Probing Roles of Ethylene in Leaf Gas Exchange, Growth and Development Using ACC-Deaminase Containing Rhizobacteria and 1-Methylcyclopropene (1-MCP).

Chen, Lin (2012) Probing Roles of Ethylene in Leaf Gas Exchange, Growth and Development Using ACC-Deaminase Containing Rhizobacteria and 1-Methylcyclopropene (1-MCP). PhD thesis, Lancaster University.

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Abstract

Ethylene is generally an inhibitor of plant growth and it is produced in increasing amounts when plants are exposed to abiotic stress. A number of rhizobacteria which contain the enzyme 1-aminocyclopropane-1-carboxylate deaminase (ACCd) can hydrolyse ACC, the immediate precursor of ethylene, and thus decrease root ACC concentration and root ethylene evolution. Whilst promotion of plant vegetative growth by ACC-d containing rhizobacteria has been observed in different plant species, only a few studies examined the influence of this group of bacteria on plant development such as flowering time. This work presented here aims to study effects of the ACC-d containing rhizobacterium (Variovorax. paradoxus 5C-2) on the growth and development of Arabidopsis, and also investigate the role of ethylene in regulating the interactions between V. paradoxus 5C-2 and plants by using wild type plants and a group of ethylene related mutants. Soil inoculation with V.paradoxus 5C-2 promoted growth of Columbia wild type (WT) and the ethylene overproducing mutant eto1-1, and also enhanced floral initiation of WT plants. However, these effects were not seen in ethylene insensitive mutants (etr1-1, ein2-l). Soil inoculation with V. paradoxus 5C-2 decreased foliar ACC concentrations of wild type plants and foliar ethylene emission in both WT and etol-1 plants. Taken together, these results suggest V. paradoxus 5C-2 inoculation promotes Arabidopsis growth and flowering via an ethylene-dependent pathway. The effect of V. paradoxus 5C-2 on wheat (Triticum aestivum cv. Ashby) was also assessed at seedling stage (with 3 leaves), but no growth promotion was observed in wheat either in well watered or drying soil. Further experiments investigated interactions between ABA and ethylene in stomatal regulation of wheat. Abscisic acid (ABA) is a key signal which regulates plant response to stress, particularly in regulating stomatal responses to drought. It is suggest that older leaves (3 weeks old) lose stomatal sensitivity to ABA. Recent studies indicate that ethylene can close stomata when ABA levels are relatively low; but antagonize ABA induced stomatal closure such that they can remain open when ABA levels increase. The work described here explored the role of ethylene in regulating stomatal responses of leaves of different growth stages to ABA, and to soil drying followed by rehydration. Furthermore, the hypothesis that lack of stomatal response to ABA in older leaves occurs via effects of leaf aging on ethylene production and/or sensitivity. Similar results as described by earlier report were obtained, whereby older, more mature leaves lost their ability to close stomata in response to exogenous ABA treatments and soil drying followed by rehydration, while young mature leaves closed stomata more fully in response to changes in water availability. Pretreating plants with 1-methylcyclopropene (1-MCP) which antagonizes ethylene receptors, or soil inoculation with rhizobacterium V. paradoxus 5C-2 restored the ability to close stomata after soil drying-rehydration treatments, indicating that ethylene is involved in the sluggish stomatal response to ABA in older leaves. Further work suggests that stomata of older leaves are more sensitive to ethylene compared to young leaves, explaining the relative insensitivity of stomatal closure to both ABA and drought/rehydration in older leaves. Therefore, improving stomatal response of aged leaves to soil drying via rhizobacteria or chemical (1-MCP) application can be useful to increase water use efficiency during plant vegetative growth period in agriculture practice.