Ultracytochemical localization of Ca2+ during the phloem ganglion development in Phyllostachys edulis

Ultracytochemical localization of Ca²⁺ was investigated using the potassium pyroantimonate precipitation method during the development of phloem ganglion. The result showed that Ca²⁺ was mainly localized in the cell wall and intercellular spaces in the initiating phase. With the development of the phloem ganglion, the distribution of Ca²⁺ transferred to the vacuole, and the Ca²⁺ deposits in the cell wall and intercellular space decreased. At the later stage of the developmental phase, Ca²⁺ was distributed in the tonoplast and vacuole phagocytosis, and the vacuole became the main calcium storage in this phase. At the early stage of maturation of the phloem ganglion, most of the phloem ganglion cells’ vacuoles cracked, and the cytoplastic Ca²⁺ content increased in large number. In the mature phloem ganglion, not only were there a few Ca²⁺ localized in the cytoplast of mature cells, but also in the differentiating cells in the vacuoles. Ca²⁺ was distributed in the tonoplast and vacuole contents; initiating cells almost had no Ca²⁺. In general, Ca²⁺ concentration in mature phloem ganglion cells was at a low level. The results indicated that the changes in Ca²⁺ distribution evoked the phloem ganglion generation, and Ca²⁺ regulated the physiological function of the phloem ganglion.     

毛竹"韧皮部结"发育过程中Ca2+-ATP酶的超微定位

用电镜和细胞化学技术对毛竹[Phyllostachys edulis（Carr.）H.De Lehaie]节部“韧皮部结”发育过程中Ca^2＋-ATP酶进行了超微细胞化学定位研究.结果显示：在“韧皮部结”形成期,仅细胞质膜和细胞核上具有很高的Ca^2＋-ATP酶活性;随着“韧皮部结”的发育,发育期细胞质膜上的Ca^2＋-ATP酶活性较形成期有所降低,而细胞核上仍保持较高的Ca^2＋-ATP酶活性,胞间连丝、运输小泡膜上都具有Ca^2＋-ATP酶活性;发育后期,液泡膜及内质网上也开始出现Ca^2＋-ATP酶沉积物;成熟期的“韧皮部结”细胞质膜上的Ca^2＋-ATP酶活性较发育期有所升高,并且在“韧皮部结”成熟的过程中,细胞核、内质网、胞间连丝、质体膜和细胞质降解物上始终都有较高的Ca^2＋-ATP酶活性.实验结果表明“韧皮部结”细胞具有活跃的生理代谢以及频繁的共质体运输和信息交流.     

毛竹茎秆纤维细胞发育过程中的细胞程序性死亡

利用TUNEL检测、细胞学及细胞化学方法,对毛竹茎秆纤维细胞发育过程中的细胞程序性死亡进行了研究。在次生壁形成的早期,纤维细胞出现染色质凝聚、细胞器膨胀、液泡膜解体和细胞质泡状化等典型的细胞程序性死亡形态学特征;TUNEL检测反应呈阳性,显示此时的纤维细胞核DNA发生了片段化。此时,在纤维细胞裂解的液泡膜、降解的细胞质和凝聚的染色质上具有ATPase活性。纤维细胞质的Ca^2＋水平会随着次生壁的形成而逐渐升高,随后Ca^2＋聚集成块状。在初生壁形成后期,纤维细胞染色质上的酸性磷酸酶（APase）活性增强。随着纤维次生壁的持续增厚,ATPase、酸性磷酸酶和Ca^2＋将在裂解的细胞质和凝聚的染色质上持续存在多年。结果表明,毛竹茎秆纤维细胞的次生壁形成过程是一个主动自溶的细胞程序性死亡过程。初生壁形成后期染色质上酸性磷酸酶活性增强及次生壁形成期胞质Ca^2＋的聚集,与纤维细胞的程序性死亡密切相关。ATPase,Ca^2＋和APase参与了纤维细胞程序性死亡过程中原生质体的降解。     

