Loureiro M F, DE Faria S M, James E K, Pott A, Franco A A
Universidade Federal Mato Grosso, Faculdade de Agronomia, Cuiabá, MT 78060-900, Brazil.
CNPAB/EMBRAPA, Seropédica, Itaguai, RJ 23851-970, Brazil.
New Phytol. 1994 Oct;128(2):283-295. doi: 10.1111/j.1469-8137.1994.tb04012.x.
We report on the structure of N -fixing nodules formed on the stem of Discolobium pulchellum Benth., an aquatic legume in the subfamily Pupilionoideae, tribe Aeschynomeneae, from the Hooded areas in the 'Pantanal Matogrossense' region of Brazil. The stern (and root) nodules were obligately aquatic, requiring permanent submergence in water or flooded soil, and receive oxygen via profuse aerenchyma covering The lower stem. Of the 69 isolates of rhizobia isolated from stem and root nodules, 70% were fast-growing acid producers and 38% were slow growers. The rhizobia were not photosynthetic. Nodules were connected to the stem, and the vascular system from the stem branched throughout the nodule, penetrating the infected, tissue within finger-like ingrowths of cortex. In both stem and root nodules, infected tissue was aeschynomenoid or desmodioid, that is, without uninfected (interstitial) cells. The infected cells in stem nodules were vacuolate, with visible infection threads. The inner cortex was rich in amyloplasts and contained the components of an oxygen diffusion barrier (a boundary cell layer without intercellular spaces and glycoprotein occlusions of intercellular spaces in other cell layers). The mid-cortex, external to the boundary layer, consisted of loosely-packed cells and these were continuous with stem aerenchyma. The outer part of the nodules was made up of phellogen-derived cells forming a periderm, or 'corky' layer of cells. The periderm formed large lenticels above cortical vascular bundles. These lenticels also connected with the stem aerenchyma. Root nodules differed only in that infected cells were not vacuolate, bacteroids were larger and contained more poly-β-hydroxybutyrate (PHB) and there was less aerenchyma/lenticellular tissue. Stem and root nodule structure is discussed in terms of adaptations to O constraints in an aquatic environment.
我们报道了在巴西“潘塔纳尔湿地马托格罗索地区” Hooded 区域的豆科蝶形花亚科合萌族水生豆科植物美丽异木棉（Discolobium pulchellum Benth.）茎上形成的固氮根瘤的结构。茎（和根）瘤为专性水生，需要永久浸没在水中或水淹土壤中，并通过覆盖茎下部的大量通气组织获取氧气。从茎和根瘤中分离出的69株根瘤菌中，70%为快速生长的产酸菌，38%为慢速生长菌。这些根瘤菌不进行光合作用。根瘤与茎相连，茎的维管系统在整个根瘤中分支，穿透皮层指状向内生长物内的受感染组织。在茎和根瘤中，受感染组织为合萌型或山蚂蝗型，即没有未受感染（间隙）细胞。茎瘤中的受感染细胞有液泡，可见感染丝。内皮层富含造粉体，并含有氧气扩散屏障的成分（一层没有细胞间隙的边界细胞层以及其他细胞层中细胞间隙的糖蛋白堵塞）。边界层外部的中皮层由松散排列的细胞组成，这些细胞与茎通气组织相连。根瘤的外部由木栓形成层衍生的细胞组成，形成周皮或“木栓质”细胞层。周皮在皮层维管束上方形成大的皮孔。这些皮孔也与茎通气组织相连。根瘤的不同之处仅在于受感染细胞没有液泡，类菌体更大且含有更多的聚-β-羟基丁酸（PHB），并且通气组织/皮孔状组织较少。本文从对水生环境中氧气限制的适应性方面讨论了茎和根瘤的结构。
