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On the Inside

The isoflavones play diverse roles in the plant-microbe interactions of the Papilionoideae. Isoflavones function as antibiotics as well as precursors for defense-related phytoalexins. They also serve as signal molecules in the induction of microbial genes involved in soybean (Glycine max) nodulation. Although isoflavones are normally present at relatively low levels in mature soybean tissues, their accumulation is strongly induced in response to pathogen attack or defense elicitors. Isoflavone synthase (IFS) is a key enzyme for the formation of the isoflavones. Subramanian et al. (pp. 1345–1353) show that the silencing of both copies of IFS genes in roots initiated from soybean cotyledons leads to the effective spread of silencing throughout the nontransformed cotyledon tissues. Distal silencing was established within 5 d of transformation and was highly efficient for a 3to 4-d period, after which it faded. Distal silencing effected a nearly complete suppression of mRNA accumulation for both the IFS1 and IFS2 genes and of isoflavone production induced by wounding or treatment with the cell wall glucan elicitor from Phytophthora sojae. Preformed isoflavone conjugates were not reduced in distal tissues, suggesting little turnover of these stored isoflavone pools. Silencing of IFS led to enhanced susceptibility to P. sojae. The soybean cotyledon system may prove to be a convenient and effective system for the functional analysis of other plant genes through gene silencing.