A splice site mutation in shrunken1-m causes the shrunken 1 mutant phenotype in maize

Abstract

Endosperm development plays an important role in the determination of grain weight in maize. In this context, a spontaneous recessive shrunken kernel mutant, shrunken1-m (sh1-m), identified from improved maize inbred line Zheng58 (Gai-Z58) was studied. Physiological and microscopic analysis revealed that the sh1-m mutant significantly increased the content of amylose, and its starch granules showed distinctive morphology. Utilizing 1877 recessive sh1-m individuals from a BC1 segregating population of B73/sh1-m and 19 molecular markers, the sh1-m gene was limited to a 2.1 kb interval within the GRMZM2G089713 gene. A 5-bp substitution in sh1-m covering the 3′ end of the thirteenth exon and the 5′ splicing site of the thirteenth intron led to a missense mutation and a mis-splicing site that resulted in early translational termination. sh1 reference mutant sh1-912A (shrunken-912A) had a single transversion at the 3′ splice junction of the second intron resulting in a mis-splicing site causing the 13-bp spliced out in the third exon and the initiation codon to shift to 118-bp downstream. Further genetic allelism tests between them confirmed that sh1-m is a novel allele of the Sh1 locus. Sh1 was constitutively expressed in all tested tissues, and its expression pattern/level was not significantly changed in developing seeds of the sh1-m mutant. Transcript mRNA patterns/levels of some other genes involved in starch biosynthesis was significantly altered in the sh1-m mutant. These results provide direct evidence that sh1-m plays a key role in starch biosynthesis and provide valuable information for grain quality research and breeding in maize.