Assessing physiological and genetic evidence for weedy rice competitiveness traits at the vegetative growth stage

Weedy rice (Oryza spp.) has successfully adapted to invasion of cultivated rice (O. sativa) fields by being a strong competitor from the early vegetative growth stages to crop harvest. While seed shattering and seed dormancy have been shown to contribute to competitiveness at the reproductive stage, much less is known about the traits enhancing weedy rice competitiveness at the vegetative stage. We examined several growth and physiological traits in five different weedy rice lineages with different ancestral origins, and found that no single vegetative phenotype characterizes all weedy rice. Divergence in growth and physiological traits between weedy rice groups and their putative cultivated ancestors has been limited, suggesting that altered vegetative traits have not been a common path to weed adaptation. There is a lack of convergence in patterns of gene expression in two independent weedy rice lineages, suggesting that there are few shared genetic mechanisms in the evolution of vegetative traits. We conclude that, among weedy rice groups, consistent weed-adaptive traits are limited to the reproductive stage, and caution against assumptions that agricultural weeds necessarily have enhanced growth or physiological mechanisms compared to their ancestors that facilitate their invasion of crop fields.