Plant growth and tissue sucrose metabolism in the system of trifoliate orange and arbuscular mycorrhizal fungi

Highlights

• Growth and sucrose metabolism in system of AMF and trifoliate orange were studied.

• Five AMF inoculations significantly increased growth performance and chlorophyll.

• AM plants had significantly lower sucrose and higher fructose in leaf and root.

• AM plants recorded notably higher leaf AI, NI and SS but lower root AI and NI.

• AM colonization correlated with root fructose but negatively with root sucrose.

Abstract

Mycorrhizal development is absolutely dependent on hexoses of the host plant from sucrose cleavage by sucrose-cleaving enzymes. A pot study evaluated effects of five arbuscular mycorrhizal fungi (AMF) species including Diversispora spurca, Funneliformis mosseae, Glomus versiforme, Rhizophagus intraradices, and Paraglomus occultum on growth performance, chlorophyll, sucrose, fructose and glucose concentrations, and sucrose-related enzyme (acid invertase, AI; neutral invertase, NI; sucrose synthase, synthetized direction, SS) activities in four-month-old trifoliate orange (Poncirus trifoliata) seedlings. All the inoculated treatments significantly increased plant height, stem diameter, leaf number, shoot and root fresh weight, total root length, root surface area, root volume, and chlorophyll a, chlorophyll b and total chlorophyll concentrations. In general, AMF inoculation markedly decreased leaf and root sucrose concentration and increased leaf and root glucose concentrations. AI, NI and SS activities in leaves were generally significantly higher in AM than in non-AM seedlings, whereas in roots AI, NI and SS activities were significantly lower in AM than in non-AM seedlings, except a significantly higher root SS activity in R. intraradices- and G. versiforme-colonized seedlings. Root AM colonization was significantly positively correlated with root glucose but negatively with root sucrose. These results indicated that AMF could regulate sucrose synthesis and cleavage of the host plant for AM development through affecting sucrose-related enzymes and chlorophyll concentration, thereby stimulating plant growth.

Introduction

Arbuscular mycorrhizal fungi (AMF), a kind of special nutritional living microorganisms, can form symbiotic associations with ∼80% of higher plants, where the fungi supply soil mineral nutrients and water to their plant hosts, in exchange of consuming host's carbohydrates for the fungal partners (Jakobsen et al., 2002, Smith and Read, 2008, Wu et al., 2011). And the most important carbon source for fungal growth derives from hexoses (glucose and/or fructose) by the sucrose cleavage (Franken, 2010). Therefore, sucrose cleavage is the most significant to mycorrhizal development and physiological functionings. In general, citrus plants represent shallow root system and no or less root hairs, and thus are strongly dependent on the AMF for uptake of water and nutrients from the soil (Wu et al., 2012). Therefore, it has important significance to clarify the mycorrhizal functioning according to the responses on sugar concentration and related enzyme activities in the host plant and analyses of relationship between AMF and sucrose metabolism.

Studies showed that AMF inoculation could improve the carbon metabolism of plants, such as the increases of plant carbon concentration and leaf photosynthesis (Jiang et al., 2008). Leake et al. (2004) found that growth of mycorrhizal fungi would consume roughly 4–20% carbohydrates (mainly sucrose) produced by photosynthesis of the host plant (Bago et al., 2003). As a result, mycorrhizal association can enhance a sink demand for carbohydrates from the host plant (Zhu and Miller, 2003). Nevertheless, Solaiman and Saito (1997) proved, in terms of isotope labeling method and nuclear magnetic resonance spectroscopy, that sucrose could not be absorbed as the direct carbon source by AMF. Further research found that sucrose in mycorrhiza could be absorbed and utilized by intraradical mycelium (IRM) of arbuscular mycorrhizas, where sucrose in mycorrhizal structures could be hydrolyzed into hexoses via invertases or sucrose synthase (SS) in host plants (Schubert et al., 2003). Therefore, sucrose-cleaving enzymes are vital for mycorrhizal formation and development in carbon demand. Meanwhile, sucrose is cleaved into glucose, as the main functioning for acid invertase (AI), to meet the rapid increasing demands of hexoses for organs’ metabolism in plants (Gao et al., 1999). And neutral invertase (NI) is to regulate the apoplastic load to keep sucrose concentration gradient between the source and the sink (Julie et al., 1996). SS, a reversible type of glycosyltransferase, being used to synthesize or cleave sucrose, was a cytosolic enzyme that played an important role in the regulation of sugar metabolism and sink strength (Schäfer et al., 2005).

Our previous study revealed that inoculation with Funneliformis mosseae could mediate activity of AI, NI and SS in three citrus genotypes, suggesting that sucrose cleavage is highly dependent on host genotypes and sucrose-cleaving enzyme types/activity (Wu et al., 2013). Schubert et al. (2003) observed that colonization of soybean roots by F. mosseae resulted in an increase in alkaline invertase activity for the fungal symbiont. On the other hand, mycorrhizal interacting with the host plant exists variation within the fungal species, though the mycorrhizal association is usually nonspecific (Zou and Wu, 2011). Knowledge about the effects of different AMF species on sucrose cleavage and relevant enzymes is poorly known.

The objectives of the study are to evaluate the effects of different AMF species on carbohydrate concentrations and sucrose-cleaving enzyme activities in leaves and roots of trifoliate orange and to clarify the efficiency of AMF species on sucrose cleavage.