温度逆境交叉适应对葡萄叶片膜脂过氧化和细胞钙分布的影响

 研究了高温锻炼对低温胁迫下和低温锻炼对高温胁迫下葡萄(Vitis vinifera)叶片中丙二醛（MDA）、谷胱甘肽(GSH)和抗坏血酸(AsA)含量变化以及细胞中Ca2+分布的影响。结果表明: 高（低）温胁迫使正常生长的叶片丙二醛含量升高, GSH和AsA含量下降,低（高）温锻炼预处理能减少MDA含量,提高GSH和AsA含量,抑制了由于温度胁迫引起MDA含量升高和GSH和AsA下降趋势。常温下葡萄叶肉细胞的Ca2+主要分布于液泡、细胞间隙中;高温胁迫和低温胁迫后,细胞质中聚集大量Ca2+沉淀颗粒,液泡中和细胞间隙Ca2+沉淀颗粒减少,叶绿体超微结构被破坏,Ca2+稳态平衡遭到破坏。高温锻炼后细胞质出现大量的Ca2+沉淀颗粒,主要来源于细胞间隙,低温锻炼后细胞质也出现大量的Ca2+沉淀颗粒,主要来源于液泡,两者的叶绿体超微结构都完整;高温锻炼的叶片经过低温胁迫和低温锻炼的叶片经过高温胁迫后,细胞间隙和液泡内Ca2+沉淀颗粒增加,细胞质中Ca2+沉淀颗粒很少,叶绿体较完整,Ca2+稳态平衡得以维持。推测高低温锻炼能够通过Ca2+启动抗逆基因表达和维持细胞中Ca2+稳态平衡来交叉适应低高温的胁迫。     

枸杞体细胞胚发生中Ca^2＋和ATPase的超微结构定位研究

研究2,4-D诱导枸杞体细胞胚发生中的作用及其与Ca^2 含量和ATPase活性时空分布动态之间的关系,以探讨2,4-D诱导植物体细胞胚发生的作用机理。采用超微细胞化学定位的方法,跟踪分析了体细胞胚发生与发育的不同时期,Ca^2 和ATPase活性的时空分布动态。结果表明：2,4-D是诱导离体培养的枸杞体细胞进入胚胎状态的关键激素。在含有2,4-D和不含2,4-D的培养条件下,分别诱导枸杞体细胞脱分化后,再转入除去2,4-D的MS培养基上,进行分化培养,结果前者可分化形成体细胞胚,因而称为胚性愈伤组织。后者在相同条件却不能分化形成胚,故称为非胚性愈伤组织。在2,4-D诱导枸杞的胚性愈伤组织中,胚性细胞分化早期的细胞间隙和细胞壁上均有Ca^2 沉淀。随着胚性细胞的分化、分裂和多细胞原胚形成,这时Ca^2 在细胞内的分布主要集中在细胞膜和液泡膜上;球形胚期在细胞核中Ca^2 呈弥散性分布。在此过程中,ATPase活性时空分布与Ca^2 的定位变化具有高度一致性,仅仅稍滞后于Ca^2 出现的时间。而在胚性细胞分化早期,ATPase活性同样位于质膜上,随后在液泡和细胞核都可见ATPase活性分布。而在非胚性愈伤组织中,则未见Ca^2 和ATPase活性呈时空动态分布,而且随着非胚性细胞的液泡化,无论是Ca^2 含量,还是ATPase活性都呈逐渐降低的趋势。表明Ca^2 和ATPase活性变化与2,4-D诱导的胚性细胞分化和发育密切相关。并由此推测,Ca^2 和ATPase的时空分布对胚性细胞分化中的信息传递和调控相关基因表达起着关键性作用。     

枸杞体细胞胚发生中Ca2+和ATPase的超微结构定位研究

研究2,4-D诱导枸杞体细胞胚发生中的作用及其与Ca~(2+)含量和ATPase活性时空分布动态之间的关系,以探讨2,4-D诱导植物体细胞胚发生的作用机理。采用超微细胞化学定位的方法,跟踪分析了体细胞胚发生与发育的不同时期,Ca~(2+)和ATPase活性的时空分布动态。结果表明:2,4-D是诱导离体培养的枸杞体细胞进入胚胎状态的关键激素。在含有2,4-D和不含2,4-D的培养条件下,分别诱导枸杞体细胞脱分化后,再转入除去2,4-D的MS培养基上,进行分化培养,结果前者可分化形成体细胞胚,因而称为胚性愈伤组织。后者在相同条件却不能分化形成胚,故称为非胚性愈伤组织。在2,4-D诱导枸杞的胚性愈伤组织中,胚性细胞分化早期的细胞间隙和细胞壁上均有Ca~(2+)沉淀。随着胚性细胞的分化、分裂和多细胞原胚形成,这时Ca~(2+)在细胞内的分布主要集中在细胞膜和液泡膜上;球形胚期在细胞核中Ca~(2+)呈弥散性分布。在此过程中,ATPase活性时空分布与Ca~(2+)的定位变化具有高度一致性,仅仅稍滞后于Ca~(2+)出现的时间。而在胚性细胞分化早期,ATPase活性同样位于质膜上,随后在液泡和细胞核都可见ATPase活性分布。而在非胚性愈伤组织中,则未见Ca~(2+)和ATPase活性呈时空动态分布,而且随着非胚性细胞的液泡化,无论是Ca~(2+)含量,还是ATPase活性都呈逐渐降低的趋势。表明Ca~(2+)和ATPase活性变化与2,4-D诱导的胚性细胞分化和发育密切相关。并由此推测,Ca~(2+)和ATPase的时空分布对胚性细胞分化中的信息传递和调控相关基因表达起着关键性作用。     

