The spikelet is a unique inflorescence structure in grasses.  However, the molecular mechanism that regulates its development remains unclear, and we therefore characterize a spikelet mutant of rice (Oryza sativa L.), aberrant-floral spikelet 1 (afs1), which was derived from treatment of Xinong 1B with ethyl methanesulfonate.  In the afs1 mutant, the spikelet developed an additional lemma-like organ alongside the other normally developed floral organs, and the paleae were degenerated to differing degrees with or without normally developed inner floral organs.  Genetic analysis revealed that the afs1 phenotype was controlled by a single recessive gene.  The AFS1 gene was mapped between the insertion/deletion (InDel) marker Indel19 and the simple sequence repeat marker RM16893, with a physical distance of 128.5 kb on chromosome 4.  Using sequence analysis, we identified the deletion of a 5-bp fragment and a transversion from G to A within LOC_Os04g32510/ LAX2, which caused early termination of translation in the afs1 mutant.  These findings suggest that AFS1 may be a new allele of LAX2, and is involved in the development of floral organs by regulating the expression of genes related to their development.  The above results provide a new view on the function of LAX2, which may also regulate the development of spikelets.

 

An elite backcrossed inbred line Z550 with increased grains per panicle was identified from advanced backcrosses between Nipponbare and Xihui 18 by simple sequence repeat (SSR) marker-assisted selection (MAS).  Z550 carries 13 substitution segments distributed on chromosomes 1, 6, 7, 8, 9, 10, and 12, with an average substitution length of 1.68 Mb.  Compared with the Nipponbare parental line, plant height, panicle length, spikelets per panicle, grains per panicle, and grain weight for Z550 were significantly increased.  While the grain width of Z550 was significantly narrower, and the seed setting ratio (81.43%) was significantly lower than that of Nipponbare, it is still sufficient for breeding purposes.  Quantitative trait loci (QTLs) mapping for important agronomic traits was conducted with the F₂ population derived from Nipponbare crossed with Z550 using the restricted maximum likelihood (REML) method.  A total of 16, including 12 previously unreported QTLs were detected, with contribution rates ranging from 1.46 to 10.49%.  Grains per panicle was controlled by 8 QTLs, 5 of which increased number of grains whereas 3 decreased it.  qGPP-1, with the largest contribution (10.49%), was estimated to increase grains per panicle by 30.67, while qGPP-9, with the minimum contribution rate (2.47%), had an effect of increasing grains per panicle by 15.79.  These results will be useful for further development of single segment substitution lines with major QTLs, and for research of their molecular functions via QTL cloning.

 

Rice (Oryza sativa L.) is one of the most important food crops worldwide and a model monocot plant for gene function analysis, so it is an ideal system for studying flower development.  This study reports a mutant, named multi-floret spikelet 3 (mfs3), which is related to the spikelet development in rice and derived from the ethylmethane sulfonate (EMS)-treated rice cultivar XIDA 1B.  In mfs3, the main body of palea (bop) was degenerated severely and only glume-like marginal regions of palea (mrp) remained, while other floral organs developed normally, indicating that the palea identity was seriously influenced by the mutation.  It was also observed that the number of floral organs was increased in some spikelets, including 2 lemmas, 4 mrp, 4 lodicules, 8–10 stamens, and 2 pistils, which meant that the spikelet determinacy was lost to some degree in mfs3.  Furthermore, genetic analysis demonstrated that the mfs3 trait was controlled by a single recessive gene.  Using 426 F₂ mutants derived from the cross between sterile line 56S and mfs3, the MULTI-FLORET SPIKELET 3 (MFS3) gene was mapped between the molecular markers RM19347 and RM19352 on Chr.6, with a physical distance of 106.3 kb.  Sequencing of candidate genes revealed that an 83-bp fragment loss and a base substitution occurred in the LOC_Os06g04540 gene in the mutant, confirming preliminarily that the LOC_Os06g04540 gene was the MFS3 candidate gene.  Subsequent qPCR analysis showed that the mutation caused the down-regulation of OsMADS1 and FON1 genes, and the up-regulation of OsIDS1 and SNB genes, which are all involved in the regulation of spikelet development.  The MFS3 mutation also significantly reduced the transcription of the REP gene, which is involved in palea development.  These results indicated that the MFS3 gene might be involved in the spikelet meristem determinacy and palea identity by regulating the expression of these related genes.