Signalling in systemic plant defence – roots put in hard graft

When it comes to keeping the world green, roots have historically been considered to be mere purveyors of water and nutrients to the shoots. However, this view has changed dramatically over the past 25 years as evidence has accumulated that roots can be the ‘movers and shakers’ in orchestrating aboveground interactions between plants and their panoply of parasites (Bezemer and van Dam, 2005; Erb et al., 2009; Pieterse et al., 2014). Through the use of ingenious combinations of functional genetics and micro-grafting, signalling mechanisms involving the root system have been identified that alter the level of resistance to aboveground attacks (Rudrappa et al., 2008; Erb et al., 2009; Nalam et al., 2012; Fragoso et al., 2014). Levels of shoot resistance can also be influenced by beneficial and harmful interactions between roots and a variety of soilinhabiting organisms (Bezemer and van Dam, 2005; Pieterse et al., 2014). In addition to regulating aboveground defences, roots serve as dynamic producers and storage facilities for defensive metabolites and nutrients that can be deployed aboveground through vascular transport (Erb et al., 2009). The importance of roots in defence against aboveground attackers has unfortunately become painfully obvious to citrus growers, who have seen their orchards become infested by a trinity of leaf-feeding herbivores with piercing–sucking lifestyles. In the Americas, leaf attacks by the Asian citrus psyllid Diaphorina citri have caused an epidemic of citrus greening disease (also known as Huanglongbing). The Candidatus Liberibacter spp. bacteria spread by the psyllid ravaged millions of commercial citrus trees. Combinations of rootstocks and scions have now been identified that show tolerance to high Huanglongbing pressure (Stover et al., 2016). These promising results will hopefully contribute to a successful disease management programme. On both sides of the Atlantic, the brown citrus aphid Toxoptera citricida has long spread citrus tristeza virus (CTV) between leaves, which has sent more than 85 million trees to an untimely retirement (Bruessow et al., 2010). Although CTV-tolerant rootstocks have been identified and deployed in the groves, this is likely to have contributed to outbreaks of the third herbivore, the two-spotted spider mite Tetranychus urticae. Heavy spider mite infestations lead to fruit scarring, chlorotic leaf spots and leaf loss (Bruessow et al., 2010).