Strigolactones are a group of plant compounds of diverse but related chemical structures. They have similar bioactivity across a broad range of plant species, act to optimize plant growth and development, and promote soil microbe interactions. Carlactone, a common precursor to strigolactones, is produced by conserved enzymes found in a number of diverse species. Versions of the MORE AXILLARY GROWTH1 (MAX1) cytochrome P450 from rice and Arabidopsis thaliana make specific subsets of strigolactones from carlactone. However, the diversity of natural strigolactones suggests that additional enzymes are involved and remain to be discovered. Here, we use an innovative method that has revealed a missing enzyme involved in strigolactone metabolism. By using a transcriptomics approach involving a range of treatments that modify strigolactone biosynthesis gene expression coupled with reverse genetics, we identified LATERAL BRANCHING OXIDOREDUCTASE (LBO), a gene encoding an oxidoreductase-like enzyme of the 2-oxoglutarate and Fe(II)-dependent dioxygenase superfamily. Arabidopsis lbo mutants exhibited increased shoot branching, but the lbo mutation did not enhance the max mutant phenotype. Grafting indicated that LBO is required for a graft-transmissible signal that, in turn, requires a product of MAX1. Mutant lbo backgrounds showed reduced responses to carlactone, the substrate of MAX1, and methyl carlactonoate (MeCLA), a product downstream of MAX1. Furthermore, lbo mutants contained increased amounts of these compounds, and the LBO protein specifically converts MeCLA to an unidentified strigolactone-like compound. Thus, LBO function may be important in the later steps of strigolactone biosynthesis to inhibit shoot branching in Arabidopsis and other seed plants.plant | branching | strigolactone | biosynthesis | Arabidopsis

Cereal Chem. 91(3):218-227Seed protein and starch composition determine the efficiency of the fermentation process in the production of grain-based ethanol. Sorghum, a highly water-and nutrient-efficient plant, provides an alternative to fuel crops with greater irrigation and fertilizer requirements, such as maize. However, sorghum grain is generally less digestible because of extensive disulfide cross-linking among sulfur-rich storage proteins in the proteinstarch matrix. Thus, the fine structure and composition of the seed endosperm directly impact grain end use, including fermentation performance. To test the hypothesis that kafirin (prolamin) seed storage proteins specifically influence the efficiency of ethanol production from sorghum, 10 diverse genetic lines with allelic variation in the β-, γ-, and δ-kafirins, including three β-kafirin null mutants, were tested for ethanol yield and fermentation efficiency. Our selected lines showed wide variation in grain biochemical features, including total protein (9.96-16.47%), starch (65.52-74.29%), and free amino nitrogen (FAN) (32.84-73.51 mg/L). Total ethanol yield (ranging from 384 to 426 L/metric ton), was positively correlated to starch content (R 2 = 0.74), and there was a slight positive correlation between protein digestibility and ethanol yield (R 2 = 0.52). Increases in FAN content enhanced fermentation efficiency (R 2 = 0.65). The highest ethanol producer was elite staygreen breeding line B923296, and the line with the highest fermentation efficiency at the 72 h time point was inbred BT×623. A large-seeded genotype, KS115, carrying a novel γ-kafirin allele, was rich in FAN and exhibited excellent short-term fermentation efficiency at 85.68% at the 20 h time point. However, the overall ethanol yield from this line was comparatively low at 384 L/metric ton, because of insufficient starch, low digestibility, and high crude protein.Multivariate analysis indicated an association between the β-kafirin allele and variation in grain digestibility (P = 0.042) and FAN (P = 0.036), with subsequent effects on ethanol yield. Reversed-phase HPLC profiling of the alcohol-soluble kafirin protein fraction revealed diversity in protein content and composition across the lines, with similarities in peak distribution profiles among β-kafirin null mutants compared with normal lines.

Grain protein composition determines quality traits, such as value for food, feedstock, and biomaterials uses. The major storage proteins in sorghum are the prolamins, known as kafirins. Located primarily on the periphery of the protein bodies surrounding starch, cysteine-rich β- and γ-kafirins may limit enzymatic access to internally positioned α-kafirins and starch. An integrated approach was used to characterize sorghum with allelic variation at the kafirin loci to determine the effects of this genetic diversity on protein expression. Reversed-phase high performance liquid chromatography and lab-on-a-chip analysis showed reductions in alcohol-soluble protein in β-kafirin null lines. Gel-based separation and liquid chromatography-tandem mass spectrometry identified a range of redox active proteins affecting storage protein biochemistry. Thioredoxin, involved in the processing of proteins at germination, has reported impacts on grain digestibility and was differentially expressed across genotypes. Thus, redox states of endosperm proteins, of which kafirins are a subset, could affect quality traits in addition to the expression of proteins.