Prashanth, K. | bioRxiv Science

bioRxiv2023

Importance of Leaf Age in Grapevines Under Salt Stress

The combined effects of salt stress and leaf age on photosynthesis and stomatal conductance are not fully understood. Salt stress develops at different rates in various plant organs and tissues, leading to osmotic and ionic stresses. We studied the development of salt stress (30 mM NaCl) in two grapevine rootstocks by examining the gas exchange, photochemistry, and salt accumulation of leaves at different ages. Our results indicate that leaf age strongly affected photosynthesis but had a minor effect on stomatal conductance in both control and salinity conditions. Surprisingly, salt stress lowered stomatal conductance but had little effect on photosynthesis, opposite to the leaf age effects. Reduction in photosystem II efficiency explained the age-associated decline in photosynthesis, suggesting that the light-dependent phase of photosynthesis is the first process affected by leaf aging. We also found that the sodium exclusion capacity decreased as leaves matured and was age and variety dependent, indicating the importance of leaf age when sampling for salt tolerance studies. Therefore, we suggest that sampling only young leaves is missing critical information in the evaluation of salt-tolerant traits in grapevines.

plant biology↗

bioRxiv2019

Darwinian selection analysis of the two-component system PmrAB indicates there could be lingering delay in emergence of colistin resistance in Acinetobacter baumannii

Investigations on the selection pressure acting on point mutations in PmrAB two-component system may provide insights into the future of colistin therapy in Acinetobacter baumannii, since mutations in pmrAB are implicated in colistin resistance. We performed adaptive selection analysis of pmrAB and compared with the available data on colistin resistant strains. We analysed PmrAB sequences in 3113 draft genomes of A. baumannii obtained from RefSeq database. Adaptive selection analysis was performed by two widely used programs namely, HyPhy and PAML. In addition, to examine the reliability of the approach, the same analysis was performed on gyrA of Escherichia coli and Salmonella enterica, since adaptive mutations on gyrA confer quinolone resistance. Mutations that had caused colistin resistance were found to be neither adaptive nor polymorphic, rather they occur at sites that are either under neutral or purifying selection. Strong negative evolutionary selection pressure is also observed at sites throughout both PmrA and PmrB. Sites with high levels of polymorphisms in PmrAB were found to be under neutral selection. Notably, there was no sign of positive selection. Some of them are rather deleterious. These conditions might be maintaining the incidence of colistin resistance in A. baumannii under check. Therefore, in the context of colistin resistance, natural selection plays only a minor role and we assert that in future, A. baumannii may not be able to sustain and successfully disseminate colistin resistance. Therefore, at present the concerns raised about continuing the usage of colistin for the treatment against A. baumannii infections appears to be unnecessary.

microbiology↗