organs will tend to grow at a defined angle from the vertical (called gravity set point
angle or GSA) after emerging from the primary organ. This distinct growth behavior,
termed plagiogravitropism, is pushed to its extreme in organs that grow horizontally (di-
agravitropism). For instance, stolons and rhizomes grow horizontally, exploring a plant’s
neighborhood and colonizing it through vegetative reproduction at their nodes.
Interestingly, GSA can also vary depending on the environmental status of a growing
organ. As discussed in Chapters 5 and 6, touch or lateral humidity gradients can tem-
porarily inhibit gravitropism, thereby allowing a root to change its growth pattern to reach
better environments despite the influence of gravity (Massa and Gilroy 2003; Takahashi
et al. 2003). Similarly, light can modulate gravitropism in addition to promoting distinct
tropic responses called phototropism (Kiss et al. 2002; Kiss et al. 2003).
The developmental stage of a plant organ will also influence its GSA. For instance, the
peanut gynophore will switch from negative (upward) to positive (downward) gravitro-
pism upon fertilization, driving the developing fruit into the sand where it has to be lo-
cated in order complete its developmental and maturation program (Moctezuma and
Feldman 1998).
Hence, not only is it necessary for a plant organ to perceive gravity, it is also impor-
tant for that organ to transduce the corresponding vectorial information into a defined
growth pattern that will ultimately allow it to reach environments that are better suited for
plant growth and development. How is this accomplished? Although new information
hints at some of the molecular mechanisms that allow distinct directional stimuli (such as
touch and humidity gradients) to affect gravitropism in roots (discussed in Chapters 5 and
6), very little is known about how a specific GSA is actually set for a defined organ based
on its developmental program or surrounding environment. Yet, the tools used in the
study of organ growth behavior in plants are becoming increasingly sophisticated, such
that we can anticipate a future that will shed light on the regulatory mechanisms that tune
gravity signal transduction to the characteristics of an organ’s endogenous and external
environment, thereby modulating overall growth and morphogenesis.
2.4 Acknowledgments
We thank John Stanga, Laura Vaughn, Jessica Will, Elison Blancaflor, and Simon Gilroy
for critical comments on this manuscript. This work was supported by grants from NSF,
NASA, UW College of Agriculture and Life Sciences Hatch funds, and UW Graduate
School Grant-in-Aid to PHM.
2.5 Literature cited
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