The year 2020 marks the 150th anniversary of the elucidation of the process of plant organ growth at the cellular level by Julius Sachs (1870). In this Addendum to a Review Article in Molecular Plant, we describe this fundamental discovery and argue that the etiolated grass coleoptile still represents the system of choice for the experimental analysis of auxin (indole-3-acetic acid, IAA)-action. With reference to the phenomenon of ‘tissue tension’, we discuss the acid-growth hypotheses of IAA-induced wall loosening and the process of vacuolar expansion, respectively. IAA-mediated elongation appears to be independent of wall acidification, and may be regulated via the secretion of glycoproteins into the outer epidermal wall, whereby turgor (and tissue) pressure provides the ‘driving force’ for growth. As predicted by the “acid growth-hypothesis”, the fungal phytotoxin Fusicoccin (Fc) induces organ elongation via the rapid secretion of protons. We conclude that “cell elongation” can only be understood at the level of the entire organ that displays biomechanical features not established by single cells. This systems-level approach can be traced back to the work of Sachs (1870).

2020年是Julius Sachs（1870）在细胞水平阐明植物器官生长过程的150周年。在此分子植物评论文章的附录中，我们描述了这一基本发现，并认为黄化的胚芽鞘胚芽仍然代表生长素（吲哚-3-乙酸，IAA）作用的实验分析的选择系统。关于“组织张力”现象，我们分别讨论了IAA引起的壁松动和液泡扩张过程的酸生长假说。IAA介导的伸长似乎与壁酸化无关，并且可以通过糖蛋白分泌到表皮外壁中来调节，由此膨胀（和组织）压力为生长提供了“驱动力”。正如“酸生长假说”所预测的那样，真菌性植物毒素岩藻球菌素（Fc）通过质子的快速分泌诱导器官伸长。我们得出的结论是，“细胞伸​​长”只能在整个器官的水平上理解，而整个器官显示出的生物力学特征不是单个细胞所建立的。这种系统级方法可以追溯到Sachs（1870）的工作。