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Cytochemical Studies on GERL, Provacuoles, and Vacuoles in Root

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Cytochemical Studies on GERL, Provacuoles, and Vacuoles in Root Proc. Natl. Acad. Sci. USA Vol. 75, No. 2, pp. 852-856, February 1978 Cell Biology Cytochemical studies on GERL, provacuoles, and vacuoles in root meristematic cells of Euphorbia (electron microscopy/enzyme localizations/lysosome/cellular autophagy/membrane flow) FRANCIS MARTY Institut de Biologie cellulaire, Unite d'Enseignement et de Recherche Scientifique de Luminy, Universit6 d'Aix-Marseille, 13288 Marseille Cedex 2, France Communicated by Alex B. Notikoff, November 9,1977 ABSTRACT The genesis of plant vacuoles has been studied M cacodylate buffer containing 0.22 M sucrose and then washed by high-voltage and conventional electron microscopy. Vacuole again with quick changes in graded buffers from pH 7.4 to 5.0 genesis is a lysosomal multistep rocess: (i) GERL (Golgi-asso- ciated Endoplasmic Reticulum from which Lysosomes appar- and finally in a citrate buffer (pH 4.8) to remove calcium salts. ently form) produces provacuoles into which lysosomal enzymes The pieces were incubated at 370 freely floating in acid phos- and probably other materials appear to be concentrated and phatase (EC 3.1.3.2) medium according to Novikoff (8, 9) or packaged; (ii) GERL-derived provacuoles cooperate to drive a in Bell and Barrnett medium for thiolacetic acid esterase (10). programmed cellular autophagy leading to young vacuoles; and Controls consisted of tissues either incubated without the sub- (iii) the young vacuoles swell and fuse together into a few large strate (sodium (3-glycerophosphate or cytidine 5'-monophos- mature vacuoles which continue to collect the GERL-derived phate (CMP) for acid phosphatase activities and thiolacetic acid provacuoles throughout the life of the cell. for the corresponding acid esterase) or in the presence of sodium Although vacuoles are the most conspicuous membrane- fluoride (0.01 M for acid phosphatase activities and 0.1 M for bounded organelles in differentiated plant cells and their bio- thiolacetic acid esterase). After the buffered rinses, subsequent chemical characterization is in progress (1, 2), the question of processing was conventional. their origin is unsettled despite many light and electron mi- Zinc Iodide-Osmium Reaction. After excision, 2-mm root croscopy studies (3-5). This report describes the use of marker tips were immediately fixed by immersion for 6-24 hr in a enzyme activities and electron microscopy, at conventional as mixture of zinc iodide and osmium tetroxide (ZIO) at room well as very high voltage, to investigate the pattern of vacu- temperature in the dark (11). Controls were fixed for the same olation in root meristematic cells. The results emphasize the role length of time in unbuffered 0.4% aqueous OsO4 at pH 5.9 of GERL* in the origin of vacuoles and establish the sequence without zinc iodide. of related lysosomal events. They suggest that the plant va- Sectioning and Microscopy. Thin sections (up to 800 A) cuome is an intracellular digestive system involved in cellular were examined in the Philips 300 electron microscope at 80kv. autophagy and in internal membrane flow. For morphological studies they were stained either with uranyl acetate and lead citrate or with 1% aqueous potassium per- MATERIAL AND METHODS manganate; sections from cytochemical specimens were ob- served without counterstain. Thick (1-10 jim) sections of tissues Germination. Seeds of Euphorbia characias L. harvested treated by selective cytochemical staining were held in 200- locally from wild plants were sterilized in 10% commercial mesh uncoated sandwich grids and were examined without bleach for 30 min. The seeds were rinsed, soaked for 18 hr. and counterstain at 2.5 MV with the Toulouse 3 MV electron mi- then placed in petri plates on filter paper moistened with croscope (11). deionized water. As soon as the root emerged, the seedlings were selected, transferred to a humid atmosphere, and grown ver- RESULTS tically at room temperature (220) in the dark. When the root Morphological Sequence. The most undifferentiated root reached 10 ± 2 mm, the seedlings were collected for processing cells just adjacent to the quiescent zone were devoid of vacuoles for electron microscopy. or their identifiable precursors. Nearby, in the youngest cells Morphological Studies. Whole seedlings were quickly im- where vacuolation is initiated, the primordial vacuole precursors mersed in an ice-cold fixative containing either 3% glutaral- or provacuoles arose in the immediate vicinity of dictyosomes dehyde/0. 