Characterization of TaSPP-5A gene associated with sucrose content in wheat (Triticum aestivum L.)

Sucrose, the major product of photosynthesis and the primary sugar transported as a soluble carbohydrate via the phloem, is a critical determinant for harvest yield in wheat crops. Sucrose-phosphatase (SPP) catalyzes the final step in the sucrose biosynthesis pathway, implying its essential role in the plant.

In this study, wheat SPP homologs genes were isolated from chromosomes 5A, 5B, and 5D, designated as TaSPP-5A, TaSPP-5B, and TaSPP-5D, respectively. Sequence alignment showed one 1-bp Insertion-deletion (InDel) and three single nucleotide polymorphisms (SNPs) at TaSPP-5A coding region, forming two haplotypes, TaSPP-5Aa and TaSPP-5Ab, respectively. A derived cleaved amplified polymorphism sequence (dCAPS) marker, TaSPP-5A-dCAPS, was developed to discriminate allelic variation based on the polymorphism at position 1242 (C-T). A total of 158 varieties were used to perform a TaSPP-5A marker-trait association analysis, where two haplotypes were significantly associated with sucrose content in two environments and with thousand-grain weight (TGW) and grain length (GL) in three environments. Quantitative real-time PCR further revealed that TaSPP-5Aa showed relatively higher expression than TaSPP-5Ab in wheat seedling leaves, generally associating with increased sucrose content and TGW. The expression of TaSPP-5A and sucrose content in TaSPP-5Aa haplotypes were also higher than those in TaSPP-5Ab haplotypes under both 20% PEG-6000 and 100 μM ABA treatment. Sequence alignment showed that the two TaSPP-5A haplotypes comprised 11 SNPs from -395 to -1962 bp at TaSPP-5A promoter locus, participating in the formation of several conserved sequences, may account for the high expression of TaSPP-5A in TaSPP-5Aa haplotypes. In addition, the distribution analysis of TaSPP-5A haplotypes revealed that TaSPP-5Aa was preferred in the natural wheat population, being strongly positively selected in breeding programs.

According to the SNPs detected in the TaSPP-5A sequence, two haplotypes, TaSPP-5Aa and TaSPP-5Ab, were identified among wheat accessions, which potential value for sucrose content selection was validated by association analysis. Our results indicate that the favorable allelic variation TaSPP-5Aa should be valuable in enhancing grain yield by improving the sucrose content. Furthermore, a functional marker, TaSPP-5A-dCAPS, can be used for marker-assisted selection to improve grain weight in wheat and provides insights into the biological function of TaSPP-5A gene.