Abstract
The plant hormone auxin regulates many processes throughout plant growth and development. Aboveground auxin biosynthesis is thought to occur exclusively in the shoot apex, where it then moves via polar auxin transport (PAT) down through developing vasculature in the stem. In contrast to the canonical view of the shoot apex as the sole site of auxin production, there is some evidence to suggest that other tissues contribute auxin to the stem. Woody plants in particular may require additional sources of auxin to support the extensive vascular development in the woody stem, but very little is known about whole-plant auxin dynamics outside of herbaceous (i.e., non-woody) plants. A series of experiments were conducted using the model woody plant hybrid poplar (Populus tremula x alba) in order to better understand the sources of auxin that supply the developing stem. First, shoot apices were removed and the auxin content of developing xylem at three positions along the stem was determined after 48 hours. Removal of the apex led to a 40% reduction in the auxin content of developing xylem located 30 cm beneath the apex but had no effect further down, suggesting either that PAT is extremely slow, or that mature leaves contribute auxin directly to the developing xylem. Plants were then defoliated along 75% of the lowermost stem. Interestingly, defoliation reduced auxin levels in the apex by about 50% but had no effect on auxin levels in the developing xylem further down the stem. However, analysis of a transformant expressing an auxin-responsive reporter construct (DR5:GUS) showed that cells associated with the vasculature (primary xylem parenchyma; PXP) in the petioles of mature leaves were indeed auxin-responsive. Application of the auxin transport inhibitor NPA to petioles confirmed that PAT from leaf to stem is occurring, as auxin levels “downstream” from the site of NPA application were reduced by about 60%. In addition, qRTPCR analysis showed that the auxin biosynthetic gene YUCCA1 is expressed in mature leaves, albeit it at low levels. Despite this evidence for the production of auxin and its export by mature leaves, more extensive application of NPA to petioles had no effect on the auxin content of PXP or developing xylem in the stem, suggesting that the auxin moving through the petiole may not enter the stem. If all of the auxin in the stem is indeed derived from the shoot apex, transport through the stem must be exceptionally slow, as the levels of auxin in the apex are much lower than in the developing xylem. Transport rates this slow would imply that auxin is remarkably long-lived, given the distance it must travel in a large woody stem. These findings underscore the need for a better understanding of the rate and mechanism of auxin turnover, in addition to its transport and biosynthesis.
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Claims 10
Claim · model-written summary
Apex removal reduced developing-xylem auxin content 30 cm below the apex by 40% after 48 hours.
- Organ / stage
- developing xyleme0
- Treatment
- Removal of the apexe0
- Outcome
- auxin content of developing xyleme0
- Direction
- reductione0
- Value
- 40%e0
PopulusNot reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Removal of the apex led to a 40% reduction in the auxin content of developing xylem located 30 cm beneath the apex”
t0“Populus”
Claim · model-written summary
Apex removal had no effect on developing-xylem auxin farther down the stem.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“had no effect further down”
e1“Removal of the apex”
t0“Populus”
Claim · model-written summary
Defoliation reduced apex auxin levels by about 50%.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“defoliation reduced auxin levels in the apex by about 50%”
t0“Populus”
Claim · model-written summary
Defoliation had no effect on developing-xylem auxin farther down the stem.
- Treatment
- defoliatione0
- Outcome
- auxin levels in the developing xylem further down the steme0
- Direction
- no effecte0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“defoliation reduced auxin levels in the apex by about 50% but had no effect on auxin levels in the developing xylem further down the stem”
t0“Populus”
Claim · model-written summary
Mature-leaf petiole primary xylem parenchyma cells were auxin responsive.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“cells associated with the vasculature (primary xylem parenchyma; PXP) in the petioles of mature leaves were indeed auxin-responsive”
t0“Populus”
Claim · model-written summary
NPA application to petioles reduced downstream auxin levels by about 60%.
- Treatment
- Application of the auxin transport inhibitor NPA to petiolese0
- Outcome
- auxin levels “downstream”e0
- Direction
- reducede0
- Value
- about 60%e0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Application of the auxin transport inhibitor NPA to petioles confirmed that PAT from leaf to stem is occurring, as auxin levels “downstream” from the site of NPA application were reduced by about 60%”
t0“Populus”
Claim · model-written summary
YUCCA1 was expressed at low levels in mature leaves.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the auxin biosynthetic gene YUCCA1 is expressed in mature leaves, albeit it at low levels”
t0“Populus”
Claim · model-written summary
Extensive NPA application to petioles had no effect on stem PXP auxin content.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“more extensive application of NPA to petioles had no effect on the auxin content of PXP or developing xylem in the stem”
t0“Populus”
Claim · model-written summary
Extensive NPA application to petioles had no effect on developing-xylem auxin content in the stem.
- Treatment
- application of NPA to petiolese0
- Outcome
- auxin content of PXP or developing xylem in the steme0
- Direction
- no effecte0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“more extensive application of NPA to petioles had no effect on the auxin content of PXP or developing xylem in the stem”
t0“Populus”
Claim · model-written summary
Auxin levels in the apex were much lower than in developing xylem.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“the levels of auxin in the apex are much lower than in the developing xylem”
t0“Populus”
The same questions in other papers
class · physiology: hormones · Populus (poplar)Results agree
1 paper in the same direction as this paper
- negative
Claims are grouped by organism, treatment and what was measured or targeted. A paper counts as positive when any of its claims in the group is positive.