Transgenic alfalfa (Medicago sativa) with increased sucrose phosphate synthase activity shows enhanced growth when grown under N2-fixing conditions

Abstract

Overexpression of SPS in alfalfa is accompanied by early flowering, increased plant growth and an increase in elemental N and protein content when grown under N2-fixing conditions. Sucrose phosphate synthase (SPS; EC 2.3.1.14) is the key enzyme in the synthesis of sucrose in plants. The outcome of overexpression of SPS in different plants using transgenic approaches has been quite varied, but the general consensus is that increased SPS activity is associated with the production of new sinks and increased sink strength. In legumes, the root nodule is a strong C sink and in this study our objective was to see how increasing SPS activity in a legume would affect nodule number and function. Here we have transformed alfalfa (Medicago sativa, cv. Regen SY), with a maize SPS gene driven by the constitutive CaMV35S promoter. Our results showed that overexpression of SPS in alfalfa, is accompanied by an increase in nodule number and mass and an overall increase in nitrogenase activity at the whole plant level. The nodules exhibited an increase in the level of key enzymes contributing to N assimilation including glutamine synthetase and asparagine synthetase. Moreover, the stems of the transformants showed higher level of the transport amino acids, Asx, indicating increased export of N from the nodules. The transformants exhibited a dramatic increase in growth both of the shoots and roots, and earlier flowering time, leading to increased yields. Moreover, the transformants showed an increase in elemental N and protein content. The overall conclusion is that increased SPS activity improves the N status and plant performance, suggesting that the availability of more C in the form of sucrose enhances N acquisition and assimilation in the nodules.

Comparison of alfalfa plants overexpressing glutamine synthetase with those overexpressing sucrose phosphate synthase demonstrates a signaling mechanism integrating carbon and nitrogen metabolism between the leaves and nodules

“…In this study, we have shown that the constitutive overexpression of GS 1 improves plant growth and performance in alfalfa just as has been reported for alfalfa plants overexpressing SPS in a constitutive manner (Gebril et al., ). The question that we have raised in this study is why an increase in the expression of these two genes with completely different functions but with key roles in primary metabolism have the same outcome in nodulated alfalfa plants—increased nodule number, growth rates, and biomass.…”

“…Carbohydrate analysis was done using the anthrone method essentially as described in our earlier papers (Aleman et al., ; Gebril et al., ). For Suc and starch analysis, plants (35–40 days post‐inoculation) were kept under supplemental light for 48 hr, and leaves and nodules were harvested at 3:00 p.m.…”

Molecular improvement of alfalfa for enhanced productivity and adaptability in a changing environment

“…In line with this, transgenic alfalfa plants expressing a maize sucrose phosphate synthase, which encodes a key enzyme of sucrose biosynthesis in plants, resulted in dramatic increases in the growth of shoots and roots. These plants exhibited an approximately 30% increase in above‐ground dry weight compared to untransformed control plants; however, this strategy also resulted in an earlier flowering time and an overall increase in protein content (Gebril et al, ), which suggests that the overexpression of sucrose phosphate synthase has the potential to cause profound effects on plant development that may not all be ideal for forage crop improvement.…”

The Use of Gene Modification and Advanced Molecular Structure Analyses towards Improving Alfalfa Forage

“…For example, Gebril et al [51] constitutively expressed the maize sucrose phosphate synthase ( SPS ) gene in alfalfa resulting in increased growth rate and enriched crude protein content in leaves. SPS catalyzes the key reaction in the synthesis of plant sucrose, which is a main stable energy product from photosynthesis.…”

“…SPS catalyzes the key reaction in the synthesis of plant sucrose, which is a main stable energy product from photosynthesis. Overexpression of SPS in alfalfa increased chlorophyll content and photosynthetic rates and increased the sucrose level in leaves, as well as root mass and nodule numbers [51]. In another study, simultaneously expressing two bacterial genes, aspartate kinase ( AK ) and adenylylsulfate reductase ( APR ), affected amino acid accumulation in alfalfa leading to an increase in levels of sulfur amino acids [52].…”