HS removal (recovery), as assessed by phenological, physiological and biochemical characteristics. In particular, the AS-NtSAMDC transgenics exhibited significantly reduced rate of photosynthesis, as well as enzymatic and non-enzymatic antioxidants. These transgenics suffered irreversible damage, which significantly reduced their growth potential upon return to normal conditions. These data reinforce the contribution of increased PA homeostasis to tolerance, and can move forward our understanding on the PA-mediated mechanism(s) conferring tolerance to HS that might be targeted via traditional or biotechnological breeding for developing HS tolerant plants.
Subject(s)
Hydrogen Peroxide , Nicotiana , Carboxy-Lyases , Gene Expression Regulation, Plant , Heat-Shock Response , Hydrogen Peroxide/metabolism , Plant Breeding , Nicotiana/metabolismABSTRACT
Polyamines (PAs) and hydrogen peroxide (H2O2), the product of PA oxidation by polyamine oxidase (PAO), are potential players affecting plant growth, development and responses to abiotic/biotic stresses. Genetically modified Nicotiana tabacum plants with altered PA/H2O2 homeostasis due to over/underexpression of the ZmPAO gene (S-ZmPAO/AS-ZmPAO, respectively) were assessed under heat stress (HS). Underexpression of ZmPAO correlates with increased thermotolerance of the photosynthetic machinery and improved biomass accumulation, accompanied by enhanced levels of the enzymatic and non-enzymatic antioxidants, whereas ZmPAO overexpressors exhibit significant impairment of thermotolerance. These data provide important clues on PA catabolism/H2O2/thermotolerance, which merit further exploitation.
Subject(s)
Gene Expression Regulation, Plant , Nicotiana/physiology , Oxidoreductases Acting on CH-NH Group Donors/genetics , Plant Growth Regulators/metabolism , Plant Proteins/genetics , Thermotolerance , Hydrogen Peroxide/metabolism , Oxidation-Reduction , Oxidoreductases Acting on CH-NH Group Donors/metabolism , Plant Proteins/metabolism , Plants, Genetically Modified/genetics , Plants, Genetically Modified/metabolism , Polyamines/metabolism , Zea mays/genetics , Polyamine OxidaseABSTRACT
We studied the structure and diversity of the phyllosphere bacterial community of a Mediterranean ecosystem, in summer, the most stressful season in this environment. To this aim, we selected nine dominant perennial species, namely Arbutus unedo, Cistus incanus, Lavandula stoechas, Myrtus communis, Phillyrea latifolia, Pistacia lentiscus, Quercus coccifera (woody), Calamintha nepeta, and Melissa officinalis (herbaceous). We also examined the extent to which airborne bacteria resemble the epiphytic ones. Genotype composition of the leaf and airborne bacteria was analysed by using denaturing gradient gel electrophoresis profiling of a 16S rDNA gene fragment; 75 bands were cloned and sequenced corresponding to 28 taxa. Of these, two were found both in the air and the phyllosphere, eight only in the air, and the remaining 18 only in the phyllosphere. Only four taxa were found on leaves of all nine plant species. Cluster analysis showed highest similarity for the five evergreen sclerophyllous species. Aromatic plants were not grouped all together: the representatives of Lamiaceae, bearing both glandular and non-glandular trichomes, formed a separate group, whereas the aromatic and evergreen sclerophyllous M. communis was grouped with the other species of the same habit. The epiphytic communities that were the richest in bacterial taxa were those of C. nepeta and M. officinalis (Lamiaceae). Our results highlight the remarkable presence of lactic acid bacteria in the phyllosphere under the harsh conditions of the Mediterranean summer, the profound dissimilarity in the structure of bacterial communities in phyllosphere and air, and the remarkable differences of leaf microbial communities on neighbouring plants subjected to similar microbial inocula; they also point to the importance of the leaf glandular trichome in determining colonization patterns.
