Plant hormones (Literature sources on phytohormones and plant signalling)

Plant Physiology (2023)

Excerpts: "In this issue of Plant Physiology, Wang et al., 2023 found two molecular pathways that regulate GA metabolism in response to touch, one dependent on JA and one dependent on an unexpected hormonal player, ethylene (ET)."

"Thus, the authors characterized touch-induced morphogenesis in two ET-insensitive lines, ein2 and ein3 eil1. As expected, repetitive touch repressed growth of wild-type plants, leading to more compact (reduced diameter) rosettes that also flowered later than their unstimulated counter parts. The ein2 and ein3 eil1 plants were also responsive to touch but, interestingly, the effect of treatment was stronger than in the wild type, suggesting that ET could be a negative regulator of thigmomorphogenesis (Fig. 1, left)."

"As breakdown of GA is necessary for expression of thigmomorphogenesis (Lange & Lange, 2015), the authors measured levels of GA4, a bioactive GA, in the ET-insensitive lines. They found that touch treatment severely lowered the concentration of GA4 in the ein2 and ein3 eil1 plants, even to a greater extent than in the wild type......Consistent with this model, supplementation of touch-induced ein2 and ein3 eil1 plants with exogenous GA4 rescued the compact growth and delay in flowering time phenotypes."

"In conclusion, Wang and collaborators explored the connection between the pathways of two known regulators of touch-induced morphogenesis, JA and GA. They found that JA activates GA breakdown, leading to decreased growth and delayed flowering. The authors also incorporated ET to the picture. ET acts as negative regulator of thigmomorphogenesis, also through an effect on GA catabolism, but in this case ET represses GA catabolism, likely to prevent exaggerated responses. (Fig. 1). These findings position GA metabolism as a central node in thigmomorphogenesis."

Science Advances (2022)

Abstract: "Plants respond to mechanical stimuli to direct their growth and counteract environmental threats. Mechanical stimulation triggers rapid gene expression changes and affects plant appearance (thigmomorphogenesis) and flowering. Previous studies reported the importance of jasmonic acid (JA) in touch signaling. Here, we used reverse genetics to further characterize the molecular mechanisms underlying touch signaling. We show that Piezo mechanosensitive ion channels have no major role in touch-induced gene expression and thigmomorphogenesis. In contrast, the receptor-like kinase Feronia acts as a strong negative regulator of the JA-dependent branch of touch signaling. Last, we show that calmodulin-binding transcriptional activators CAMTA1/2/3 are key regulators of JA-independent touch signaling. CAMTA1/2/3 cooperate to directly bind the promoters and activate gene expression of JA-independent touch marker genes like TCH2 and TCH4. In agreement, camta3 mutants show a near complete loss of thigmomorphogenesis and touch-induced delay of flowering. In conclusion, we have now identified key regulators of two independent touch-signaling pathways."

Plant Physiology (2023)

Abstract: "Touch induces marked morphological changes in plants, including reduced rosette diameters and delayed flowering, a process called thigmomorphogenesis. Previous studies have revealed that thigmomorphogenesis in Arabidopsis (Arabidopsis thaliana) results from touch-induced accumulation of jasmonic acid (JA) and GIBBERELLIN 2-OXIDASE7 (GA2ox7) transcripts, which encode a gibberellin (GA) catabolism enzyme, leading to reduced levels of active GAs. However, the mechanisms underlying thigmomorphogenesis remain uncharacterized. Here we showed that touch induces ethylene (ET) production in Arabidopsis. After touch treatment, ET biosynthesis and signaling mutants exhibited even greater thigmomorphogenic changes and more decreased GA4 contents than did wild-type plants. Biochemical analysis indicated that the transcription factor ETHYLENE INSENSITIVE3 (EIN3) of the ET pathway binds to the promoter of GA2ox8 (encoding another GA 2-oxidase performing the same GA modification as GA2ox7) and represses GA2ox8 transcription. Moreover, MYC2, the master regulator of JA signaling, directly promoted GA2ox7 expression by binding the MYC2 promoter. Further genetic analysis suggested that the ET and JA pathways independently control the expression of GA2ox8 and GA2ox7, respectively. This study reveals that the ET pathway is a novel repressor of touch-induced thigmomorphogenesis and highlights that the ET and JA pathway converge on GA catabolism but play opposite roles to fine-tune GA4 content during thigmomorphogenesis."