1 M cacodylate buffer, pH 7.4 or 2.5% glutaral- (Fig. 1 A-D). The Golgi stacks consisted of five or six evenly dehyde/2% formaldehyde (prepared from paraformaldehyde spaced elements. Rough endoplasmic reticulum (ER) sheets immediately before use)/0.09 M cacodylate buffer, pH 7.4. approached the adjacent Golgi stack but provacuoles were not After 30 min, 1.5-mm apical root tips were excised while in seen budding from Golgi elements or from ER cisternae. Rather fixative, left immersed for an additional 90 min, and then they appeared to bud from GERL. GERL was at the "trans" processed as usual for observation by electron microscopy. side (12) of the dictyosomes and consisted of a twisted, Enzyme Activities. Preliminary processing included a short smooth-surfaced, polygonal meshwork of anastomosing tubules fixation in gfutaraldehyde or glutaraldehyde/formaldehyde and saccules, the saccules facing the Golgi stacks. The saccular followed by thorough rinses in 0.1 M cacodylatebuffer (pH 7.4). portions of GERL were separated from the trans element of the Root tips were generally cut axially and, in each experiment, Golgi stack by a distance greater than that between elements some tips were sectioned with the Sorvall TC 2 tissue chopper in the stack. Despite numerous small gaps between smooth ER at approximately 50 Am. Specimens were collected in cold 0.1 Abbreviations: CMP, cytidine 5'-monophosphate; ZIO, zinc iodide- The costs of publication of this article were defrayed in part by the osmium tetroxide; ER, endoplasmic reticulum. payment of page charges. This article must therefore be hereby marked * The acronym GERL has been given by Novikoff (6, 7) to the region "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate of smooth ER that is located at the trans aspect of the Golgi apparatus this fact. and that appears to produce Lysosomes. 852 Downloaded by guest on October 1, 2021 Cell Biology: Marty Proc. Natl. Acad. Sci. USA 75 (1978) 853 1.4 d FIG 1. Portions of meristematic cells from the root tip. AV, autophagic vacuole; ER, endoplasmic reticulum; G. Golgi stack; GE, GERL; M, mitochondrion; PV, provacuole; V, vacuole. (A) A dictyosome in a meristematic cell. (Not incubated; X57,600.) (B) Meristematic cell. (Incubated with f,-glycerophosphate 45 min. X64,000.) (C) Detailed view of a nascent provacuole. (ZIO stain 12 hr; X51,500.) (D) GERL portion in a highly meristematic cell. (Incubated with CMP 60 mmn section 1 ,um thick; X46,000.) (E) Elongated provacuoles in a vacuolating cell. (Not incubated; X31,250.) (F) Provacuolar channels in a cell. (Incubated with CMP 75 min, section 1 ,um thick; X25,000.) and GERL, direct continuities of rough and smooth ER sug- the bars of cages, around portions of the cytoplasm. It was dif- gested that rough ER gives rise to smooth elements of GERL. ficult to appreciate this architecture in ordinary thin sections In thin sections, nascent provacuoles were seen budding from where equatorial sections through the cage show only short or lying adjacent to GERL as if they arose from GERL at circumferentially arranged profiles (Fig. 2A, arrows). Although junctions of the anastomosing tubules. they would generally be erroneously interpreted as vesicles, In slightly more differentiated cells, provacuoles were ZIO-stained thick specimens (Fig. 2B) made it clear that each elongated into tubes 0. I Am in diameter (Fig. 1E). Their tri- short profile actually was a transverse section through one of dimensional aspect could be ascertained from sections 1 to 10 the tubes forming the bars of a cage. Subsequently, the tubes ,um thick studied by high-voltage electron microscopy (Fig. IF). of each sequestering cage merged laterally to entrap the pre- The branched provacuolar tubes formed continuous tracts viously demarcated portion of cytoplasm in a double-mem- throughout the cytoplasm. Because the tubes were cut at diverse brane system containing a continuous exoplasmic space. This angles, the observed profiles in thin sections were those of sequence of events led to autophagy but, initially, the seques- random sections through the sinuous and interconnected tubes tered cytoplasm was morphologically unaltered and might have (Fig. 1E). contained mitochondria, plastids, dictyosomes, ER, or peroxi- In the next stage, the provacuole tubes were wrapped, like somes. A webbed aspect of the forming double-membrane Downloaded by guest on October 1, 2021 854 Cell Biology: Marty Proc. Natl. Acad. Sci. USA 75 (1978) Gv*1 *lz.".... f 4 P V so *:O .S: .60 ma,i.... ..i,#, [ i F A: AlK *!:" # .. .. An.'... .>||;> AltA r 2. B i.ll ;$ %jo,~~~~~~~~ s .. "'I- -It' 4;7vIVW': 4 s,;fp .4:. ,.*.. ..1. f,,:I., I * esA, F: .^r:, >.. "I' ., 4 .t, An_ A.allude;> t~txtt;AlStak> /eJq e edil ee ,rAW E rk o A. As,... ti*W f t*9,~R ? e~~~~~~~4If~~~~~~~~~~~~go = ..S_ AM. .. A_ . .. e 2+<;sE:; \ti jot HEN ala ;'r'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ _ elf. < *Arts0s>9i;- * X t is jiFT 9 ?4,+f C A AL - n FIG. 2. Portions of meristematic cells from the root tip. Abbreviations as in Fig. 1. (A) Thin equatorial section through a.Rsequestration .R cage. (Not incubated; X18,900.) (B) Section 1 Asm thick including part of sequestration cage. (ZIO stain 12 h; X18,000.) (C) Typical.. ring-like.. ,.x@.:autophagic vacuole in a vacuolating cell. (Not incubated; X37,100.) (D) Autophagic vacuole in a root cell. (Incubated for the demonstration of thiolacetic acid esterase activity; X41,200.) .....t: :_>......
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