Immunocytochemical Analysis Shows that Glyoxysomes Are Directly Transformed to Leaf Peroxisomes during Greening of Pumpkin Cotyledons

Abstract

The functional transition of glyoxysomes to leaf peroxisomes occurs during greening of germinating pumpkin cotyledons (Cucurbita sp.Amakuri Nankin). The immunocytochemical protein A-gold method was employed in the analysis of the transition using glyoxysomal specific citrate synthase immunoglobulin G and leaf peroxisomal specific glycolate oxidase immunoglobulin G. The labeling density of citrate synthase was decreased in the microbodies during the greening, whereas that of glycolate oxidase was dramatically increased. Double labeling experiments using different sizes of protein A-gold particles show that both the glyoxysomal and the leaf peroxisomal enzymes coexist in the microbody of the transitional stage indicating that glyoxysomes are directly transformed to leaf peroxisomes during greening.There exist two functionally different types of microbody in plant tissues. Glyoxysomes are unique organelles engaged in the degradation of reserve oil via #-oxidation and glyoxylate cycle in germinated fatty seeds such as pumpkin (Cucurbita sp.) (1, 5). Upon greening, the cotyledons become photosynthetic organs; functional leaf peroxisomes, with a crucial role in photorespiration, appear together with chloroplasts. The important question is raised of whether the functional transition of microbodies proceeds in a linear and continuous fashion or whether a discontinuous, two-step sequence operates therein. Indeed, two drastically opposing hypotheses, i.e. 'one-population' and 'two-population' models, have been proposed (1). According to the onepopulation hypothesis, glyoxysomes are directly transformed to leaf peroxisomes during greening of cotyledons, accompanying the insertion of newly synthesized leaf peroxisome-specific enzymes and the concomitant breakdown of glyoxysome-specific enzymes (2, 12). On the contrary, the two-population hypothesis postulates that glyoxysomes are broken down and leaf peroxisomes are newly synthesized de novo (6, 7). As schematically illustrated in Figure 1, according to the one-population hypoth-

Autophagy-Related Proteins Are Required for Degradation of Peroxisomes inArabidopsisHypocotyls during Seedling Growth

“…The transition of seedling peroxisomes to leaf peroxisomes appears to be a gradual process, as several reports described a transitional form of peroxisomes containing both glyoxylate cycle and photorespiratory enzymes (Titus and Becker, 1985;Nishimura et al, 1986;Sautter, 1986). We do not know which of these three types of peroxisomes is subjected to autophagy ( Figure 5).…”

“…These two enzymes, which are found in various tissues in addition to the oil-storing tissues of germinating seeds (reviewed by Pracharoenwattana and Smith, 2008), rapidly disappear during postgerminative growth (Nishimura et al, 1986;. The degradation of the enzymes of the glyoxylate cycle is concomitant with the transition of ICL-positive seedling peroxisomes (formerly called glyoxysomes) to ICLnegative leaf peroxisomes, which participate in photorespiration.…”

Disrupting Autophagy Restores Peroxisome Function to anArabidopsis lon2Mutant and Reveals a Role for the LON2 Protease in Peroxisomal Matrix Protein Degradation

“…Immunolabeling experiments in greening cucurbit cotyledons directly demonstrate the presence of both photorespiration enzymes and glyoxylate cycle enzymes in the same peroxisomes (Titus and Becker, 1985;Nishimura et al, 1986;Sautter, 1986). Moreover, MLS is stable following in vitro import into early pumpkin (Cucurbita sp Amakuri Nankin) seedling peroxisomes but degraded after import into transitional peroxisomes (Mori and Nishimura, 1989).…”

Peroxisome Function, Biogenesis, and Dynamics in Plants

“…During seedling development, peroxisome functions shift from fatty acid utilization to photorespiration (Titus and Becker, 1985;Nishimura et al, 1986;Sautter, 1986;. Autophagy mutants accumulate peroxisomal proteins (Shibata et al, 2013;Yoshimoto et al, 2014) and peroxisomes Yoshimoto et al, 2014) during this transition, suggesting a role for pexophagy in clearing obsolete peroxisomes.…”

“…In addition to the core processes of b-oxidation and ROS detoxification, plant peroxisomes house diverse specialized functions (for review, see Reumann and Bartel, 2016) that may change during development (Titus and Becker, 1985;Nishimura et al, 1986;Sautter, 1986; or in response to environmental cues (for review, see Goto-Yamada et al, 2015). For example, plant peroxisomes sequester enzymes acting in photorespiration, which is important when ribulose-1,5-bisphosphate carboxylase/oxygenase fixes O 2 instead of CO 2 .…”