Roles of MAP65-1 and BPP1 in Gravity Resistance of Arabidopsis hypocotyls

The present experiment aims to clarify the roles of 65 kDa microtubule-associated protein-1 (MAP65-1) and basic proline-rich protein1 (BPP1), which are involved in the maintenance of transverse microtubule orientation, in gravity resistance, using green fluorescent protein (GFP)-expressing Arabidopsis lines. Hypergravity at 300 G inhibited elongation growth and promoted lateral expansion of epidermal cells in the subapical region of hypocotyls in GFP-MAP65-1 line expressing by native promoter and BPP1-GFP line expressing by a constitutive cauliflower mosaic virus 35S promoter. In BPP1-GFP line, hypergravity showed smaller effects on modification of growth anisotropy than wild type. Also, hypergravity induced reorientation of cortical microtubules from transverse to longitudinal directions in both lines. However, in BPP1-GFP line, hypergravity showed smaller effects on reorientation of cortical microtubules. When the expression levels of MAP65-1 were determined by analyzing GFP fluorescence in hypocotyls of GFP-MAP65-1 line, hypergravity decreased the levels of MAP65-1 in the subapical region, where hypergravity modified growth anisotropy and orientation of cortical microtubules. These results indicate that the regulation of levels of MAP65-1 and BPP1 is involved in the hypergravity-induced reorientation of cortical microtubules, which may lead to modification of growth anisotropy, thereby developing a tough body against the gravitational force in Arabidopsis hypocotyls.