Pectin, an integral component of the plant cell wall, is a recalcitrant substrate against enzymatic challenges by most animals. In characterizing the source of a leaf beetle's (Cassida rubiginosa) pectin-degrading phenotype, we demonstrate its dependency on an extracellular bacterium housed in specialized organs connected to the foregut. Despite possessing the smallest genome (0.27 Mb) of any organism not subsisting within a host cell, the symbiont nonetheless retained a functional pectinolytic metabolism targeting the polysaccharide's two most abundant classes: homogalacturonan and rhamnogalacturonan I. Comparative transcriptomics revealed pectinase expression to be enriched in the symbiotic organs, consistent with enzymatic buildup in these structures following immunostaining with pectinase-targeting antibodies. Symbiont elimination results in a drastically reduced host survivorship and a diminished capacity to degrade pectin. Collectively, our findings highlight symbiosis as a strategy for an herbivore to metabolize one of nature's most complex polysaccharides and a universal component of plant tissues.

中文翻译：

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草食动物的果胶分解共生体的急剧基因组减少。

果胶是植物细胞壁不可或缺的组成部分，是抵抗大多数动物进行酶攻​​击的顽强底物。在表征叶甲虫（Cassida rubiginosa）果胶降解表型的来源时，我们证明了它对容纳在与前肠连接的专门器官中的细胞外细菌的依赖性。尽管拥有不存在于宿主细胞内的任何生物体中最小的基因组（0.27 Mb），但共生体仍保留了针对多糖的两个最丰富类别的功能性果胶分解代谢：高半乳糖醛酸聚糖和鼠李糖半乳糖醛酸聚糖I。共生器官，与果胶酶靶向抗体免疫染色后在这些结构中的酶促积累相一致。共生生物的消除导致宿主的存活率大大降低，果胶的降解能力降低。总的来说，我们的发现突出了共生作为草食动物代谢自然界中最复杂的多糖之一和植物组织的通用成分的一种策略。