Background

Sucrose, as the major product of photosynthesis in plants, is transported from sources to sinks by three steps, phloem loading, long-distance transport in phloem, and phloem unloading, which are mediated by both plasmodesmata and sugar transporters (Zhu et al., 2013; Milne et al., 2018). Sucrose is then transformed into other sugars such as glucose and fructose. These soluble sugars are involved in energy and material metabolism and signaling, plant growth and development, and plant responses to biotic and abiotic stresses (Bezrutczyk et al., 2018; Yamada and Osakabe, 2018; Wei et al., 2020). On the other hand, pests regulate plant sugar partitioning to facilitate their infection of, and damage to, plants (Hofmann et al., 2009; Ibraheem et al., 2014; Sosso et al., 2019). It is highly beneficial for researchers to understand the mechanisms of plant-pest interactions in order to investigate changes in soluble sugar content of plant tissues during pest infection or damage.

Several methods have been developed to quantitate soluble sugars, such as the anthrone-sulfuric acid colorimetric method, high-performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS), and gas chromatography-mass spectrometry (GC-MS) (Montero et al., 2004; Jan et al., 2006; Magwaza and Opara, 2015; Ye et al., 2019). Among these methods, LC-MS and GC-MS are widely used for sugar quantitation. In comparison with LC, GC shows higher resolution at the high-performance level; although, a dervatization step is needed to convert sugars into volatile compounds (Romani et al., 1994; Cataldi et al., 2000). It has been reported that approximately 50 mg fresh weight of leaves and roots from two-year-old olive plants can be used to quantitate the soluble sugar content of these tissues by GC-MS (Cataldi et al., 2000), which, as we learned, is the highest sensitivity of GC-MS used in characterizing soluble sugar content of plant tissues. Here, we describe in detail a new GC-MS protocol, with some important modifications, that is able to quantitate the three soluble sugars, sucrose, glucose, and fructose, in plant tissues with approximately 15 mg fresh weight. This method was modified by us and has been successfully applied in our current work.