Characterization of the spatial and temporal expression of the OsSSII-3 gene encoding a key soluble starch synthase in rice

Genetic Dissection and Functional Differentiation of ALKa and ALKb, Two Natural Alleles of the ALK/SSIIa Gene, Responding to Low Gelatinization Temperature in Rice

BACKGROUND

ALK is the key gene controlling rice gelatinization temperature (GT), which is closely associated with the eating and cooking quality (ECQ) in rice (Oryza sativa L.). To date, at least three ALK alleles are thought to be responsible for the diversity of GT among rice cultivars. The ALK^c/SSIIaⁱ allele with high activity of the soluble starch synthase IIa (SSIIa) controls high GT, but the accurate functional difference between ALK^a and ALK^b alleles, both controlling low GT, is not clearly elucidated. Thus, we generated rice near-isogenic lines (NILs) by introducing different ALK alleles into the japonica cultivar Nipponbare (Nip) to clarify the discrepant effects of the two low-GT ALK alleles.

RESULTS

The results showed that the function of two low-GT alleles (ALK^a and ALK^b) was different, and a much lower GT was observed in NIL(ALK^b) rice grains compared with that of Nip(ALK^a). Moreover, the starches of NIL(ALK^b) grains had a higher degree of branching, higher setback, consistence and higher cool pasting viscosity than those of Nip(ALK^a). The lower expression level of ALK^b, compared with ALK^a, resulted in depleted intermediate chains and increased short chains of amylopectin, thus affected the thermal and pasting properties of NILs' grains. Also, the data revealed both low-GT alleles were mainly found in temperate japonica, but more ALK^b was found in other subpopulations such as indica as compared to ALK^a.

CONCLUSIONS

Overall, all the results suggested that the function between two low-GT alleles was different, and the distribution of ALK^b was much wider than that of ALK^a among the subpopulations of cultivated rice.

Down-Regulation of SSSII-2 Gene Expression Results in Novel Low-Amylose Rice with Soft, Transparent Grains

Although soft rice, with low amylose content (AC), has high eating and cooking quality (ECQ), its appearance is poor due to the opaque endosperm. Here, a novel soft rice with low AC but a transparent appearance was generated by knocking-down the expression of SSSII-2, a gene encoding one isoform of soluble starch synthase (SSS). The physicochemical properties of the SSSII-2 RNAi rice are quite different from the control but more like the popular soft rice "Nanjing 46". The taste value assay further demonstrated that the ECQ of SSSII-2 RNAi rice was as high as "Nanjing 46", but only SSSII-2 RNAi rice retained the transparent endosperm under low moisture conditions. Further examination showed that the different morphologies and fine structures of the starch granules may contribute to the specific properties of SSSII-2 RNAi rice. Therefore, SSSII-2 has potential application in future high quality rice breeding programs.

Rice tissue-specific promoters and condition-dependent promoters for effective translational application

Rice (Oryza sativa) is one of the most important staple food crops for more than half of the world's population. The demand is increasing for food security because of population growth and environmental challenges triggered by climate changes. This scenario has led to more interest in developing crops with greater productivity and sustainability. The process of genetic transformation, a major tool for crop improvement, utilizes promoters as one of its key elements. Those promoters are generally divided into three types: constitutive, spatiotemporal, and condition-dependent. Transcriptional control of a constitutive promoter often leads to reduced plant growth, due to a negative effect of accumulated molecules during cellular functions or energy consumption. To maximize the effect of a transgene on transgenic plants, it is better to use condition-dependent or tissue-specific promoters. However, until now, those types have not been as widely applied in crop biotechnology. In this review, we introduce and discuss four groups of tissue-specific promoters (50 promoters in total) and six groups of condition-dependent promoters (27 promoters). These promoters can be utilized to fine-tune desirable agronomic traits and develop crops with tolerance to various stresses, enhanced nutritional value, and advanced productivity.