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.Arabidopsis natural variation ͉ sucrose synthase ͉ hypocotyls T otal plant yield depends on the acquisition of raw material, i.e., photosynthesis and mineral (plus water) uptake, and on the ability of the plant to cope with stress. However, the economic yield of a crop is to a large extent also determined by the partitioning of dry matter over the harvestable and nonharvestable parts of the plant. The molecular and physiological basis of the regulation of assimilate partitioning in plants is still poorly understood. In terms of biomass, the most important components in assimilate partitioning and in total yield are carbohydrates. There is increasing evidence that a limited number of key enzymes, involved in primary (carbohydrate) metabolism, might be pivotal in this process (1).Functionally, a plant can be divided into sources (the sites of assimilate production) and sinks (the sites of use and͞or storage). Sinks can either be rapidly growing, expanding organs, such as elongating stems and roots, or storage sinks accumulating reserves, such as fruits, seeds, or tubers (2).In most plant species, carbon is transported from source to sink in the form of the disaccharide sucrose. Upon arrival in the sink, sucrose has to be hydrolyzed. In plants, two pathways are available for sucrose cleaving: via invertase (Inv), yielding glucose and fructose, and via sucrose synthase (Susy), yielding fructose and UDP-glucose. In several cases, it has been suggested that sink strength might depend on the activities of these sucrose-splitting enzymes. There is increasing evidence that in storage sinks, the predominant pathway is via Susy, whereas in growing sinks, the Inv route is most important. In potatoes, the elongating rhizomes (stolons) exhibit high Inv activity, whereas a switch toward Susycatalyzed sucrose bre...

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Sweetpotato yield depends on a change in the developmental fate of adventitious roots into storage-roots. The mechanisms underlying this developmental switch are still unclear. We examined the hypothesis claiming that regulation of root lignification determines storage-root formation. We show that application of the plant hormone gibberellin increased stem elongation and root gibberellin levels, while having inhibitory effects on root system parameters, decreasing lateral root number and length, and significantly reducing storage-root number and diameter. Furthermore, gibberellin enhanced root xylem development, caused increased lignin deposition, and, at the same time, decreased root starch accumulation. In accordance with these developmental effects, gibberellin application upregulated expression levels of sweetpotato orthologues of Arabidopsis vascular development regulators (IbNA075, IbVND7, and IbSND2) and of lignin biosynthesis genes (IbPAL, IbC4H, Ib4CL, IbCCoAOMT, and IbCAD), while downregulating starch biosynthesis genes (IbAGPase and IbGBSS) in the roots. Interestingly, gibberellin downregulated root expression levels of orthologues of the Arabidopsis BREVIPEDICELLUS transcription factor (IbKN2 and IbKN3), regulator of meristem maintenance. The results substantiate our hypothesis and mark gibberellin as an important player in regulation of sweetpotato root development, suggesting that increased fiber formation and lignification inhibit storage-root formation and yield. Taken together, our findings provide insight into the mechanisms underlying sweetpotato storage-root formation and provide a valuable database of genes for further research.

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.

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 ϫ 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 Ͻ 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.Carbohydrates constitute the major part of plant biomass, and carbohydrate metabolism is one of the central biochemical pathways in plant cells. Hence understanding carbohydrate metabolism and its regulation is of crucial importance. Over the last few decades, most of the genes encoding the enzymes catalyzing the various steps of carbohydrate metabolic routes, have been unraveled. However, this does not automatically imply that the regulatory mechanisms of these routes have become clear as well. Carbohydrate metabolism as a whole, but also the individual steps of the various pathways, are likely to be controlled by a plethora of genes, encoding the enzymes and regulators at different levels.Quantitative trait locus (QTL) analysis is a powerful approach to identify genes involved in processes controlled by many genes when genetic variation for these genes is present. This has been shown in Arabidopsis for traits such as flowering time (Koornneef et al., 1998; Ungerer et al., 2002), nitrogen use efficiency (Loudet et al., 2003), and salt tolerance (Quesada et al., 2002). A few papers have shown the potential of this approach in genetic mapping of enzyme activities. Suggestions for possible candidate genes have often been inferred from a similar map position of these QTLs with genes known to be involved in the process under study (Arabidopsis [Mitchell-Olds and Pedersen, 1998]; maize [Prioul et al., 1999]; tomato [Fridman et al., 2002]; rice [Hu et al., 2001]).Now that the genome of Arabidopsis has been sequenced (Arabidopsis Genome Initiative, 2000), the map position of most genes encoding the enzymes involved in primary metabolism are know...

SummaryThe evidence for a role of gibberellins in the regulation of potato tuber formation is reviewed. Endogenous gibberellin levels in plants are high under non-inducing conditions and decrease under inducing conditions. Exogenously applied gibberellins inhibit tuber formation, whereas applying inhibitors of gibberellin biosynthesis has the opposite effect. Cellular events involved in tuberization, viz., cell division, cell enlargement and orientation of micro-tubules, are also reviewed. Based on available evidence, a major regulatory role of gibberellins is suggested. However, it is also argued that tuber formation is not simply regulated by gibberellins acting as the sole signal between above-ground and below-ground parts, since stolon tips are able to synthesize their own gibberellins, and the phenotype of phytochrome B-antisense plants cannot be explained only by altered levels of GAs.

Drought reduces the availability of soil water and the mobility of nutrients, thereby limiting the growth and productivity of rice. Under drought, arbuscular mycorrhizal fungi (AMF) increase P uptake and sustain rice growth. However, we lack knowledge of how the AMF symbiosis contributes to drought tolerance of rice. In the greenhouse, we investigated mechanisms of AMF symbiosis that confer drought tolerance, such as enhanced nutrient uptake, stomatal conductance, chlorophyll fluorescence, and hormonal balance (abscisic acid (ABA) and indole acetic acid (IAA)). Two greenhouse pot experiments comprised three factors in a full factorial design with two AMF treatments (low-and high-AMF colonization), two water treatments (well-watered and drought), and three rice varieties. Soil water potential was maintained at 0 kPa in the well-watered treatment. In the drought treatment, we reduced soil water potential to − 40 kPa in experiment 1 (Expt 1) and to − 80 kPa in experiment 2 (Expt 2). Drought reduced shoot and root dry biomass and grain yield of rice in both experiments. The reduction of grain yield was less with higher AMF colonization. Plants with higher AMF colonization showed higher leaf P concentrations than plants with lower colonization in Expt 1, but not in Expt 2. Plants with higher AMF colonization exhibited higher stomatal conductance and chlorophyll fluorescence than plants with lower colonization, especially under drought. Drought increased the levels of ABA and IAA, and AMF colonization also resulted in higher levels of IAA. The results suggest both nutrient-driven and plant hormone-driven pathways through which AMF confer drought tolerance to rice.