2015年3月19日由德国的Irmer 等人发表在PNAS上的一篇论文：New aspect of plant-rhizobia interaction: Alkaloid biosynthesis in Crotalaria depends on nodulation，报道了猪屎豆中生物碱的合成必须依赖于根瘤菌的结瘤固氮激活，否则不能合成。十分有意思，推荐大家阅读。

猪屎豆合成的生物碱为含氮的吡咯里西啶生物碱(pyrrolizidine alkaloids, PAs)，它是该植物产生的化学防御物质，可以抵御牲畜和昆虫的采食。该生物碱是含氮的，而氮从什么地方来，作者证实它的氮只来自于与根瘤菌的共生固氮，而不是来自于土壤中的氮素。作者证实根瘤是PAs生物碱合成的起始位置，合成之后再运输到植物其它部位，以起到化学防御作用。

正是因为猪屎豆的生物碱对动物有毒杀作用，因此，该生物碱可以用做杀虫剂使用，特别它对线虫有一定杀灭作用，多用于土壤线虫的治理。

苦参、苦豆子等也产生生物碱，也有杀虫作用，不知道苦参和苦豆子的生物碱合成是否依赖于根瘤菌的结瘤固氮呢？

文章思路十分清楚，也不复杂，但论据确凿，结论明晰。

论文见这里：

http://www.pnas.org/content/early/2015/03/13/1423457112.abstract

New aspect of plant–rhizobia interaction: Alkaloid biosynthesis in Crotalaria depends on nodulation

Abstract

Infection of legume hosts by rhizobial bacteria results in the formation of a specialized organ, the nodule, in which atmospheric                              nitrogen is reduced to ammonia. Nodulation requires the reprogramming of the plant cell, allowing the microsymbiont to enter                              the plant tissue in a highly controlled manner. We have found that, in Crotalaria (Fabaceae), this reprogramming is associated with the biosynthesis of pyrrolizidine alkaloids (PAs). These compounds are                              part of the plant’s chemical defense against herbivores and cannot be regarded as being functionally involved in the symbiosis.                              PAs in Crotalaria are detectable only when the plants form nodules after infection with their rhizobial partner. The identification of a plant-derived                              sequence encoding homospermidine synthase (HSS), the first pathway-specific enzyme of PA biosynthesis, suggests that the plant                              and not the microbiont is the producer of PAs. Transcripts of HSS are detectable exclusively in the nodules, the tissue with                              the highest concentration of PAs, indicating that PA biosynthesis is restricted to the nodules and that the nodules are the                              source from which the alkaloids are transported to the above ground parts of the plant. The link between nodulation and the                              biosynthesis of nitrogen-containing alkaloids in Crotalaria highlights a further facet of the effect of symbiosis with rhizobia on the ecologically important trait of the plant’s chemical                              defense.