**6. Hd3a homolog in plant development**

The role of Hd3a as flowering mobile signal in rice and *Arabidopsis* leads to investigation of its function in another plants. The diverse role of FT was identified in many plants. FT protein involved in vernalization, stomatal opening, dormancy and tuberization [45–48]. Navarro et al. [49] provided a new insight in understanding the florigen as graft-transmissible signal and designated its role as tuberi. Hd3a homolog were overexpressed in potato under noninductive long-day condition and resulting in tuberization and flower development. A grafting experiment using tobacco florigen as scion and potato as rootstock to test the hypothesis confirmed that FT proteins were transmitted from scion to rootstock and induced tuberization. This experiment suggests that FT/Hd3a is a strong tuberization inducer.

The flowering and tuberization pathway are regulated in different pathways, indicating that FT has many homolog in potato. This hypothesis was confirmed by Navarro et al. [49]. In potato, there are three homolog of FT. StSP3A regulate the tuber formation, StSP3D regulate the flowering, and StSP5G as the repressor of tuberization. Expression of *StSP6A* gene is strong in leaves and stolon of short-day induced plants. Interestingly, 30-minute night break was reported to inhibit tuberization. The *StSP3D* expression is necessary for flowering induction. However, this gene is not required for normal tuberization. Interestingly, the StSP5G is an antagonist for StSP6A. The expression of StSP5G was high in long-day condition [47]. The role of Hd3a/FT protein in potato flowering and tuberization is well understood. This photoperiodic pathway is conserved in potato, rice, and *Arabidopsis*.

Recent study in onion tuberization reveals that its induction is controlled by Hd3a/FT homolog in the photoperiodic pathway. Onion is a biennial plant that forms tuber during the summer and flower after the winter in the next summer. There are many Hd3a/FT homologs found in onion and the pattern is the same with potato. There are Hd3a/FT homologs that regulate the tuber/bulb formation (AcFT1 and AcFT4) and flowering (AcFT2) [50]. As seen in potato tuberization, AcFT4 in onion has the same role as StSP5G in potato that inhibits onion tuber formation; while AcFT1 is a strong inducer for tuberization (tuberigen) under long-day condition. This finding suggests that Hd3a/FT has a versatile role in plant development.

Tuberigen and florigen are known to be mobile/graft-transmissible signals. Several experiments showed that this signal is interchangeable between species in Solanaceae family (potato/tobacco, and potato/tomato) and can induce tuberization of potato and onion, but cannot in other plant species. As example, the ability to form a tuber is limited in potato species among Solanaceae family. Due to those fact, there must be an unknown mechanism present in the tuberization process in potato. The underlying mechanism that involves FT roles needs further investigation.

Another Hd3a role in rice as mobile signal that induced branching was reported by Tsuji et al. [18]. Their findings suggest that Hd3a was moved along the xylem to promote branching in rice lateral meristem. The formation of florigen activation complex (FAC) for the developmental process other than flowering is not well understood. The experiment proved that Hd3a protein was transported to lateral meristem in the lateral bud and its transport is required for branching. This finding suggests that Hd3a has the function besides flowering.

Our work in local black rice namely "Cempo Ireng" which has very long harvest period also reveal that Hd3a gene is conserved. The regulation of flowering pathway of rice in short- and long-day condition is well understood. However, when rice are planted in neutral day condition such as tropical condition, the photoperiodic pathway might be affected. Our result suggests that Hd3a, which normally regulate flowering under short-day condition, is expressed in neutral day condition earlier compared to its homolog RFT1, which is responsible for flowering induction in long-day condition. In "Cempo Ireng," two flowering genes are expressed. The expression pattern of both genes are redundant to induce the flowering under neutral day condition. RFT1 and Hd3a are known to have different induction pathways. Our result suggests that RFT1 and Hd3a may have independent pathway in rice flowering regulation under neutral day condition according to their expression pattern (data not shown). The Hd3a/RFT1 interactor of black rice that involved in flowering induction is of interest for future study to understand the flowering mechanism under neutral day condition and shortening the harvest period of black rice "Cempo Ireng."