脱硫废弃物对碱胁迫下水稻叶片钙分布、Ca2+-ATPase活性及抗氧化特征的影响

为了探讨脱硫废弃物提高水稻抗盐碱的作用机制,采用盆栽法,研究脱硫废弃物对碱胁迫下水稻幼苗叶片总钙含量、Ca2+分布、细胞膜Ca2+-ATPase活性及活性氧含量等的变化.结果表明： 对照处理的细胞中钙颗粒零星分布于细胞壁和叶绿体中,添加脱硫废弃物和CaSO4处理的细胞质膜、细胞间隙、细胞壁和液泡中有大量的钙颗粒分布;随着脱硫废弃物和CaSO4添加量的增加,叶片总钙含量增加,质膜和液泡膜Ca2+-ATPase活性呈上升趋势,质膜透性、MDA含量和活性氧O2〖SX(B-*3〗-〖〗·〖SX)〗产生速率呈下降趋势,SOD、POD等保护酶活性升高.添加脱硫废弃物在一定程度上能够减缓碱胁迫对水稻造成的细胞伤害,起主要作用的物质可能是其主要成分CaSO4.     

脱硫废弃物对碱胁迫下油葵幼叶细胞钙分布及Ca2+-ATPase活性的影响

利用脱硫废弃物改良盐碱地对于确保国家粮食安全和生态安全,发展循环经济具有重要意义。为了探索脱硫废弃物提高植物抗盐碱机理,采用盆栽试验法, 研究了施入不同量脱硫废弃物和CaSO₄对碱胁迫下油葵叶片细胞钙分布、总钙含量以及质膜和液泡膜Ca²⁺-ATPase活性的影响。结果表明:在碱胁迫下(CK),Ca²⁺与焦锑酸钾结合成黑色颗粒成团零星分布于叶绿体和液泡中,叶绿体超微结构受到不同程度的破坏。施入脱硫废弃物和CaSO₄,叶绿体结构完整,细胞间隙、细胞壁和液泡中的钙颗粒逐渐增多,同时,质膜和液泡膜Ca²⁺-ATPase活性随脱硫废弃物和纯品硫酸钙施量的增加而增加,其中液泡膜Ca²⁺-ATPase活性无论是对照(CK)还是处理的活性均高于质膜Ca²⁺-ATPase活性。叶片细胞内总钙含量也随脱硫废弃物和CaSO₄施用量的增加呈升高趋势。说明脱硫废弃物和CaSO₄通过增加Ca²⁺-ATPase活性,有利于钙通过质膜和液泡膜进入细胞内,维持膜结构的稳定性,缓解碱对油葵的胁迫。     

ATPase in mature and differentiating phloem and xylem

A cytochemical study using a lead precipitation technique has been made of the distribution of adenosine triphosphatase (ATPase) in mature and differentiating phloem and xylem cells of Nicotiana tabacum and Pisum sativum. The sites of ATPase localization in tobacco phloem were the plasma membrane, endoplasmic reticulum, mitochondria, dictyosomes, plasmodesmata, and the dispersed P proteins of mature sieve elements. In pea phloem sieve elements ATPase was localized in the endoplasmic reticulum, but was not associated with the P proteins or plasma membranes at any stage of their differentiation. In pea transfer cells ATPase activity was associated with the endoplasmic reticulum at all stages of their differentiation and with the plasma membrane of transfer cells that had formed wall ingrowths. In xylem cells of both tobacco and pea the patterns of ATPase activity was similar. At early stages of differentiation ATPase activity was associated with the plasma membrane and the endoplasmic reticulum. At intermediate stages of differentiation ATPase activity continued to be associated with the endoplasmic reticulum, but was no longer associated with the plasma membrane. At later stages of xylem element differentiation ATPase activity was associated with disintegrating organelles and with the hydrolyzing cell walls.     

