Alternate dwarfing system in wheat
Genes influencing plant height have revolutionized agriculture in the late 1960s by almost doubling the wheat and rice production. The so-called ‘green revolution’ genes in wheat and rice encode proteins that either regulate synthesis of plant growth hormone gibberellins (GA) or modulate its signaling pathway. Under conditions of environmental stress, the most commonly used wheat dwarfing mutants, rht1 and rht2, were reported to exhibit adverse effect on a number of plant attributes including coleoptile and first leaf length, and primary and secondary roots and root biomass. Reduced coleoptile and first leaf length adversely affect seedling emergence from deep planting depths that is a common practice for low rainfall production areas of the world. About 86% of the US wheat and about 50% of the wheat producing area of the world is drought prone.
In addition to GA dwarfs, agronomically relevant reduction in plant height can also be achieved by mutating genes involved in biosynthesis or transportation of other plant hormones like brassinosteroids and auxins. We are currently exploring few candidate genes which may lead to reduction in plant height without lowering seedling emergence under drought conditions.
Characterization of genes involved in soybean fertility
In soybean W4 controls the anthocyanin pigment biosynthesis in flowers and hypocotyls. Mutations in the W4 gene result in altered pigment accumulation patterns in petals and reduced levels of purple pigments in flowers and hypocotyls. The mutant allele, w4-m, is characterized by variegated flowers. Recently, the W4 gene has been cloned. Loss of function in the w4-m mutant was shown to be due to insertion of a 20,548 base pair long active transposable element, which resulted variegated flower phenotype.
In collaboration with Dr. Reid Palmer at USDA and Dr Madan Bhattacharyya at Iowa State University, we started a study using the w4-m system, where we looked for reversion of variegated flower to normal purple color flower. The hypothesis was that the occurrence of new mutations would be maximized by searching among progenies of revertant (wild-type; purple flower) plants descended from mutable plants. If the reversion of the unstable allele is the result of an excision of the transposable element from the w4-m locus, new mutations might be detected among the progenies of these revertants. Such mutants would be expected if the excised element were to insert at a second locus. We have identified several male-sterile female sterile mutants from the progeny of w4-m line.
We are studying association between mutation and transposable element and utilizing transposon tagging to clone male-sterility, female-sterility genes in soybean.
Systemic Acquired Resistance (SAR) in soybean
SAR is one of the most common inducible defense mechanisms that can be triggered by infection with certain pathogenic strains. The induced resistance is typically effective against a wide range of pathogens including those taxonomically unrelated to the SAR inducing organism. The development of SAR is associated with the induction of pathogenesis related (PR) gene expression. non-expressor of PR1(NPR1) is a regulatory gene of the SA signal pathway. NPR1 is required for the SAR signal transduction pathway.
Recently, we have shown that SAR in soybean is regulated by a pathway, similar to the one characterized in Arabidospsis. There are two orthologous NPR1 copies in soybean. Arabidopsis npr1 mutant transformed with either of the soybean genes (GmNPR1-1 or GmNPR1-2) showed complementation for lost NPR1 function and showed induction of SAR.
Arabidopsis NPR1 protein interacts with TGA transcription factors in the nucleus to activate the expression of PR1. Transportation of NPR1 protein into nucleus is stimulated by SAR inducer. Although, we are able to show indirectly that treatment with SAR inducer leads to induction of SAR, conclusive evidence that NPR1 becomes a monomer and moves to the nucleus need to be shown.
In Arabidopsis, 5 cysteine residues are shown to be involved in oligomer-monomer transition. In our investigation, we showed that four of these residues are conserved in soybean; however, Cys216 is not conserved. This may suggest that there may be a difference between regulatory mechanisms of GmNPR1 and Arabidopsis NPR1.