Transcriptome analysis of Pará rubber tree (H. brasiliensis) seedlings under ethylene stimulation.

BACKGROUND: Natural rubber (cis-1,4-polyioprene, NR) is an indispensable industrial raw material obtained from the Pará rubber tree (H. brasiliensis). Natural rubber cannot be replaced by synthetic rubber compounds because of the superior resilience, elasticity, abrasion resistance, efficient heat dispersion, and impact resistance of NR. In NR production, latex is harvested by periodical tapping of the trunk bark. Ethylene enhances and prolongs latex flow and latex regeneration. Ethephon, which is an ethylene-releasing compound, applied to the trunk before tapping usually results in a 1.5- to 2-fold increase in latex yield. However, intense mechanical damage to bark tissues by excessive tapping and/or over-stimulation with ethephon induces severe oxidative stress in laticifer cells, which often causes tapping panel dryness (TPD) syndrome. To enhance NR production without causing TPD, an improved understanding of the molecular mechanism of the ethylene response in the Pará rubber tree is required. Therefore, we investigated gene expression in response to ethephon treatment using Pará rubber tree seedlings as a model system. RESULTS: After ethephon treatment, 3270 genes showed significant differences in expression compared with the mock treatment. Genes associated with carotenoids, flavonoids, and abscisic acid biosynthesis were significantly upregulated by ethephon treatment, which might contribute to an increase in latex flow. Genes associated with secondary cell wall formation were downregulated, which might be because of the reduced sugar supply. Given that sucrose is an important molecule for NR production, a trade-off may arise between NR production and cell wall formation for plant growth and for wound healing at the tapping panel. CONCLUSIONS: Dynamic changes in gene expression occur specifically in response to ethephon treatment. Certain genes identified may potentially contribute to latex production or TPD suppression. These data provide valuable information to understand the mechanism of ethylene stimulation, and will contribute to improved management practices and/or molecular breeding to attain higher yields of latex from Pará rubber trees.

How kinesin waits between steps.

Kinesin-1 (conventional kinesin) is a dimeric motor protein that carries cellular cargoes along microtubules by hydrolysing ATP and moving processively in 8-nm steps. The mechanism of processive motility involves the hand-over-hand motion of the two motor domains ('heads'), a process driven by a conformational change in the neck-linker domain of kinesin. However, the 'waiting conformation' of kinesin between steps remains controversial-some models propose that kinesin adopts a one-head-bound intermediate, whereas others suggest that both the kinesin heads are bound to adjacent tubulin subunits. Addressing this question has proved challenging, in part because of a lack of tools to measure structural states of the kinesin dimer as it moves along a microtubule. Here we develop two different single-molecule fluorescence resonance energy transfer (smFRET) sensors to detect whether kinesin is bound to its microtubule track by one or two heads. Our FRET results indicate that, while moving in the presence of saturating ATP, kinesin spends most of its time bound to the microtubule with both heads. However, when nucleotide binding becomes rate-limiting at low ATP concentrations, kinesin waits for ATP in a one-head-bound state and makes brief transitions to a two-head-bound intermediate as it walks along the microtubule. On the basis of these results, we suggest a model for how transitions in the ATPase cycle position the two kinesin heads and drive their hand-over-hand motion.