Coincidence of potato CONSTANS (StCOL1) expression and light cannot explain night‐break repression of tuberization

In the obligate short‐day potato Solanum tuberosum group Andigena (Solanum andigena), short days, or actually long nights, induce tuberization. Applying a night break in the middle of this long night represses tuberization. However, it is not yet understood how this repression takes place. We suggest a coincidence model, similar to the model explaining photoperiodic flowering in Arabidopsis. We hypothesize that potato CONSTANS (StCOL1), expressed in the night of a short day, is stabilized by the light of the night break. This allows for StCOL1 to repress tuberization through induction of StSP5G, which represses the tuberization signal StSP6A. We grew S. andigena plants in short days, with night breaks applied at different time points during the dark period, either coinciding with StCOL1 expression or not. StCOL1 protein presence, StCOL1 expression and expression of downstream targets StSP5G and StSP6A were measured during a 24‐h time course. Our results show that a night break applied during peak StCOL1 expression is unable to delay tuberization, while coincidence with low or no StCOL1 expression leads to severely repressed tuberization. These results imply that coincidence between StCOL1 expression and light does not explain why a night break represses tuberization in short days. Furthermore, stable StCOL1 did not always induce StSP5G, and upregulated StSP5G did not always lead to fully repressed StSP6A. Our findings suggest there is a yet unknown level of control between StCOL1, StSP5G and StSP6A expression, which determines whether a plant tuberizes.     

StGA2ox1 is induced prior to stolon swelling and controls GA levels during potato tuber development

The formation and growth of a potato ( Solanum tuberosum ) tuber is a complex process regulated by different environmental signals and plant hormones. In particular, the action of gibberellins (GAs) has been implicated in different aspects of potato tuber formation. Here we report on the isolation and functional analysis of a potato GA 2-oxidase gene ( StGA2ox1 ) and its role in tuber formation. StGA2ox1 is upregulated during the early stages of potato tuber development prior to visible swelling and is predominantly expressed in the subapical region of the stolon and growing tuber. 35S-over-expression transformants exhibit a dwarf phenotype, reduced stolon growth and earlier in vitro tuberization. Transgenic plants with reduced expression levels of StGA2ox1 showed normal plant growth, an altered stolon swelling phenotype and delayed in vitro tuberization. Tubers of the StGA2ox1 suppression clones contain increased levels of GA₂₀, indicating altered GA metabolism. We propose a role for StGA2ox1 in early tuber initiation by modifying GA levels in the subapical stolon region at the onset of tuberization, thereby facilitating normal tuber development and growth.     

Tuber Formation and Growth of in vitro Cultivated Transgenic Potato Plants Overproducing Phytochrome B

We studied the effect of the ectopic expression of the Arabidopsis PHYB gene, which encodes the phytochrome B (phyB) apoprotein, under the control of cauliflower mosaic virus 35S promoter on the photoperiodic response of tuberization and growth of potato (Solanum tuberosum L., cv. Désirée) transformed lines. Stem cuttings of transformed and control plants were cultured on Murashige and Skoog nutrient medium containing 5 or 8% sucrose in the phytotron chambers at 20°C under conditions of a long day (16 h), a short day (10 h), or in darkness. We showed that the overexpression of the PHYB gene enhanced the inhibitory effect of the long day on tuberization. In addition, tuber initiation in these transformed plants occurred at a higher sucrose concentration. The insertion of the PHYB gene decreased plant and tuber weights and shortened stems and internodes. Thus, we demonstrated the complex result of the PHYB gene insertion: it affected the photoperiodic response of tuberization, the control of tuber initiation by sucrose, and the growth of potato vegetative organs.     

Comparative proteomics of tuber induction, development and maturation reveal the complexity of tuberization process in potato (Solanum tuberosum L.)

Tuberization in potato ( Solanum tuberosum L.) is a developmental process that serves a double function, as a storage organ and as a vegetative propagation system. It is a multistep, complex process and the underlying mechanisms governing these overlapping steps are not fully understood. To understand the molecular basis of tuberization in potato, a comparative proteomic approach has been applied to monitor differentially expressed proteins at different development stages using two-dimensional gel electrophoresis (2-DE). The differentially displayed proteomes revealed 219 protein spots that change their intensities more than 2.5-fold. The LC-ES-MS/MS analyses led to the identification of 97 differentially regulated proteins that include predicted and novel tuber-specific proteins. Nonhierarchical clustering revealed coexpression patterns of functionally similar proteins. The expression of reactive oxygen species catabolizing enzymes, viz., superoxide dismutase, ascorbate peroxidase and catalase, were induced by more than 2-fold indicating their possible role during the developmental transition from stolons into tubers. We demonstrate that nearly 100 proteins, some presumably associated with tuber cell differentiation, regulate diverse functions like protein biogenesis and storage, bioenergy and metabolism, and cell defense and rescue impinge on the complexity of tuber development in potato.     

Sucrose application causes hormonal changes associated with potato tuber induction

Stems of potato plants (Solanum tuberosum L. cv. Dianella) were immersed in solutions containing water (control), sucrose, glucose, paclobutrazol, and gibberellic acid. The effects of these treatments on the ethylene release, levels of endogenous gibberellins, glucose, sucrose, and starch were correlated with tuberization of nodal cuttings, excised from potato stems. Paclobutrazol and sucrose improved the percent of tuberization and/or increased tuber weight. In contrast, GA₃ inhibited tuber formation compared with the control. The level of endogenous free GAs was negatively correlated with percent tuberization. However, the level of conjugated GAs was positively correlated with both percent tuberization and tuber weight. The effect of sucrose on potato tuber induction in relation to the possible role of sucrose in GA-conjugate formation is discussed.     

Overexpression of jasmonic acid carboxyl methyltransferase increases tuber yield and size in transgenic potato

Jasmonates control diverse plant developmental processes, such as seed germination, flower, fruit and seed development, senescence and tuberization in potato. To understand the role of methyl jasmonate (MeJA) in potato tuberization, the Arabidopsis JMT gene encoding jasmonic acid carboxyl methyltransferase was constitutively overexpressed in transgenic potato plants. Increases in tuber yield and size as well as in vitro tuberization frequency were observed in transgenic plants. These were correlated with JMT mRNA level––the higher expression level, the higher the tuber yield and size. The levels of jasmonic acid (JA), MeJA and tuberonic acid (TA) were also higher than those in control plants. Transgenic plants also exhibited higher expression of jasmonate-responsive genes such as those for allene oxide cyclase (AOC) and proteinase inhibitor II (PINII). These results indicate that JMT overexpression induces jasmonate biosynthesis genes and thus JA and TA pools in transgenic potatoes. This results in enhanced tuber yield and size in transgenic potato plants.     

Morphological and molecular characterization of a spontaneously tuberizing potato mutant: an insight into the regulatory mechanisms of tuber induction

Background  

Tuberization in potato (Solanum tuberosum L.) represents a morphogenetic transition of stolon growth to tuber formation, which is under complex environmental and endogenous regulation. In the present work, we studied the regulatory mechanisms and the role of different morphogenetic factors in a newly isolated potato mutant, which exhibited spontaneous tuberization (ST). The ST mutant was characterized in detail at morphological, physiological and biochemical levels.