MORPHO-PHYSIOLOGICAL AND MOLECULAR EVALUATION OF KENYAN BRACHIARIA ECOTYPES IN RELATION TO DROUGHT AND SPIDER MITE (Tetranychus urticae Koch) STRESS

Abstract

Brachiaria (Urochloa decumbens) also known as signal grass, is a C4 plant that is tolerant to drought. The objective of this study was to evaluate Kenyan Brachiaria ecotypes through morpho-physiological and molecular methods to aid in identification of tolerant ecotypes to both drought and red spider mite (Tetranychus urticae Koch) stresses and to give insight on the differentially expressed, up regulated and down regulated genes. The study adopted a Randomized Complete Block Design (RCBD) in a 3 x 25 factorial arrangement. Results showed differences (P< 0.05) between biomass and plant height (r=0.2386), internode length (r=0.2367) for the stress experiment. Based on the morpho-physiological traits, BrK 1, BrK 3, BrK 10, BrK 15 and BrK 20 showed tolerance to drought. On the other hand, based on 11 drought stress indices and overall ranking, the most tolerant ecotypes were BrK 7-1, BrK 6-2, BrK 18-3, BrK1-4 and BrK 13-5 in stressed condition. Percentage functional plant loss index after 24 days infestation with red spider mite indicated that BrK 21-43.99%, BrK 7-40.89%, BrK 8-30.94%, BrK 18-30.19% and BrK 22-29.39% were tolerant to spider mite. Using Gene Ontology (GO), Ribosome was the most significantly affected cellular components (CC), oxidoreductase activity and structural molecular activity were the most significantly affected molecular functions (MF) while ribosome biogenesis and carbohydrate metabolic process represented significantly affected biological processes (BP) in the presence of the stressors. Differential expression profiling and enrichment using Kyoto Encyclopedia of Genes and Genomes (KEGG) identified significantly up-regulated ribosome biosynthesis, phenyl propanoid biosynthesis, fatty acid, cutin, suberin and wax biosynthesis, fatty acid elongation and amino acid and nucleotide biosynthesis. There was significant down-regulation of photosynthesis, cyanoaminoacids metabolism, glycerophospholipid biosynthesis, diterpenoid biosynthesis, circadian rhythm and Zeatin biosynthesis. Among the putative differentially expressed genes significantly up or down regulated included, RPL, RPS, ATPFoA, CYP71E1, CYP75B1, CYP98A, UGT85B1, GA2OX, CHS, CKX-2, GDP1 and DPP1. The study also discovered Simple Sequence Repeats (SSRs) and Single Nucleotide Polymorphisms (SNPs) regions that need to be validated, develop markers and high throughput tools for functional genomics, genetic and population diversity studies.