Sugars en route to the roots. Transport,metabolism and storage within plant roots and towards microorganisms of the rhizosphere

In plants, the root is a typical sink organ that relies exclusively on the import of sugar from the aerial parts. Sucrose is delivered by the phloem to the most distant root tips and, en route to the tip, is used by the different root tissues for metabolism and storage. Besides, a certain portion of this carbon is exuded in the rhizosphere, supplied to beneficial microorganisms and diverted by parasitic microbes. The transport of sugars toward these numerous sinks either occurs symplastically through cell connections (plasmodesmata) or is apoplastically mediated through membrane transporters (MST, mononsaccharide tranporters, SUT/SUC, H+/sucrose transporters and SWEET, Sugar will eventually be exported transporters) that control monosaccharide and sucrose fluxes. Here, we review recent progresses on carbon partitioning within and outside roots, discussing membrane transporters involved in plant responses to biotic and abiotic factors.     

From Squalene to Brassinolide： The Steroid Metabolic and Signaling Pathways across the Plant Kingdom

The plant steroid hormones, brassinosteroids （BRs）, and their precursors, phytosterols, play major roles in plant growth, development, and stress tolerance. Here, we review the impressive progress made during recent years in elucidating the components of the sterol and BR metabolic and signaling pathways, and in understanding their mecha- nism of action in both model plants and crops, such as Arabidopsis and rice. We also discuss emerging insights into the regulations of these pathways, their interactions with other hormonal pathways and multiple environmental signals, and the putative nature of sterols as signaling molecules.     

Root Starch Reserves Are Necessary for Vigorous Re-Growth following Cutting Back in Lotus japonicus

Perenniality and vegetative re-growth vigour represent key agronomic traits in forage legume (Fabaceae) species. The known determinants of perenniality include the conservation of the vegetative meristem during and after the flowering phase, and the separation of flowering from senescence. The ability of the plants to store nutrient resources in perennial organs and remobilize them may also play an important role in the perennial growth habit, and in determining the capacity of the plant to re-grow following grazing or from one season to the next. To examine the importance of stored starch, we examined the vegetative re-growth vigour following cutting back of a unique collection of Lotus japonicus mutants impaired in their ability to synthesize or degrade starch. Our results establish that starch stored in the roots is important for re-growth vigour in Lotus japonicus. We extended this analysis to a collection of Lotus (trefoil) species and two ecotypes of Lotus japonicus displaying a large variation in their carbohydrate resource allocation. There was a positive correlation between root starch content and re-growth vigour in these natural variants, and a good general correlation between high re-growth vigour and the perennial life-form. We discuss the relationship between perenniality and the availability of root carbohydrates for re-growth.     

A Suite of Lotus japonicus Starch Mutants Reveals Both Conserved and Novel Features of Starch Metabolism

