Newly Formed Vacuoles in Root Meristems of Barley and Pea Seedlings Have Characteristics of Both Protein Storage and Lytic Vacuoles

Abstract

Plant cells are considered to possess functionally different types of vacuoles in the same cell. One of the papers cited in support of this concept reported that protein storage and lytic vacuoles in root tips of barley (Hordeum vulgare) and pea (Pisum sativum) seedlings were initially separate compartments that later fused to form a central vacuole during cell elongation. We have reinvestigated the situation in these two roots using immunogold electron microscopy as well as immunofluorescence microscopy of histological sections. Using antisera generated against the whole protein of a-tonoplast intrinsic protein (TIP) as well as specific C-terminal TIP peptide antisera against a-, g-, and d-TIP, together with antisera against the storage proteins barley lectin and pea legumin and vicilin, we were unable to obtain evidence for separate vacuole populations. Instead, our observations point to the formation of a single type of vacuole in cells differentiating both proximally and distally from the root meristem. This is a hybridtype vacuole containing storage proteins and having both a-and g-TIPs, but not d-TIP, in its tonoplast. As cells differentiate toward the zone of elongation, their vacuoles are characterized by increasing amounts of g-TIP and decreasing amounts of a-TIP.

Protein Storage Vacuoles Are Transformed into Lytic Vacuoles in Root Meristematic Cells of Germinating Seedlings by Multiple, Cell Type-Specific Mechanisms

“…The importance of these studies lies in the fact that they contradict the predictions of the vacuole biogenesis hypothesis of Paris et al (1996), which postulates that the large LVs of vegetative cells develop by the merging of separate PSVs and LVs. The immunolabeling results of our investigation are in complete agreement with those of Olbrich et al (2007), and our ultrastructural results demonstrate that the seedling root cells of tobacco contain only one type of vacuole at any one time of development. Thus, upon rehydration, the radicle cells only contain PSVs, but during subsequent root development, the PSVs are systematically transformed into LVs via cell typespecific pathways.…”

“…As documented by Hinz et al (1995) and by Olbrich et al (2007), the storage proteins in chemically fixed PSVs and barley (Hordeum vulgare) seedlings appear clumped and often aggregated along the deformed inner surface of the vacuole membrane. We observed the same types of darkly stained storage protein aggregates in the PSVs of chemically fixed root tips of tobacco (Supplemental Fig.…”

“…The main contribution of the Olbrich et al (2007) and the Hunter et al (2007) vacuole biogenesis studies was the demonstration that all of the developing vacuoles in root cells contain both PSV and LV marker proteins. The importance of these studies lies in the fact that they contradict the predictions of the vacuole biogenesis hypothesis of Paris et al (1996), which postulates that the large LVs of vegetative cells develop by the merging of separate PSVs and LVs.…”

“…The hypothesis that PSVs have an ontogenetic relationship to the central vacuole has a long history (Hara and Matsubara, 1980;Van der Wilden et al, 1980;Melroy and Herman, 1991;Olbrich et al, 2007), but the general mechanism of the PSV-to-LV transformation Figure 9. Schematic diagram illustrating the different pathways involved in the transformation of PSVs into LVs in different types of root cells during seed germination and early root development.…”

“…The multivacuole concept of Paris et al (1996) postulates that young root tip cells contain two separate types of vacuoles, PSVs and LVs, which fuse with each other to form the large central vacuole. This hypothesis has been challenged by Olbrich et al (2007) and by Hunter et al (2007), whose PSV and LV marker protein studies have demonstrated that root cells contain only one type of vacuole.…”