1
00:00:05.320 --> 00:00:09.193
Now, I want to go back to our
experiments on gravitropism.

2
00:00:09.193 --> 00:00:12.879
What we had seen earlier was
that the gravitropism is

3
00:00:12.879 --> 00:00:17.420
sensed in specific cells in the root cap,
but we still didn't.

4
00:00:18.760 --> 00:00:22.070
Come to the conclusion what's
sensing the gravity or

5
00:00:22.070 --> 00:00:24.480
how the gravity is being sensed.

6
00:00:24.480 --> 00:00:29.170
I want to look a little bit more into the
structure of the cells in the root cap.

7
00:00:29.170 --> 00:00:34.360
And if you look at it with a microscope,
what we see is that these cells contain

8
00:00:34.360 --> 00:00:40.220
specific organelles, specific subcellular
structures, which are called amyloplasts.

9
00:00:41.690 --> 00:00:48.655
Amyloplasts are similar to chloroplasts,
except that amyloplasts contain starch.

10
00:00:48.655 --> 00:00:52.877
They don't do photosynthesis,
they accumulate starch and

11
00:00:52.877 --> 00:00:58.520
what we can see is that these amyloplasts
are found on the bottom side of the cell.

12
00:01:00.360 --> 00:01:04.670
Such that if you take a root and
turn it onto its side,

13
00:01:04.670 --> 00:01:10.040
the amyloplasts then fall to
the bottom of the cell and

14
00:01:10.040 --> 00:01:13.050
are defining the downside of the cells.

15
00:01:14.300 --> 00:01:19.577
This is somewhat similar to what we
described in the beginning of the lecture

16
00:01:19.577 --> 00:01:24.947
of the otoliths, of the crystals in our
ear organs which respond to gravity.

17
00:01:24.947 --> 00:01:27.250
They're always found on
the bottom of the vestibule.

18
00:01:28.790 --> 00:01:32.034
Now, these crystals in our ears and

19
00:01:32.034 --> 00:01:36.585
the amyloplast roots
are not the same structure.

20
00:01:36.585 --> 00:01:39.700
They're completely different structurally,
but

21
00:01:39.700 --> 00:01:43.460
it seems that their function may
be similar to detect gravity.

22
00:01:44.860 --> 00:01:48.330
So once the scientists discovered,

23
00:01:48.330 --> 00:01:52.360
once they saw that these amyloplast are
always falling to the bottom of the cells,

24
00:01:53.380 --> 00:01:58.240
this led to the hypothesis that maybe
these amyloplasts are the gravity receptor

25
00:01:58.240 --> 00:02:03.390
themselves, but does this prove
that they're the gravity receptors?

26
00:02:04.820 --> 00:02:09.926
What experiments could you do to
test the hypothesis that the falling

27
00:02:09.926 --> 00:02:15.572
of the amyloplasts to the bottom side
of the cell is what is sensing gravity?

28
00:02:15.572 --> 00:02:16.753
I want to describe two or

29
00:02:16.753 --> 00:02:20.310
three sets of experiments that have
been done to test this hypothesis.

30
00:02:22.020 --> 00:02:26.100
The first one utilized the very
strong magnetic field.

31
00:02:27.350 --> 00:02:31.640
If you put a plant in a very
strong magnetic field,

32
00:02:31.640 --> 00:02:37.370
you can induce the amyloplasts
to move to one side of the cell.

33
00:02:37.370 --> 00:02:42.680
As you can see here in this picture, when
they put a seedling in between two huge

34
00:02:42.680 --> 00:02:48.170
magnets, you can get the amyloplasts
to go to one side of the cell and

35
00:02:48.170 --> 00:02:50.740
what ended up happening is that

36
00:02:50.740 --> 00:02:56.350
the shoot would then bend according
to where the amyloplasts are.

37
00:02:56.350 --> 00:03:00.990
If the imunoplasts were on
the right side of the cell,

38
00:03:00.990 --> 00:03:04.450
the whole seeding bent to the right.

39
00:03:04.450 --> 00:03:05.820
If the magnet was reversed and

40
00:03:05.820 --> 00:03:11.210
the amyloplasts were on the left side of
the cell, the seedling bent to the left.

41
00:03:11.210 --> 00:03:12.210
In other words,

42
00:03:12.210 --> 00:03:17.700
the direction of the bending correlated
with the position of the amyloplasts.

43
00:03:19.610 --> 00:03:23.090
So, does this prove unequivocally
that the amyloplasts are what's

44
00:03:23.090 --> 00:03:25.020
sensing the gravity?

45
00:03:25.020 --> 00:03:29.617
Not unequivocally, but
it gives quite strong support, I think.

46
00:03:29.617 --> 00:03:32.290
Let's talk about a few other
experiments that were done.

47
00:03:32.290 --> 00:03:37.712
If you wanted to test gravitropism and
the effect on amylopast, maybe the best

48
00:03:37.712 --> 00:03:43.400
place to test such a hypothesis would be
in conditions where there's no gravity.

