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Summary: Shade affects all aspects of plant growth and development, including seed germination, hypocotyl elongation, petiole growth, leaf hyponasty, and flowering time. Here, we found that mutations in the key Arabidopsis karrikins signal perception‐associated KARRIKIN INSENSITIVE 2 (KAI2) gene, encoding an α/β‐fold hydrolase, and the MORE AXILLARY GROWTH 2 (MAX2) gene, encoding an F‐box protein, led to greater hypocotyl elongation under shade avoidance conditions.We further verified that these phenotypes were caused by perception of the endogenous KAI2‐ligands (KLs), and that this phenotype is independent of strigolactone biosynthetic or signaling pathways. Upon perception of a KL, it is probable that the target protein forms a complex with the KAI2/MAX2 proteins, which are degraded through the action of the 26S proteasome.We demonstrated that SUPPRESSOR OF MAX2‐1 (SMAX1) is the degradation target for the KAI2/MAX2 complex in the context of shade avoidance. KAI2 and MAX2 require SMAX1 to limit the hypocotyl growth associated with shade avoidance. Treatment with l‐kynurenine, an inhibitor of auxin accumulation, partially restored elongation of kai2 mutant hypocotyls under simulated shade. Furthermore, KAI2 is involved in regulating auxin accumulation and polar auxin transport, which may contribute to the hypocotyl shade response.In addition, SMAX1 gene overexpression promoted the hypocotyl shade response. RNA‐sequencing analysis revealed that SMAX1‐overexpression affected the expression of many auxin homeostasis genes, especially under simulated shade. Altogether, our data support the conclusion that KL signaling regulates shade avoidance by modulating auxin homeostasis in the hypocotyl. [ABSTRACT FROM AUTHOR]

Eleven undescribed diterpenoids possessing labdane, 3,18-cyclo-labdane, 19 (4 → 3)-labdane and 12- nor -labdane skeletons, named leucolactones A-K, were isolated from the heartwood of a large woody Lamiaceae plant, Leucosceptrum canum. Their structures were determined by NMR, MS, and in the case of leucolactones A by single crystal X-ray diffraction analysis. Plausible biosynthetic pathway of leucolactones were proposed. Leucolactones showed significant inhibitory effects against seed germination and root elongation of Arabidopsis thaliana in the Petri dish bioassay. Among them, the diastereomeric leucolactones G and H were the most potent, with EC 50 values for root elongation of 6.53 ± 1.35 and 9.75 ± 1.25 μM, respectively. The preliminary structure-activity relationship of leucolactones was discussed. The increase of auxin reporter activity in A. thaliana DR5::GUS roots by leucolactone H was observed, indicating that leucolactones altered auxin accumulation and distribution. These findings suggested that leucolactones might be involved in regulation of plant growth and development through altering auxin accumulation and distribution, presumably contributing to the heartwood formation in L. canum. Eleven undescribed labdane diterpenoids with inhibitory effect on root elongation and seed germination of Arabidopsis thaliana were isolated from the heartwood of a large woody Lamiaceae plant Leucosceptrum canum. [Display omitted] • Eleven undescribed diterpenoids were isolated from the heartwood of Leucosceptrum canum. • Leucolactones showed significant inhibitory effects on root elongation and seed germination of Arabidopsis thaliana. • Leucolactone H increased the activity of auxin reporter DR5::GUS. [ABSTRACT FROM AUTHOR]

Auxin is important in the development of plant vascular tissues. Reconnection of vascular bundles between scion and stock is a primary aim of grafting, and polar auxin transport greatly affects the formation of a continuous vascular model. The role of auxin in the process of graft-union development was studied by grafting the seedlings of Arabidopsis thaliana (L.) Heynh. DR5:GUS marker plants, which exert the auxinspecific responses. Auxin induced the DR5:GUS expression in the vascular bundles around graft surface and stimulated the formation of multiple vascular bundle reconnections on the third day after grafting (DAG). DR5:GUS expression was delayed for one day in both scion and stock and dramatically declined by the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA). Vascular bundle reconnection was observed only on the 4th DAG. These results suggest that auxin stimulates the reconnection of the vascular bundles, whereas NPA inhibits it. We studied the role of PIN proteins in graft development by grafting seedlings of PIN:GUS plants. PIN had different expression patterns in the graft process. Expression levels of PIN genes were analyzed by real-time PCR. All PIN genes had the higher expression level at the third DAG. We conclude that auxin stimulates the development of graft unions, and the patterns of expressions of PIN family genes can affect the development of graft-union by controlling the auxin flow. [ABSTRACT FROM AUTHOR]

In Arabidopsis thaliana in vitro culture, shoots were induced from the shoot apical meristem (SAM) of germinating seeds in the presence of 2,4-dichlorophenoxyacetic acid. Primary shoot primordia developed leaf-like structures, from which secondary shoot primordia were produced. Regenerated shoots were recovered when the material was transferred to a medium lacking auxin. Adventitious roots formed from a callusing basal region of the secondary shoots. The CUC1 transcription factor was expressed at the apex of the primary shoot primordium and at the boundary between the regenerated SAM and the developing leaf primordia. The DR5::GUS transgene was used to localize sites of maximum auxin occurrence. Auxin was firstly detected in the dividing cells beneath the SAM epidermis, which coincided with sites where primary shoot primordia were initiated. In the regenerated shoots, auxin response was not detected in the basal region of the stem, suggesting that the regenerating structures were shoots rather than somatic embryos. Direct shoot regeneration from the A. thaliana SAM requires a localized accumulation of auxin. [ABSTRACT FROM AUTHOR]