Antisense-mediated S-adenosyl-L-methionine decarboxylase silencing affects heat stress responses of tobacco plants.

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.

Underexpression of apoplastic polyamine oxidase improves thermotolerance in Nicotiana tabacum.

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.

Deregulation of apoplastic polyamine oxidase affects development and salt response of tobacco plants.

Polyamine (PA) homeostasis is associated with plant development, growth and responses to biotic/abiotic stresses. Apoplastic PA oxidase (PAO) catalyzes the oxidation of PAs contributing to cellular homeostasis of reactive oxygen species (ROS) and PAs. In tobacco, PAs decrease with plant age, while apoplastic PAO activity increases. Our previous results with young transgenic tobacco plants with enhanced/reduced apoplastic PAO activity (S-ZmPAO/AS-ZmPAO, respectively) established the importance of apoplastic PAO in controlling tolerance to short-term salt stress. However, it remains unclear if the apoplastic PAO pathway is important for salt tolerance at later stages of plant development. In this work, we examined whether apoplastic PAO controls also plant development and tolerance of adult plants during long-term salt stress. The AS-ZmPAO plants contained higher Ca2+ during salt stress, showing also reduced chlorophyll content index (CCI), leaf area and biomass but taller phenotype compared to the wild-type plants during salt. On the contrary, the S-ZmPAO had more leaves with slightly greater size compared to the AS-ZmPAO and higher antioxidant genes/enzyme activities. Accumulation of proline in the roots was evident at prolonged stress and correlated negatively with PAO deregulation as did the transcripts of genes mediating ethylene biosynthesis. In contrast to the strong effect of apoplastic PAO to salt tolerance in young plants described previously, the effect it exerts at later stages of development is rather moderate. However, the different phenotypes observed in plants deregulating PAO reinforce the view that apoplastic PAO exerts multifaceted roles on plant growth and stress responses. Our data suggest that deregulation of the apoplastic PAO can be further examined as a potential approach to breed plants with enhanced/reduced tolerance to abiotic stress with minimal associated trade-offs.

Silencing S-Adenosyl-L-Methionine Decarboxylase (SAMDC) in Nicotiana tabacum Points at a Polyamine-Dependent Trade-Off between Growth and Tolerance Responses.

Polyamines (PAs) are nitrogenous molecules that are indispensable for cell viability and with an agreed-on role in the modulation of stress responses. Tobacco plants with downregulated SAMDC (AS-SAMDC) exhibit reduced PAs synthesis but normal levels of PA catabolism. We used AS-SAMDC to increase our understanding on the role of PAs in stress responses. Surprisingly, at control conditions AS-SAMDC plants showed increased biomass and altered developmental characteristics, such as increased height and leaf number. On the contrary, during salt stress AS-SAMDC plants showed reduced vigor when compared to the WT. During salt stress, the AS-SAMDC plants although showing compensatory readjustments of the antioxidant machinery and of photosynthetic apparatus, they failed to sustain their vigor. AS-SAMDC sensitivity was accompanied by inability to effectively control H2O2 levels and concentrations of monovalent and divalent cations. In accordance with these findings, we suggest that PAs may regulate the trade-off between growth and tolerance responses.

Exploring biodiversity in the bacterial community of the Mediterranean phyllosphere and its relationship with airborne bacteria.

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.