Inositol 1,4,5-trisphosphate releases Ca2+ from vacuolar membrane vesicles of oat roots

In plant cells, transient changes in cytoplasmic Ca2+ levels can modulate numerous developmental processes. Ca2+ is accumulated in the vacuole via a H+/Ca2+ antiport system that is energized by the tonoplast H+-pumping ATPase. Inositol 1,4,5-triphosphate (InsP3), but not inositol 1,4-bisphosphate, myo-inositol 1-phosphate, or fructose 2,6-bisphosphate, caused a transient reduction of Ca2+ levels in tonoplast vesicles. The decrease was dependent on InsP3 concentration (Km apparent = 0.6 microM). The InsP3-induced Ca2+ release was blocked by the Ca2+ antagonist, 8-(N,N-diethylamino)-octyl 3,4,5-trimethoxybenzoate-HCl. These results suggest that the vacuolar membrane is one target site for InsP3 action and that InsP3 may operate as a second messenger in the mobilization of intracellular Ca2+ in plant cells.     

鹤顶兰胚囊发育过程中Ca~（2＋）分布的超微细胞化学定位（英文）

用焦锑酸盐沉淀法对鹤顶兰（Phaius tankervilliae）胚囊发育过程中的Ca2＋状态进行超微细胞化学定位。观察结果发现：功能大孢子时期,珠孔端的胚囊壁上开始出现小颗粒的Ca2＋沉淀,但功能大孢子细胞内未见明显的Ca2＋标记;四核胚囊时期胚囊壁上的Ca2＋沉淀明显增多,液泡膜上有Ca2＋沉淀出现,珠孔处的Ca2＋沉淀颗粒较大;成熟胚囊时期,胚囊壁上的Ca2＋沉淀进一步增多,且胚囊内Ca2＋分布明显增多,且极性明显,珠孔端助细胞、卵细胞比合点端反足细胞有更多的Ca2＋沉淀。鹤顶兰成熟胚囊内Ca2＋积累的来源有：（1）在胚囊成熟前主要由珠被细胞、珠细胞通过胞间连丝向胚囊运输;（2）以沉淀有大量Ca2＋的小泡形式跨过胚囊壁进入胚囊。     

New insights into the tonoplast architecture of plant vacuoles and vacuolar dynamics during osmotic stress

Background  

The vegetative plant vacuole occupies >90% of the volume in mature plant cells. Vacuoles play fundamental roles in adjusting cellular homeostasis and allowing cell growth. The composition of the vacuole and the regulation of its volume depend on the coordinated activities of the transporters and channels localized in the membrane (named tonoplast) surrounding the vacuole. While the tonoplast protein complexes are well studied, the tonoplast itself is less well described. To extend our knowledge of how the vacuole folds inside the plant cell, we present three-dimensional reconstructions of vacuoles from tobacco suspension cells expressing the tonoplast aquaporin fusion gene BobTIP26-1::gfp.     

Immunohistochemical localization of hemicelluloses and pectins varies during tissue development in the bamboo culm

The distribution of hemicelluloses and pectins in bamboo internodes was studied immunocytochemistrically at various stages of development. The ultra-structures of bamboo cell walls have been reported previously at various stages. The internodes were identically classified into three developmental phases: primary wall stage (phase I), unlignified secondary wall stage (phase II) and lignified wall stage (phase III), using the same bamboo culm. (1-->3, 1-->4)-Beta-glucans were distributed in nearly all tissues in an actively elongating stage. Limited amounts of beta-glucans were deposited in primary walls and the middle lamellae, but were limited to the phloem in secondary walls. This suggests that the function of beta-glucans might be different in phloem vis-à-vis other tissues. Highly-substituted xylans were located in nearly all tissues of early phase I, but had disappeared in all tissues immediately prior to lignification. In contrast, low-branched xylan epitopes were present only in the protoxylem in phase I, but were present in all tissues immediately prior to lignification in phase II. In phase III, the epitopes were densely localized in lignified walls, suggesting that the substitution of xylans is closely related to maturation. Methyl-esterified (but not unesterified) pectins were present in all tissues of early phase I. Just before and after lignification, both types of pectins were concentrated in the phloem and protoxylem. Xyloglucans were largely distributed in the phloem and in lignified tissues, suggesting that they might be closely correlated with maturation. This represents the first account of the distribution of hemicelluloses and pectins at the tissue and ultrastructural level in bamboo internodes at various stages of development.     

