Abstract
Plants respond to and resist gravitational acceleration, but the mechanism of signal perception in the response is unknown. We studied the role of MCA (mid1-complementing activity) proteins in gravity perception by analyzing the expression of the MCA1 and MCA2 genes, and the growth of hypocotyls of mca mutants, under hypergravity conditions in the dark. An MCA1 promoter::GUS fusion reporter gene construct (MCA1p::GUS) and MCA2p::GUS were expressed almost universally in etiolated seedlings. Under hypergravity conditions, the expression levels of both genes increased compared with that under the 1 g condition, and remained higher, especially in the basal supporting region. On the other hand, mca-null and MCA-overexpressing seedlings showed normal growth under the 1 g condition. Hypergravity suppressed elongation growth of hypocotyls, but this effect was reduced in hypocotyls of mca-null mutants compared with the wild type. In contrast, MCA-overexpressing seedlings were hypersensitive to increased gravity; suppression of elongation growth was detected at a lower gravity level than that in the wild type. These results suggest that MCAs are involved in the perception of gravity signals in plants, and may be responsible for resistance to hypergravity.
Highlights
Plants are exposed to a variety of mechanical stresses such as wind and gravity, and they have developed efficient mechanisms to respond to these stressors over evolutionary time, in particular after their emergence on land
The expression patterns of MCA1 and MCA2 in etiolated seedlings grown at 300 g were analyzed using an MCA1 promoter::GUS fusion reporter gene construct (MCA1p::GUS) and MCA2p::GUS
MCA1p::GUS was expressed in the cotyledons and hypocotyls, regardless of age or gravitational conditions (Figure 1)
Summary
Plants are exposed to a variety of mechanical stresses such as wind and gravity, and they have developed efficient mechanisms to respond to these stressors over evolutionary time, in particular after their emergence on land. We have previously studied the mechanisms of plant responses to gravitational acceleration, mainly with basipetal hypergravity produced by centrifugation [1,2]. Hypergravity generally suppresses elongation growth and increases the rigidity of plant organs. The cell wall is an important source of mechanical strength in plants; it is likely that the properties of the cell wall are modified under hypergravity conditions. Hypergravity induces changes in cell wall metabolism, such as accumulation of its constituents, polymerization of certain matrix polysaccharides due to breakdown suppression, stimulation of cross-link formation, and modifications to the wall environment, which lead to increased cell wall rigidity [3]. Hypergravity promotes the lateral expansion of plant organs, with concomitant suppression of elongation growth [4]. Hypergravity induces reorientation of Plants 2020, 9, 590; doi:10.3390/plants9050590 www.mdpi.com/journal/plants
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