The sweet and the sticky: a new paradigm for divergent phloem function.

韧皮部是所有维管植物中两个主要的运输系统之一，主要功能是将糖和其他营养物质和代谢产物运输到整个植物中。高营养的韧皮部汁液也使其成为害虫和病原体非常有吸引力的目标。此外，许多主要的虫媒病毒性疾病在韧皮部传播。因此，了解韧皮部的许多组成部分的功能将对制定改善植物防御的战略产生广泛的影响。例如，保护可以来自抗微生物或抗昆虫蛋白质，或来自归类为次级代谢物的小分子，如抗氧化剂或摄食抑制剂。对韧皮部的研究主要集中在南瓜科（包括黄瓜和甜瓜）的物种上，在这些物种中，可以很容易地从切割的茎和叶中收集到韧皮部的汁液。在大多数物种中，阻断机制迅速封闭伤口。同样不寻常的是，南瓜汁的糖含量比其他植物低30倍，这一点困扰了研究人员几十年。我们现在已经解决了这个悖论，证明了出血的汁液来自第二个韧皮部系统，长期以来一直被认为是南瓜家族的一个特征，但被认为是一个次要的贡献者。事实上，主要的韧皮部确实含有高糖，但在切割时立即堵塞（像大多数物种一样），而次要的出血韧皮部含有低糖。这两个系统中的蛋白质也完全不同，这是以前从未在任何植物中描述过的。许多问题出现了：为什么这种双重制度是有利的？它在南瓜家族中是独一无二的吗？蚜虫等昆虫以两种韧皮部中的哪一种为食？既然我们知道了大韧皮部运输大部分的糖，那么小韧皮部的功能是什么呢？葫芦科植物两个韧皮部的功能之和与其他植物单个韧皮部的功能相同吗？在这里，我们解决了一些迷人的和重要的问题。我们的长期目标是更好地了解所有植物的韧皮部功能，例如，提高对蚜虫的抗性。为此，我们将首先比较主要和次要韧皮部中的蛋白质。这些蛋白质将具有多种功能，从伤口密封到在整个植物中携带信号信息，再到直接发挥防御作用。然后，我们将特别关注一个丰富的蛋白质类，我们已经发现广泛分布在高等植物科，并密切相关的蛋白质已经知道阻止韧皮部伤口的豆类物种。我们将研究这些蛋白质的生化特性，以了解阻断机制。我们还将在模式植物拟南芥中表达相应的基因，使我们能够看到葫芦蛋白质是否在其他物种中具有相同的功能。基于氨基酸序列的相似性，我们已经在许多物种中发现了南瓜蛋白的亲属，但这不足以判断功能是否相同。最后，我们将在携带（或缺乏）一个重要的蚜虫抗性基因的甜瓜植株上测试不同毒力的蚜虫。这将包括蚜虫行为的电子记录，蚜虫生长速度的测量，显微镜和摄入韧皮部汁液的化学分析。这些实验将揭示蚜虫是喜欢吃主要的糖韧皮部还是次要的流血韧皮部。独特的双重韧皮部将使我们能够更好地确定是什么决定了害虫的成功或失败，从中我们可以开始设计作物昆虫防御的新策略。

Phloem is one of the two major transport systems in all vascular plants, functioning mainly to transport sugars and other nutrients and metabolites throughout the plant. The highly nutritious phloem sap also makes it a very attractive target for pests and pathogens. In addition, many major insect-borne viral diseases spread in the phloem. Knowledge of the functions of the many components of phloem will therefore have wide-reaching consequences for development of strategies in improving plant defence. Protection can, for example, come from antimicrobial or anti-insect proteins, or from small molecules classed as secondary metabolites such as antioxidants or feeding deterrents. Research into phloem has mainly been on species such as members of the pumpkin family (including cucumber and melon) where bleeding phloem sap can be readily collected from cut stems and leaves. In most species a blocking mechanism rapidly seals the wound. Also unusual is the fact that pumpkin sap has a sugar content about 30 times less than in other plants, something that has perplexed researchers for decades. We have now resolved this paradox by showing that the bleeding sap comes from a second phloem system that has long been known as a feature in the pumpkin family but was thought to be a minor contributor. In fact the major phloem does contain high sugar but blocks immediately on cutting (like most species) whereas the minor bleeding phloem has low sugar. The proteins in the two systems are also completely different, something never previously described for any plant. Many questions arise: why is this dual system advantageous? Did it evolve uniquely in the pumpkin family? Which of the two phloems do insects such as aphids feed on? Now that we know the major phloem transports most of the sugar, what functions can be suggested for the minor phloem? Is the sum of functions of the two cucurbit phloems the same as the functions of the single phloem of other plants? Here we address some of these fascinating and important issues. Our long-term aim is to develop a better understanding of phloem function for all plants, with a view, for example, to improving resistance to aphids. To do this we will first compare the proteins in major and minor phloem. These proteins will have diverse functions from wound sealing to carrying signalling information throughout the plant, to acting directly in defence. We will then focus especially on one abundant protein class that we have found to be distributed widely across higher plant families, and is closely related to proteins already known to block phloem wounds in legume species. We will examine the biochemical properties of these proteins to understand the blocking mechanism. We will also express the corresponding gene(s) in the model plant Arabidopsis, allowing us to see if the cucurbit proteins may have the same function in other species. Based on amino acid sequence similarity, we have found relatives of the pumpkin protein in many species, but this is insufficient to judge whether the functions are identical. Finally, we will test aphids that vary in their virulence on melon plants that carry (or lack) an important aphid resistance gene. This will include electrical recordings of aphid behaviour, measurement of aphid growth rates, microscopy and chemical analysis of ingested phloem sap. These experiments will reveal whether aphids prefer to feed on the major sugar phloem or the minor bleeding phloem. The unique dual phloem will allow us to define better what determines success or failure of the pest, from which we can start to design new strategies for insect defence in crops.