Abstract
Strigolactones (SLs) are carotenoid-derived phytohormones that are known to influence various aspects of plant growth and development. As root-derived signals, SLs can enhance symbiosis between plants and arbuscular mycorrhizal fungi (AMF). However, little is known about the roles of SLs in plant defense against soil-borne pathogens. Here, we determined that infection with root-knot nematodes (RKNs; Meloidogyne incognita) induced SL biosynthesis in roots of tomato (Solanum lycopersicum). Silencing of SL biosynthesis genes compromised plant defense against RKNs, whilst application of the SL analog racGR24 enhanced it. Accumulation of endogenous jasmonic acid (JA) and abscisic acid (ABA) in the roots in response to RKN infection was enhanced by silencing of SL biosynthetic genes and was suppressed by application of racGR24. Genetic evidence showed that JA was a positive regulator of defense against RKNs while ABA was a negative regulator. In addition, racGR24 enhanced the defense against nematode in a JA-deficient mutant but not in an ABA-deficient mutant. Silencing of SL biosynthetic genes resulted in up-regulation of MYC2, which negatively regulated defense against RKNs. Our results demonstrate that SLs play a positive role in nematode defense in tomato and that MYC2 negatively regulates this defense, potentially by mediating hormone crosstalk among SLs, ABA and JA.
Highlights
Strigolactones (SLs) are plant hormones that were first identified in 1966 as a germination stimulant in the parasitic weed genus Striga (Cook et al.,1966).SLs are derived from carotenoids via sequential oxidative cleavage by carotenoid-cleavage dioxygenases, and belong to the apocarotenoid class of phytohormones, which includes abscisic acid (ABA; Matusova et al, 2005; López-Ráez et al, 2008)
To determine whether SL biosynthesis is involved in the defense against root-knot nematodes (RKNs) (M. incognita) in tomato, we first examined the time-course of gene transcripts involved in SL biosynthesis (CCD7, CCD8, and MORE AXILLARY GROWTH1 (MAX1)) in response to RKN infection in a susceptible genotype
UPLC-MS/MS analysis indicated that RKN infection induced the accumulation of orobanchol and didehydro-orobanchol but did not alter the accumulation of solanacol in the roots at 1 dpi or 2 dpi (Fig. 1B).The transcripts of PLANT DEFENSE FACTOR (PDF), PROTEINASE INHIBITOR1 (PI-1), and Proteinase inhibitors (PIs)-2, which are involved in the defense response, were up-regulated whereas that of MYC2 was down-regulated at 3 h after the RKN infection (Supplementary Fig. S1)
Summary
Strigolactones (SLs) are plant hormones that were first identified in 1966 as a germination stimulant in the parasitic weed genus Striga (Cook et al.,1966).SLs are derived from carotenoids via sequential oxidative cleavage by carotenoid-cleavage dioxygenases, and belong to the apocarotenoid class of phytohormones, which includes abscisic acid (ABA; Matusova et al, 2005; López-Ráez et al, 2008). SLs play pivotal roles in modulating the coordinated development of roots and shoots, in plant–microbe symbiosis, and in stress responses (Akiyama et al, 2005; Gomez-Roldan et al, 2008; Kapulnik et al, 2011a; Ha et al, 2014). SLs have been suggested to have a positive effect on root-hair elongation (Kapulnik et al, 2011b).With regards to shoot architecture, a lack of SL biosynthesis or signaling components results in increased numbers of lateral shoot branches (Gomez-Roldan et al, 2008; Umehara et al, 2008). RNAi silencing of tomato CCD8 has been shown to result in increased susceptibility to the pathogens Botrytis cinerea and Alternaria alternata, and this is associated with decreased levels of the defense-related hormones abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA) in leaves (Torres-Vera et al, 2014)
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