玉米叶片主脉及其鞘组织的早期发育和超微结构

EARLY DEVELOPMENT AND ULTRASTRUCTURE OF THE MAJOR VEINS AND THEIR SHEATH TISSUES IN THE MAIZE LEAVES 

研究了玉米（ＺｅａｍａｙｓＬ．）叶片中层基本分生组织３个相邻细胞形成主脉原形成层及维管束鞘（ＢＳ）的早期发育过程和超微结构。结果表明，在由３个相邻细胞组成的３细胞单元中，位于中间的细胞发生２次平周分裂，产生１个较小的和１个较大的细胞，前者分化为近轴端ＢＳ细胞前体，后者再发生１次不均等平周分裂产生１个远轴端ＢＳ细胞前体和１个原形成层起始细胞；位于两侧的细胞其中有１个也发生１次平周分裂，所产生的２个子细胞与另１个侧向细胞共同构成侧向ＢＳ细胞前体。在原生韧皮部启动分化之前，所有原形成层细胞均显示与原形成层起始细胞基本相似的超微结构特征：细胞核大，具电子致密的异染色质；细胞质致密，具丰富的核糖体、原质体和线粒体；细胞壁薄，有许多胞间连丝。通常只有短的粗面内质网和少量小液泡存在。此外在原形成层发育过程中也存在细胞质解体导致新液泡形成的现象，某些内质网参与了这一过程。ＢＳ和原形成层之间的超微结构差异是在叶脉启动以后逐渐出现的。呈分生状态的ＢＳ细胞在分化过程中所出现的超微结构变化主要有：（１）原质体经历原片层体阶段向叶绿体转化；（２）多层同心膜复合体出现，它们在ＢＳ细胞的分化过程中可能具有降解某些核糖体及其它细胞质组分?

The report described the ultrastructural changes that occurred in the major veins and their associated bundle sheaths (BS) of the maize (Zea mays L.) leaf blade in the process of their differentiation from three adjacent cells in the middle layer of the ground meristem, the minimal number of cells involved with the initiation of a procambial strand and the associated BS. The inner cell underwent two successive unequal periclinal divisions: a smaller cell that later differentiated into the adaxial BS cell precursor, and a larger one that divided once again periclinally yielding an abaxial BS cell precursor and a centrally located procambial initial cell. One of the two lateral cells immediately adjacent to either side of the inner cell also divided periclinally; these derivatives, along with another lateral cell of the original three celled unit formed the precursor cells of the lateral BS. Prior to the initiation of protophloem differentiation, all of the procambial cells showed ultrastructural characteristics basically similar to the procambial initial. They possessed a prominent nucleus with electron dense aggregates of heterochromatin, a dense cytoplasm rich in ribosomes, proplastids and mitochondria; also a thin wall containing numerous plasmodesmata. In many cases, only short pieces of rough endoplasmic reticulum cisternae and a few small sized vacuoles were present. In addition, evidence of cytoplasmic disintegration leading to new vacuole formation was noted in the process of procambium development. It was observed that certain endoplasmic reticulum was engaged in the sequestration and lysis of cytoplasm. No apparent ultrastructural difference was found between the BS cell precursors and the procambial initials, that was, the distinction between the procambium and the surrounding BS cells occurred gradually after vein initiation. The major ultrastructural changes which occurred during the differentiation of the meristematic BS cells into the vacuolated cells were (1) a proplastid to chloroplast transformation going through a prolamellar body stage, and (2) the appearance of the multi concentric membrane complex which might play a role in the degradation of some ribosomes and other cytoplasmic components during the differentiation of BS cells.