Long-range signalling in plant reproductive development

Animals and plants produce regulatory signals at specific places of their bodies, in order to regulate developmental events which take place at a distance. Plants use this mechanism to adjust their development to the changing environment. Flowering and tuber formation are controlled by signals generated in the leaves that travel throughout the plant to reach their target tissues: the shoot apical meristem for flowering and the underground stolons for tuberization. Although the existence of these long-distance plant messengers was postulated almost seventy years ago, their chemical nature is still not clear. These leaf-derived signals are graft-transmissible and move through the plant vascular system. Presumably they are very similar or even identical for flowering and tuberization and common to most plant species. It is generally accepted that their composition is complex and includes positive and negative regulators. Many different substances, including classical plant hormones and metabolites have been postulated to be components of these mobile signals, but conclusive evidence of this is still lacking. Recent work has positioned these signals within the genetic network that regulates flowering time and suggests roles for specific genes in the generation, transport or response to the signalling molecules. Current knowledge of long-range signalling mechanisms in other physiological and developmental events, together with the finding of common regulators involved in flowering, tuberization and other processes like pathogen and wound responses, should help to establish the biochemical composition of these elusive messenger signals.     

Key players associated with tuberization in potato: potential candidates for genetic engineering

Tuberization in potato (Solanum tuberosum L.) is a complex biological phenomenon which is affected by several environmental cues, genetic factors and plant nutrition. Understanding the regulation of tuber induction is essential to devise strategies to improve tuber yield and quality. It is well established that short-day photoperiods promote tuberization, whereas long days and high-temperatures inhibit or delay tuberization. Worldwide research on this complex biological process has yielded information on the important bio-molecules (proteins, RNAs, plant growth regulators) associated with the tuberization process in potato. Key proteins involved in the regulation of tuberization include StSP6A, POTH1, StBEL5, StPHYB, StCONSTANS, Sucrose transporter StSUT4, StSP5G, etc. Biomolecules that become transported from “source to sink” have also been suggested to be important signaling candidates regulating the tuberization process in potatos. Four molecules, namely StSP6A protein, StBEL5 RNA, miR172 and GAs, have been found to be the main candidates acting as mobile signals for tuberization. These biomolecules can be manipulated (overexpressed/inhibited) for improving the tuberization in commercial varieties/cultivars of potato. In this review, information about the genes/proteins and their mechanism of action associated with the tuberization process is discussed.     

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.     

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.     

Relation Between Environmental Factors and the LOX Activities Upon Potato Tuber Formation and Flower-bud Formation in Morning Glory

Life cycles of plants including tuberization and flowering are strongly related to environmental factors such as photoperiod and temperature. Theobroxide induces potato tuber formation and flower bud formation of morning glory under non-inductive conditions and stimulates the activity of lipoxygenase (LOX). In this study, to understand the LOX activity more systematically, the relationships between LOX activity and light and temperature, which effects potato tuber and flower-bud formation, have been investigated. The results showed that LOX activity in morning glory was greatly enhanced up to 30 min and then declined after switching from the light to the dark condition, while the activity did not vary when switching from the dark to the light condition. In addition, the temperature profile of measured LOX activity in the potato and morning glory plants was nearly consistent with the time taken to form potato tubers and flower buds in morning glory, respectively, at different growing temperatures. These results strongly suggest that LOX activity is directly connected with light and temperature to regulate the formation of tubers and flower-buds.     

马铃薯块茎发育机理及其基因表达

马铃薯（Solanum tuberosum L.)块茎是有块茎马铃薯植物的地下变态器官,它由匍匐茎顶端膨大形成,对于马铃薯块茎形成的生理机制已有许多研究,这些研究表明,块茎发生受许多因素的影响,总体来讲短日 照,较低的温度以及离体条件下培养基较高的蔗糖浓度等有利于块茎形成,同时,块茎形成过程中内源激素亦发生一系列变化,然而,对于块茎形成中相关基因表达,进而调控块茎形成的系统研究目前还较滞后,已有研究显示,块茎形成与膨大涉及到一系列基因的表达与关闭,同时它也与淀粉合成和块茎储藏蛋白基因的表达有关,综述了这一领域现有的研究进展。     

马铃薯块茎发育机理及其基因表达

马铃薯(Solanum tuberosum L.)块茎是有块茎马铃薯植物的地下变态器官,它由匍匐茎顶端膨大形成。对于马铃薯块茎形成的生理机制已有许多研究,这些研究表明,块茎发生受许多因素的影响,总体来讲短日照、较低的温度以及离体条件下培养基较高的蔗糖浓度等有利于块茎形成,同时,块茎形成过程中内源激素亦发生一系列变化。然而,对于块茎形成中相关基因表达,进而调控块茎形成的系统研究目前还较滞后。已有研究显示,块茎形成与膨大涉及到一系列基因的表达与关闭,同时它也与淀粉合成和块茎储藏蛋白基因的表达有关。综述了这一领域现有的研究进展。     

Phytochrome B mediates the photoperiodic control of tuber formation in potato

To determine whether phytochrome B is involved in the response of potato plants to photoperiod, a potato PHYB cDNA fragment was inserted in the antisense orientation behind the 35S CaMV promoter in Bin19 and this construct was transformed into Solanum tuberosum ssp. andigena plants which normally require short days for tuberization. Two independent transformants were obtained that had much lower levels of PHYB mRNA and protein, and which exhibited phenotypes characteristic of phyB mutants, for example, elongated stems and decreased chlorophyll content. The level of phyA, and of several phytochrome A-controlled responses, was unaffected in these plants. The photoperiodic control of tuberization in these antisense PHYB plants was abolished, the plants tuberizing in short day, long day, or short day plus night break conditions. This result shows that phytochrome B is required for the photoperiodic control of tuberization in potato ( Solanum tuberosum ssp. andigena ) and that it regulates this developmental process by preventing tuber formation in non-inductive photoperiods rather than by promoting tuberization in inductive photoperiods.     

