SYMBIOTIC TISSUE DEGRADATION PATTERN IN THE INEFFECTIVE NODULES OF 3 NODULATION MUTANTS OF PEA (PISUM-SATIVUM L)

Three symbiotic mutants of pea (Pisum sativum L.) forming ineffective nodules were investigated for aberrations in nodule structure using li ght and transmission electron microscopy. The mutants were ordered acc ording to the timing of the nodule development block. In the mutant Ri sfixO, symbiotic tissue development is arrested before the late symbio tic zone (LSZ) forms, while the infected cells of the LSZ of Risfix T lose the wild-type structure after full differentiation. In contrast t o the bacteroid degradation via an electron-dense stage in RisfixO, ly sis of symbiosome contents prevails in Risfix T nodules. Enhancement o f the lytic function of symbiosomes in RisfixO may be interpreted in t erms of the symbiosome-lysosome homology. The weakened control over sy mbiotic development in RisfixO may be responsible for the abundant spr ead of the infection threads and their enlargement. Cells from the LSZ of Risfix V undergo fast collapse, resembling defence necrosis, after differentiation. In contrast to the nodules of RisfixO and Risfix T, degraded nodules of Risfix V do not function as a sink for photosyntha tes and a source of the nodulation regulatory factor. This is indicate d by the absence of further starch accumulation after collapse, and by hypernodulation. (C) 1995 Annals of Botany Company