In this study, the role of the recently identified class of phytohormones, strigolactones, in shaping root architecture was addressed. Primary root lengths of strigolactone-deficient and -insensitive Arabidopsis (Arabidopsis thaliana) plants were shorter than those of wild-type plants. This was accompanied by a reduction in meristem cell number, which could be rescued by application of the synthetic strigolactone analog GR24 in all genotypes except in the strigolactone-insensitive mutant. Upon GR24 treatment, cells in the transition zone showed a gradual increase in cell length, resulting in a vague transition point and an increase in transition zone size. PIN1/3/7-green fluorescent protein intensities in provascular tissue of the primary root tip were decreased, whereas PIN3-green fluorescent protein intensity in the columella was not affected. During phosphatesufficient conditions, GR24 application to the roots suppressed lateral root primordial development and lateral root forming potential, leading to a reduction in lateral root density. Moreover, auxin levels in leaf tissue were reduced. When auxin levels were increased by exogenous application of naphthylacetic acid, GR24 application had a stimulatory effect on lateral root development instead. Similarly, under phosphate-limiting conditions, endogenous strigolactones present in wild-type plants stimulated a more rapid outgrowth of lateral root primordia when compared with strigolactone-deficient mutants. These results suggest that strigolactones are able to modulate local auxin levels and that the net result of strigolactone action is dependent on the auxin status of the plant. We postulate that the tightly balanced auxin-strigolactone interaction is the basis for the mechanism of the regulation of the plants' root-to-shoot ratio.Strigolactones, exuded from plants, have been known for a long time to act as germination stimulants for seeds of root parasitic plants such as Orobanche and Striga spp. (for review, see Bouwmeester et al., 2003Bouwmeester et al., , 2007. As root parasitic plants consume a large proportion of the host plants' solutes, they cause wilting and early plant death. Initially, the discovery that strigolactones are also involved in the symbiotic interaction with arbuscular mycorrhizal fungi (Akiyama et al., 2005) was believed to provide an explanation for why the host plants' capacity to produce strigolactones was not lost during evolution. Because arbuscular mycorrhizal fungi are potent providers of nutrients such as phosphate (Pi) and nitrogen to their host, the observation that Pi starvation induced strigolactone biosynthesis in host plants' roots was not surprising (Yoneyama et al., 2007;Ló pez-Ráez et al., 2008). The recent discovery that strigolactones, or closely related compounds, also act as phytohormones inside the host plants and are involved in the inhibition of axillary bud outgrowth (Gomez-Roldan et al., 2008;Umehara et al., 2008) is an additional explanation why plants continue to produce these fatal ger-

Autophagy-related proteins Atg5 and Atg7 are rate-limiting components of autophagic flux in Arabidopsis. Overexpression of ATG5 or ATG7 genes stimulates Atg8 lipidation, autophagosome formation, and autophagic flux, leading to improved plant fitness.

Jamil M, Charnikhova T, Cardoso C, Jamil T, Ueno K, Verstappen F, Asami T & Bouwmeester HJ (2011). Quantification of the relationship between strigolactones and Striga hermonthica infection in rice under varying levels of nitrogen and phosphorus. Weed Research51, 373–385. Summary Strigolactone exudation, as well as Striga hermonthica germination and attachment, was studied under different levels of nitrogen (N) and phosphorus (P) in two cultivars of rice (IAC 165 and TN 1). Exudation of strigolactones by rice was the highest under mineral‐deficient conditions, whereas increasing N and P dose reduced the amount of strigolactones in the exudates. Deficiency of P led to the highest strigolactone exudation, when compared with N or NP deficiency. Production of strigolactones differed strongly between the two cultivars. IAC 165 produced about 100‐fold higher amounts than TN 1 of 2′‐epi‐5‐deoxystrigol, orobanchol and three new strigolactones. Across all N and P treatments, a positive relationship was found between the amount of strigolactones in the exudates of both cultivars and in vitro S. hermonthica germination. These results show that the positive effect of fertiliser application in S. hermonthica control is, at least partly, because of the suppression of strigolactone production and hence of S. hermonthica germination and subsequent attachment. This warrants further research into practical application. Maintaining suitable N and P nutrient status of soil through fertiliser use might be a promising strategy to reduce damage in cereals by this notorious weed.

Significance Strigolactones are a new class of plant hormones regulating plant shoot and root architecture in response to the environment. Also present in root exudates, strigolactones stimulate the germination of parasitic plant seeds. This report describes a genomic polymorphism—associated with the Indica / Japonica subspecies divide in rice that has a major impact on the biosynthesis of strigolactones, plant tillering, and germination of the parasitic plant Striga hermonthica —consisting of the deletion of two strigolactone biosynthetic genes orthologous to Arabidopsis MAX1 . Both of these genes rescued the Arabidopsis max1-1 highly branched mutant phenotype and increased the strigolactone level when overexpressed in the Indica rice variety Bala. This finding is of great interest for plant physiologists, plant evolutionary biologists, and breeders.