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Abiotic stresses such as heat, drought or salinity have been widely studied individually. Nevertheless, in the nature and in the field, plants and crops are commonly exposed to a different combination of stresses, which often result in a synergistic response mediated by the activation of several molecular pathways that cannot be inferred from the response to each individual stress. By screening microarray data obtained from different plant species and under different stresses, we identified several conserved stress-responsive genes whose expression was differentially regulated in tomato (Solanum lycopersicum L.) roots in response to one or several stresses. We validated 10 of these genes as reliable biomarkers whose expression levels are related to different signalling pathways involved in adaptive stress responses. In addition, the genes identified in this work could be used as general salt-stress biomarkers to rapidly evaluate the response of salt-tolerant cultivars and wild species for which sufficient genetic information is not yet available. [ABSTRACT FROM AUTHOR]

Summary: Nitrogen availability is an important limiting factor for plant growth. Although NH4 + assimilation is energetically more favorable than NO3 −, it is usually toxic for plants. In order to study if an improved ammonium assimilatory metabolism could increase the plant tolerance to ammonium nutrition, tomato (Solanum lycopersicum L. cv P-73) plants were transformed with an NH4 +-dependent asparagine synthetase (AS-A) gene from Escherichia coli (asnA) under the control of a PCpea promoter (pea isolated constitutive promotor). Homozygous (Hom), azygous (Az) asnA and wild type (WT) plants were grown hydroponically for 6 weeks with normal Hoagland nutrition (NO3 −/NH4 + =6/0.5) and high ammonium nutrition (NO3 −/NH4 + =3.5/3). Under Hoagland''s conditions, Hom plants produced 40–50% less biomass than WT and Az plants. However, under NO3 −/NH4 + =3.5/3 the biomass of Hom was not affected while it was reduced by 40–70% in WT and Az plants compared to Hoagland, respectively. The Hom plants accumulated 1.5–4 times more asparagine, glycine, serine and soluble proteins and registered higher glutamine synthetase (GS) and glutamate synthase (GOGAT) activities in the light-adapted leaves than the other genotypes, but had similar NH4 + and NO3 − levels in all conditions. In the dark-adapted leaves, a protein catabolism occurred in the Hom plants with a concomitant 25–40% increase in organic acid concentration, while asparagine accumulation registered the highest values. The aforementioned processes might be responsible for a positive energetic balance as regards the futile cycle of the transgenic protein synthesis and catabolism. This explains growth penalty under standard nutrition and growth stability under NO3 −/NH4 + =3.5/3, respectively. [Copyright &y& Elsevier]

Salinity limits crop productivity, in part by decreasing shoot concentrations of the growth-promoting and senescence-delaying hormones cytokinins. Since constitutive cytokinin overproduction may have pleiotropic effects on plant development, two approaches assessed whether specific root-localized transgenic IPT (a key enzyme for cytokinin biosynthesis) gene expression could substantially improve tomato plant growth and yield under salinity: transient root IPT induction (HSP70::IPT) and grafting wild-type (WT) shoots onto a constitutive IPT-expressing rootstock (WT/35S::IPT). Transient root IPT induction increased root, xylem sap, and leaf bioactive cytokinin concentrations 2- to 3-fold without shoot IPT gene expression. Although IPT induction reduced root biomass (by 15%) in control (non-salinized) plants, in salinized plants (100 mM NaCl for 22 d), increased cytokinin concentrations delayed stomatal closure and leaf senescence and almost doubled shoot growth (compared with WT plants), with concomitant increases in the essential nutrient K+ (20%) and decreases in the toxic ion Na+ (by 30%) and abscisic acid (by 20–40%) concentrations in transpiring mature leaves. Similarly, WT/35S::IPT plants (scion/rootstock) grown with 75 mM NaCl for 90 d had higher fruit trans-zeatin concentrations (1.5- to 2-fold) and yielded 30% more than WT/non-transformed plants. Enhancing root cytokinin synthesis modified both shoot hormonal and ionic status, thus ameliorating salinity-induced decreases in growth and yield. [ABSTRACT FROM PUBLISHER]

• Constitutive ABA overproduction reduces shoot and root growth and close stomata, under optimal conditions. • Constitutive ABA overproduction reduces the percentage loss in shoot and root growth, under salinity conditions. • Constitutive ABA overproduction increases the total root length, under salinity conditions. • Constitutive ABA overproduction limited plant growth under optimal conditions by altering growth regulatory gene networks. • Salinity differentially altered the growth regulatory gene networks in ABA overproducing plants, thereby sustaining growth. To better understand abscisic acid (ABA)'s role in the salinity response of tomato (Solanum lycopersicum L.), two independent transgenic lines, sp5 and sp12, constitutively overexpressing the LeNCED1 gene (encoding 9- cis -epoxycarotenoid dioxygenase, a key enzyme in ABA biosynthesis) and the wild type (WT) cv. Ailsa Craig, were cultivated hydroponically with or without the addition of 100 mM NaCl. Independent of salinity, LeNCED1 overexpression (OE) increased ABA concentration in leaves and xylem sap, and salinity interacted with the LeNCED1 transgene to enhance ABA accumulation in xylem sap and roots. Under control conditions, LeNCED1 OE limited root and shoot biomass accumulation, which was correlated with decreased leaf gas exchange. In salinized plants, LeNCED1 OE reduced the percentage loss in shoot and root biomass accumulation, leading to a greater total root length than WT. Root qPCR analysis of the sp12 line under control conditions revealed upregulated genes related to ABA, jasmonic acid and ethylene synthesis and signalling, gibberellin and auxin homeostasis and osmoregulation processes. Under salinity, LeNCED1 OE prevented the induction of genes involved in ABA metabolism and GA and auxin deactivation that occurred in WT, but the induction of ABA signalling and stress-adaptive genes was maintained. Thus, complex changes in phytohormone and stress-related gene expression are associated with constitutive upregulation of a single ABA biosynthesis gene, alleviating salinity-dependent growth limitation. [ABSTRACT FROM AUTHOR]