Herbivore attack elicits costly defenses that are known to decrease plant fitness by using resources that are normally slated for growth and reproduction. Additionally, plants have evolved mechanisms for tolerating attack, which are not understood on a molecular level. Using 11 C-photosynthate labeling as well as sugar and enzyme measurements, we found rapid changes in sink-source relations in the annual Nicotiana attenuata after simulated herbivore attacks, which increased the allocation of sugars to roots. This herbivore-induced response is regulated by the ␤-subunit of an SnRK1 (SNF1-related kinase) protein kinase, GAL83, transcripts of which are rapidly down-regulated in source leaves after herbivore attack and, when silenced, increase assimilate transport to roots. This C diversion response is activated by herbivore-specific elicitors and is independent of jasmonate signaling, which regulates most of the plant's defense responses. Herbivore attack during early stages of development increases root reserves, which, in turn, delays senescence and prolongs flowering. That attacked GAL83-silenced plants use their enhanced root reserves to prolong reproduction demonstrates that SnRK1 alters resource allocation so that plants better tolerate herbivory. This tolerance mechanism complements the likely defensive value of diverting resources to a less vulnerable location within the plant.carbon-11 ͉ defense ͉ plant-herbivore interactions ͉ tolerance P lants have evolved a variety of mechanisms for reducing the negative impact of herbivore attack on fitness; these mechanisms include direct and indirect defenses and tolerance (1). Defenses are costly, expending energy and resources that could otherwise be used to grow and generate offspring. Inducible defenses allow plants to invest resources into defense only when needed. Although defenses limit the extent of damage, even well defended plants lose large amounts of tissue when attacked by herbivores that have adapted to their defenses. Then, plants would benefit from tolerance, which minimizes the fitness consequences of tissue loss to herbivores (2-4). Defense against, and tolerance of, herbivory are not mutually exclusive; most plantinsect interactions likely combine both (5, 6). In contrast to the rapid advances in our understanding of defense mechanisms, little is known about the traits that allow plants to tolerate herbivore damage.Tolerance, which is measured by comparing the fitness of a genotype in environments with and without attackers, remains uncharacterized at the molecular level (2, 7). At a physiological level, increases in photosynthetic rate, branching, and storage in belowground tissues are thought to be involved (8-10). These responses require the tuning of primary metabolism, for which mutant screens and other reverse genetic approaches with model plants have yet to yield molecular regulators. Host plants that have coevolved with adapted herbivores likely have elaborate defense and tolerance responses to minimize the fitness consequences of herbivory...

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SummaryMost current models of assimilate carbohydrate partitioning are based on growth patterns observed under a range of experimental conditions, from which a set of empirical rules are derived to simulate partitioning. As a result, they are not good at extrapolating to other conditions; this requires a mechanistic approach, which only transport-resistance (TR) models currently provide. We examine an approach to incorporating recent progress in phloem physiology into the TR approach, which leads to a 'minimalist' Münch model of a branched system with competing sinks. In vivo whole-plant measurements have demonstrated that C-flow rates are dependent not only on the properties of the sink, but also on the properties of the whole transport system, and the detailed dynamics of this behaviour is mimicked by the proposed model. This model provides a sound theoretical framework for an unambiguous definition of sink and source strengths, with sink priority being an emergent property of the model. Further developments are proposed, some of which have already had limited application, to cope with the complexity of plants; the emphasis is on a modular approach, together with the importance of choosing the appropriate scale level for both structure and function. Whole-plant experiments with in vivo measurement of the phloem dynamics will be needed to help with this choice.

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