Cyanobacterial branching enzymes bind to α-glucan via surface binding sites.

Besides their catalysis, specific interactions between starch/glycogen processing enzymes and their substrates have been reported. Multiple branching enzyme (BE) isoforms, BE1, BE2, and BE3, have been found in a limited number of cyanobacterial species that are characterized by amylopectin accumulation. Seven surface binding sites (SBSs) located away from the active site have been identified in crystal structures of cyanobacterial BE1 from Crocosphaera subtropica (Cyanothece sp.) ATCC 51142 (51142BE1). In the present study, binding affinity toward amylopectin, amylose, and glycogen was investigated for wild-type 51142BE1 and its mutants (residues at SBSs important for sugar-binding were replaced by alanine). These enzymes showed retarded mobility during electrophoresis in non-denaturing polyacrylamide gels in the presence of polysaccharides. This was caused by interactions between the enzymes and the polysaccharides, enabling calculation of the dissociation constants (Kd values) of the enzymes toward the polysaccharides. Mutational analysis indicated that particular domains of the protein (domains A and C) were involved in the polysaccharide binding. Kd values toward the polysaccharides were also measured for 10 BE isoforms (five BE1, three BE2, and two BE3) from 5 cyanobacterial strains. All BEs displayed much lower Kd values (higher affinity) toward amylopectin and amylose than toward glycogen, as described for plant BEs. In addition, one BE2 displayed exceptionally high Kd values (low affinity), while two BE3 exhibited multiple Kd values to all polysaccharides. These results could be ascribed to sequence variations in the SBSs, irrespective of the catalytic specificity.

The NAC Transcription Factor Gene OsY37 (ONAC011) Promotes Leaf Senescence and Accelerates Heading Time in Rice.

Leaf senescence is an important physiological process involving the degradation of a number of metabolites and their remobilization to new reproductive and storage organs. NAC (NAM, ATAF, and CUC) transcription factors are reported as important regulators of the senescence process. Here, we describe the identification and functional characterization of the NAC transcription factor gene, OsY37 (Oryza sativa Yellow37, ONAC011) obtained from Oryza sativa cv. indica, and japonica. We created transgenic plants expressing the OsY37 gene under the control of a strong and constitutive CaMV35S promoter. The resulting transgenic plants overexpressing OsY37 gene showed early heading and precocious senescence phenotype of flag leaves compared with wild-type plants. By contrast, blocking the function of this gene via RNAi (RNA interference) and CRES-T (Chimeric Repressor Silencing Technology) technology, delayed both heading time and leaf senescence. Furthermore, knockdown of OsY37 expression caused dwarfism and high accumulation of chlorophyll during the vegetative phase. Irrespective of early or delayed senescence, transgenic plants showed reduced grain yields. Our results indicate that OsY37 acts as a positive regulator of heading and senescence during the reproductive phase in rice. In addition, OsY37 may be involved in plant development and grain yield.

Mapping of QTLs underlying flowering time in sorghum [Sorghum bicolor (L.) Moench].

Due to its critical importance in crop yield, the photoperiodic regulation of flowering time is considered an important trait in sorghum breeding programs. In this study, quantitative trait loci for flowering time were detected using an F(2) population derived from a cross between Kikuchi Zairai, a late-flowering cultivar originating from Japan and SC112, an early-flowering cultivar originating from Ethiopia. F(2) plants were grown with their parents under a natural day length and a 12 h day length. Two linkage maps were constructed using 213 simple sequence repeats markers. Nine quantitative trait loci controlling flowering time were identified in F(2) plants grown under a natural day length, whereas 7 QTLs were identified under a 12 h day length. Five QTLs controlling flowering time were shared under both of the day length conditions.