Immunohistochemical Localization of Hemicelluloses and Pectins Varies During Tissue Development in the Bamboo Culm

The distribution of hemicelluloses and pectins in bamboo internodes was studied immunocytochemistrically at various stages of development. The ultra-structures of bamboo cell walls have been reported previously at various stages. The internodes were identically classified into three developmental phases: primary wall stage (phase I), unlignified secondary wall stage (phase II) and lignified wall stage (phase III), using the same bamboo culm. (1→,1→4)-β-Glucans were distributed in nearly all tissues in an actively elongating stage. Limited amounts of β-glucans were deposited in primary walls and the middle lamellae, but were limited to the phloem in secondary walls. This suggests that the function of β-glucans might be different in phloem vis-à-vis other tissues. Highly-substituted xylans were located in nearly all tissues of early phase I, but had disappeared in all tissues immediately prior to lignification. In contrast, low-branched xylan epitopes were present only in the protoxylem in phase I, but were present in all tissues immediately prior to lignification in phase II. In phase III, the epitopes were densely localized in lignified walls, suggesting that the substitution of xylans is closely related to maturation. Methyl-esterified (but not unesterified) pectins were present in all tissues of early phase I. Just before and after lignification, both types of pectins were concentrated in the phloem and protoxylem. Xyloglucans were largely distributed in the phloem and in lignified tissues, suggesting that they might be closely correlated with maturation. This represents the first account of the distribution of hemicelluloses and pectins at the tissue and ultrastructural level in bamboo internodes at various stages of development.     

Increased Expression of Vacuolar Aquaporin and H+-ATPase Related to Motor Cell Function in Mimosa pudica L

Mature motor cells of Mimosa pudica that exhibit large and rapid turgor variations in response to external stimuli are characterized by two distinct types of vacuoles, one containing large amounts of tannins (tannin vacuole) and one without tannins (colloidal or aqueous vacuole). In these highly specialized cells we measured the abundance of two tonoplast proteins, a putative water-channel protein (aquaporin belonging to the [gamma]-TIPs [tonoplast intrinsic proteins]) and the catalytic A-subunit of H+-ATPase, using either high-pressure freezing or chemical fixation and immunolocalization. [gamma]-TIP aquaporin was detected almost exclusively in the tonoplast of the colloidal vacuole, and the H+-ATPase was also mainly localized in the membrane of the same vacuole. Cortex cells of young pulvini cannot change shape rapidly. Development of the pulvinus into a motor organ was accompanied by a more than 3-fold increase per length unit of membrane in the abundance of both aquaporin and H+-ATPase cross-reacting protein. These results indicate that facilitated water fluxes across the vacuolar membrane and energization of the vacuole play a central role in these motor cells.     

Immunogold localization of callose and other cell wall components in pea nodule transfer cells

Summary Transfer cells are located adjacent to xylem and phloem elements in pea nodule vascular tissues. The composition of the labyrinthine wall intrusions was investigated by immunogold labeling using specific antibody probes. Callose antigen was found at the base of newly formed cell wall intrusions and also in adjacent plasmodesmata. Sections through developed labyrinthine intrusions revealed that wall ingrowths had an internal structure with small domains of callose suggesting the presence of channels or vents. Xyloglucan and pectin antigens were uniformly distributed within the wall, but the distribution of extensin antigens was variable, with different antigens being detected in different regions of the wall ingrowth. A lectinlike glycoprotein, PsNLEC-1, was localized in intercellular spaces associated with nodule transfer cells. Previously, expression of this component was observed in other types of cells showing complex involution of the plasma membrane, namely root cortical cells harboring arbuscular mycorrhizae and nodule cells harboring nitrogen-fixing rhizobia.     