The metabolism of starch is of central importance for many aspects of plant growth and development. Information on leaf starch metabolism other than in Arabidopsis (Arabidopsis thaliana) is scarce. Furthermore, its importance in several agronomically important traits exemplified by legumes remains to be investigated. To address this issue, we have provided detailed information on the genes involved in starch metabolism in Lotus japonicus and have characterized a comprehensive collection of forward and TILLING (for Targeting Induced Local Lesions IN Genomes) reverse genetics mutants affecting five enzymes of starch synthesis and two enzymes of starch degradation. The mutants provide new insights into the structure-function relationships of ADP-glucose pyrophosphorylase and glucan, water dikinase1 in particular. Analyses of the mutant phenotypes indicate that the pathways of leaf starch metabolism in L. japonicus and Arabidopsis are largely conserved. However, the importance of these pathways for plant growth and development differs substantially between the two species. Whereas essentially starchless Arabidopsis plants lacking plastidial phosphoglucomutase grow slowly relative to wild-type plants, the equivalent mutant of L. japonicus grows normally even in a 12-h photoperiod. In contrast, the loss of GLUCAN, WATER DIKINASE1, required for starch degradation, has a far greater effect on plant growth and fertility in L. japonicus than in Arabidopsis. Moreover, we have also identified several mutants likely to be affected in new components or regulators of the pathways of starch metabolism. This suite of mutants provides a substantial new resource for further investigations of the partitioning of carbon and its importance for symbiotic nitrogen fixation, legume seed development, and perenniality and vegetative regrowth.Recent studies in Arabidopsis (Arabidopsis thaliana) have greatly enhanced our knowledge about pathways of transitory starch metabolism (Zeeman et al., 2007; Keeling and Myers, 2010; Kötting et al., 2010; Zeeman et al., 2010). The pathway of synthesis is well established for several species, but the degradative pathway is understood only in Arabidopsis. During synthesis, the plastidial isoforms of phosphoglucoisomerase (PGI1) and phosphoglucomutase (PGM1), together with ADP-Glc pyrophosphorylase (AGPase), catalyze the conversion of the Calvin cycle intermediate Fru 6-P to ADPGlc, the substrate for starch synthases (Supplemental Fig. S1). Leaves of mutants lacking any of these three enzymes either have strongly reduced starch contents or lack starch almost completely (Caspar et al., 1985; Hanson and McHale, 1988; Lin et al., 1988a, 1988b; Kruckeberg et al., 1989; Harrison et al., 1998; Yu et al., 2000; Streb et al., 2009). In contrast, the phenotypes of mutants lacking individual enzymes that convert ADPGlc into starch vary between species and are often much less pronounced (starch synthases [Delvallé et al., 2005; Zhang et al., 2005] and starch-branching enzymes [Tomlinson et al., 1997; Blauth et al., 2001; Dumez et al., 2006]).The degradation of the starch granule in Arabidopsis leaves is catalyzed primarily by β-amylases and isoamylase 3 (Wattebled et al., 2005; Delatte et al., 2006; Fulton et al., 2008). Normal rates of degradation require phosphorylation of the starch polymers by two glucan, water dikinases, GWD1 (Ritte et al., 2002) and GWD3 (or PWD, for phosphoglucan water, dikinase; Baunsgaard et al., 2005; Kötting et al., 2005), followed by dephosphorylation by a phosphoglucan phosphatase, STARCH EXCESS4 (SEX4; Kötting et al., 2009). Maltose produced by starch degradation is exported from the chloroplast by a maltose transporter and further metabolized to hexose phosphates in the cytosol (Zeeman et al., 2007; Supplemental Fig. S1). Mutations in numerous components of this pathway result in a starch-excess phenotype, in which the starch content of leaves at the end of the night is higher than that of wild-type plants.These studies have also revealed the importance of starch turnover for the productivity of the plant. Mutants of Arabidopsis that are essentially unable to synthesize transitory starch, or with reduced rates of starch degradation at night, have a reduced rate of growth and delayed flowering time relative to wild-type plants under most conditions (Caspar et al., 1985, 1991; Eimert et al., 1995; Corbesier et al., 1998; Smith and Stitt, 2007). However, it is not known whether information about the nature and importance of starch turnover in Arabidopsis is widely applicable. Plant species differ considerably in the extent to which starch is stored in leaves at night as well as in diurnal patterns of growth and metabolic demand. The function and regulation of starch metabolism in heterotrophic organs and its importance in major physiological and developmental processes such as perenniality, vegetative regrowth, symbiotic nitrogen fixation, and the accumulation of seed storage reserves cannot be studied easily in Arabidopsis and remain largely unknown. These processes represent traits of agronomic value in legumes (Fabaceae), a family that includes some of the most agriculturally important forage (e.g. alfalfa [Medicago sativa] and clover [Trifolium spp.]), grain (e.g. pea [Pisum sativum] and common bean [Phaseolus vulgaris]), and oilseed (e.g. soybean [Glycine max]) crops.Some information is already available about starch metabolism in pea and other legume crops (Martin and Smith, 1995; Wang et al., 1998b, and refs. therein). However, characteristics including large genome sizes and recalcitrant transformation and regeneration have limited progress on these species. There is insufficient information to allow either an overview of the nature and importance of starch metabolism in legumes or a meaningful comparison with the detailed picture emerging for Arabidopsis. The development of both Lotus japonicus and Medicago truncatula as legume model systems, and the wide range of genetic and genomic resources generated for them, offer the opportunity for a systematic analysis.To elucidate the pathway of starch synthesis and degradation in legumes and provide resources for future experimentation, we screened an ethyl methanesulfonate (EMS)-mutagenized population of L. japonicus (Perry et al., 2003) for mutants altered in transitory starch metabolism and carried out genetic mapping to identify the mutation responsible for their phenotype. We also used TILLING (for Targeting Induced Local Lesions IN Genomes; McCallum et al., 2000) to confirm that the mutations identified were indeed responsible for the mutant phenotype and to obtain additional mutations in genes known to affect leaf starch content in other species. We present the results of molecular and phenotypic analyses on the mutants that provide novel insights into the structure-function relationship of the AGPase and GWD1 enzymes. In addition, our analyses reveal new information on the nature and importance of starch metabolism for plant growth and development in L. japonicus. The importance of starch accumulation and degradation and a comparison with pathways in other plant species are also discussed.     

A molecular basis for the endo-beta 1,3-glucanase activity of the thaumatin-like proteins from edible fruits

Fruit-specific thaumatin-like proteins were isolated from cherry, apple and banana, and their enzymatic and antifungal activities compared. Both the apple and cherry possess a moderate endo-beta 1,3-glucanase activity but are devoid of antifugal activity. In contrast, the banana thaumatin-like protein inhibits the in vitro hyphal growth of Verticillium albo-atrum but is virtually devoid of endo-beta 1,3-glucanase activity. Both structural and molecular modeling studies showed that all three thaumatin-like proteins possess an extended electronegatively charged cleft at their surface, which is believed to be a prerequisite for endo-beta 1,3-glucanase activity. Docking experiments showed that the positioning of linear (1,3)-beta-D-glucans in the cleft of the apple and cherry proteins allows an interaction with the glutamic acid residues that are responsible for the hydrolytic cleavage of the glucan. Due to a different positioning in the cleft of the banana thaumatin-like protein, the linear beta-glucans cannot properly interact with the catalytic glutamic acid residues and as a result the protein possesses no enzymatic activity. The possible function of the fruit-specific thaumatin-like proteins is discussed in view of the observed biological activities and structural features.     

Boosting crop yields with plant steroids

Plant sterols and steroid hormones, the brassinosteroids (BRs), are compounds that exert a wide range of biological activities. They are essential for plant growth, reproduction, and responses to various abiotic and biotic stresses. Given the importance of sterols and BRs in these processes, engineering their biosynthetic and signaling pathways offers exciting potentials for enhancing crop yield. In this review, we focus on how alterations in components of sterol and BR metabolism and signaling or application of exogenous steroids and steroid inhibitors affect traits of agronomic importance. We also discuss areas for future research and identify the fine-tuning modulation of endogenous BR content as a promising strategy for crop improvement.