Carotenoid cleavage dioxygenases (CCDs) form hormones and signaling molecules. Here we show that a member of an overlooked plant CCD subfamily from rice, that we name Zaxinone Synthase (ZAS), can produce zaxinone, a novel apocarotenoid metabolite in vitro. Loss-of-function mutants (zas) contain less zaxinone, exhibit retarded growth and showed elevated levels of strigolactones (SLs), a hormone that determines plant architecture, mediates mycorrhization and facilitates infestation by root parasitic weeds, such as Striga spp. Application of zaxinone can rescue zas phenotypes, decrease SL content and release and promote root growth in wild-type seedlings. In conclusion, we show that zaxinone is a key regulator of rice development and biotic interactions and has potential for increasing crop growth and combating Striga, a severe threat to global food security.

In response to herbivore (Spodoptera littoralis) attack, lima bean (Phaseolus lunatus) leaves produced hydrogen peroxide (H 2 O 2 ) in concentrations that were higher when compared to mechanically damaged (MD) leaves. Cellular and subcellular localization analyses revealed that H 2 O 2 was mainly localized in MD and herbivore-wounded (HW) zones and spread throughout the veins and tissues. Preferentially, H 2 O 2 was found in cell walls of spongy and mesophyll cells facing intercellular spaces, even though confocal laser scanning microscopy analyses also revealed the presence of H 2 O 2 in mitochondria/peroxisomes. Increased gene and enzyme activations of superoxide dismutase after HW were in agreement with confocal laser scanning microscopy data. After MD, additional application of H 2 O 2 prompted a transient transmembrane potential (V m ) depolarization, with a V m depolarization rate that was higher when compared to HW leaves. In transgenic soybean (Glycine max) suspension cells expressing the Ca 21 -sensing aequorin system, increasing amounts of added H 2 O 2 correlated with a higher cytosolic calcium ([Ca 21 ] cyt ) concentration. In MD and HW leaves, H 2 O 2 also triggered the increase of [Ca 21 ] cyt , but MD-elicited [Ca 21 ] cyt increase was more pronounced when compared to HW leaves after addition of exogenous H 2 O 2 . The results clearly indicate that V m depolarization caused by HW makes the membrane potential more positive and reduces the ability of lima bean leaves to react to signaling molecules.

(V.V., M.E.M.)Continuous mechanical damage initiates the rhythmic emission of volatiles in lima bean (Phaseolus lunatus) leaves; the emission resembles that induced by herbivore damage. The effect of diurnal versus nocturnal damage on the initiation of plant defense responses was investigated using MecWorm, a robotic device designed to reproduce tissue damage caused by herbivore attack. Lima bean leaves that were damaged by MecWorm during the photophase emitted maximal levels of b-ocimene and (Z)-3-hexenyl acetate in the late photophase. Leaves damaged during the dark phase responded with the nocturnal emission of (Z)-3-hexenyl acetate, but with only low amounts of b-ocimene; this emission was followed by an emission burst directly after the onset of light. In the presence of 13 CO 2 , this light-dependent synthesis of b-ocimene resulted in incorporation of 75% to 85% of 13 C, demonstrating that biosynthesis of b-ocimene is almost exclusively fueled by the photosynthetic fixation of CO 2 along the plastidial 2-C-methyl-Derythritol 4-P pathway. Jasmonic acid (JA) accumulated locally in direct response to the damage and led to immediate up-regulation of the P. lunatus b-ocimene synthase gene (PlOS) independent of the phase, that is, light or dark. Nocturnal damage caused significantly higher concentrations of JA (approximately 2-3 times) along with enhanced expression levels of PlOS. Transgenic Arabidopsis thaliana transformed with PlOS promoterTb-glucuronidase fusion constructs confirmed expression of the enzyme at the wounded sites. In summary, damage-dependent JA levels directly control the expression level of PlOS, regardless of light or dark conditions, and photosynthesis is the major source for the early precursors of the 2-C-methyl-D-erythritol 4-P pathway.