Abstract
Glycinebetaine is an important non-toxic osmoprotectant, which is accumulated in higher plants under various stresses. The biosynthesis of glycinebetaine achieved via is a two-step oxidation from choline and betaine aldehyde, catalyzed by choline monooxygenase (CMO) and betaine aldehyde dehydrogenase (BADH), respectively. Up-regulated gene expression of BADH and CMO induced by stress is clearly observed, but the signal transduction is poorly understood. Here, glycinebetaine accumulation in response to osmotic stress and growth recovery induced by exogenous glycinebetaine were observed in a watermelon cell line. When tracing back to the genome sequence of watermelon, it shows that there exists only one member of ClCMO or ClBADH corresponding to glycinebetaine biosynthesis. Both genes harbor a CGTCA-motif in their promoter region which is involved in methyl jasmonate (MeJA)-responsiveness. Amongst MeJA, Ethephon, abscisic acid (ABA), and salicylic acid (SA), MeJA was most effective in gene inducing the expression of ClCMO and ClBADH, and the accumulation of glycinebetaine could also reach an amount comparable to that after osmotic stress by mannitol. Moreover, when ibuprofen (IBU), a JA biosynthesis inhibitor, was pre-perfused into the cells before osmotic stress, glycinebetaine accumulation was suppressed significantly. Interestingly, newly grown cells can keep a high content of glycinebetaine when they are sub-cultured from osmotic stressed cells. This study suggests that osmotic stress induced glycinebetaine biosynthesis occurs via JA signal transduction and not only plays a key role in osmotic stress resistance but also contributes to osmotic stress hardening.
Highlights
Plants are subject to various stress conditions in their life cycle
From the growth curve we could see that the subculture cycle of the suspension cells was 7 days, and the cells on the third and fourth day were in the exuberant exponential mitotic phase (Figures 2D,E)
When compared with the treatment of 100 mM mannitol, only methyl jasmonate (MeJA) treatment caused a comparable increase in glycinebetaine content, while other treatments could only induce weaker accumulation (Figure 4E). These results indicate that the glycinebetaine biosynthesis can be mainly intermediated by jasmonic acid signaling pathway
Summary
Plants are subject to various stress conditions in their life cycle. Osmotic stress is one of the most common adverse environments encountered by higher plants. ABA can induce the expression of genes that associate with stress response and tolerance (Zhu et al, 1997) and promote the accumulation of compounds such as proline (Chen and Kao, 1993). Exogenous glycinebetaine treated bean plants improved the recovery ability, enhanced CO2 absorption and chlorophyll fluorescence under water deficit (Xing and Rajashekar, 1999) It can alleviate the salt-induced inhibition of shoot growth and maintain the ultra-structure in rice seedlings (Rahman et al, 2002). Exogenous signal molecules combined with inhibitors were used to screen the signal pathway of the osmotic stress induced glycinebetaine biosynthesis These results were discussed in context of the crosstalk of endogenous signals, abiotic stress resistance hardening and plant osmoprotectants
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