Characterization of drr1, an alkamide-resistant mutant of Arabidopsis, reveals an important role for small lipid amides in lateral root development and plant senescence.

Alkamides belong to a class of small lipid signals of wide distribution in plants, which are structurally related to the bacterial quorum-sensing signals N-acyl-l-homoserine lactones. Arabidopsis (Arabidopsis thaliana) seedlings display a number of root developmental responses to alkamides, including primary root growth inhibition and greater formation of lateral roots. To gain insight into the regulatory mechanisms by which these compounds alter plant development, we performed a mutant screen for identifying Arabidopsis mutants that fail to inhibit primary root growth when grown under a high concentration of N-isobutyl decanamide. A recessive N-isobutyl decanamide-resistant mutant (decanamide resistant root [drr1]) was isolated because of its continued primary root growth and reduced lateral root formation in response to this alkamide. Detailed characterization of lateral root primordia development in the wild type and drr1 mutants revealed that DRR1 is required at an early stage of pericycle cell activation to form lateral root primordia in response to both N-isobutyl decanamide and N-decanoyl-l-homoserine lactone, a highly active bacterial quorum-sensing signal. Exogenously supplied auxin similarly inhibited primary root growth and promoted lateral root formation in wild-type and drr1 seedlings, suggesting that alkamides and auxin act by different mechanisms to alter root system architecture. When grown both in vitro and in soil, drr1 mutants showed dramatically increased longevity and reduced hormone- and age-dependent senescence, which were related to reduced lateral root formation when exposed to stimulatory concentrations of jasmonic acid. Taken together, our results provide genetic evidence indicating that alkamides and N-acyl-l-homoserine lactones can be perceived by plants to modulate root architecture and senescence-related processes possibly by interacting with jasmonic acid signaling.

Cytokinin receptors are involved in alkamide regulation of root and shoot development in Arabidopsis.

Alkamides and N-acilethanolamides are a class of lipid compounds related to animal endocannabinoids of wide distribution in plants. We investigated the structural features required for alkamides to regulate plant development by comparing the root responses of Arabidopsis (Arabidopsis thaliana) seedlings to a range of natural and synthetic compounds. The length of the acyl chain and the amide moiety were found to play a crucial role in their biological activity. From the different compounds tested, N-isobutyl decanamide, a small saturated alkamide, was found to be the most active in regulating primary root growth and lateral root formation. Proliferative-promoting activity of alkamide treatment was evidenced by formation of callus-like structures in primary roots, ectopic blades along petioles of rosette leaves, and disorganized tumorous tissue originating from the leaf lamina. Ectopic organ formation by N-isobutyl decanamide treatment was related to altered expression of the cell division marker CycB1:uidA and an enhanced expression of the cytokinin-inducible marker ARR5:uidA both in roots and in shoots. The involvement of cytokinins in mediating the observed activity of alkamides was tested using Arabidopsis mutants lacking one, two, or three of the putative cytokinin receptors CRE1, AHK2, and AHK3. The triple cytokinin receptor mutant was insensitive to N-isobutyl decanamide treatment, showing absence of callus-like structures in roots, the lack of lateral root proliferation, and absence of ectopic outgrowths in leaves under elevated levels of this alkamide. Taken together our results suggest that alkamides and N-acylethanolamides may belong to a class of endogenous signaling compounds that interact with a cytokinin-signaling pathway to control meristematic activity and differentiation processes during plant development.