The transition to flowering is governed by different pathways integrating endogenous and exogenous signals. Here, we evaluated the role of the phytohormone cytokinin (CK) in regulating Arabidopsis thaliana flowering time. By analyzing key mutants in CK metabolism, transport and signalling, we found that the hormone promotes flowering under both long-day (LD) and short-day (SD) conditions, with a stronger impact on flowering under SDs. Genetic analyses indicated that both trans- and cis-zeatin regulate the floral transition, while isopentenyladenine plays a minor role. Blocking CK export from roots and reciprocal grafting experiments revealed that root-derived CK is an important flowering signal. Perception and transmission of the CK flowering signal depended on distinct CK receptors, phosphotransmitter proteins and several B-type response regulators. Further, CK functioned through floral integrators such as OVEREXPRESSION OF CONSTANS1 (SOC1) and components of the age pathway. The CK status of plants affected the levels of the age pathway microRNAs miR156 and miR172. Cytokinin-promoted flowering required the miR156-target SQUAMOSA PROMOTER BINDING PROTEIN-LIKE15 (SPL15) and miR172, and the late-flowering phenotype of LD-grown CK-deficient plants depended on miR172-targeted APETALA2 (AP2)-like genes encoding floral repressors. Collectively, this study shows that CK regulates flowering time through the two-component signaling system and components of the age pathway, providing a genetic framework for future investigations.

Herbivores sharing host plants are often temporally and spatially separated, limiting direct interactions between them. Nevertheless, as observed in numerous aboveground study systems, they can reciprocally influence each other via systemically induced plant responses. In contrast, examples of such plant-mediated interactions between belowground herbivores are scarce; however, we postulated that they similarly occur, given the large diversity of root-interacting soil organisms. To test this hypothesis, we analyzed the performance of cabbage root fly (Delia radicum) larvae feeding on the main roots of field mustard (Brassica rapa) plants whose fine roots were infected by the root-knot nematode (Meloidogyne incognita). Simultaneously, we studied the effects of M. incognita on D. radicum-induced defense responses and the accumulation of primary metabolites in the main root. We observed that almost 1.5 times as many D. radicum adults emerged from nematode-infected plants, indicating a facilitation effect of M. incognita infection. Although we observed increases in the accumulation of proteins and two essential amino acids, the strongest effect of nematode infection was visible in the defense response to D. radicum. We observed a 1.5 times higher accumulation of the defense-related phytohormone JA-Ile in response to D. radicum on nematode-infected plants, coinciding with a 75% increase in indole glucosinolate concentrations. Contrastingly, concentrations of aliphatic glucosinolates, secondary metabolites negatively affecting D. radicum, were 10-25% lower in nematode-infected plants. We hypothesize that the attenuated aliphatic glucosinolate concentrations result from antagonistic interactions between biosynthetic pathways of both glucosinolate classes, which was reflected in the expression of key biosynthesis genes. Our results provide explicit evidence of plant-mediated interactions between belowground organisms, likely via systemically induced responses in roots.

The transition to flowering is governed by different pathways integrating endogenous and exogenous signals. Here, we evaluated the role of the phytohormone cytokinin (CK) in regulating Arabidopsis thaliana flowering time. By analyzing key mutants in CK metabolism, transport and signalling, we found that the hormone promotes flowering under both long-day (LD) and short-day (SD) conditions, with a stronger impact on flowering under SDs. Genetic analyses indicated that both trans- and cis-zeatin regulate the floral transition, while isopentenyladenine plays a minor role. Blocking CK export from roots and reciprocal grafting experiments revealed that root-derived CK is an important flowering signal. Perception and transmission of the CK flowering signal depended on distinct CK receptors, phosphotransmitter proteins and several B-type response regulators. Further, CK functioned through floral integrators such as OVEREXPRESSION OF CONSTANS1 (SOC1) and components of the age pathway. The CK status of plants affected the levels of the age pathway microRNAs miR156 and miR172. Cytokinin-promoted flowering required the miR156-target SQUAMOSA PROMOTER BINDING PROTEIN-LIKE15 (SPL15) and miR172, and the late-flowering phenotype of LD-grown CK-deficient plants depended on miR172-targeted APETALA2 (AP2)-like genes encoding floral repressors. Collectively, this study shows that CK regulates flowering time through the two-component signaling system and components of the age pathway, providing a genetic framework for future investigations.

Herbivores sharing host plants are often temporally and spatially separated, limiting direct interactions between them. Nevertheless, as observed in numerous aboveground study systems, they can reciprocally influence each other via systemically induced plant responses. In contrast, examples of such plant-mediated interactions between belowground herbivores are scarce; however, we postulated that they similarly occur, given the large diversity of root-interacting soil organisms. To test this hypothesis, we analyzed the performance of cabbage root fly (Delia radicum) larvae feeding on the main roots of field mustard (Brassica rapa) plants whose fine roots were infected by the root-knot nematode (Meloidogyne incognita). Simultaneously, we studied the effects of M. incognita on D. radicum-induced defense responses and the accumulation of primary metabolites in the main root. We observed that almost 1.5 times as many D. radicum adults emerged from nematode-infected plants, indicating a facilitation effect of M. incognita infection. Although we observed increases in the accumulation of proteins and two essential amino acids, the strongest effect of nematode infection was visible in the defense response to D. radicum. We observed a 1.5 times higher accumulation of the defense-related phytohormone JA-Ile in response to D. radicum on nematode-infected plants, coinciding with a 75% increase in indole glucosinolate concentrations. Contrastingly, concentrations of aliphatic glucosinolates, secondary metabolites negatively affecting D. radicum, were 10-25% lower in nematode-infected plants. We hypothesize that the attenuated aliphatic glucosinolate concentrations result from antagonistic interactions between biosynthetic pathways of both glucosinolate classes, which was reflected in the expression of key biosynthesis genes. Our results provide explicit evidence of plant-mediated interactions between belowground organisms, likely via systemically induced responses in roots.

