**3. Conclusions**

*Plant Science - Structure, Anatomy and Physiology in Plants Cultured in Vivo and in Vitro*

tion of the G1 to S transition.

**2.3 Other phytohormones**

in root growth and development.

ment of LR in osmotic/salt stress condition.

BRs interact with auxin to promote LR development.

emerged roots in deepwater rice.

the regulation of JA homeostasis.

through downregulating *PIN* expression and preventing the establishment of auxin gradient in LR founder cells [77]. Mutants with reduced cytokinin level or deficient cytokinin signaling increased the number of LRs [45, 58, 60, 78], while cytokinin treatment suppresses LR initiation and development [77, 79, 80]. Li et al. [80] reported that cytokinins inhibit LR initiation by blocking the cell cycle of pericycle founder cell at G2 to M transition phase while promoting LR elongation by stimula-

Through mutant analysis Chang et al. [81] showed that cytokinin biosynthesis genes *IPT3* and *IPT5* and all three cytokinin receptor genes *AHK2*, *AHK3*, and *CRE1/AHK4* act redundantly during LR initiation, and early stages of lateral root primordia (LRP) formation are particularly cytokinin sensitive. They suggest that cytokinin may serve as a positional signal for new LRP formation. In rice, ERF3 interacts with WOX11 to promote crown root initiation and elongation by regulating the cytokinin-responsive gene *RR2* [82]. Cytokinin has also been shown to modulate LR formation by mediating environmental cues. Jeon et al. [83] showed that CYTOKININ RESPONSE FACTOR 2 (CRF2) and CRF3 encoding APETALA2

Other phytohormones, like abscisic acid (ABA), gibberellic acid (GA), brassinosteroid (BR), jasmonic acid (JA), ethylene, and strigolactone (SL), also participate

Signora et al. [84] showed that ABA plays an important role in mediating the effects of nitrate on LR formation in *Arabidopsis*. Brady et al. [85] reported that *ABSCISIC ACID INSENSITIVE 3* (*ABI3*) is involved in auxin signaling and LR development. De Smet et al. [86] reported that ABA application leads to the inhibition of LR development immediately after the emergence of the LRP from the parent root and prior to the activation of the LR meristem in an auxin-independent manner. Shkolnik-Inbar and Bar-Zvi [87] showed that *ABI4*, which encodes an ABA-regulated AP2 domain transcription factor, mediates ABA and cytokinin inhibition of LR formation through the reduction of PAT. Ding et al. [88] reported that ABA signaling and auxin homeostasis regulate WRKY46 to modulate the develop-

Hansen [89] reported on the GA-mediated light dependent promotion and inhibition of AR formation. Through mutant analysis, Yaxley et al. [90] showed that GA is important for normal root elongation in pea. Fu and Harberd [91] showed that auxin regulates root growth through GA-mediated DELLA protein destabilization. Steffens et al. [92] showed that GA is ineffective on its own but acts synergistically with ethylene to promote the number of penetrating roots and the growth rate of

BR is a positive regulator of root development [93]. Bao et al. [94] showed that

JA, a crucial plant defense hormone, also participated in the regulation of root development. Raya-González et al. [95] observed that low concentrations of JA inhibited PR growth through an auxin-independent manner and promoted LR formation auxin-dependently, and JA receptor COI1 is involved in JA-induced LR formation and LR positioning. Cross-talk between JA and auxin has been frequently reported. JA has been reported to be implicated in YUC9-mediated auxin biosynthesis in wounded leaves in *Arabidopsis* [96]. Cai et al. [97] also reported a crosstalk between JA and auxin biosynthesis during LR formation mediated by ERF109. Gutierrez et al. [98] showed that auxin controls *Arabidopsis* AR initiation through

transcription factors regulate *Arabidopsis* LR initiation under cold stress.

**24**

The root system of higher plants is modified by intrinsic developmental signals and diverse environmental cues. Both the internal and the external signals converged on phytohormones to regulate the formation of a highly plastic and adaptive RSA, which sustains the growth of plants even in adverse conditions. Several lines of evidences suggest that cross-talks among different phytohormones are essential for the regulation of root development, and auxin plays a central role in these processes. Although auxin and cytokinin as the key regulators of root development have been extensively studied, the roles of other phytohormones still need to be further characterized to give us a full view of plant root development.

### **Conflict of interest**

The authors declare no conflict of interest.

#### **Acronyms and abbreviations**

