CsAGA1 and CsAGA2 Mediate RFO Hydrolysis in Partially Distinct Manner in Cucumber Fruits.

Mengying Zhang,Zhiping Zhang,Minmin Miao,Haibo Dai,Jinji Zhang,Bing Hua

doi:10.3390/ijms222413285

Abstract

A Raffinose family oligosaccharides (RFOs) is one of the major translocated sugars in the vascular bundle of cucumber, but little RFOs can be detected in fruits. Alpha-galactosidases (α-Gals) catalyze the first catabolism step of RFOs. Six α-Gal genes exist in a cucumber genome, but their spatial functions in fruits remain unclear. Here, we found that RFOs were highly accumulated in vascular tissues. In phloem sap, the stachyose and raffinose content was gradually decreased, whereas the content of sucrose, glucose and fructose was increased from pedicel to fruit top. Three alkaline forms instead of acid forms of α-Gals were preferentially expressed in fruit vascular tissues and alkaline forms have stronger RFO-hydrolysing activity than acid forms. By inducible gene silencing of three alkaline forms of α-Gals, stachyose was highly accumulated in RNAi-CsAGA2 plants, while raffinose and stachyose were highly accumulated in RNAi-CsAGA1 plants. The content of sucrose, glucose and fructose was decreased in both RNAi-CsAGA1 and RNAi-CsAGA2 plants after β-estradiol treatment. In addition, the fresh- and dry-weight of fruits were significantly decreased in RNAi-CsAGA1 and RNAi-CsAGA2 plants. In cucurbitaceous plants, the non-sweet motif within the promoter of ClAGA2 is widely distributed in the promoter of its homologous genes. Taken together, we found RFOs hydrolysis occurred in the vascular tissues of fruits. CsAGA1 and CsAGA2 played key but partly distinct roles in the hydrolysis of RFOs.

Highlights

Cucumber (Cucumis sativus L.) is an important vegetable worldwide with great economic and nutritional value [1,2]
In which sucrose (Suc) is the predominant sugar transported in the vascular bundle, raffinose (Raf) family oligosaccharides (RFOs) are the major sugars transported between “source” organs and “sink”
To exclude the potential nonspecific inhibition of CsAGA1 and CsAGA3 expression, we examined the expression of both CsAGA1 and CsAGA3, and found no significant difference in RNAi-CsAGA2 plants with β-estradiol treatment compared with wild type or RNAi-CsAGA2 plants without β-estradiol treatment (Figure 4A,C)

Summary

Introduction

Cucumber (Cucumis sativus L.) is an important vegetable worldwide with great economic and nutritional value [1,2]. In which sucrose (Suc) is the predominant sugar transported in the vascular bundle, raffinose (Raf) family oligosaccharides (RFOs) are the major sugars transported between “source” organs and “sink”. Fruit is one of the most important sink organs in cucumber plants. RFOs consisting of stachyose (Sta) and Raf are translocated into sieve elements and hydrolysed into sucrose (Suc) in sink tissues after long-distance translocation [10,11]. Some researchers believe that the catabolism of Sta and Raf occurs in the pedicel. Evidence supporting this view include little RFOs, which were detected in fruits [3,12,13]. While the activity of enzymes catalysing RFOs catabolism, such as α-galactosidase, Suc synthase (SS, synthetic activity), SPS and UDP- galactose pyrophosphorylase were detected in the pedicel [6,14]

Methods

Findings

Discussion

Conclusion