The Interaction of Gibberellins and Photoperiod in the Control of Potato Tuberization

Solanum tuberosum ssp. andigena plants require a short-day (SD) photoperiod for tuber formation, a process that is also affected by gibberellins (GAs). Grafting experiments have confirmed that the photoperiod is perceived in the leaves. Tuber formation, however, usually takes place in the underground stolons. In this review, photoperiod-dependent tuberization has been divided into five chronological events: SD photoperiod perception, short-term adaptive responses to SD conditions, generation and transport of tuber-inducing signal(s), tuber formation, and long-term adaptive responses to tuber growth. Within this frame of study, the interaction of GAs and photoperiod is revised. Similar to the flowering process in Arabidopsis, we suggest the existence of two independent pathways that control tuber formation: a photoperiod-dependent pathway and a GA-dependent pathway. Nevertheless, photoperiod-dependent tuber formation requires the action of GAs at specific stages to orchestrate this complex process of development.     

Phytochrome F plays critical roles in potato photoperiodic tuberization

The transition to tuberization contributes greatly to the adaptability of potato to a wide range of environments. Phytochromes are important light receptors for the growth and development of plants, but the detailed functions of phytochromes remain unclear in potato. In this study, we first confirmed that phytochrome F ( St PHYF ) played essential roles in photoperiodic tuberization in potato. By suppressing the St PHYF gene, the strict short‐day potato genotype exhibited normal tuber formation under long‐day ( LD ) conditions, together with the degradation of the CONSTANTS protein St COL 1 and modulation of two FLOWERING LOCUS  T ( FT ) paralogs, as demonstrated by the repression of St SP 5G and by the activation of St SP 6A during the light period. The function of St PHYF was further confirmed through grafting the scion of St PHYF ‐silenced lines, which induced the tuberization of untransformed stock under LD s, suggesting that St PHYF was involved in the production of mobile signals for tuberization in potato. We also identified that St PHYF exhibited substantial interaction with St PHYB both in vitro and in vivo . Therefore, our results indicate that St PHYF plays a role in potato photoperiodic tuberization, possibly by forming a heterodimer with St PHYB .     

Tuberization in Potato at High Temperatures: Promotion by Disbudding

The role of the terminal and axillary buds, as presumptive organsof gibberellin synthesis, in the control of tuberization inpotato (Solanum tuberosum L., cv. Sebago) at high temperatureswas studied. Decapitation alone strongly promoted the outgrowthof axillary buds, but did not promote tubenzation. When growthof the axillary buds was suppressed by the use of chemical pruningagents (MH, TIBA or 1-decanol), tuberization was promoted. Manualremoval of the buds promoted tuberization to a similar extent.The results are consistent with the hypothesis that the budsare major sites of gibberellin synthesis in the potato, andthat high temperatures stimulate the synthesis of gibberellinsand their export to the stolons, where they inhibit tuber formation. Solarium tuberosum L., potato, tuberization, temperature, disbudding, chemical pruning, gibberellins     

Sucrose increases calcium-dependent protein kinase and phosphatase activities in potato plants.

The effect of sucrose on tuber formation, calcium-dependent protein kinase (CDPK) and phosphatase activities was analysed using in vitro cultured potato plants. In short treatments, sucrose induced CDPK and phosphatase activities. In long treatments, sucrose induced tuber formation in the absence of other tuber inducing stimuli. Sorbitol caused a minor increase in CDPK activity and affected plant morphology but did not induce tuber development. The addition of the protein kinase inhibitor Staurosporine precluded sucrose-induced tuberization. Altogether, our results suggest that phosphorylation/dephosphorylation events are involved in sucrose-induced tuber development.     

The multifaceted roles of FLOWERING LOCUS T in plant development

One of the key developmental processes in flowering plants is the differentiation of the shoot apical meristem into a floral meristem. This transition is regulated through the integration of environmental and endogenous stimuli, involving a complex, hierarchical signalling network. In arabidopsis, the FLOWERING LOCUS T (FT) protein, a mobile signal recognized as a major component of florigen, has a central position in mediating the onset of flowering. FT-like genes seem to be involved in regulating the floral transition in all angiosperms examined to date. Evidence from molecular evolution studies suggests that the emergence of FT-like genes coincided with the evolution of the flowering plants. Hence, the role of FT in floral promotion is conserved, but appears to be restricted to the angiosperms. Besides flowering, FT-like proteins have also been identified as major regulatory factors in a wide range of developmental processes including fruit set, vegetative growth, stomatal control and tuberization. These multifaceted roles of FT-like proteins have resulted from extensive gene duplication events, which occurred independently in nearly all modern angiosperm lineages, followed by sub- or neo-functionalization. This review assesses the plethora of roles that FT-like genes have acquired during evolution and their implications in plant diversity, adaptation and domestication.