Low Salicylic Acid Level Improves Pollen Development Under Long-Term Mild Heat Conditions in Tomato.

Exposure to high temperatures leads to failure in pollen development, which may have significant implications for food security with ongoing climate change. We hypothesized that the stress response-associated hormone salicylic acid (SA) affects pollen tolerance to long-term mild heat (LTMH) (≥14 days exposure to day-/nighttime temperature of 30-34/24-28°C, depending on the genotype), either positively, by inducing acclimation, or negatively, by reducing investment in reproductive development. Here, we investigated these hypotheses assessing the pollen thermotolerance of a 35S:nahG tomato line, which has low SA levels. We found that reducing the SA level resulted in increased pollen viability of plants grown in LTMH and further characterized this line by transcriptome, carbohydrate, and hormone analyses. Low expression of JAZ genes in 35S:nahG and LTMH hypersensitivity of low-jasmonic acid (JA) genotypes together suggest that the increased pollen thermotolerance in the low-SA line involves enhanced JA signal in developing anthers in LTMH. These findings have potential application in the development of more thermotolerant crops.

Quantitative trait loci analysis of hormone levels in Arabidopsis roots.

Quantitative trait loci (QTL) analyses for five groups of hormones, including cytokinins in Arabidopsis roots were performed using recombinant inbred lines (Ler×Cvi). Significant QTLs were detected for cytokinins, jasmonic acid and salicylic acid. Separate analysis of two sub-populations, viz., vegetative and flowering plants revealed that many of the QTLs were development-specific. Using near-isogenic lines, several significant QTLs were confirmed; three co-localized QTL regions were responsible for determining several cytokinin metabolites. Using a knock-out plant, a functional role of zeatin N-glucosyltransferase gene (UGT76C2) underlying a large-effect QTL for levels of tZ-N-glucosides and tZRMP was evaluated in the metabolism of cytokinins. Pleotropic effects of this gene were found for cytokinin levels in both roots and leaves, but significant changes of morphological traits were observed only in roots. Hormone QTL analysis reveals development-specific and organ-dependent aspects of the regulation of plant hormone content and metabolism.

NADP-MALIC ENZYME 1 Affects Germination after Seed Storage in Arabidopsis thaliana.

Aging decreases the quality of seeds and results in agricultural and economic losses. The damage that occurs at the biochemical level can alter the seed physiological status. Although loss of viability has been investigated frequently, little information exists on the molecular and biochemical factors involved in seed deterioration and loss of viability. Oxidative stress has been implicated as a major contributor to seed deterioration, and several pathways are involved in protection against this. In this study, we show that seeds of Arabidopsis thaliana lacking a functional NADP-MALIC ENZYME 1 (NADP-ME1) have reduced seed viability relative to the wild type. Seeds of the NADP-ME1 loss-of-function mutant display higher levels of protein carbonylation than those of the wild type. NADP-ME1 catalyzes the oxidative decarboxylation of malate to pyruvate with the simultaneous production of CO2 and NADPH. Upon seed imbibition, malate and amino acids accumulate in embryos of aged seeds of the NADP-ME1 loss-of-function mutant compared with those of the wild type. NADP-ME1 expression is increased in imbibed aged as compared with non-aged seeds. NADP-ME1 activity at testa rupture promotes normal germination of aged seeds. In seedlings of aged seeds, NADP-ME1 is specifically active in the root meristematic zone. We propose that NADP-ME1 activity is required for protecting seeds against oxidation during seed dry storage.

Auxin synthesis gene tms1 driven by tuber-specific promoter alters hormonal status of transgenic potato plants and their responses to exogenous phytohormones.

KEY MESSAGE: Ectopic auxin overproduction in transgenic potato leads to enhanced productivity accompanied with concerted and occasional changes in hormonal status, and causing altered response of transformants to exogenous auxin or cytokinin. Previously, we generated potato transformants expressing Agrobacterium-derived auxin synthesis gene tms1 driven by tuber-specific patatin gene promoter (B33-promoter). Here, we studied the endogenous hormonal status and the response to exogenous phytohormones in tms1 transformants cultured in vitro. Adding indole-3-acetic acid (IAA) or kinetin to culture medium affected differently tuberization of tms1-transformed and control plants, depending also on sucrose content in the medium. Exogenous phytohormones ceased to stimulate the tuber initiation in transformants at high (5-8%) sucrose concentration, while in control plants the stimulation was observed in all experimental settings. Furthermore, exogenous auxin partly inhibited the tuber initiation, and exogenous cytokinin reduced the average tuber weight in most transformants at high sucrose content. The elevated auxin level in tubers of the transformants was accompanied with a decrease in content of cytokinin bases and their ribosides in tubers and most shoots. No concerted changes in contents of abscisic, jasmonic, salicylic acids and gibberellins in tubers were detected. The data on hormonal status indicated that the enhanced productivity of tms1 transformants was due to auxin and not mediated by other phytohormones. In addition, exogenous cytokinin was shown to upregulate the expression of genes encoding orthologs of auxin receptors. Overall, the results showed that tms1 expression and local increase in IAA level in transformants affect both the balance of endogenous cytokinins and the dynamics of tuberization in response to exogenous hormones (auxin, cytokinin), the latter reaction depending also on the carbohydrate supply. We introduce a basic model for the hormonal network controlling tuberization.

Thermodynamic and structural properties of tuber starches from transgenic potato plants grown in vitro and in vivo.

