In plants with two embryonic leaves (cotyledons), the apical hook protects the shoot meristem as the seedling pushes through the soil towards the surface. This hook, which forms at the top of the hypocotyl (the part of the seedling below the cotyledons) soon after germination, opens when the seedling is exposed to light, allowing the cotyledons to expand. In this issue, two papers by Eva Benková, Dominique Van Der Straeten and colleagues use real-time analysis to investigate the roles of the plant hormones auxins and ethylene during apical hook development in Arabidopsis thaliana seedlings.
On p. 597, Van Der Straeten and colleagues describe how the auxin influx carriers AUX1 and LIKE AUX1 3 (LAX3) are involved in auxin-ethylene interactions during apical hook development. By localising auxin biosynthesis enzymes and influx carriers and measuring auxin levels and auxin reporter expression, they show that auxin biosynthesis and translocation from the cotyledon and meristem into the hypocotyl is necessary for hook development. Ethylene treatment, they report, increases auxin accumulation in the meristem, cotyledons and hypocotyl, and a strong ethylene signal enhances auxin biosynthesis at the inner side of the hook. Finally, mutant analysis shows that LAX3 is essential for proper hook formation, whereas AUX1 is involved in an ethylene-induced hook exaggeration phenotype.
On p. 607, Benková and co-workers report that PIN proteins, a family of auxin efflux carriers, also play a role in apical hook development. The authors show that several PIN proteins have specific, partly overlapping spatial and temporal expression patterns in the hook. Elimination of these PIN proteins by mutation, they report, interferes with particular phases of hook development. Furthermore, ethylene-induced enhancement of apical hook formation and subsequent hook curvature exaggeration is accompanied by changes in auxin distribution and in the expression of several PIN genes.
Based on these combined results, Benková, Van Der Straeten and colleagues propose a model in which the coordinated activities of LAX3 and of the potentially ethylene-regulated AUX1 and PIN proteins regulate the transport of auxins made in the cotyledons, meristem and hypocotyl tip of the seedling basipetally towards the root and laterally towards the outer hypocotyl tissue layers, thereby establishing an auxin gradient that controls apical hook development.