lec 32- roots part 2

Question 1

what is the purpose of lateral roots?

Answer 1

to increase surface area in the soil to get more nutrients up

Question 2

are lateral roots far away from the root apical meristem?

Answer 2

yes

Question 3

how is lateral root production initiated?

Answer 3

by lateral root founder cells

Question 4

where are lateral root founder cells located?

Answer 4

in the pericycle, they enlarge and divide to form a lateral root primordium that eventually emerges through the overlying layers

Question 5

after the lateral root emerges, what develops?

Answer 5

a lateral root meristem

Question 6

where are pericycle cells primed?

Answer 6

where the meristem transitions into the zone of elongation

Question 7

do oscillations of auxin prime the pericycle cells? do they also re-specify?

Answer 7

yes, they also re-specify primed cells into LR founder cells

Question 8

is auxin transport needed for the development of the primordium?

Answer 8

yes

Question 9

what is the evidence behind auxin being needed to develop lateral roots?

Answer 9

-when roots are incubated in auxin, the number of lateral roots produced increases, it promotes increased cell division in pericycle cells

-when an auxin transport inhibitor is applied to roots, the number of lateral roots decreases, pericycle cells divisions are also reduced

-this shows that auxin positively regulates lateral root formation by promoting pericycle cell divisions

Question 10

how do roots respond to environmental cues? what is the response?

Answer 10

-roots grow downwards, in parallel to the force of gravity
-when a root is reoriented with respect to gravity, the change is detected in the root cap, converted to a biochemical cue, and transmitted to the cells in the elongation zone where the response occurs

-the response is a change in the rates at which cells on the upper and lower flanks of the root elongate, cells on the upper flank elongate more than the cells on the lower flanks because the cells are held together and do not move from one another, the change in cell elongation causes root to bend down

Question 11

what are the two models behind how roots sense gravity?

Answer 11

1. the starch-statolith hypothesis: proposes that gravity is detected by specialized statocytes, in these cells starch-filled organelles called amyloplasts or statoliths sediment to the bottoms of the cells in response to gravity
2. the protoplast-pressure model: the hydrostatic pressure of the entire protoplast is thought to trigger conformational changes of gravity receptors at the plasma membrane, the direction of gravity would be perceived by sensing the differences in tension and compression between plasma membrane and the extracellular matrix at the top and bottom of the cell

Question 12

how do statocytes detect gravity?

Answer 12

-gravity settles statoliths to the bottom of the cell, causing some sort of calcium release into the cytoplasm to act as a signal

Question 13

what are the two pieces of evidence that support the starch-statolith hypothesis?

Answer 13

1. when roots are vertically oriented, the pressure the statoliths exert is uniformly distributed, while in horizontally oriented roots the pressure on the “new bottom” is unequal
2. mutants that deposit little or no starch into their statoliths take a much longer time to respond to gravity than wildtype, the mutant is caused by a mutation in the gene PGM needed for starch synthesis

Question 14

how do roots respond to gravity?

Answer 14

-settling of the statoliths/amyloplasts causes auxin exporters in root columella cells to change their cellular distribution, PIN3 and PIN7 auxin export proteins localize to the new bottoms side
-PIN2 becomes more abundant in epidermal cells on the lower side of the root, reinforcing the movement of extra auxin along the lower flank