The transition to flowering is governed by different pathways integrating endogenous and exogenous signals. Here, we evaluated the role of the phytohormone cytokinin (CK) in regulating Arabidopsis thaliana flowering time. By analyzing key mutants in CK metabolism, transport and signalling, we found that the hormone promotes flowering under both long-day (LD) and short-day (SD) conditions, with a stronger impact on flowering under SDs. Genetic analyses indicated that both trans- and cis-zeatin regulate the floral transition, while isopentenyladenine plays a minor role. Blocking CK export from roots and reciprocal grafting experiments revealed that root-derived CK is an important flowering signal. Perception and transmission of the CK flowering signal depended on distinct CK receptors, phosphotransmitter proteins and several B-type response regulators. Further, CK functioned through floral integrators such as OVEREXPRESSION OF CONSTANS1 (SOC1) and components of the age pathway. The CK status of plants affected the levels of the age pathway microRNAs miR156 and miR172. Cytokinin-promoted flowering required the miR156-target SQUAMOSA PROMOTER BINDING PROTEIN-LIKE15 (SPL15) and miR172, and the late-flowering phenotype of LD-grown CK-deficient plants depended on miR172-targeted APETALA2 (AP2)-like genes encoding floral repressors. Collectively, this study shows that CK regulates flowering time through the two-component signaling system and components of the age pathway, providing a genetic framework for future investigations.

Herbivores sharing host plants are often temporally and spatially separated, limiting direct interactions between them. Nevertheless, as observed in numerous aboveground study systems, they can reciprocally influence each other via systemically induced plant responses. In contrast, examples of such plant-mediated interactions between belowground herbivores are scarce; however, we postulated that they similarly occur, given the large diversity of root-interacting soil organisms. To test this hypothesis, we analyzed the performance of cabbage root fly (Delia radicum) larvae feeding on the main roots of field mustard (Brassica rapa) plants whose fine roots were infected by the root-knot nematode (Meloidogyne incognita). Simultaneously, we studied the effects of M. incognita on D. radicum-induced defense responses and the accumulation of primary metabolites in the main root. We observed that almost 1.5 times as many D. radicum adults emerged from nematode-infected plants, indicating a facilitation effect of M. incognita infection. Although we observed increases in the accumulation of proteins and two essential amino acids, the strongest effect of nematode infection was visible in the defense response to D. radicum. We observed a 1.5 times higher accumulation of the defense-related phytohormone JA-Ile in response to D. radicum on nematode-infected plants, coinciding with a 75% increase in indole glucosinolate concentrations. Contrastingly, concentrations of aliphatic glucosinolates, secondary metabolites negatively affecting D. radicum, were 10-25% lower in nematode-infected plants. We hypothesize that the attenuated aliphatic glucosinolate concentrations result from antagonistic interactions between biosynthetic pathways of both glucosinolate classes, which was reflected in the expression of key biosynthesis genes. Our results provide explicit evidence of plant-mediated interactions between belowground organisms, likely via systemically induced responses in roots.

Jasmonates (JAs) are a family of oxylipin phytohormones regulating plant development and growth and mediating ‘defense versus growth’ responses. The upstream JA biosynthetic precursor cis-(+)-12-oxo-phytodienoic acid (cis-OPDA) acts independently of CORONATIVE INSENSITIVE 1 (COI1)-mediated JA signaling in several stress-induced and developmental processes. However, its perception and metabolism are only partially understood. A few years ago, a low abundant isoleucine analog of the biologically active JA-Ile, OPDA-Ile, was detected years ago in wounded leaves of flowering plants, opening up the possibility that conjugation of cis-OPDA to amino acids might be a relevant mechanism for cis-OPDA regulation. Here, we extended the analysis of amino acid conjugates of cis-OPDA and identified naturally occurring OPDA-Val, OPDA-Phe, OPDA-Ala, OPDA-Glu, and OPDA-Asp accumulating in response to biotic and abiotic stress in Arabidopsis (Arabidopsis thaliana). The OPDA-amino acid conjugates displayed cis-OPDA-related plant responses in a JA-Ile-dependent manner. We also showed that the synthesis and hydrolysis of cis-OPDA amino acid conjugates are mediated by members of the amidosynthetase GRETCHEN HAGEN 3 (GH3) and the amidohydrolase INDOLE-3-ACETYL-LEUCINE RESISTANT 1 (ILR1)/ILR1-like (ILL) families. Thus, OPDA amino acid conjugates function in the catabolism or temporary storage of cis-OPDA in stress responses instead of acting as chemical signals per se.