Diageotropica and Lateral Rooting, the Rest of the Story

Plant roots have long been of practical and scientific interest for their role in plant stature, and water and nutrient uptake. One of the first classes of root to be singled out and explored were the lateral roots. Much of the study of lateral roots has been with seedling plants due to their early development and relatively easy access. McCully (1975) and Charlton (1991) have reviewed lateral root initiation and out growth with a focus on anatomy and patterns. Torrey (1976) reviewed the physiological control of lateral root initiation, and Ormon-Ligeza et al. (2013) provide a more recent molecular review. Lateral root initiation in Arabidopsis has been described anatomically (Dubrovsky et al., 2000, 2006) and some of the related molecular aspects (Ivanchenko et al., 2006) have been studied, leading to eventual description of the pathways of control of lateral root initiation and outgrowth. One of the tools used in the study of lateral root initiation and its control has been plant mutants, which lack or have modified patterns of lateral root initiation. One such mutant, diageotropica (dgt) has recently been used to characterize a non-auxin compound, which is required for the localization of lateral root initiation in the pericycle (Ivanchenko et al., 2015; Spiegelman et al., 2015). These recent results suggest that the pathway involved is conditioned by a cyclophilin which is produced in the shoot and transported to the root where it effects the localization of auxin induced cellular division of pericycle cells in a parent root (Ivanchenko et al., 2015; Spiegelman et al., 2015). But is that the end of the story? As Retzer and Luschnig (2015) and Su et al. (2015) suggest, the identification and role of the mutant cyclophilin adds an interesting twist to the auxin signaling story. However, there is an additional complicating twist that has been all but ignored because of early attempts to discredit an initial hypothesis. The original mutant (diageotropica, dgt or dgt1-1) is a single gene point mutation resulting in an amino acid substitution (G 137 changed out) in a protein called a cyclophilin (Oh et al., 2002, 2006). This mutation, though relatively simple, conditions a series of pleiotropic phenes (see Lynch and Brown, 2012, for a definition of a phene) resulting in a complex phenotype: lack of lateral rooting, diageotropic growth habit of roots, and shoots, folded (curled) leaves and cotyledons, open hypocotyl hook, and modified vascular system first discovered, …