Acropetal Auxin Transport Inhibition Is Involved in Indeterminate But Not Determinate Nodule Formation

Legumes enter into a symbiotic relationship with nitrogen-fixing rhizobia, leading to nodule development. Two main types of nodules have been widely studied, indeterminate and determinate, which differ in the location of the first cell division in the root cortex, and persistency of the nodule meristem. Here, we compared the control of auxin transport, content, and response during the early stages of indeterminate and determinate nodule development in the model legumes Medicago truncatula and Lotus japonicus, respectively, to investigate whether differences in auxin transport control could explain the differences in the location of cortical cell divisions. While auxin responses were activated in dividing cortical cells during nodulation of both nodule types, auxin (indole-3-acetic acid) content at the nodule initiation site was transiently increased in M. truncatula, but transiently reduced in L. japonicus. Root acropetal auxin transport was reduced in M. truncatula at the very start of nodule initiation, in contrast to a prolonged increase in acropetal auxin transport in L. japonicus. The auxin transport inhibitors 2,3,5-triiodobenzoic acid and 1-N-naphthylphthalamic acid (NPA) only induced pseudonodules in legume species forming indeterminate nodules, but failed to elicit such structures in a range of species forming determinate nodules. The development of these pseudonodules in M. truncatula exhibited increased auxin responses in a small primordium formed from the pericycle, endodermis, and inner cortex, similar to rhizobia-induced nodule primordia. In contrast, a diffuse cortical auxin response and no associated cortical cell divisions were found in L. japonicus. Collectively, we hypothesize that a step of acropetal auxin transport inhibition is unique to the process of indeterminate nodule development, leading to auxin responses in pericycle, endodermis, and inner cortex cells, while increased auxin responses in outer cortex cells likely require a different mechanism during the formation of determinate nodules.

Precocious progression of tissue maturation instructs basipetal initiation of leaflets in Chelidonium majus subsp. asiaticum (Papaveraceae)

PREMISE OF THE STUDY

On a compound leaf, leaflet primordia are repetitively formed along the apical-basal axis, with the direction varying among taxa. Why and how the directions vary among species is yet to be solved, although a change in a single factor was proposed to cause the variation. In this study, we compared two species in the Papaveraceae with different directions of leaflet initiation, Chelidonium majus subsp. asiaticum (basipetal) and Eschscholzia californica (acropetal). Because E. californica has been studied in some detail, we focused on C. majus and asked how basipetal pattern is achieved. •

METHODS

Since only immature leaf primordial tissue has leaflet-generating competency, we performed histological and gene expression analyses on markers of the tissue maturation state. In addition, we performed a time-course analysis of leaf primordial growth. •

KEY RESULTS

Quantitative reverse transcription-PCR analysis demonstrated that a putative regulator of tissue maturation in C. majus, the CINCINNATA homolog, had higher expression in apical parts than in basal parts during the organogenetic phase. In contrast, expression of the CIN homolog was not elevated in either the apical or basal parts in E. californica during the organogenetic phase. •

CONCLUSIONS

In C. majus, apical parts of leaf primordia have already lost leaflet-generating competency during the organogenetic phase. We propose that precocious progression of the maturation process instructs basipetal progression of leaflet initiation in C. majus. This is not the mirror image of data on E. californica, which shows the opposite direction in leaflet formation, indicating that variation in direction is not attributable to a change in a single factor.