49
00:03:45.260 --> 00:03:48.440
Now, where can we find conditions with
no gravity to carry out an experiment?

50
00:03:49.950 --> 00:03:52.660
Well actually, we do have a place that
we could do experiments like that and

51
00:03:52.660 --> 00:03:54.429
it's called
the International Space Station.

52
00:03:55.540 --> 00:04:00.782
Actually, since the 1980s, plants
have been flown into space, even with

53
00:04:00.782 --> 00:04:05.790
the space shuttle to test the effect of
gravity on plant growth and I'm going to

54
00:04:05.790 --> 00:04:11.291
show you here one experiment that was
done in 2009 that was used with rapeseed.

55
00:04:11.291 --> 00:04:12.516
And as you can see here,

56
00:04:12.516 --> 00:04:16.820
these are rapeseeds which were
germinated in space for two days.

57
00:04:16.820 --> 00:04:19.915
It's actually not in the absence in
gravity, we call it microgravity.

58
00:04:19.915 --> 00:04:24.510
Because even in space, there's still very,
very small gravitational pull.

59
00:04:24.510 --> 00:04:27.336
We could see that their roots,
don't know where to go.

60
00:04:27.336 --> 00:04:28.280
They're going to the side.

61
00:04:28.280 --> 00:04:28.837
They're going up.

62
00:04:28.837 --> 00:04:30.090
They're going down.

63
00:04:30.090 --> 00:04:34.750
There's no gravitational signal for
them to know where down is.

64
00:04:34.750 --> 00:04:39.958
Now in the same experiment up on the space
station, they put these rapeseed

65
00:04:39.958 --> 00:04:45.370
seeds on a centrifuge similar to what
Knight did back in the 19th century.

66
00:04:45.370 --> 00:04:49.130
Of course, without a water wheel,
this is on an electric centrifuge.

67
00:04:49.130 --> 00:04:54.030
When these seedlings are put on
the centrifuge, the centrifugical force

68
00:04:54.030 --> 00:04:59.520
emulates a gravitational force and
then the seedlings, the roots grow down.

69
00:05:00.720 --> 00:05:05.070
So this is clearly showing,
I think that the seedlings need gravity

70
00:05:05.070 --> 00:05:07.720
to know where the root is down and
where it's not down.

71
00:05:09.260 --> 00:05:11.870
So, whats happening here in
space to their amyloplast?

72
00:05:13.720 --> 00:05:17.520
When you look at the amyloplasts
in the absence of gravity,

73
00:05:17.520 --> 00:05:21.010
the amyloplasts are distributed
evenly throughout the cytoplasm.

74
00:05:21.010 --> 00:05:24.209
They don't fall to the bottom.

75
00:05:24.209 --> 00:05:28.241
But when you put them in the centrifuge,
the amyloplasts go to what would be

76
00:05:28.241 --> 00:05:32.220
the bottom side of the cell and that
helps the plant know which way to grow.

77
00:05:34.570 --> 00:05:37.684
The last approach I want to
talk about is genetics.

78
00:05:37.684 --> 00:05:42.675
We cited in a few of the earlier
experiments in our earlier lectures that

79
00:05:42.675 --> 00:05:47.501
we can utilize mutants in arabidopsis
to understand plant senses and

80
00:05:47.501 --> 00:05:51.742
it ends up just like there
are plants that are blind to light,

81
00:05:51.742 --> 00:05:55.420
scientists have isolated
mutants that are blind.

82
00:05:55.420 --> 00:05:56.797
That's actually not the right word.

83
00:05:56.797 --> 00:06:02.670
That are agravitropic, mutants that have
lost the ability to respond to gravity.

84
00:06:02.670 --> 00:06:06.440
I just want to show you one mutant here,
which I find fascinating.

85
00:06:06.440 --> 00:06:13.908
Because this mutant, when you put it on
its side, it stems do know how to grow up.

86
00:06:13.908 --> 00:06:17.870
But if you look at its roots,
its roots still know how to grow down.

87
00:06:19.810 --> 00:06:23.600
So, what we learn from this is
that some of the mechanisms

88
00:06:23.600 --> 00:06:28.100
of gravitropism may be different in
the stems than they are in the roots.

89
00:06:28.100 --> 00:06:30.550
Now, I'm not going to go
into all of the biology.

90
00:06:30.550 --> 00:06:33.420
Soon, you'll find some
links to the articles that

91
00:06:33.420 --> 00:06:38.380
are explaining what's going on, but
the take home messages that we can find

92
00:06:38.380 --> 00:06:43.260
mutants that are agravitropic for
both roots and for shoots.

93
00:06:43.260 --> 00:06:46.410
Mutants that are agravitropic for
just shoots or

94
00:06:46.410 --> 00:06:48.930
mutants that are agravitropic for
just roots.