Seasonal dynamics of δ(13) C of C-rich fractions from Picea abies (Norway spruce) and Fagus sylvatica (European beech) fine roots.

The (13/12) C ratio in plant roots is likely dynamic depending on root function (storage versus uptake), but to date, little is known about the effect of season and root order (an indicator of root function) on the isotopic composition of C-rich fractions in roots. To address this, we monitored the stable isotopic composition of one evergreen (Picea abies) and one deciduous (Fagus sylvatica), tree species' roots by measuring δ(13) C of bulk, respired and labile C, and starch from first/second and third/fourth order roots during spring and fall root production periods. In both species, root order differences in δ(13) C were observed in bulk organic matter, labile, and respired C fractions. Beech exhibited distinct seasonal trends in δ(13) C of respired C, while spruce did not. In fall, first/second order beech roots were significantly depleted in (13) C, whereas spruce roots were enriched compared to higher order roots. Species variation in δ (13) C of respired C may be partially explained by seasonal shifts from enriched to depleted C substrates in deciduous beech roots. Regardless of species identity, differences in stable C isotopic composition of at least two root order groupings (first/second, third/fourth) were apparent, and should hereafter be separated in belowground C-supply-chain inquiry.

Root cap angle and gravitropic response rate are uncoupled in the Arabidopsis pgm-1 mutant.

The sedimentation of starch-filled plastids is thought to be the primary mechanism by which gravity is perceived in roots. Following gravity perception, auxin redistribution toward the lower flank of roots, initiated in the root cap, is believed to play a role in regulation of the gravity response. Amyloplast sedimentation and auxin flux, however, have never been directly linked. The overall aim of this study was to investigate the relationship among plastid sedimentation, gravitropism and auxin flux. Our data show that pgm-1 roots respond to gravity at one-third the rate of wild-type (WT) roots. Maintaining the root tip at a constant angle using image analysis coupled to a rotating stage resulted in a constant rate of response regardless of the angle of tip orientation in pgm-1 mutants, in contrast to the responses of WT and pin3-1 mutants, which showed increasing response rates as the tip was constrained at greater angles. To indirectly visualize auxin flux following reorientation, we generated a pgm-1 mutant line expressing the DR5::GFPm reporter gene. In WT roots a GFP gradient was observed with a maximum along the lower flank, whereas pgm-1 roots formed a GFP maximum in the central columella but lacked any observable gradient up to 6 h following reorientation. Our study suggests that the relationship between root cap angle and gravitropic response depends upon plastid sedimentation-based gravity sensing and supports the idea that there are multiple, overlapping sensory response networks involved in gravitropism.