Expression of auxin transporter genes in flax (Linum usitatissimum) fibers during gravity response

Abstract

Gravitropism is an adaptive reaction of plants associated with the ability of various plant organs to be located and to grow in a certain direction relative to the gravity vector, while usually the asymmetric distribution of the phytohormone auxin is a necessary condition for the gravitropical bending of plant organs. Earlier, we described significant morphological changes in phloem fibers with a thickened cell wall located on different sides of the stem in the area of the gravitropic curvature. The present study is the first work devoted to the identification of genes encoding auxin transporters in cells at different stages of development and during gravity response. In this study, the flax genes encoding the AUX1/LAX, PIN-FORMED, PIN-LIKES, and ABCB auxin transporters were identified. A comparative analysis of the expression of these genes in flax phloem fibers at different stages of development revealed increased expression of some of these genes at the stage of intrusive growth (LusLAX2 (A, B), LuxPIN1-D, LusPILS7 (C, D)), at the early stage of tertiary cell wall formation (LusAUX1 (A, D), LusABCB1 (A, B), LusABCB15-A, LusPIN1 (A, B), LusPIN4-A, and LusPIN5-A), and at the late stage of tertiary cell wall development (LusLAX3 (A, B)). It was shown that in the course of gravitropism, the expression of many genes, including those responsible for the influx of auxin in cells (LusAUX1-D), in the studied families increased. Differential expression of auxin transporter genes was revealed during gravity response in fibers located on different sides of the stem (upper (PUL) and lower (OPP)). The difference was observed due to the expression of genes, the products of which are responsible for auxin intracellular transport (LusPILS3, LusPILS7-A) and its efflux (LusABCB15-B, LusABCB19-B). It was noted that the increased expression of PIN genes and ABCB genes was more typical of fibers on the opposite side. The results obtained allow us to make an assumption about the presence of differential auxin content in the fibers of different sides of gravistimulated flax plants, which may be determined by an uneven outflow of auxin. This study gives an idea of auxin carriers in flax and lays the foundation for further studies of their functions in the development of phloem fiber and in gravity response.

Keywords: Linum usitatissimum L.; auxin transport; fiber; flax; gene expression; gravitropism.

Table 1.. List and some characteristics of LusPIN and LusPILS sequences

The corrected sequences are highlighted in bold. * Incomplete sequences. Hereinafter: LUS – Linum usitatissimum; AT – Arabidopsis thaliana; AA – number of amino acids; Mw – molecular weight, kDa; pI – isoelectric point; TMH – number of transmembrane domains.

Fig. 1.. Phylogenetic tree for amino acid sequences PF03547 in A. thaliana and L. usitatissimum

Maximum Likelihood method, model Le_Gascuel_2008 (LG+G). Bootstrap support 1000. Performed in the MEGA 7 program. The red marker indicates the sequences of A. thaliana.

Fig. 2.. Expression of LusPIN/PILS with an increased level of expression only in intrusively growing fibers.

Here and in Fig. 3–9: iFIBa – intrusively growing fibers; tFIBa – the early stage of tertiary cell wall (TCW) formation; tFIBb – the late stage of TCW formation. 8, 24, 96 hours – the time of fixation of the samples after the stem inclination; TGR – total gene reads; PUL – pulling side; OPP – opposite to the PUL-side. Without dividing – the plants were not subjected to gravistimulation, and the stem was not divided into PUL and OPP.

Fig. 3.. Expression of LusPIN1-A, B, C, LusPIN4-A, B, LusPILS2-B in flax fibers under normal conditions and in gravity response.

Fig. 4.. Expression of LusPIN5-A and LusPILS3 in flax fibers under normal conditions and in gravity response.

Fig. 5.. Expression of LusPIN8-B, LusPILS1-B, LusPILS7-A in flax fibers under normal conditions and in gravity response.

Table 2.. List and some characteristics of LusAUX1/LAX sequences

Fig. 6.. Expression of LusLAX2-A, B in fibers under normal conditions and in gravity response.

Fig. 7.. Expression of LusLAX3-A, and B in flax fibers under normal conditions and in gravity response.

Fig. 8.. Expression of LusAUX1-A, B, C, D in flax fibers under normal conditions and in gravity response.

Table 3.. List and some characteristics of LusABCB sequences

* The sequence may be incorrect.

Fig. 9.. Expression of some LusABCB1, 15, and 19 gene isoforms in fibers under normal conditions and in gravity response.