Histochemical Examination of Invertase Activities in the Growing Tip of the Plant Root

Abstract

Background:A growing part of the root is one of the most active sinks for sucrose coming from source leaves through the phloem. In the root, sucrose is unloaded from conducting bundles and is distributed among the surrounding cells. To be involved in the metabolism, sucrose should disintegrate into hexoses by means of degrading enzymes. Aims:The aim of this research was to explore the possibility of the involvement of one such enzymes, invertase, in phloem unloading as well as distribution of its activity in the functionally different tissues of the plant root tips. Method:To estimate the enzyme activities in root tissues, we applied two techniques: the histochemical method using nitro blue tetrazolium. The localization of phloem unloading was studied with carboxyfluorescein, a fluorescent marker for symplastic transport. Results:Invertase activity was not detected in the apical part of the meristem. It appeared only between the basal part of this zone and the beginning of the elongation zone. There is the root phloem unloading in that area. Invertase activity increased with increasing the distance from the root tip and reached the highest values in the region of cell transition to elongation and in the elongation zone. The activities of the enzyme varied in different tissues of the same zone and sometimes in the neighboring cells of the same tissue. Biochemical determination of invertase activity was made in the maize root segments coincident to the zones of meristem, cell elongation and differentiation. The results of both methods of determination of invertase activity were in agreement. Conclusion:It was concluded that phloem unloading correlated with invertase activity, possibly because of the activation of invertase by unloaded sucrose. Invertase is one of the factors involved in the processes preparing the cells for their transition to elongation because the concentration of osmotically active hexoses increases after cleavage of sucrose, that stimulates water entry into the cells, which is necessary for elongation growth.

An in Vivo Imaging Assay Detects Spatial Variability in Glucose Release from Plant Roots

“…Suc metabolism could occur within the cytoplasm, with the Glc subsequently exported into the rhizosphere, or Suc could be first exported into the apoplast, where it could be hydrolyzed by cell wall invertases, leading to the release of Glc into the rhizosphere. Recently, Lunkova et al (2017) showed that maize roots have substantial vacuolar invertase activity in the region of cell elongation (∼2-7 mm from the root tip), with the highest activity in the cortical and epidermal cells, and that the activity was reduced in the meristem and more basal region of the roots. These results suggest that Suc is hydrolyzed within the symplasm and that Glc is released into the rhizosphere.…”