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

Abstract

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;Kö tting 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., 1988aLin et al., , 1988bKruckeberg 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....

Root starch accumulation in response to arbuscular mycorrhizal colonization differs among Lotus japonicus starch mutants

“…The Lotus japonicus mutants, SL5035-7, SL4841-4 and pgm1-4 have been described previously ( Table 1; Vriet et al 2010). …”

“…To test this hypothesis, we chose the Lotus japonicus mutants SL5035-7 and SL4841-4 that are disturbed in starch breakdown, and SL4725-4 that cannot produce starch in any parts of the plant (Vriet et al 2010). SL5035-7 and SL4841-4 have been mapped to two different loci on chromosome 1 but the mutated genes are unknown.…”

“…SL5035-7 and SL4841-4 have been mapped to two different loci on chromosome 1 but the mutated genes are unknown. SL 4725-4 carries a mutation in PHOSPHOGLUCOMUTASE 1 and was named pgm1-4 (Vriet et al 2010). All mutants were subjected to colonization by G. margarita for 4 weeks, to reach full colonization (Novero et al 2002).…”

“…Starch accumulation in the root Vriet et al (2010) focussed on starch accumulation in leaves for most mutants described in their study and investigated starch accumulation in roots only in selected mutants such as pgm1-4. Furthermore, starch dynamics in roots in response to stimuli other than the diurnal light cycle have not been investigated.…”

“…However, it remains unclear whether starch accumulation is absolutely required for AM colonization. To investigate the importance of root starch metabolism for AM development we took advantage of the availability of an extensive series of novel Lotus japonicus mutants that are either defective in starch synthesis or breakdown (Vriet et al 2010). Here we show that AM colonization does not depend on root starch accumulation and plant sugars can be supplied to the AM fungus without a detour through starch.…”