TheologisEthylene biosynthesis and action: a case of conservation.
Adding even more complexity to the crosstalk between light signaling and regulation of ethylene biosynthesis in plants, it is currently known that LONG HYPOCOTYL IN FAR-RED1 (HFR1) is particularly relevant during low F/FR responses. Accordingly, HFR1 is a nuclear protein structurally similar to PIF3 (), which is phosphorylated by COP1 and usually marked for degradation under complete darkness (Figure ). However, in the presence of light, especially under BL- or FRL-rich radiation, this protein remains much more stable due to the presence of active PHYA and CRYs. Some FRL-induced responses that are associated with modifications in ethylene emission (e.g., repression of ACS8 transcription) are apparently influenced by the biochemical interaction between PHYA-HFR1 or via the heterodimerization of HFR1 with PIF3 (). However, it is also possible that such FRL-induced responses associated with modifications in ethylene evolution occurs through a more indirect pathway involving the HFR1-induced repression of genes and/or enzymes of other plant hormone biosynthetic pathways, such as GA and auxin (). Recent work has also suggested that PIFs and COP1 complexes synergistically repress photomorphogenesis in the dark, indicating that PIF proteins might inhibit HY5 by direct and indirect mechanisms (). Furthermore, COP1 was reported as capable of physically interact with PIF3-LIKE1 (PIL1) and promote PIL1 degradation via the 26S proteasome, whereas PHYB physically interacts with PIL1 and enhances PIL1 protein accumulation upon RL irradiation, possibly via suppressing the COP1–PIL1 association (; ).
biosynthesis and action – a case of conservation.
In conclusion, a relationship between ABA and ethylene during ripening and senescence was indicated in the tomato fruit: (i) the expression of the ABA biosynthetic gene (LeNCED1) occurs before that of ethylene biosynthesis genes; (ii) ABA content also preceded the climacteric increase in ethylene production; (iii) ABA may induce ethylene biosynthesis via the regulation of ACS and ACO gene expression; (iv) exogenous ABA accelerates fruit ripening, and fluridone or NDGA treatment delayed fruit ripening by inhibition of ABA; and (v) ethylene plays a key role in the later stages of fruit ripening.
Although very little information is available on CDPK signaling mechanisms during ACS regulation, it is widely accepted that the MPK3/6 module of MPK kinase (MKK) cascades plays relevant roles in the regulation of ethylene biosynthesis (; ; ; ; ). Accordingly, the MPK3/6 module is mostly under control of MKK4, MKK5, and MKK9, constituting an important step in various signaling pathways involved in stress-induced responses in plants (; ; ). For example, the stress-activated MKK4/5 signaling cascade can positively regulate ethylene biosynthesis by activating MPK6 (). In addition, ACS6 was initially identified as a potential substrate for MPK3/6 phosphorylation () and further studies have confirmed that the MPK3/6 module is associated with enhanced stability of both type I ACS2 and ACS6 (; ; Figure ). Interestingly, the autocatalytic ethylene production is often stress related and at least in Arabidopsis relies on post-translational regulation of type I ACSs by MPK3/6 cascade (). Furthermore, the MKK9–MPK3/6 module is also suggested as a potential regulatory mechanism of ACS2/6 stabilization in some particular biological contexts (), supporting previous suggestions of the possible role of MKK9 and MPKs in ethylene biosynthesis control (; ).
Ethylene biosynthesis and action in tomato: a model …
Ethylene biosynthesis in higher plants is now well characterized by a relatively simple metabolic pathway, which is, however, coordinated with some other equally important synthetic pathways involved in the plant metabolism regulation (e.g., polyamines). The identification of the intermediate components in ethylene biosynthesis allowed further elucidation of the two committed reactions in this pathway, which comprise the rate-limiting enzymes 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS, EC 22.214.171.124) and ACC oxidase (ACO, EC 1.4.3; ; ; ; ; ; ). Both ACS and ACO are encoded by multigene families whose members have been well characterized in some plant species and they are recognized as major players in ethylene biosynthetic regulation. However, as we will discuss in more detail below, the regulatory mechanisms of ethylene biosynthesis usually converge on the modulation of ACS proteins (; ).
Ethylene biosynthesis and action in ..
Cloning and characterization of NCEDs in fruits facilitate understanding of the role of ABA in fruit ripening. Two NCED genes have been isolated from avocado (Persea Americana Mill.), a climacteric fruit. PaNCED1 and PaNCED3 were expressed consistently with ethylene production during fruit ripening but earlier than ABA accumulation. Thus it has been proposed that ethylene induces NCED gene expression and ABA accumulation, which results in post-ripeness (). Recently, two NCED genes, CsNCED1 and CsNCED2, have been isolated from Citrus sinensis L. Osbeck, a non-climacteric fruit (). Only CsNCED1 was expressed in fruit in accordance with ABA accumulation, suggesting that CsNCED1 may play a key role in ABA biosynthesis.