Reduction of indole-3-acetic acid methyltransferase activity compensates for high-temperature male sterility in Arabidopsis

Abstract

High temperature is a general stress factor that causes a decrease in crop yield. It has been shown that auxin application reduces the male sterility caused by exposure to higher temperatures. However, widespread application of a hormone with vast effects on plant physiology may be discouraged in many cases. Therefore, the generation of new plant varieties that locally enhance auxin in reproductive organs may represent an alternative strategy. We have explored the possibility of increasing indole-3-acetic acid (IAA) in ovaries by reducing IAA methyltransferase1 (IAMT1) activity in Arabidopsis thaliana. The iamt1 mutant showed increased auxin signalling in funiculi, which correlated with a higher growth rate of wild-type pollen in contact with mutant ovaries and premature ovule fertilization. While the production of seeds per fruit was similar in the wild type and the mutant at 20 °C, exposure to 29 °C caused a more severe decrease in fertility in the wild type than in the mutant. Loss of IAMT1 activity was also associated with the production of more nodes after flowering and higher tolerance of the shoot apical meristem to higher temperatures. As a consequence, the productivity of the iamt1 mutant under higher temperatures was more than double of that of the wild type, with almost no apparent trade-off.

Keywords: auxin; fertility; pollen.

Figure 1

IAMT1 is expressed during pistil development. (a) Expression of IAMT1 in flower tissues according to transcriptomic data from AtGenExpress Database. Dark grey color indicates no data available. (b) Beanplot view normalized expression values of the first 30 IAMT1 co‐expressed genes in floral tissues.

Figure 2

Faster pollen tube growth in iamt1 ovaries. Progress of pollen tubes was visualized with callose (a, b) and GA20ox1::GUS (c, d) staining of wild‐type and iamt1 ovaries 6 h after manual application of wild‐type pollen. (e) Pollen tube growth rate determined after pollination of wild‐type and iamt1 mutant ovaries with wild‐type pollen. Arrowheads in (a) and (b) mark the point reached by pollen tubes. Scale bars in (a) and (c) are 0.4 and 0.2 mm, respectively.

Figure 3

Fertilization occurs earlier in iamt1 mutant ovaries. Pictures of representative embryo development stages in wild‐type (a–c) and iamt1 ovaries (d–f) 2, 4 and 6 days after manual application of the corresponding pollen. (g) Quantification of embryo stages as a function of time after manual pollination (at least 250 embryos were analysed per time point).

Figure 4

Increased seed production by iamt1 fruits at higher temperature. (a) Representation of siliques measured in our approach (numbers 4–8). (b) Silique fertility phenotype discrimination threshold set up based in a seeds‐per‐silique dot‐blot of wild‐type and iamt1 plants grown at 20 °C. (c) Seeds per silique measurements of wild‐type and iamt1 plants grown under different temperature regimes. (d) Distribution of silique fertility phenotypes from plants grown at different temperature conditions. ‘Unfertilized’ stands for siliques with no mature seeds. Asterisk indicates P < 0.01 (Student's t‐test).

Figure 5

Total productivity of iamt1 plants. Node (a) and silique (b) formation rates. Relative variation in total number of seeds (c) and siliques (d) produced by wild‐type and iamt1 plants grown at different conditions compared to 20 °C grown wild‐type production. To calculate relative values, total number of seeds (or siliques) was counted from a total of nine plants per genotype and condition as explained in Experimental procedures, the mean values were obtained in each case, and then all values were referred to that of wild type at 20 °C. One and two asterisks indicate P < 0.01 and P < 0.001, respectively (Student's t‐test).

Figure 6

Phenotypic characterization of the iamt1 mutant. (a) Picture of the plants. (b) Size of leaves. (c) Chlorophyll levels. (d) Flowering time in long days. Bar length, 1 cm.