Factors responsible for deep-sowing tolerance in wheat seedlings: varietal differences in cell proliferation and the co-ordinated synchronization of epidermal cell expansion and cortical cell division for the gibberellin-mediated elongation of first internodes.

BACKGROUND AND AIMS: A wheat cultivar, Triticum aestivum 'Hong Mang Mai', shows tolerance to deep-sowing conditions by extreme elongation of the first internode, likely mediated by the gibberellin (GA) response. To understand factors involved in the response of this deep-sowing-tolerant cultivar, cell expansion and division that confer elongation on the first internodes of wheat seedlings were investigated. METHODS: The lengths and numbers of epidermal and cortical cells of the first internodes in three wheat cultivars were measured. These parameters were compared in wheat seedlings treated with gibberellin A(3) (GA(3)) or an inhibitor of GA biosynthesis, uniconazole. KEY RESULTS: The varietal differences in the elongation of the first internodes were due to differences in cell numbers resulting from the different abilities of cell division, but not cell expansion. In seedlings treated with GA(3), the first internode of 'Hong Mang Mai' was 2-fold longer than the control. The GA-stimulated elongation of the first internodes was attributed to 2-fold increases in the number of cortical cells and length of epidermal cells. The different GA-responses observed in these two tissues were also detected in other cultivars, although the response was much lower than that noted in 'Hong Mang Mai'. The seedlings treated with uniconazole exhibited reduced numbers of cortical cells and reduced lengths of epidermal cells, with both of these effects being more pronounced in 'Hong Mang Mai'. CONCLUSIONS: The deep-sowing-tolerant cultivar 'Hong Mang Mai' is able to elongate the first internode to a greater degree due to enhanced cell division and a heightened response to GA. In addition, cell expansion in the epidermis and cell division in the cortex are synchronized for the elongation of the first internodes. In response to GA, this well-co-ordinated synchronization yields the rapid elongation of the first internodes in wheat seedlings.

Shoot circumnutation and winding movements require gravisensing cells.

Circumnutation and winding in plants are universal growth movements that allow plants to survive despite their sessile nature. However, the detailed molecular mechanisms controlling these phenomena remain unclear. We previously found that a gravitropic mutant of Japanese morning glory (Pharbitis nil or Ipomoea nil), Shidare-asagao (weeping), is defective not only in circumnutation but also in the winding response. This phenotype is similar to that of the Arabidopsis SCARECROW (SCR) mutant. We therefore investigated whether morning glory SCR (PnSCR) is involved in the weeping phenotype. We found that one amino acid was inserted into the highly conserved VHIID motif in weeping-type PnSCR; this mutation caused abnormal endodermal differentiation. We introduced either the mutant or WT PnSCR into Arabidopsis scr mutants for complementation tests. PnSCR of the WT, but not of weeping, rescued the shoot gravitropism and circumnutation of scr. These results show that both the abnormal gravitropism and the circumnutation defect in weeping are attributable to a loss of PnSCR function. Thus, our data show that gravisensing endodermal cells are indispensable for shoot circumnutation and the winding response and that PnSCR is responsible for the abnormal phenotypes of weeping.

Effect of gravity on apical dominance in Pharbitis nil.

When the upper part of main shoot of morning glory (Pharbitis nil) is gently bent down, lateral bud on the bending region is released from apical dominance and starts to elongate. But, clinorotating the bending shoots prevents the release of the lateral bud from apical dominance. These results suggest that gravity affects apical dominance in morning glory. Here we verified the gravity-regulated apical dominance by using a weeping morning glory defective in gravitropic response due to abnormal differentiation of endodermis. That is, bending main shoot of the weeping morning glory hardly caused the lateral bud to elongate. In addition, decapitation of apical bud released the lateral bud from apical dominance, and exogenous auxin applied to the cut surface of the decapitated stem was inhibitory to the outgrowth of the lateral bud in the wild type. However, the effect of auxin was much less in the weeping morning glory. Thus, apical dominance of the weeping morning glory was weaker and less influenced by gravity than that of the wild type, which could occur due to abnormal differentiation of endodermis required for graviperception.

Spiral growth and cell wall properties of the gibberellin-treated first internodes in the seedlings of a wheat cultivar tolerant to deep-sowing conditions.

The Hong Mang Mai wheat cultivar is tolerant to deep-sowing conditions because it has an elongated first internode that is sensitive to gibberellin (GA3). The cells in the GA-treated first internodes were approximately 4.2 mm long, twice as long as the untreated Hong Mang Mai first internode cells. The elongation of the first internode of Hong Mang Mai, particularly when treated with GA3, was accompanied by remarkable spiral growth. In contrast, the first internodes of the GA-insensitive cultivar Norin 10 did not exhibit GA3-induced elongation or spiral growth. The walls of the first internode cells of GA3-treated Hong Mang Mai seedlings showed increased extensibility and higher (1-->3), (1-->4)-beta-d-glucanase activity, autolysis and glucan contents than the cell walls of untreated Hong Mang Mai first internodes. The changes in the cell wall extensibility due to GA3 treatment correlated strongly with the GA3-induced changes in cell wall glucan content, autolysis, and glucanase activity. GA3-treated Hong Mang Mai seedlings showed elevated expression of Glucanase EI gene in the first internode compared to GA3-treated Norin 10. Thus, GA aids first internode elongation in Hong Mang Mai by enhancing glucan turnover and thus increasing cell wall loosening. The spiral growth of the first internode also helps the plant elongate against soil resistance, thereby promoting the deep-sowing tolerance of this cultivar.