New auxin analogs with growth-promoting effects in intact plants reveal a chemical strategy to improve hormone delivery

Abstract

Plant growth depends on the integration of environmental cues and phytohormone-signaling pathways. During seedling emergence, elongation of the embryonic stem (hypocotyl) serves as a readout for light and hormone-dependent responses. We screened 10,000 chemicals provided exogenously to light-grown seedlings and identified 100 compounds that promote hypocotyl elongation. Notably, one subset of these chemicals shares structural characteristics with the synthetic auxins, 2,4-dichlorophenoxyacetic acid (2,4-D), and 1-naphthaleneacetic acid (1-NAA); however, traditional auxins (e.g., indole-3-acetic acid [IAA], 2,4-D, 1-NAA) have no effect on hypocotyl elongation. We show that the new compounds act as ''proauxins'' akin to prodrugs. Our data suggest that these compounds diffuse efficiently to the hypocotyls, where they undergo cleavage at varying rates, releasing functional auxins. To investigate this principle, we applied a masking strategy and designed a pro-2,4-D. Unlike 2,4-D alone, this pro-2,4-D enhanced hypocotyl elongation. We further demonstrated the utility of the proauxins by characterizing auxin responses in light-grown hypocotyls of several auxin receptor mutants. These new compounds thus provide experimental access to a tissue previously inaccessible to exogenous application of auxins. Our studies exemplify the combined power of chemical genetics and biochemical analyses for discovering and refining prohormone analogs with selective activity in specific plant tissues. In addition to the utility of these compounds for addressing questions related to auxin and lightsignaling interactions, one can envision using these simple principles to study other plant hormone and small molecule responses in temporally and spatially controlled ways. chemical genetics ͉ light

Biochemical analyses of indole-3-acetaldoxime-dependent auxin biosynthesis in Arabidopsis

“…S6). In contrast, application of IAA or IAOx to cyp79b2 cyp79b3 mutants resulted in the inhibition of root growth and did not complement the phenotype under the same conditions (data not shown), presumably because of the difference in diffusion and/or metabolic rate between IAM and these compounds in plants as suggested previously (26). Together, all of these results indicated that IAM is an intermediate of IAOx-dependent IAA biosynthesis in Arabidopsis (Fig.…”

Rapid Auxin-Induced Cell Expansion and Gene Expression: A Four-Decade-Old Question Revisited

“…We hypothesized that a primary reason for the impaired 2,4-D activity and for the residual delayed response to the endogenous auxin IAA in the mutant is altered uptake and transport of the hormone in the mutant tissue. To overcome this possible complication, we used 2,4-D methyl ester, a synthetic, lipophilic proauxin (Christian et al, 2008;Savaldi-Goldstein et al, 2008) that rapidly enters plant cells by an unfacilitated pathway and then becomes cleaved intracellularly by esterases to yield active free 2,4-D. As shown in Figure 2C, 2,4-D methyl ester induces the rapid change in growth in both wild-type and mutant hypocotyls similarly. This is consistent with the idea that one of the reasons for the developmental phenotypes in the afb mutants is altered auxin uptake or transport.…”

Arabidopsis SAURs are critical for differential light regulation of the development of various organs

“…If true, supplementing with exogenous auxin should weaken the inhibition of hypocotyl elongation by light. To test this hypothesis, we used the auxin analog picloram because it is more effective than IAA at inducing hypocotyl elongation under light (35). SAUR50 and SAUR65 were selected for the test, which are representative lirSAURs whose expression declines in hypocotyls after light treatment.…”