A standardized method for analysis of Medicago truncatula phenotypic development

Medicago truncatula has become a model system to study legume biology. It is imperative that detailed growth characteristics of the most commonly used cultivar, line A17 cv Jemalong, be documented. Such analysis creates a basis to analyze phenotypic alterations due to genetic lesions or environmental stress and is essential to characterize gene function and its relationship to morphological development. We have documented morphological development of M. truncatula to characterize its temporal developmental growth pattern; developed a numerical nomenclature coding system that identifies stages in morphological development; tested the coding system to identify phenotypic differences under phosphorus (P) and nitrogen (N) deprivation; and created visual models using the L-system formalism. The numerical nomenclature coding system, based on a series of defined growth units, represents incremental steps in morphological development. Included is a decimal component dividing growth units into nine substages. A measurement component helps distinguish alterations that may be missed by the coding system. Growth under N and P deprivation produced morphological alterations that were distinguishable using the coding system and its measurement component. N and P deprivation resulted in delayed leaf development and expansion, delayed axillary shoot emergence and elongation, decreased leaf and shoot size, and altered root growth. Timing and frequency of flower emergence in P-deprived plants was affected. This numerical coding system may be used as a standardized method to analyze phenotypic variation in M. truncatula due to nutrient stress, genetic lesions, or other factors and should allow valid growth comparisons across geographically distant laboratories.