枸杞花药发育过程中钙的分布

在枸杞花药发育过程中,用焦锑酸钾沉淀的钙颗粒显示出了一个与花药发育事件有关的分布特征：在孢原细胞时期的花药中钙颗粒很少。在造孢细胞到小孢子母细胞时期,花药中钙颗粒增加。当花粉母细胞进行减数分裂时,花药中的钙颗粒进一步增加,尤其是在小孢子母细胞的胼胝质壁中。在小孢子发育早期,花药药隔部位的绒毡层细胞质中钙颗粒也明显增加并特异性地分布在其内切向壁上。当小孢子被释放出后,钙颗粒开始特异性积累在正在形成的花粉外壁中,尤其在萌发孔的部位聚集了大量的钙颗粒。当小孢子形成大液泡时,其细胞质中的钙颗粒明显减少。在小孢子分裂形成二胞花粉后,在二胞花粉的大液泡中又特异性地出现许多细小钙颗粒。随着二胞花粉的大液泡完全消失,其细胞质中又出现了许多钙颗粒。接近开花时的成熟花粉粒细胞质中,细小的钙颗粒主要分布在营养细胞和生殖细胞中。枸杞花药发育过程中钙的分布特征反映了其参与调控花粉发育过程。     

脱硫废弃物对碱胁迫下水稻叶片钙分布、Ca2+-ATPase活性及抗氧化特征的影响

为了探讨脱硫废弃物提高水稻抗盐碱的作用机制,采用盆栽法,研究脱硫废弃物对碱胁迫下水稻幼苗叶片总钙含量、Ca2+分布、细胞膜Ca2+-ATPase活性及活性氧含量等的变化.结果表明:对照处理的细胞中钙颗粒零星分布于细胞壁和叶绿体中,添加脱硫废弃物和CaSO4处理的细胞质膜、细胞间隙、细胞壁和液泡中有大量的钙颗粒分布;随着脱硫废弃物和CaSO4添加量的增加,叶片总钙含量增加,质膜和液泡膜Ca2+-ATPase活性呈上升趋势,质膜透性、MDA含量和活性氧O2-产生速率呈下降趋势,SOD、POD等保护酶活性升高.添加脱硫废弃物在一定程度上能够减缓碱胁迫对水稻造成的细胞伤害,起主要作用的物质可能是其主要成分CaSO4.     

Localization of Phloem Oxidases

Among oxidases, cytochrome oxidase has been localized in mitochondria of all phloem cells, catalase has been visualized in parenchyma peroxisomes and peroxidase has been localized in cell walls and in several cell organelles. In angiosperms, peroxidase is present in all phloem cell walls; it is sensitive to cyanide inhibition excepted in sieve areas and around plasmodesmata between sieve tubes and companion cells. In some species, this cyanide resistant oxidasic activity can be localized without exogenous H₂O₂. Peroxidase is localized on ribosomes, inside vacuoles, on the tonoplast and often on the plasmalemma in companion cells and differentiating sieve elements. In young sieve cells some dictyosomes can exhibit a strong peroxidasic activity. In mature parenchyma cells peroxidase can be associated with ER cisternae but not with vacuoles.     

Acid invertase is predominantly localized to cell walls of both the practically symplasmically isolated sieve element/companion cell complex and parenchyma cells in developing apple fruits

The acid invertase (β‐fructosidase, EC 3·2·1·26) was localized at subcellular level via immunogold electron microscopy in the phloem‐unloading zone of developing apple fruit. The enzyme (immunogold particles) was found to reside predominantly in the cell walls of the sieve element/companion cell (SE/CC) complex, phloem parenchyma cells and other parenchyma cells. There was almost no gold particle found in cytoplasm and vacuole. This distribution pattern remained unchanged throughout the growing season, but the enzyme numbers varied. The density of immunogold particles increased during fruit development. The immunoblotting of soluble and insoluble acid invertases provided a supporting proof for the assays of immunolocalization. The biochemical analysis showed a predominantly cell‐wall‐distributed activity of acid invertase that corresponds essentially with its amount distribution. The ultrastructural observations showed that there were numerous plasmodesmata between the parenchyma cells, but almost no plasmodesmium between the SE/CC complex and its surrounding parenchyma cells, practically resulting in the symplasmic isolation of the SE/CC complex. It is therefore suggested that the unloading pathway of sucrose from the SE/CC complex may be predominantly apoplasmic in the developing apple fruit, and that the unloaded sucrose may be hydrolysed by the functional acid invertase localized in the cell wall before it is loaded in sink cells.