Abstract
Both nitric oxide (NO) and strigolactone (SL) are growth regulating signal components in plants; however, regarding their possible interplay our knowledge is limited. Therefore, this study aims to provide new evidence for the signal interplay between NO and SL in the formation of root system architecture using complementary pharmacological and molecular biological approaches in the model Arabidopsis thaliana grown under stress-free conditions. Deficiency of SL synthesis or signaling (max1-1 and max2-1) resulted in elevated NO and S-nitrosothiol (SNO) levels due to decreased S-nitrosoglutathione (GSNO) reductase (GSNOR) protein abundance and activity indicating that there is a signal interaction between SLs and GSNOR-regulated levels of NO/SNO. This was further supported by the down-regulation of SL biosynthetic genes (CCD7, CCD8 and MAX1) in GSNOR-deficient gsnor1-3. Based on the more pronounced sensitivity of gsnor1-3 to exogenous SL (rac-GR24, 2 µM), we suspected that functional GSNOR is needed to control NO/SNO levels during SL-induced primary root (PR) elongation. Additionally, SLs may be involved in GSNO-regulated PR shortening as suggested by the relative insensitivity of max1-1 and max2-1 mutants to exogenous GSNO (250 µM). Collectively, our results indicate a connection between SL and GSNOR-regulated NO/SNO signals in roots of A. thaliana grown in stress-free environment. As this work used max2-1 mutant and rac-GR24 exerting unspecific effects to both SL and karrikin signaling, it cannot be ruled out that karrikins are partly responsible for the observed effects, and this issue needs further clarification in the future.
Highlights
Compared to the wild-type (Col-0), the primary root (PR) of gsnor1-3 mutant was by 57% shorter; its root system contained very few LRs, and its LR density was low (Figure 1) indicating that GSNOR activity is necessary for normal root development (Lee et al, 2008; Holzmeister et al, 2011; Kwon et al, 2012; Shi et al, 2015)
The majority of the articles dealing with SL–nitric oxide (NO) interplay uses pharmacological approach and focuses on the root system of crops grown with special nutrient supply
This study combines molecular biological and pharmacological approaches in order to reveal interactions between NO and SLs as growth regulating signals in the model plant Arabidopsis thaliana grown in stress-free conditions
Summary
Strigolactones (SLs) have been first identified as germination inducers of parasite plants in the 1960s (Cook et al, 1966) and since they have been found to be phytohormones due to their multiple roles in regulating growth and developmental processes of higher plants (Gomez-Roldan et al, 2008; Umehara et al, 2008; Zwanenburg and Blanco-Ania, 2018; Bouwmeester et al, 2019).SLs as terpenoid lactones can be categorized as canonical SLs containing ABC ring and noncanonical SLs lacking such a ring (Al-Babili and Bouwmeester, 2015; Waters et al, 2017). SLs are synthetized from carotenoids in the plastids with the involvement of enzymes such as beta-carotene-isomerase (D27), two carotenoid cleavage dioxygenases (CCD7/MAX3 and CCD8/MAX4), cytochrome P450 (MAX1), and SL–NO Interplay in Arabidopsis Roots. Following its transport into the cytoplasm, carlactone is converted into carlactonoic acid which is the common precursor of the naturally occurring SLs (Jia et al, 2019). A cytochrome P450 and a 2-oxoglutarate-dependent dioxygenase genes were identified being involved in SL synthesis in Lotus japonicus (Mori et al, 2020), and hydroxyl carlactone derivatives as relevant intermediaries in SL synthesis have been identified in Arabidopsis (Yoneyama et al, 2020). Our knowledge about the details of SL biosynthesis after carlactone is still limited (Bouwmeester et al, 2019). It has been shown that SLs are synthetized in both the root and the shoot and that the SL signal can spread from the root to the shoot system (Foo et al, 2001)
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