Phloem Biology of the Cucurbitaceae

Abstract

Cucurbit phloem is especially interesting and unusual in several respects, which has made these plants the subject of intensive study by microscopists, physi- ologists and molecular biologists. Early light microscopists were attracted to cucur- bits due to the large dimensions of the sieve tubes and sieve pores, which made them relatively easy to see (Esau, The phloem. Berlin/Stuttgart: Gebrüder Borntraeger, 1969). Hartig, the forest botanist who discovered the sieve element, turned to Cucurbita in his studies, describing many aspects of phloem structure including sieve plates, callose and “slime plugs.” (See Esau, The phloem. Berlin/Stuttgart: Gebrüder Borntraeger, 1969 for the early history of phloem research). Cucurbita has also featured strongly in the analysis of phloem development. For example, Esau et al. (Bot Gaz 123:233–243, 1962) described the development of the sieve pores and the involvement of callose deposition in this process in Cucurbita max- ima. A striking feature of cucurbit phloem is the presence of a unique array of extrafascicular (outside the vascular bundles) sieve tubes, which have been studied by several groups in recent years (Zhang et al. Proc Nat Acad Sci U S A 107:13532– 13537, 2010; Gaupels et al. Plant Physiol 160:2285–2299, 2012; Zhang et al. Plant Physiol 158:1873–1882, 2012; Gaupels and Ghirardo, Front Plant Sci 4:187, 2013; Gaupels et al. Front Plant Sci 7:154, 2016). Another interesting quirk is that cucurbit “phloem” exudes copiously when cut, which has led to its use in many metabolomic and proteomic analyses. It should be noted, however, that as early as 1944 Crafts and Lorenz thought the high nitrogen content of cucurbit exudate so unusual as to make it suspect as true, mobile phloem sap, a caution that has borne out in recent years (see below). In this chapter I will leave much of the early history of cucurbit phloem biology to Esau (The phloem. Berlin/Stuttgart: Gebrüder Borntraeger, 1969) and focus for the most part on several aspects of the subject addressed since the publication of her monumental work