Activity and expression of ADP-glucose pyrophosphorylase during rhizome formation in lotus (Nelumbo nucifera Gaertn.).

BACKGROUND

Lotus root is a traditional and popular aquatic vegetable in China. Starch is an important component of the rhizome and directly affects the quality of processed products. ADP -glucose pyrophosphorylase (AGPase) is a rate-limiting enzyme associated with starch biosynthesis in plants. Therefore, in the present study, AGPase activity and NnAGP expression during rhizome development of lotus were analyzed.

RESULTS

Among 15 cultivars analyzed, the contents of amylose and total starch in the rhizome were highest in 'Mei Ren Hong'. 'Su Zhou' and 'Zhen Zhu' showed the lowest amylose, amylopectin and total starch contents. In the rhizome, activity of AGPase was highest at the middle swelling stage of development, and higher activity was observed in the 'Hou ba' leaf and terminational leaf at the same stage. Three AGPase genes, comprising two large subunit genes (NnAGPL1 and NnAGPL2) and one small subunit gene (NnAGPS), were isolated and identified. The deduced amino acid sequences showed 40.5 % similarity among the three genes. Full-length genomic DNA sequences of NnAGPL1, NnAGPL2, and NnAGPS were 4841, 11,346 and 4169 bp, respectively. Analysis of the temporal and spatial expression patterns revealed that the transcription levels of NnAGPL1 and NnAGPS were higher in the rhizome, followed by the 'Hou ba' leaf, whereas NnAGPL2 was significantly detected in the 'Hou ba' leaf and terminational leaf. The initial swelling stage of rhizome development was accompanied by the highest accumulation of mRNAs of NnAGPL1, whereas expression of NnAGPL2 was not detected during rhizome development. The transcript level of NnAGPS was highest at the initial swelling stage compared with the other rhizome developmental stages. Transcription of NnAGPL1, NnAGPL2, and NnAGPS was induced within 24 h after treatment with exogenous sucrose. The mRNA level of NnAGPL1 and NnAGPS was increased by exogenous ABA, whereas transcription of NnAGPL2 was not affected by ABA.

CONCLUSIONS

The three AGPase genes display marked differences in spatial and temporal expression patterns. Regulation of AGPase in relation to starch synthesis in lotus is indicated to be complex.