Potato plants harboring Phytochrome B (PHYB) gene from Arabidopsis thaliana or rol genes from Agrobacterium rhizogenes were used to study the effect of transgene expression on structure and properties of starch in tubers. Thermodynamic characteristics of starch (melting temperature, enthalpy of melting, thickness of crystalline lamellae) were shown to be variable depending on the transgene expression and plant culturing mode: in vitro or in soil. The expression of rolB or rolC genes in in vitro cultured plants evoked opposite effects on starch melting temperature and crystalline lamellae thickness. AtPHYB or rolB expression in the soil-grown potato led to the formation of more defective or more ordered starch structures, respectively, in comparison with starches of the same lines grown in vitro. On the whole, our study revealed genotype-dependent differences between starches extracted from tubers of in vitro or in vivo grown plants.

Expression of auxin synthesis gene tms1 under control of tuber-specific promoter enhances potato tuberization in vitro.

Phytohormones, auxins in particular, play an important role in plant development and productivity. Earlier data showed positive impact of exogenous auxin on potato (Solanum tuberosum L.) tuberization. The aim of this study was to generate potato plants with increased auxin level predominantly in tubers. To this end, a pBinB33-tms1 vector was constructed harboring the Agrobacterium auxin biosynthesis gene tms1 fused to tuber-specific promoter of the class I patatin gene (B33-promoter) of potato. Among numerous independently generated B33:tms1 lines, those without visible differences from control were selected for detailed studies. In the majority of transgenic lines, tms1 gene transcription was detected, mostly in tubers rather than in shoots. Indoleacetic acid (IAA) content in tubers and the auxin tuber-to-shoot ratio were increased in tms1-expressing transformants. The organ-specific increase in auxin synthesis in B33:tms1-transformants accelerated and intensified the process of tuber formation, reduced the dose of carbohydrate supply required for in vitro tuberization, and decreased the photoperiodic dependence of tuber initiation. Overall, a positive correlation was observed between tms1 expression, IAA content in tubers, and stimulation of tuber formation. The revealed properties of B33:tms1 transformants imply an important role for auxin in potato tuberization and offer prospects to magnify potato productivity by a moderate organ-specific enhancement of auxin content.

Vacuolar invertase regulates elongation of Arabidopsis thaliana roots as revealed by QTL and mutant analysis.

The possible role of the sucrose-splitting enzymes sucrose synthase and invertase in elongating roots and hypocotyls of Arabidopsis was tested by using a combination of histochemical methods and quantitative trait locus (QTL) analysis. Lengths of roots and hypocotyls correlated better with invertase activities than with sucrose synthase activities. The highest correlations were observed with activities in the elongating zones of roots. The genetic basis of these correlations was studied by using QTL analysis. Several loci, affecting invertase activity, colocated with loci that had an effect on root or hypocotyl length. Further fine mapping of a major locus for root length, but not for hypocotyl length (top chromosome 1), consistently showed colocation with the locus for invertase activity containing a gene coding for a vacuolar invertase. The analysis of a functional knockout line confirmed the role of this invertase in root elongation, whereas other invertase genes might play a role in hypocotyl elongation. Thus, we show the power of QTL analysis, combined for morphological and biochemical traits, followed by fine-mapping and mutant analysis, in unraveling the function of genes and their role in growth and development.

Histochemical analysis reveals organ-specific quantitative trait loci for enzyme activities in Arabidopsis.

To identify genetic loci involved in the regulation of organ-specific enzyme activities, a specific histochemical staining protocol was used in combination with quantitative trait locus (QTL) analysis. Using phosphoglucomutase (PGM) as an example, it is shown that enzyme activity can specifically, and with high resolution, be visualized in non-sectioned seedlings of Arabidopsis. The intensities of staining were converted to quantitative data and used as trait for QTL analysis using Landsberg erecta x Cape Verde Islands recombinant inbred lines. Independently, PGM activities were quantified in whole-seedling extracts, and these data were also used for QTL analysis. On the basis of extract data, six significant (P < 0.05) loci affecting PGM activity were found. From the histochemical data, one or more specific QTLs were found for each organ analyzed (cotyledons, shoot apex, hypocotyl, root, root neck, root tip, and root hairs). Loci detected for PGM activity in extracts colocated with loci for histochemical staining. QTLs were found coinciding with positions of (putative) PGM genes but also at other positions, the latter ones supposedly pointing toward regulatory genes. Some of this type of loci were also organ specific. It is concluded that QTL analysis based on histochemical data is feasible and may reveal organ-specific loci involved in the regulation of metabolic pathways.

In situ staining of activities of enzymes involved in carbohydrate metabolism in plant tissues.

A powerful technique is described to localize the activities of a range of enzymes in a wide variety of plant tissues. The method is based on the coupling of the enzymatic reaction to the reduction of NAD and subsequent reduction and precipitation of nitroblue tetrazolium. Enzymes that did not reduce NAD could be visualized by coupling their activities to glucose-6-phosphate dehydrogenase activity via one or more intermediary 'coupling' enzymes. The method is shown to be applicable for the detection of the activities of hexokinase, fructokinase, sucrose synthase, uridine 5'-diphospho-glucose pyrophosphorylase, ADP-glucose pyrophosphorylase, phosphoglucomutase, and phosphoglucose isomerase. It could be used for all tissues tested, including green leaves, stems, roots, fruits, and seeds. The method is specific, very sensitive, and has a high spatial resolution, giving information at the cellular and the subcellular level. The localization of sucrose synthase, invertase, and uridine 5'-diphospho-glucose pyrophosphorylase in transgenic potato plants, carrying a cytokinin biosynthesis gene, is studied and compared with wild-type plants.