Abstract:

Ammopiptanthus mongolicus is one of the only two ever-green shrub species in the north-west China deserts, and is able to form nodules with rhizobia as an ancient legume. In this study, histology of root nodules of A. mongolicus was analyzed. Nitrogen-fixing nodules showed typical characteristics of indeterminate nodules, and from the distal (apical) part of a nodule to the proximal (basal) region, four central zones were able to be distinguished: the bacteria-free meristematic zone I; infection zone II, in which the cells began to differentiate; nitrogen-fixing zone III; and senescent zone IV. Exception for the apical meristem, the other three zones were surrounded by lateral nodular tissues such as the parenchyma (i.e ., the inner cortex), the vascular bundles, the endodermis and the cortex, successively. Intracellular infection was observed in A. mongolicus normal nodules, while intercellular infection characteristic was shown in abnormal nodules. Ultrastructural observation showed that the region between zone II and III contained diversified cell types, exhibiting diverse cell processes: 1) After bacteria were released, host cells initiated the assembling process of perbacteriod membrane, during which vesiculars were engaged in transporting membrane materials to bacteria vicinity from endomembrane reticulum. Intact perbacteriod membrane was formed and contained a single bacterium, afterwards adjacent membranes were fused and enlarged to form a large peripheral membrane which wrapped several bacteria. 2) Compared to other regions, interzone II-III contained much more starch granules. 3)Many plasmodesmata existed between contiguous infected cells. Electron microscopy cytochemistry studies showed that glycoprotein grains in a loose form were located at internal wall of parenchyma cells. After being infected by bacteria the glycoprotein was translocated into the intercellular spaces between infected cells or between infected and un-infected cells, where most glycoprotein grains were attached to the external wall with some dissociated.

Key words: Ammopiptanthus mongolicus, nodule, histology, ultrastructure, glycoprotein